Crotty, S.M., Pinton, D., Canestrelli, A., Fischman, H., Ortals, C., Dahl, N.R., Williams, S.L., Bouma, T.J. and Angelini, C. 2023. Faunal engineering stimulates landscape-scale accretion in southeastern US salt marshes. Nature Communications. 14(881).
The fate of coastal ecosystems depends on their ability to keep pace with sea-level rise—yet projections of accretion widely ignore effects of engineering fauna. Here, we quantify effects of the mussel, Geukensia demissa, on southeastern US saltmarsh accretion. Multi-season and -tidal stage surveys, in combination with field experiments, reveal that deposition is 2.8-10.7-times greater on mussel aggregations than any other marsh location. Our Delft-3D-BIVALVES model further predicts that mussels drive substantial changes to both the magnitude (±200,000 mussels and find that this faunal engineer drives far greater changes to relative marsh accretion rates than predicted (±>0.4 cm·yr−1). Thus, we highlight an urgent need for empirical, experimental, and modeling work to resolve the importance of faunal engineers in directly and indirectly modifying the persistence of coastal ecosystems globally.
Bice, K., Sheldon, J.E., Schalles, J.F., Alber, M. and Meile, C. 2023. Temporal patterns and causal drivers of aboveground plant biomass in a coastal wetland: insights from time-series analyses . Frontiers of Marine Sciences. 10(1130958).
Salt marshes play a crucial role in coastal biogeochemical cycles and provide unique ecosystem services. Salt marsh biomass, which can strongly influence such services, varies over time in response to hydrologic conditions and other environmental drivers. We used gap-filled monthly observations of Spartina alterniflora aboveground biomass derived from Landsat 5 and Landsat 8 satellite imagery from 1984-2018 to analyze temporal patterns in biomass in comparison to air temperature, precipitation, river discharge, nutrient input, sea level, and drought index for a southeastern US salt marsh. Wavelet analysis and ensemble empirical mode decomposition identified month to multi-year periodicities in both plant biomass and environmental drivers. Wavelet coherence detected cross-correlations between annual biomass cycles and precipitation, temperature, river discharge, nutrient concentrations (NOx and PO43–) and sea level. At longer periods we detected coherence between biomass and all variables except precipitation. Through empirical dynamic modeling we showed that temperature, river discharge, drought, sea level, and river nutrient concentrations were causally connected to salt marsh biomass and exceeded the confounding effect of seasonality. This study demonstrated the insights into biomass dynamics and causal connections that can be gained through the analysis of long-term data.
Hawman, P., Mishra, D. and O’Connell, J.L. 2023. Dynamic emergent leaf area in tidal wetlands: Implications for satellite-derived regional and global blue carbon estimates. Remote Sensing of Environment. 290(15).
The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate highlights the importance of blue carbon in tidal wetlands in combating climate change. In this study, we highlight the uncertainty associated with leaf area index (LAI) estimations in tidal wetlands, specifically salt marshes, a key vegetation parameter for productivity models and Earth System Models (ESM). LAI, derived from satellite reflectance data, is linked to atmospheric carbon exchange and gross primary production (GPP) across vegetative ecosystems. However, estimating salt marsh LAI is challenging because canopy height and density vary across short distances, and tidal flooding alters the atmosphere-exposed leaf area, hereafter called emergent leaf area index (ELAI), at short time scales. Further, in tidal wetlands dominated by species such as Spartina alterniflora, canopy height and density vary across short distances. We present a novel approach for measuring spatiotemporal dynamics in tidal wetland ELAI. We modeled ELAI from vertical LAI profiles and created spatial estimates across tidal periods. We then linked ELAI with eddy covariance carbon (C) fluxes through footprint modeling and revealed correlations between emergent leaf area and C fluxes. Next, we demonstrated that ELAI can be readily estimated across 10-m spatial scales using Sentinel-2 satellite data, even during high tides (R2 = 0.89; NRMSE = 10%). Finally, we showed a common product, MODIS MYD15A2H, underestimated (20%) LAI during dry conditions but overestimated (7–93%) during high flooding. Dynamic ELAI could reduce uncertainties in satellite-derived global GPP products when developing blue carbon budgets for ecosystems threatened by accelerated sea level rise.
Mao, L., Mishra, D., Hawman, P., Narron, C., O’Connell, J.L. and Cotten, D.L. 2023. Photosynthetic Performance of Tidally Flooded Spartina Alterniflora Salt Marshes. JGR Biogeosciences. 128(3).
Spartina alterniflora has a distinct flood-adapted morphology, and its physiological responses are likely to vary with differences in tidal submergence. To understand these responses, we examined the impacts of tidal inundation on the efficiency of Photosystem II (φPSII) photochemistry and leaf-level photosynthesis at different canopy heights through a combination of in situ chlorophyll fluorescence (ChlF), incident photosynthetically active radiation, and tide levels. Our result showed small declines (7%–8.3%) in φPSII for air-exposed leaves when the bottom canopies were tidally submerged. Submerged leaves produced large reductions (30.3%–41%) in φPSII. Our results suggest that when submerged, PSII reaction centers in S. alterniflora leaves are still active and able to transfer electrons, but only at ∼20% of the typical daily rate. We attribute this reduction in φPSII to the decrease in the fraction of “open” PSII reaction centers (10% of the total) and the stomatal conductance rate caused by the tidal submergence. To our knowledge, this flooding induced leaf-level reduction of φPSII for S. alterniflora in field settings has not been reported before. Our findings suggest that canopy-level φPSII is dependent on the proportion of submerged versus emerged leaves and highlight the complexities involved in estimating the photosynthetic efficiency of tidal marshes.
Craft, C.B. 2023. Tidal Marsh Restoration on Sapelo Island: A Legacy of R.J. Reynolds, Jr., Eugene Odum and the University of Georgia Marine Institute. Special Issue: Wetlands30. Ecological Engineering.
Restoration of tidal marshes throughout the 20th century have attempted to bring back important functions of natural tidal systems. In this study, vertical accretion, organic carbon (C) sequestration, and nitrogen burial were compared between a natural, never diked tidal salt marsh and a hydrologically restored tidal salt marsh on Sapelo Island, Georgia to examine the impacts of restoration years later. 64 years after hydrologic restoration in 1956, the restored marsh studied had higher rates of accretion based on 137Cs and 210Pb (4.8–5.1 mm/yr), C sequestration (118–125 g C/m2/yr) and N burial (8.3–8.8.g N/m2/yr) than the never diked marsh (2.9–3.4 mm/yr, 75–85 g C/m2/yr, 4.8–5.6 g N/m2/yr).
Since maximum 137Cs deposition in 1964, approximately 30 cm of accretion has occurred in the restored marsh while the never diked marsh had approximately 10–30 cm of new soil deposited. The accumulated soil in the restored marsh was comparable to the natural marsh soil in terms of bulk density, percent C and N. However, below this depth, legacy effects from diking could be found through the higher soil bulk density and lower percent organic C and N relative to soils of the natural marsh.
Vertical accretion in the natural marsh appears to be keeping pace with the current rate of sea level rise (SLR) (3.4 mm/yr) while accretion in the restored marsh exceeds SLR as the marsh compensates for subsidence that occurred when it was diked. Under current SLR and accretion rates, ecosystem functions of continual sequestration of C and burial of N will be supported. However, as SLR accelerates, the ability of both marshes to sequester C and bury N will depend on their ability to keep pace. If not, the marshes will eventually convert to mudflats or open water with a concurrent loss of these and other ecosystem services.
Historical ecology has revolutionized our understanding of fisheries and cultural landscapes, demonstrating the value of historical data for evaluating the past, present, and future of Earth’s ecosystems. Despite several important studies, Indigenous fisheries generally receive less attention from scholars and managers than the 17th–20th century capitalist commercial fisheries that decimated many keystone species, including oysters. We investigate Indigenous oyster harvest through time in North America and Australia, placing these data in the context of sea level histories and historical catch records. Indigenous oyster fisheries were pervasive across space and through time, persisting for 5000–10,000 years or more. Oysters were likely managed and sometimes “farmed,” and are woven into broader cultural, ritual, and social traditions. Effective stewardship of oyster reefs and other marine fisheries around the world must center Indigenous histories and include Indigenous community members to co-develop more inclusive, just, and successful strategies for restoration, harvest, and management.
Densities of Orthoptera typically vary greatly over space and time. The most important salt marsh orthopteran on the East Coast of the US is Orchelimum fidicinium, an omnivore that feeds on cordgrass (Spartina alterniflora) and arthropods. We examined spatial (34 sites) and temporal (5 sites, 17 years) variation in O. fidicinium density in coastal Georgia. Sites with considerable adjacent upland habitat had higher densities of O. fidicinium than sites with little adjacent upland. Grasshoppers fed S. alterniflora from both types of sites did not differ in growth rates, ruling out food quality as an explanation. We speculate instead that O. fidicinium require terrestrial habitat for reproduction or escape from predators during extreme high tides. At five sites where O. fidicinium was common, densities varied greatly among years. Regression models indicated that current year plant biomass (three sites) or previous year plant biomass (one site) was the best predictor of O. fidicinium density. Relationships between O. fidicinium and current year plant biomass were typically negative (more grasshoppers in years with lower plant biomass). A possible explanation for this pattern is that plant nutrients may be diluted in years with high plant biomass. We found little evidence that density of animal prey (Prokelisia spp.) or abiotic factors affected O. fidicinium densities. Our study illustrates the value of examining population densities across multiple sites and years, because results from any one site or year would likely have mischaracterized the spatial and temporal distribution of this common salt marsh consumer.
Circular shell rings along the South Atlantic Coast of North America are the remnants of some of the earliest villages that emerged during the Late Archaic (5000–3000 BP). Many of these villages, however, were abandoned during the Terminal Late Archaic (ca 3800–3000 BP). We combine Bayesian chronological modeling with mollusk shell geochemistry and oyster paleobiology to understand the nature and timing of environmental change associated with the emergence and abandonment of circular shell ring villages on Sapelo Island, Georgia. Our Bayesian models indicate that Native Americans occupied the three Sapelo shell rings at varying times with some generational overlap. By the end of the complex’s occupation, only Ring III was occupied before abandonment ca. 3845 BP. Ring III also consists of statistically smaller oysters harvested from less saline estuaries compared to earlier occupations. Integrating shell biochemical and paleobiological data with recent tree ring analyses shows a clear pattern of environmental fluctuations throughout the period in which the rings were occupied. We argue that as the environment became unstable around 4300 BP, aggregation at villages provided a way to effectively manage fisheries that are highly sensitive to environmental change. However, with the eventual collapse of oyster fisheries and subsequent rebound in environmental conditions ca. post-3800 BP, people dispersed from shell rings, and shifted to non-marine subsistence economies and other types of settlements. This study provides the most comprehensive evidence for correlations between large-scale environmental change and societal transformations on the Georgia coast during the Late Archaic period.
Narratives of resilience to sea-level rise too often perpetuate social violence. An abolitionist climate justice praxis necessitates seeing beyond physical inundation to sea-level rise as the sole issue. We argue that sea-level rise is environmental racism, if not always in its racialized outcomes of disproportionate harms, then always in its racialized production of differential value. More than mitigating flood risk when developing Black land futures strategies, to mitigate coastal land loss in the face of rising seas necessitates dismantling “racial regimes of ownership” that are imbued in property relations. Recognizing such, we argue that land futures strategies in the face of rising seas must account for multiple capitalist modes of accumulation but specifically the primitive accumulation process connected to heirs’ property. Here, we argue that racial coastal formations are still underway in how sea-level rise and gentrification are leading to a double dispossession of land and displacing Gullah/Geechee people on Sapelo Island. Although such displacement precipitates loss through harm and violence, in this article we share strategies from abolition ecology praxis to “blockade” these double dispossession processes and, to a degree, even the narrative of loss. To situate the double dispossession narrative within a broader praxis that imagines alternative futures, we till the soil, trace the archives, and tread the marsh and drainage ditches with each other, and in solidarity with other residents, to work toward achieving agricultural revival, property retention, and flood risk mitigation.
Remote sensing can provide critical information about the health and productivity of coastal wetland ecosystems, including extent, phenology, and carbon sequestration potential. Unfortunately, periodic inundation from tides dampens the spectral signal and, in turn, causes remote sensing-based models to produce unreliable results, altering estimates of ecosystem function and services. We created the Flooding in Landsat Across Tidal Systems (FLATS) index to identify flooded pixels in Landsat 8 30-meter data and provide an inundated pixel filtering method. Novel applications of FLATS including inundation frequency and pattern detection are also demonstrated. The FLATS index was developed to identify flooding in Spartina alterniflora tidal marshes. We used ground truth inundation data from a PhenoCam and Landsat 8 pixels within the PhenoCam field of view on Sapelo Island, GA, USA to create the index. The FLATS index incorporates a normalized difference water index (NDWI) and a phenology-related variable into a generalized linear model (GLM) that predicted the presence or absence of marsh flooding. The FLATS equation for predicting flooding is , and we found that a cutoff 0.1 was the optimized value for separating flooded and non-flooded pixel classes. FLATS identified flooded pixels with an overall accuracy of 96% and 93% across training data and novel testing data, respectively. FLATS correctly identified true flooded pixels with a sensitivity of 97% and 81%, across training and testing data, respectively. We established the need to apply FLATS when conducting vegetation time-series analysis in coastal marshes in order to reduce the per-pixel reflectance variations attributed to tidal flooding. We found that FLATS identified 12.5% of pixels as flooded in Landsat 8 tidal marsh vegetation time-series from 2013 to 2020, after traditional quality control and preprocessing steps were conducted, which could then be filtered out or modeled separately in order to conduct remotely sensed vegetation assessments. Therefore, in tidal wetlands, we recommend incorporating FLATS into Landsat 8 preprocessing prior to vegetation analysis. We also demonstrated innovative applications for the FLATS index, particularly in detecting flooding frequency and flooding patterns relevant to the broader biophysical modeling framework, including mapping marsh vulnerability due to fluctuation in inundation frequency. The FLATS index represents advancements in the understanding and application of inundation indices for coastal marshes.
Marshes are valuable intertidal habitats that respond to changes in their environment, and their perimeters can rapidly advance or retreat over time. This study used the analyzing moving boundaries using R (AMBUR) tool kit to measure approximately 70 years of edge change at salt marshes within three Long-Term Ecological Research sites along the U.S. East Coast: Georgia Coastal Ecosystems (GCE), Virginia Coast Reserve (VCR), and Plum Island Ecosystems (PIE). At each site, changes were assessed at the open-fetch marsh outer perimeter as well as throughout interior channels of varying sizes. At the open-fetch marsh outer perimeter, both the PIE and VCR study marshes exhibited significant net retreat, with the fastest rates in areas exposed to high fetch where wave action is strong, whereas the GCE marsh
exhibited significant net advance. Changes in the sinuous interior channels were smaller, with channels often retreating on one edge but were balanced by advance on the opposite bank. When advance and retreat in the interior channels were considered along with the outer perimeter, the GCE and VCR study marshes exhibited dynamic stability in which overall marsh edge showed no significant net change, and the overall rate of marsh retreat at PIE, although still significant with respect to the uncertainty of the analysis, was considerably reduced. This study demonstrates the importance of assessing shoreline changes throughout the marsh, as rates of retreat and advance at the open-fetch marsh perimeter
may differ greatly from those in the interior, and not be indicative of the overall change in marsh edge.
Marine Group II Euryarchaeota (Candidatus Poseidoniales), abundant but yet-uncultivated members of marine microbial communities, are thought to be (photo)heterotrophs that metabolize dissolved organic matter (DOM), such as lipids and peptides. However, little is known about their transcriptional activity. We mapped reads from a metatranscriptomic time series collected at Sapelo Island (GA, USA) to metagenome-assembled genomes to determine the diversity of transcriptionally active Ca. Poseidoniales. Summer metatranscriptomes had the highest abundance of Ca. Poseidoniales transcripts, mostly from the O1 and O3 genera within Ca. Thalassarchaeaceae (MGIIb). In contrast, transcripts from fall and winter samples were predominantly from Ca. Poseidoniaceae (MGIIa). Genes encoding proteorhodopsin, membrane-bound pyrophosphatase, peptidase/proteases, and part of the ß-oxidation pathway were highly transcribed across abundant genera. Highly transcribed genes specific to Ca. Thalassarchaeaceae included xanthine/uracil permease and receptors for amino acid transporters. Enrichment of Ca. Thalassarchaeaceae transcript reads related to protein/peptide, nucleic acid, and amino acid transport and metabolism, as well as transcript depletion during dark incubations, provided further evidence of heterotrophic metabolism. Quantitative PCR analysis of South Atlantic Bight samples indicated consistently abundant Ca. Poseidoniales in nearshore and inshore waters. Together, our data suggest that Ca. Thalassarchaeaceae are important photoheterotrophs potentially linking DOM and nitrogen cycling in coastal waters.
Human-altered shorelines make up approximately 14% of the coastline in the United States, with consequences for marsh ecosystems ranging from altered physical and biological variables, to direct loss of intertidal marsh habitat, to diminished land–sea connectivity. Trophically transmitted parasites that require connectivity between upland host species and marsh host species to complete their complex lifecycles could be particularly sensitive to the effects of shoreline alterations. They can additionally respond to gradients in natural physical and biological factors, including the host communities, that are often sharp at the land–sea ecotone. Across 27 salt marshes over 45 km, we evaluated the effects of environmental variables and three types of land use (undeveloped; single-house adjacent to the marsh with small-scale shoreline armoring; and single-house adjacent to the marsh without shoreline armoring), on infection prevalence and intensity of the trematode Microphallus basodactylophallus in its second intermediate crab host, Minuca pugnax. The first intermediate hosts of M. basodactylophallus are Hydrobiid snail species that are obligate marsh residents, while the definitive hosts are terrestrial rodents and raccoons. Thus, trematode transmission must depend on cross-boundary movement by the definitive hosts. We found that although there was a trend of lower infection prevalence at undeveloped forested sites, there was no significant effect of adjacent land development on infection prevalence or intensity. Instead host, biotic and abiotic factors were correlated with infection; namely, larger M. pugnax had higher prevalence and intensity of M. basodactylophallus, and higher soil moisture and lower density of the ribbed mussel (Geukensia demissa) were associated with increased M. basodactylophallus prevalence. The small, indirect influence of upland development suggests that movement of definite hosts across the ecotone may be largely unaffected. Further, the robust trematode levels signify the ecosystem and the species interactions, upon which its complex lifecycle depends, are largely intact.
Light use efficiency (LUE) of salt marshes has not been well studied but is central to production efficiency models (PEMs) used for estimating gross primary production (GPP). Salt marshes are typically dominated by a species monoculture, resulting in large areas with distinct morphology and physiology. We measured eddy covariance atmospheric CO2 fluxes for two marshes dominated by a different species: Juncus roemerianus in Mississippi and Spartina alterniflora in Georgia. LUE for the Juncus marsh (mean = 0.160 ± 0.004 g C mol−1 photon), reported here for the first time, was on average similar to the Spartina marsh (mean = 0.164 ± 0.003 g C mol−1 photon). However, Juncus LUE had a greater range (0.073–0.49 g C mol−1 photon) and higher variability (15.2%) than the Spartina marsh (range: 0.035–0.36 g C mol−1 photon; variability: 12.7%). We compared the responses of LUE across six environmental gradients. Juncus LUE was predominantly driven by cloudiness, photosynthetically active radiation (PAR), soil temperature, water table, and vapor pressure deficit. Spartina LUE was driven by water table, air temperature, and cloudiness. We also tested how the definition of LUE (incident PAR vs. absorbed PAR) affected the magnitude of LUE and its response. We found LUE estimations using incident PAR underestimated LUE and masked day‐to‐day variability. Our findings suggest that salt marsh LUE parametrization should be species‐specific due to plant morphology and physiology and their geographic context. These findings can be used to improve PEMs for modeling blue carbon productivity.
Salt marshes rely on sufficient sediment inputs and room for lateral migration to maintain vertical and lateral stability under sea-level rise. As the global rate of sea-level rise accelerates, marshes unable to keep pace become vulnerable to drowning. We evaluated the long-term response of a salt marsh in Georgia, USA, to historical (1935–2018) and future projected rates of sea-level rise. We expected the marsh to be resilient because it receives high sediment inputs and has room to migrate landward. However, sediment cores show marsh accretion (1.55 mm y−1) is lower than the historical rate of sea-level rise (3.25 mm y−1) and that rates are independent of elevation. Results from a vertical accretion model show that while marsh area is stable through 2100 under historical and high sea-level rise scenarios, the marsh relies on elevation capital to maintain its extent under a high rate of sea-level rise. The marsh rapidly loses area beyond 2100 as it depletes its elevation reserve. By 2160, only 12% of the initial marsh area remains. Our results demonstrate that while elevation capital can extend the period of time a marsh maintains its areal extent, it does not remove the long-term threat of drowning when marsh accretion cannot keep pace with sea-level rise.
The interconnected estuarine complex of the Altamaha River and adjacent sounds located in Georgia (USA) functions as a hotspot for organic matter transformation as it is transported to the Atlantic Ocean. Here, we investigated how dissolved organic matter (DOM) composition changes both spatially and seasonally along the estuary and how it influences bacterial processing. Surface samples were collected during high tide at fifteen stations throughout the estuary in April, July, October 2017, and January 2018. Bulk, optical, and molecular analyses were conducted on samples before and after dark incubations to assess DOM sources and transformation patterns in the system. The dominant driver of change in DOM composition was found to be the terrigenous-marine gradient in organic matter sources. Six distinct clusters were identified based on the terrigenous signature of the DOM pool, explaining 45% of the variance in DOM composition in the system. Bacterial consumption of dissolved organic carbon (DOC) was strongly influenced by DOM composition, with increased degradation rates for DOM with a larger terrigenous character. However, changes in optical properties suggested that less aromatic DOM that co-varied with the terrigenous material was preferentially degraded. The passage of Hurricane Irma in September 2017 resulted in a 27% ± 7% increase in DOC content, likely due to inundation associated with storm surge and increased local precipitation, and DOC biodegradation was 17% ± 8% higher than during summer. These effects lasted for at least one month after the storm, revealing that hurricanes can have a large impact on DOM composition and cycling in coastal systems.
Decomposition of vegetal detritus is one of the most fundamental ecosystem processes. In complex landscapes, the fate of litter of terrestrial plants may depend on whether it ends up decomposing in terrestrial or aquatic conditions. However, (1) to what extent decomposition rates are controlled by environmental conditions or by detritus type, and (2) how important the composition of the detritivorous fauna is in mediating decomposition in different habitats, remain as unanswered questions. We incubated two contrasting detritus types in three distinct habitat types in Coastal Georgia, USA, to test the hypotheses that (1) the litter fauna composition depends on the habitat and the litter type available, and (2) litter mass loss (as a proxy for decomposition) depends on environmental conditions (habitat) and the litter type. We found that the abundance of most taxa of the litter fauna depends primarily on habitat. Litter type became a stronger driver for some taxa over time, but the overall faunal composition was only weakly affected by litter type. Decomposition also depends strongly on habitat, with up to ca. 80% of the initial detrital mass lost over 25 months in the marsh and forest habitats, but less than 50% lost in the creek bank habitat. Mass loss rates of oak versus pine litter differed initially but converged within habitat types within 12 months. We conclude that, although the habitat type is the principle driver of the community composition of the litter fauna, litter type is a significant driver of litter mass loss in the early stages of the decomposition process. With time, however, litter types become more and more similar, and habitat becomes the dominating factor in determining decomposition of older litter. Thus, the major driver of litter mass loss changes over time from being the litter type in the early stages to the habitat (environmental conditions) in later stages.
Benthic foraminifera are important environmental indicators of heavy-metal contaminants in marine environments because of their unique sensitivity to environmental change. However, this sensitivity can make parsing the effect of contaminants from other factors such as salinity and temperature difficult. To address this problem, this study individually compares the effects of heavy metals nickel and zinc on temperate rotalids and subtropical miliolids under different temperature and salinity regimes, including intermediate (22°C, 32 psu), elevated temperature (30°C, 32 psu), reduced temperature (18°C, 32 psu), elevated salinity (22°C, 40 psu), and reduced salinity (22°C, 12 psu). Assemblages of foraminifera were grown experimentally from propagules (small juveniles) collected from two shallow marine sites: Sapelo Island, Georgia, and Little Duck Key, Florida. Surface sediment was collected from both locations and sieved immediately after collection. Using the propagule method, assemblages of foraminifera were grown in a controlled setting from propagules in these sediment samples. Either nickel or zinc was added to each assemblage. Experimental conditions were held constant with only metal concentration, salinity, and temperature varying.
Exposure to elevated concentrations of nickel and zinc led to limited foraminiferal abundances compared to the controls under all temperatures and salinity conditions examined. In addition, high concentrations of nickel and especially zinc resulted in an increase of deformed tests of Sapelo Island foraminifera under intermediate (22°C, 32 psu) and high salinity (22°C, 40 psu) conditions. Far fewer deformities occurred in Sapelo Island assemblages in higher or lower salinities and temperatures. These results support the usefulness of foraminiferal abundance and species abundance as tools for environmental analysis. Consistent with previous work, results also identify the problems associated with using test deformities alone as a bioindicator tool.
Consumers often deplete local resources and aggregate along edges of remaining resources, forming “consumer fronts.” We examined the factors that promote Sesarma reticulatum crab aggregations at saltmarsh creek heads to explain the directional but slow movement of these fronts. We also created artificial creek heads to test the hypothesis that hydrological conditions at creek heads create superior habitat for crabs. Soil temperatures were ˜11–12% cooler, hydrogen sulfide concentrations lower (0.0 vs. ˜0.58 mg/L), and dissolved oxygen concentrations twofold higher at the creek head versus the marsh platform. In the artificial creek-head experiment, altering hydrological conditions led to lower dissolved sulfide levels, higher dissolved oxygen levels, and increased densities of crab burrows and Sesarma crabs. Moreover, the elevation of the soil surface declined rapidly at artificial creek heads versus controls, suggesting that crabs were increasing erosion. Our results suggest that abiotic conditions for crabs are better at the leading edge of the creek head than the trailing edge, explaining the directional movement of the front. Moreover, the speed at which the front propagates appears to be limited by the rate at which the creekhead erodes, rather than by crab mobility. The directional and slow movement of Sesarma fronts compared to consumer fronts of other invertebrates appears to result from the inextricable link between Sesarma and marsh geomorphology, whereas other consumer fronts are associated mostly with food resources.
Wu, F., Pennings, S.C., Ortals, C., Ruiz, J., Farrell, W.R., McNichol, S.M., Angelini, C., Spivak, A.C., Alber, M., and Tong, C. 2021. Disturbance is complicated: Headward-eroding saltmarsh creeks produce multiple responses and recovery trajectories. Limnology and Oceanography. https://doi.org/10.1002/lno.11867
Disturbances are one of the most important processes affecting natural systems, but there is a gap between simple conceptual models of disturbance and complex empirical studies. We studied the perturbation caused by headward-eroding creeks in southeastern USA salt marshes. We measured disturbance responses (magnitude and recovery trajectory) of 19 variables. Some variables (shoot density, root biomass, snail density, soil pH, soil strength, soil temperature, elevation) declined sharply, while other variables (crab burrow density, soil organic matter, soil redox) increased sharply, in response to the burrowed and grazed conditions at the creek head. These variables recovered over subsequent years or decades. Other variables (shoot height, aboveground biomass, rhizome biomass, light interception) declined sharply in the creek head, then overshot control values before recovering. Some variables (benthic algae, soil salinity) did not appear to be disturbed by the creek head. As hypothesized, plants recovered before soils and snails. Disturbance magnitude and time to recovery were often greater directly adjacent to the new creekbank than for the same variables in a parallel transect further away from the creekbank, and in some cases variables never converged with control values, indicating a persistent state change. Reducing the dimensionality of the data set into principal component axes obscured the diverse ways in which different aspects of the system responded to and recovered from the perturbation. Our study illustrates the challenges in moving from simple conceptual models of disturbance to empirical studies in which multiple variables are likely to be affected differently and follow different recovery trajectories.
Salt marshes are valuable ecosystems, and there is concern that increases in the rate of sea level rise along with anthropogenic activities are leading to the loss of vegetated habitat. The area of vegetated marsh can change not only through advance and retreat of the open fetch edge, but also due to channel widening and contracting, formation and drainage of interior ponds, formation and revegetation of interior mud flats, and marsh migration onto upland areas, each of which is influenced by different processes. This study used historical aerial photographs to measure changes in the extent of vegetated marsh over approximately 70 years at study marshes located in three long-term ecological research (LTER) sites along the US East coast: Georgia Coastal Ecosystems (GCE), Virginia Coast Reserve (VCR), and Plum Island Ecosystems (PIE). Marsh features were categorized into vegetated marsh, ponds, interior mud flats, and channels for three time periods at each site. The three sites showed different patterns of change in vegetated marsh extent over time. At the GCE study site, losses in vegetated marsh, which were primarily due to channel widening, were largely offset by channel contraction in other areas, such that there was little to no net change over the study period. The study marsh at VCR experienced extensive vegetated marsh loss to interior mud flat expansion, which occurred largely in low-lying areas. However, this loss was counterbalanced by marsh gain due to migration onto the upland, resulting in a net increase in vegetated marsh area over time. Vegetated marsh at PIE decreased over time due to losses from ponding, channel widening, and erosion at the open fetch marsh edge. Digital elevation models revealed that the vegetated areas of the three marshes were positioned at differing elevations relative to the tidal frame, with PIE at the highest and VCR at the lowest elevation. Understanding the patterns of vegetation loss and gain at a given site provides insight into what factors are important in controlling marsh dynamics and serves as a guide to potential management actions for marsh protection.
Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Here, we reveal that a historically innocuous grazer—the marsh crab Sesarma reticulatum—is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). Aerial-image analyses reveal that resulting “Sesarma-grazed” creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Field experiments further demonstrate that Sesarma-grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions.
In the nutrient-rich region surrounding marine phytoplankton cells, heterotrophic bacterioplankton transform a major fraction of recently fixed carbon through the uptake and catabolism of phytoplankton metabolites. We sought to understand the rules by which marine bacterial communities assemble in these nutrient-enhanced phycospheres, specifically addressing the role of host resources in driving community coalescence. Synthetic systems with varying combinations of known exometabolites of marine phytoplankton were inoculated with seawater bacterial assemblages, and communities were transferred daily to mimic the average duration of natural phycospheres. We found that bacterial community assembly was predictable from linear combinations of the taxa maintained on each individual metabolite in the mixture, weighted for the growth each supported. Deviations from this simple additive resource model were observed but also attributed to resource-based factors via enhanced bacterial growth when host metabolites were available concurrently. The ability of photosynthetic hosts to shape bacterial associates through excreted metabolites represents a mechanism by which microbiomes with beneficial effects on host growth could be recruited. In the surface ocean, resource-based assembly of host-associated communities may underpin the evolution and maintenance of microbial interactions and determine the fate of a substantial portion of Earth’s primary production.
Tidal freshwater marshes can protect downstream ecosystems from eutrophication by intercepting excess nutrient loads, but recent studies in salt marshes suggest nutrient loading compromises their structural and functional integrity. Here, we present data on changes in plant biomass, microbial biomass and activity, and soil chemistry from plots in a tidal freshwater marsh on the Altamaha River (GA) fertilized for 10 yr with nitrogen (+N), phosphorus (+P), or nitrogen and phosphorus (+NP). Nitrogen alone doubled aboveground biomass and enhanced microbial activity, specifically rates of potential nitrification, denitrification, and methane production measured in laboratory incubations. Phosphorus alone increased soil P and doubled microbial biomass but did not affect microbial processes. Nitrogen or P alone decreased belowground biomass and soil carbon (C) whereas +NP increased aboveground biomass, microbial biomass and N cycling, and N, P, and C assimilation and burial more than either nutrient alone. Our findings suggest differential nutrient limitation of tidal freshwater macrophytes by N and microbes by P, similar to what has been observed in salt marshes. Macrophytes outcompete microbes for P in response to long‐term N and P additions, leading to increased soil C storage through increased inputs of belowground biomass relative to N and P added singly. The susceptibility of tidal freshwater marshes to long‐term nutrient enrichment and, hence their ability to mitigate eutrophication will depend on the quantity and relative proportion of N vs. P entering estuaries and tidal wetlands.
Tidal freshwater marshes (TFMs) are threatened by seawater intrusion, which can affect microbial communities and alter biogeochemical processes. Here, we report on a long‐term, large‐scale manipulative field experiment that investigated continuous (press) and episodic (pulse, 2 months/yr) inputs of brackish water on microbial communities in a TFM. After 2.5 yr, microbial diversity was lower in press treatments than in control (untreated) plots whereas diversity in pulse plots was unaffected by brackish water additions. Sulfate reducer abundance increased in response to both press and pulse treatments whereas methanogens did not differ among treatments. Our results, along with other lab and field measurements that show reduced soil respiration and extracellular enzyme activity suggest that continuous seawater intrusion will decrease macrophyte C inputs that reduce bacterial diversity in ways that also diminish ecosystem carbon cycling.
Benthic foraminifera are valuable indicators in environmental studies, including those on marine pollution monitoring. While a great deal of foraminiferal biomonitoring research utilizes abundance and distributional data, further value resides in better understanding the incorporation of heavy metal pollutants in foraminiferal calcite. By experimentally growing assemblages of foraminifera from propagules (small juveniles) gathered from Sapelo Island, Georgia and Little Duck Key, Florida, this study examines foraminiferal incorporation of the heavy metals arsenic, cadmium, nickel, and zinc over a range of concentrations.
Surface sediment was collected and sieved to concentrate the propagules. The propagules were then used to experimentally grow assemblages with each exposed to a different heavy metal. After one month, the experimentally grown foraminifera were harvested and samples of the two most common species from each location, Ammonia tepida (Cushman) and Haynesina germanica (Ehrenberg) from Sapelo Island and Quinqueloculina sabulosa (Cushman) and Triloculina oblonga (Montagu) from Little Duck Key, were selected for trace element analysis. Calcite of the tests was analyzed using LA-ICP-MS to quantify the heavy metal incorporation.
Rotalid species A. tepida and H. germanica incorporated more cadmium as its concentration in the surrounding water increased, whereas miliolid species Q. sabulosa and T. oblonga incorporated more of the metals zinc and nickel. This study shows that while foraminiferal incorporation of heavy metals has great potential as a biomonitoring tool, multiple factors (especially inter-clade variation) must be considered carefully. In future marine environmental research, these factors may help to create a more targeted assessment of environmental pollution.
Thompson, V.D., Rick, T., Garland, C.J., Thomas, D.H., Smith, K.Y., Bergh, S., Sanger, M., Tucker, B., Lulewicz, I.H., Semon, A.M., Schalles, J.F., Hladik, C.M., Alexander, C.R. Jr. and Ritchison, B.T. 2020. Ecosystem stability and Native American oyster harvesting along the Atlantic Coast of the United States. Science Advances. 6.
The eastern oyster (Crassostrea virginica) is an important proxy for examining historical trajectories of coastal ecosystems. Measurement of ~40,000 oyster shells from archaeological sites along the Atlantic Coast of the United States provides a long-term record of oyster abundance and size. The data demonstrate increases in oyster size across time and a nonrandom pattern in their distributions across sites. We attribute this variation to processes related to Native American fishing rights and environmental variability. Mean oyster length is correlated with total oyster bed length within foraging radii (5 and 10 km) as mapped in 1889 and 1890. These data demonstrate the stability of oyster reefs despite different population densities and environmental shifts and have implications for oyster reef restoration in an age of global climate change.
Aim: Introduced species may display or foster novel latitudinal clines because they are not well adapted to their new habitats. We tested the hypothesis that the latitudinal cline in nematode diversity in salt marshes would differ between the native (United States) and introduced (China) ranges of Spartina alterniflora.
Location: East Coasts of the United States (30.32–43.33°N) and China
Methods: We extracted nematodes from soil samples collected at 32 sites along the United States East Coast and 41 sites along the Chinese coast. We compared latitudinal patterns in nematode diversity and composition between the native and introduced ranges.
Results: In the native range of S. alterniflora, nematode richness at lower latitudes was almost twice as high as that at higher latitudes. In contrast, we found no latitudinal pattern in nematode richness or diversity in the introduced range of S. alterniflora. Nematode genus richness at all sites in China was about half that at lower latitudes in the United States. Beta diversity of nematodes increased with geographic distance in the United States, but not China.
Main conclusions: Nematode diversity did not show latitudinal clines in salt marshes dominated by introduced S. alterniflora in China. A likely explanation is that the recently introduced populations are still relatively genetically homogenous, whereas in the native range, genetic variation in plant populations across latitude drives different nematode communities. We suggest that future studies of introduced species will gain additional insights by taking an explicitly geographic perspective.
Zinnert, J.C., Nippert, J.B., Rudgers, J.A., Pennings, S.C., Gonzalez, G., Alber, M., Baer, S.G., Blair, J.M., Burd, A.B., Collins, S.L., Craft, C.B., Di Iorio, D., Dodds, W.K., Groffman, P.M., Herbert, E., Hladik, C.M., Li, F., Litvak, M., Newsome, S., O’Donnell, J., Pockman, W.T., Schalles, J.F. and Young, D.R. State Changes: Insights from the U.S. Long Term Ecological Research Network. Ecosphere.
Understanding the complex and unpredictable ways ecosystems are changing and predicting the state of ecosystems and the services they will provide in the future requires coordinated, long term research. This paper is a product of a U.S. National Science Foundation funded Long Term Ecological Research (LTER) network synthesis effort that addressed anticipated changes in future populations and communities. Each LTER site described what their site would look like in 50 or 100 years based on long term patterns and responses to global change drivers in each ecosystem. Common themes emerged and predictions were grouped into state change, connectivity, resilience, time lags, and cascading effects. Here, we report on the
“state change” theme, which includes examples from the Georgia Coastal (coastal marsh), Konza Prairie (mesic grassland), Luquillo (tropical forest), Sevilleta (arid grassland) and Virginia Coastal (coastal grassland) sites. Ecological thresholds (the point at which small changes in an environmental driver can produce an abrupt and persistent state change in an ecosystem quality, property or phenomenon) were most commonly predicted. For example, in coastal ecosystems, sea level rise and climate change could convert salt marsh to mangroves and coastal barrier dunes to shrub thicket. Reduced fire frequency has converted grassland to shrubland in mesic prairie, whereas overgrazing combined with drought drive shrub encroachment in arid grasslands. Lastly, tropical cloud forests are susceptible to climate-induced changes in cloud base altitude leading to shifts in species distributions. Overall, these examples reveal that state change is a likely outcome of global environmental change across a diverse range of ecosystems and highlight the need for long term studies to sort out the causes and consequences of state change. The diversity of sites within the LTER network facilitates the emergence of overarching concepts about state changes as an important driver of ecosystem structure, function, services, and futures.
Elevation differences in salt marshes result in numerous ecological consequences as a result of variation in tidal flooding. We demonstrate here that elevation differences are also negatively correlated with soil temperature on the marsh platform, irrespective of tidal flooding. Field observations of soil temperature at 10‐cm depth in a Georgia marsh showed that elevation increases of 0.5 m corresponded to decreases in average soil temperature of 0.9–1.7°C during both winter and summer. Landsat 8 estimates of land surface temperatures across the marsh in dry (nonflooded) scenes also showed that temperature decreased with increasing elevation, which was consistent with soil observations. Similar satellite results were also found in a test marsh in Virginia. Biological reactions are temperature‐dependent, and these findings indicate that metabolic processes will vary over short distances. This is important for accurately estimating marsh metabolism and predicting how changes in temperature will affect future productivity.
Climate change is altering consumer−plant interactions in ecosystems worldwide. How consumers alter their spatial distribution, grazing activities, and functional morphology in response to climate stress can determine whether their effects on plants intensify or relax. Few studies have considered multiple
consumer response metrics to elucidate the mechanisms underpinning the resulting changes in consumer−plant interactions. Here, we tested how drought stress influences the interaction between the dominant consumer, the fungal-farming periwinkle snail Littoraria irrorata, and a foundational plant, cordgrass Spartina alterniflora, in a southeastern US salt marsh. In a 4 mo
field experiment, we maintained moderate snail densities in mesh control chambers and clear plastic climate chambers that simulated drought by elevating temperatures and drying soils. Monitoring revealed that snails
more often congregated on cordgrass stems than leaves in climate chambers than in controls. Image analyses indicated that this behavioral shift corresponded to snails inflicting shorter, but more numerous, fungal-infested scars on cordgrass leaves, and causing less plant damage in climate chambers than controls. Coincident with their net reduction in grazing, snails maintained longer radulae, whose central teeth were blunter and lateral teeth were sharper, in climate chambers compared to controls. These results suggest that under drought, snail radulae may experience less frictional wear and that, at inter mediate densities, snail−cordgrass interactions re lax. Together with prior
research showing that at high densities, snails can denude cordgrass during drought, we conclude that consumer density, behavior, and morphological responses must be integrated in predictions of how climate change will affect the direction, strength, and stability of consumer−plant interactions.
- Salt marshes suffered large‐scale degradation in recent decades. Extreme events such as hot and dry spells contributed significantly to this, and are predicted to increase not only in intensity, but also in frequency under future climate scenarios. Such repetitive extreme events may generate cumulative effects on ecosystem resilience. It is therefore important to elucidate how marsh vegetation responds to repetitive stress, and whether changes in key species interactions can modulate vegetation resilience.
- In this study, we investigated how moderate but repetitive desiccation events, caused by the combined effects of drought and high temperatures, affect cordgrass (Spartina alterniflora), the dominant habitat‐forming grass in southeastern US salt marshes. In a 4‐month field experiment, we simulated four consecutive desiccation events by periodically excluding tidal flooding and rainfall, while raising temperature. We crossed this desiccation treatment with the presence/absence of ribbed mussels (Geukensia demissa) – a mutualist of cordgrass known to enhance its desiccation resilience – and with grazing pressure by the marsh periwinkle (Littoraria irrorata) that is known to suppress cordgrass’ desiccation resilience.
- We found that each subsequent desiccation event deteriorated sediment porewater conditions, resulting in high salinity (53 ppt), low pH‐levels (3.7) and increased porewater Al and Fe concentrations (≈800 μmol/L and ≈1,500 μmol/L) upon rewetting. No effects on porewater chemistry were found as a result of snail grazing, while ribbed mussels strongly mitigated desiccation effects almost to control levels and increased cordgrass biomass by approximately 128%. Importantly, although cordgrass generally appeared healthy above‐ground at the end of the experiment, we found clear negative responses of the repetitive desiccation treatment on cordgrass below‐ground biomass, on proline (osmolyte) levels in shoots and on the number of tillers (−40%), regardless of mussel and/or snail presence.
- Synthesis. Even though the mutualism with mussels strongly mitigated chemical effects in the sediment porewater throughout the experiment, mussels could not buffer the adverse ecophysiological effects observed in cordgrass tissue. Our results therefore suggest that although mussels may alleviate desiccation stress, the predicted increased frequency and intensity of hot dry spells may eventually affect saltmarsh resilience by stressing the mutualism beyond its buffering capacity.
Restoration efforts have been escalating worldwide in response to widespread habitat degradation. However, coastal restoration attempts notoriously vary in their ability to establish resilient, high-functioning ecosystems. Conventional restoration attempts disperse transplants in competition-minimizing arrays, yet recent studies suggest that clumping transplants to maximize facilitative interactions may improve restoration success. Here, we modify the stress gradient hypothesis to generate predictions about where each restoration design will perform best across environmental stress gradients. We then test this conceptual model with field experiments manipulating transplant density and configuration across dune elevations and latitudes. In hurricane-damaged Georgia (USA) dunes, grass transplanted in competition-minimizing (low-density, dispersed) arrays exhibited the highest growth, resilience to disturbance and dune formation in low-stress conditions. In contrast, transplants survived best in facilitation-maximizing (high-density, clumped) arrays in high-stress conditions, but these benefits did not translate to higher transplant growth or resilience. In a parallel experiment in Massachusetts where dune grasses experience frequent saltwater inundation, fewer transplants survived, suggesting that there are thresholds above which intraspecific facilitation cannot overcome local stressors. These results suggest that ecological theory can be used to guide restoration strategies based on local stress regimes, maximizing potential restoration success and return-on-investment of future efforts.
Dissolved organic matter (DOM) is a large and complex mixture of compounds with source inputs that differ with location, season and environmental conditions. Here, we investigated drivers of DOM composition changes in a marsh‐dominated estuary off the southeastern U.S. Monthly water samples were collected at a riverine and estuarine site from September 2015 to September 2016, and bulk, optical, and molecular analyses were conducted on samples before and after dark incubations. Results showed that river discharge was the primary driver changing the DOM composition at the mouth of the Altamaha River. For discharge higher than ~ 150 m3 s‐1, DOC concentrations and the terrigenous character of the DOM increased approximately linearly with river flow. For low discharge conditions, a clear signature of salt marsh‐derived compounds was observed in the river. At the head of Sapelo Sound, changes in DOM composition were primarily driven by river discharge and possibly by summer algae blooms. Microbial consumption of DOC was larger during periods of high discharge at both sites, potentially due to the higher mobilization and influx of fresh material to the system. The Georgia coast was hit by Hurricane Matthew in October 2016, which resulted in a large input of carbon to the estuary. The DOC concentration was ~ 2 times higher and DOM composition was more aromatic with a stronger terrigenous signature compared to the seasonal maximum observed earlier in the year during peak river discharge conditions. This suggests that extreme events notably impact DOM quantity and quality in estuarine regions.
In estuaries, future variation in sea level and river discharge will lead to saline intrusion into low-salinity tidal marshes. To investigate the processes that control the differential response and recovery of tidal freshwater marsh plant communities to saline pulses, a 3 × 5 factorial greenhouse experiment was conducted to examine the effects of a range of salinity levels (3, 5, and 10 practical salinity units (PSU)) and pulse durations (5, 10, 15, 20, and 30 days per month) on community composition of tidal freshwater marsh vegetation. Recovery of perturbed communities was also examined after 10 months. The results showed that community composition was increasingly affected by the more-saline and longer-duration treatments. The increasing suppression of salt-sensitive species resulted in species reordering, decreased species richness, and decreased aboveground biomass. Most of the plant species were able to recover from low-salinity, short-duration saline pulses in less than 1 year. However, because not all species recovered in the heavily salinized treatments, species richness at the end of the recovery period remained low for treatments that were heavily salinized during the treatment period. In contrast, plant aboveground biomass fully recovered in the heavily salinized treatments. Although the magnitude and duration of pulsed environmental changes had strong effects on community composition, shifts in community composition prevented long-term reductions in productivity. Thus, in this study system, environmental change affected species composition more strongly than it did ecosystem processes.
- Plants adjust their size and reproductive effort in response to numerous selection pressures and constraints. The self‐thinning law describes a well‐known trade‐off between size and density. Plants also trade‐off investment into growth vs. sexual reproduction, as described by life‐history theory.
- We build on past work on plant allometry and life history by examining both self‐thinning and size‐dependent reproduction in a single plant species, the saltmarsh grass Spartina alterniflora, across a wide range of settings: three landscape positions, two habitats and eight sites, across sixteen years.
- Plants in different landscape positions and years varied tremendously in size and shoot density. However, all this variation could be explained by a single allometric relationship consistent with the self‐thinning law, but with a lower slope. Flowering was size‐dependent, and the size at which plants had a 50% probability of flowering varied among habitat, sites and years. Plants that were stressed reproduced at a smaller size than plants that were growing under good conditions, and this pattern was consistent among habitat, sites and years. Finally, reproductive biomass and the proportion of shoots flowering increased with increasing vegetative size (plant height or shoot biomass). Combining these two patterns, S. alterniflora plants growing high density are small and reproduce at a smaller size than large plants growing at low density.
- Although there is tremendous spatial and temporal variation in S. alterniflora growth and reproductive patterns, all this variation can be understood as resulting from two simple allometric trade‐offs. Because saltmarsh plants often occur in monospecific stands, they may serve as simple, model systems for studies of plant life history.
Phenology studies mostly focus on variation across time or landscapes. However, phenology can vary at fine spatial scales, and these differences may be as important as long-term change from climate warming. We used high frequency “PhenoCam” data to examine phenology of Spartina alterniflora, a foundation species native to salt marshes on the US East and Gulf coasts, and a common colonizer elsewhere. We examined phenology across three microhabitats from 2013 to 2017 and used this information to create the first spring green-up model for S. alterniflora. We then compared modern spatial variation to that exhibited over a 60-year climate record. Marsh interior plants initiated spring growth 17 days earlier than channel edge plants and spent 35 days more in the green-up phenophase and 25 days less in the maturity phenophase. The start of green-up varied by 17 days among 3 years. The best spring green-up model was based on winter soil total growing degree days. Across microhabitats, spring green-up differences were caused by small elevation changes (15 cm) that drove soil temperature variation of 0.8°C. Preliminary evidence indicated that high winter belowground biomass depletion triggered early green-up. Long-term change was similar: winter soil temperatures warmed 1.7 ± 0.3°C since 1958, and green-up advanced 11 ± 6 days, whereas contemporary microhabitat differences were 17 ± 4 days. Incorporating local spatial variation into plant phenology models may provide an early warning of climate vulnerability and improve understanding of ecosystem-scale productivity. Microscale phenology variation likely exists in other systems and has been unappreciated.
Exchange of groundwater is an important transfer mechanism for nutrients and pollutants between coastal aquifers and surface waters. Constraining such exchange in salt marshes – where biological productivity and biogeochemical cycling rates are among the highest of all coastal ecosystems – is vital for understanding ecosystem function and vulnerability. Here, we quantify groundwater discharge into the tidal Duplin River from the adjoining salt marsh near Sapelo Island, Georgia using high spatial and temporal resolution field measurements of radon-222. Field campaigns occurred for several weeks each summer during 2013, 2015, and 2016. Spatial surveys reveal a general increase in radon activity upstream through the Duplin River, which may result from either higher groundwater discharge or lower mixing rates in the headwaters. To distinguish between these possibilities, we use a radon mass balance model to determine groundwater input. We find that groundwater discharge (normalized to inundated marsh surface area) to the headwaters average 5.1–5.8 cm3/cm2marsh/day across all three field campaigns, which are comparable to those to the main channel (averaging 6.0–6.5 cm3/cm2 marsh/day across all three field campaigns). Our work reveals a positive relationship between aerial extent of marsh inundation and groundwater discharge into the Duplin River. Discharge is generally maximal during falling tide, reflecting a hydraulic gradient driver, but also is significant prior to high tide, indicative of sediment compression as a driver of groundwater inputs. Constraining the relationship between marsh inundation and resulting groundwater dynamics is an integral aspect to assessing how salt marsh circulation processes may respond to intensifying inundation (from reduced sediment supply, subsidence, and/or rising sea levels).
Benthic foraminifera have long served as indicators of environmental conditions – both natural and anthropogenically impacted. To better understand the responses of benthic foraminifera to specific heavy metal contaminants (Cd, Pb, Zn), assemblages of coastal benthic foraminifera were grown from propagules (tiny juveniles) in the lab with exposure to a single heavy metal over a range of concentrations, based on the US Environmental Protection Agency’s Critical Maximum Concentration (CMC) values. Foraminiferal propagule banks were collected from relatively pristine mudflats, located on the southern end of Sapelo Island, Georgia (USA). Consistent with the findings of numerous field-based studies, foraminifera were found to respond negatively to Cd, Pb, and Zn. Overall, assemblages grown with exposure to higher concentrations of these metals are characterized by decreased abundances, species richness, and evenness. All of the acute responses observed in these metrics occur at concentrations equal to or somewhat higher than the USEPA’s CMC values. Foraminiferal responses vary by metal, though the four most common species (two monothalamids: Ovammina opacaDahlgren, Psammophaga sapela Altin Ballero, Habura, Goldstein; and two rotaliids: Haynesina germanica (Ehrenberg), Ammonia tepida (Cushman)) responded in a broadly similar fashion. The monothalamid species however may be more sensitive to high concentrations of each metal. Of the metals examined, exposure to Pb had the most deleterious effect, followed by Zn, then Cd. These four most abundant species appear to be more tolerant of Cd than the other metals. Zn was the only metal in the study that produced abundant aberrant test morphologies. Ammonia tepida grew abnormally more frequently than any other species encountered and exhibited a distinctive enlarged aperture as well as aberrant patterns of calcification, chamber arrangement, and enlarged pores. Abnormal tests were also found in H. germanica and a miliolid. The monothalamid species did not produce aberrant test morphologies. Results support the application of foraminifera as bio-indicators in polluted environments.
Habitat patch composition and configuration mediate the fitness and distribution of many species. However, we know little about how this landscape complementation may influence the distribution of an invasive species’ ecological impacts and, in turn, how this affects ecosystem resilience to disturbance. We surveyed > 820 km of coastline to evaluate how landscape complementation mediates patterns in invasive feral hog (Sus scrofa) rooting, trampling and wallowing disturbances in southeastern US salt marshes and assessed marsh resilience to these behaviors in an 8-site survey and 13-month field experiment. We discovered that hog rooting and trampling most often occur where hardwood forest comprises > 30% and salt marsh < 22% of habitat surrounding each surveyed site, respectively, while wallowing correlated most strongly with salt marsh invertebrate densities. At the 8 survey sites, vegetation cover, soil organic carbon, and surface elevation were consistently lower, and soil anoxia and porewater ammonium-nitrogen higher, in hog-disturbed relative to undisturbed areas. The experiment revealed that vegetation can recover when rooted or trampled, but remains depressed when wallowed or repeatedly disturbed. Together, these findings provide novel evidence that habitat patch composition at landscape scales can act together with local habitat attributes to dictate invasive species’ disturbance patterns and highlight areas most vulnerable to invaders. In salt marshes, insights gleaned from such consideration of landscape complementation can inform conservation and management strategies for curbing the impact of this prolific, global invader.
Benthic foraminifera are valuable environmental indicators of heavy metal contaminants in marine environments. To broaden their effectiveness as bioindicators, this study compares individually the effects of selected heavy metal contaminants, including both metabolically essential and non-essential elements, on temperate rotalids and subtropical miliolids, as well as associated monothalamid foraminifera. To accomplish these aims, assemblages of foraminifera were grown experimentally from propagules (small juveniles) collected from two coastal sites: Sapelo Island, Georgia, and Little Duck Key, Florida, that provide an effective comparison between environments and types of foraminifera. Surface sediment was collected from both locations and sieved immediately after collection. Using the propagule method, assemblages of foraminifera were grown in the laboratory from propagules in the sediment samples. Two metabolically essential trace elements, nickel, and zinc, and two non-essential elements, arsenic and cadmium were used to represent both types of heavy metal. Experimental conditions were held constant while varying only the metal concentrations.
In treatments from both origins, increasing concentrations of cadmium, nickel, and zinc led to decreases in abundance and diversity for the foraminifera. In addition, zinc, and to a lesser extent cadmium and nickel above certain concentrations, resulted in an increase of deformed tests among the foraminifera. Deformities occurred amongst the most common calcareous species from Sapelo island: Ammonia tepida and Haynesina germanica. Fewer deformities were observed in common calcareous species from Little Duck Key, the miliolids Quinqueloculina sabulosa and Quinqueloculina bosciana featured few deformities. Notably, monothalamid species such as Psammophaga sapelaremained present at high metal concentrations. These results support previous research and reinforce the usefulness of rotalids such as A. tepida and H. germanica as bioindicators of heavy metal contamination as well as suggesting a possible use of monothalamids such as P. sapela in this manner.
Sea level rise is expected to increase inundation and saltwater intrusion into many tidal freshwater marshes and forests. Saltwater intrusion may be long-term, as with rising seas, or episodic, as with low river flow or storm surge. We applied continuous (press) and episodic (pulse) treatments of dilute seawater to replicate 2.5 × 2.5 m field plots for three years and measured soil attributes, including soil porewater, oxidation-reduction potential, soil carbon (C), and nitrogen (N) to investigate the effects of continuous and episodic saltwater intrusion and increased inundation on tidal freshwater marsh elemental cycling and soil processes. Continuous additions of dilute seawater resulted in increased porewater chloride, sulfate, sulfide, ammonium, and nitrate concentrations. Plots that received press additions also had lower soil oxidation-reduction potentials beginning in the second year. Episodic additions of dilute seawater during typical low flow conditions (Sept.-Oct.) resulted in transient increases in porewater chloride and sulfate that returned to baseline conditions once dosing ceased. Freshwater additions did not affect porewater inorganic N or soil C or N. Persistent saltwater intrusion in freshwater marshes alters the N cycle by releasing ammonium-N from sorption sites, increasing nitrification and severely reducing N storage in macrophyte biomass. Chronic saltwater intrusion, as is expected with rising seas, is likely to shift tidal freshwater marshes from a sink to a source of N whereas intermittent intrusion from drought may have no long term effect on N cycling.
Tidal salt marshes sequester and store blue carbon at both short and long time scales. Marsh soils shape and maintain the ecosystem by supporting complex biogeochemical reactions, deposition of sediment, and accumulation of organic matter. In this study, we examined the potential of imaging spectroscopy techniques to indirectly quantify and map tidal marsh soil properties at a National Estuarine Research Reserve in Georgia, USA. A framework was developed to combine modern digital image processing techniques for marsh soil mapping, including object-based image analysis (OBIA), machine learning modeling, and ensemble analysis. We also evaluated the efficacy of airborne hyperspectral sensors in estimating marsh soil properties compared to spaceborne multispectral sensors, WorldView-2 and QuickBird. The pros and cons of object-based modeling and mapping were assessed and compared with traditional pixel-based mapping methods. The results showed that the designed framework was effective in quantifying and mapping three marsh soil properties using the composite reflectance from salt marsh environment: soil salinity, soil water content, and soil organic matter content. Multispectral sensors were successful in quantifying soil salinity and soil water content but failed to model soil organic matter. The study also demonstrated the value of minimum noise fraction transformation and ensemble analysis techniques for marsh soil mapping. The results suggest that imaging spectroscopy based modeling is a promising tool to quantify and map marsh soil properties at a local scale, and is a potential alternative to traditional soil data acquisition to support carbon cycle research and the conservation and restoration of tidal marshes.
Climate change and consumer outbreaks are driving ecosystem collapse worldwide. Although much research has demonstrated that these factors can interact, how heterogeneity in top–down control intensity and physical forcing modulates ecosystem resilience to climate stress remains poorly understood. Here, we explore whether the nocturnal herbivorous crab Sesarma reticulatum can control spatially dominant cordgrass (Spartina alterniflora) growth and how its top–down effects vary with crab density, drought stress, and large-scale disturbance in southeastern US salt marshes. In multiple field experiments and surveys, we show that Sesarma depresses cordgrass growth and that its effects increase in a saturating manner with increasing crab density, such that the highest naturally occurring densities of this consumer can trigger local cordgrass die-off. This top–down effect of Sesarma is similar in magnitude to what is thought to be the dominant grazer in the system, the marsh periwinkle snail Littoraria irrorata. In a drought stress by Sesarma density experiment, we further show that salinity stress and intensive crab herbivory additively suppress cordgrass drought resistance. After drought subsides, surveys and experiments reveal that Sesarma also stifles cordgrass re-growth into existing die-off areas. Together, these results show that multiple grazers powerfully regulate the productivity and drought resilience of these intertidal grasslands and that heterogeneity in physical stress and consumer density can dictate when and where top–down forcing is important. More generally, this work provides a rare, experimental demonstration of the critical role top–down control can play across the initiation and recovery stages of ecosystem die-off.
Knowledge of light partitioning into different optically active constituents, particularly chromophoric dissolved organic matter (CDOM) in the ultraviolet (UV) is indispensable for understanding UV dependent biogeochemical issues including photochemical processes in optically complex waters. Herein a new approach is presented to investigate photochemistry by blending two ocean color algorithms, namely the composite SeaUV (Cao et al., 2014) and the SeaCDOM (Cao and Miller, 2015) algorithms, and applying them to visible remote sensing reflectance (Rrs) measured using the Hyperspectral Imager for the Coastal Ocean (HICO). As illustrated using photochemical carbon monoxide (CO) production from CDOM, this model approach allows high resolution examination of UV optical details with estimates of both depth-specific and depth-integrated photoproduction rates in a dynamic estuarine/coastal environment. Decoupled retrievals of inherent and apparent optical properties (i.e. diffuse attenuation coefficient (Kd) and CDOM absorption coefficient (ag)) using two distinct ocean color algorithms over the entire UV spectrum allow a synoptically dynamic view of CDOM’s contribution to light attenuation (ag/Kd). This provides new potential to probe UV processes in complex coastal waters on regional as well as global scales using remote sensing of ocean color.
Dissolved organic nitrogen (DON) can account for a large fraction of the dissolved nitrogen (N) pool in the ocean, but the cycling of marine DON is poorly understood. Recent discoveries that urea‐ and cyanate‐N can be oxidized by some strains of Thaumarchaeota suggest that these abundant microbes may be able to access and oxidize a fraction of the DON pool. However, measurements of the oxidation of N supplied as DON compounds are scarce. Here, we compare oxidation rates of N supplied as a variety of DON compounds in samples from Georgia coastal waters, where nitrifier communities are numerically dominated by Thaumarchaeota. Our data indicate that polyamine‐N is particularly amenable to oxidation compared to the other DON compounds tested. Oxidation of N supplied as putrescine (1,4‐diaminobutane) was generally higher than that of N supplied as glutamate, arginine, or urea, and was consistently 5–10% of the ammonia oxidation rate. Our data also suggest that the oxidation rate of polyamine‐N may increase as the length of the carbon skeleton increases. Oxidation of N supplied as putrescine, urea, and glutamate were all highest near the coast and lower further offshore, consistent with patterns of ammonia oxidation in these waters. Though it is unclear whether oxidation of polyamine‐N reflects direct oxidation by Thaumarchaeota or combines remineralization and subsequent ammonia oxidation, more rapid oxidation of N from putrescine compared to amino acids or urea suggests that polyamine‐N may contribute significantly to nitrification in the ocean.
Small-scale armoring placed near the marsh-upland interface to protect single-family homes is widespread but understudied. Using a nested, spatially blocked sampling design on the coast of Georgia, USA, we compared the biota and environmental characteristics of 60 marshes adjacent to either a bulkhead, a residential backyard with no armoring, or an intact forest. We found that marshes adjacent to bulkheads were at lower tidal elevations and had features typical of lower elevation marsh habitats: high coverage of the marsh grass Spartina alterniflora, high density of crab burrows, and muddy sediments. Marshes adjacent to unarmored residential sites had higher soil water content and lower porewater salinities than the armored or forested sites, suggesting that there may be increased freshwater input to the marsh at these sites. Deposition of Spartina wrack on the marsh-upland ecotone was negatively related to elevation at armored sites and positively related at unarmored residential and forested sites. Armored and unarmored residential sites had reduced densities of the high marsh crab Armases cinereum, a species that moves readily across the ecotone at forested sites, using both upland and high marsh habitats. Distance from the upland to the nearest creek was longest at forested sites. The effects observed here were subtle, perhaps because of the small-scale, scattered nature of development. Continued installation of bulkheads in the southeast could lead to greater impacts such as those reported in more densely armored areas like the northeastern USA. Moreover, bulkheads provide a barrier to inland marsh migration in the face of sea level rise. Retaining some forest vegetation at the marsh-upland interface and discouraging armoring except in cases of demonstrated need could minimize these impacts.
Cascading consequences of predator extinctions are well documented, but impacts of perturbations to predator size‐structure and how these vary across species remain unclear. Body size is hypothesized to be a key trait governing individual predators’ impact on ecosystems. Therefore, shifts in predator size-structure should trigger ecosystem ramifications which are consistent across functionally similar species. Using a US salt marsh as a model system, we tested this hypothesis by manipulating size class (small, medium, and large) and size diversity (combination of all three size classes) within two closely related and functionally similar predatory crab species over 4 months. Across treatments, predators suppressed densities of a dominant grazer and an ecosystem engineer, enhanced plant biomass, and altered sediment properties (redox potential and saturation). Over the metabolically equivalent experimental predator treatments, small size class predators had stronger average impacts on response variables, and size class interacted with predator species identity to drive engineer suppression. Within both predator species, size diversity increased
cannibalism and slightly weakened the average impact. These results show that predator impacts in a salt marsh ecosystem are determined by both size class and size diversity; they also highlight that size class can have species‐dependent and response‐dependent effects, underlining the challenge of generalizing trait effects.
Tidal freshwater ecosystems experience acute seawater intrusion associated with periodic droughts, but are expected to become chronically salinized as sea level rises. Here we report the results from an experimental manipulation in a tidal freshwater Zizaniopsis miliacea marsh on the Altamaha River, GA where diluted seawater was added to replicate marsh plots on either a press (constant) or pulse (2 months per year) basis. We measured changes in porewater chemistry (SO42−, Cl−, organic C, inorganic nitrogen and phosphorus), ecosystem CO2 and CH4exchange, and microbial extracellular enzyme activity. We found that press (chronic) seawater additions increased porewater chloride and sulfate almost immediately, and ammonium and phosphate after 2–4 months. Chronic increases in salinity also decreased net ecosystem exchange, resulting in reduced CO2 and CH4 emissions from press plots. Our pulse treatment, designed to mimic natural salinity incursion in the Altamaha River (September and October), temporarily increased porewater ammonium concentrations but had few lasting effects on porewater chemistry or ecosystem carbon balance. Our findings suggest that long-term, chronic saltwater intrusion will lead to reduced C fixation and the potential for increased nutrient (N, P) export while acute pulses of saltwater will have temporary effects.
Coastal low-salinity marshes are increasingly experiencing periodic to extended periods of elevated salinities due to the combined effects of sea level rise and altered hydrological and climatic conditions. However, we lack the ability to predict detailed vegetation responses, especially for saline pulses that are more realistic in nature than permanent saline presses. In this study, we exposed common freshwater and brackish plants to different durations (1–31 days per month for 3 months) of saline water (salinity of 5). We found that Zizaniopsis miliacea was more tolerant to salinity than the other two freshwater species, Polygonum hydropiperoides and Pontederia cordata. We also found that Zizaniopsis miliaceabelowground and total biomass appeared to increase with salinity pulses up to 16 days in length, although this relationship was quite variable. Brackish plants, Spartina cynosuroides, Schoenoplectus americanus and Juncus roemerianus, were unaffected by the experimental treatments. Our experiment did not evaluate how competitive interactions would further affect responses to salinity but our results suggest the hypothesis that short pulses of saline water will increase the cover of Zizaniopsis miliacea and decrease the cover of Polygonum hydropiperoides and Pontederia cordata in tidal freshwater marshes, thereby reducing diversity without necessarily affecting total plant biomass.
Tidal fresh marshes are at least as productive as nearby salt marshes, but much less is known about controls on primary production in tidal fresh vs. salt marshes. We studied a tidal fresh marsh in Georgia, U.S.A., dominated by the C3 grass Zizaniopsis miliacea. We documented seasonal variation in Z. miliacea above‐ground biomass and below‐ground macro‐organic matter over 1 yr, and annual variation in end‐of‐season aboveground biomass over 15 yr in creekbank and midmarsh zones. Aboveground biomass showed a distinct peak in July and October. Belowground macro‐organic matter was much greater than aboveground biomass and peaked in October. Overall productivity was similar to that of salt marshes downstream. Z. miliacea end‐of‐season above‐ground biomass showed a classic hump‐shaped “subsidy‐stress” relationship with plot elevation, but on average the creekbank supported about twofold more above‐ground biomass than the midmarsh, and both zones varied in biomass about 1.7‐fold among years. Annual variation in above‐ground biomass was negatively correlated with maximum and mean temperature in both zones, and positively with river discharge in the creekbank zone. Sea level, precipitation and water column salinity showed biologically plausible trends with respect to biomass. The responses of Z. miliacea to abiotic drivers were muted compared with the responses of nearby salt marshes dominated by Spartina alterniflora. Temperature was more important for Z. miliacea, whereas drivers of porewater salinity were more important in the salt marsh. Likely future changes in temperature, precipitation, and river discharge may pose a threat to the high productivity of tidal fresh marshes.
The outcome of species interactions may manifest differently at different spatial scales; therefore, our interpretation of observed interactions will depend on the scale at which observations are made. For example, in ladybeetle–aphid systems, the results from small‐scale cage experiments usually cannot be extrapolated to landscape‐scale field observations. To understand how ladybeetle–aphid interactions change across spatial scales, we evaluated predator–prey interactions in an experimental system. The experimental habitat consisted of 81 potted plants and was manipulated to facilitate analysis across four spatial scales. We also simulated a spatially explicit metacommunity model parallel to the experiment. In the experiment, we found that the negative effect of ladybeetles on aphids decreased with increasing spatial scales. This pattern can be explained by ladybeetles strongly suppressing aphids at small scales, but not colonizing distant patches fast enough to suppress aphids at larger scales. In the experiment, the positive effects of aphids on ladybeetles were strongest at three‐plant scale. In a model scenario where predators did not have demographic dynamics, we found, consistent with the experiment, that both the effects of ladybeetles on aphids and the effects of aphids on ladybeetles decreased with increasing spatial scales. These patterns suggest that dispersal was the primary cause of ladybeetle population dynamics in our experiment: aphids increased ladybeetle numbers at smaller scales because ladybeetles stayed in a patch longer and performed area‐restricted searches after encountering aphids; these behaviors did not affect ladybeetle numbers at larger spatial scales. The parallel experimental and model results illustrate how predator–prey interactions can change across spatial scales, suggesting that our interpretation of observed predator–prey dynamics would differ if observations were made at different scales. This study demonstrates how studying ecological interactions at a range of scales can help link the results of small‐scale ecological experiments to landscape‐scale ecological problems.
Mid-summer peaks in the abundance of Thaumarchaeota and nitrite concentration observed on the Georgia, USA, coast could result from in situ activity or advection of populations from another source. We collected data on the distribution of Thaumarchaeota, ammonia-oxidizing betaproteobacteria (AOB), Nitrospina, environmental variables and rates of ammonia oxidation during six cruises in the South Atlantic Bight (SAB) from April to November 2014. These data were used to examine seasonality of nitrification in offshore waters and to test the hypothesis that the bloom was localized to inshore waters. The abundance of Thaumarchaeota marker genes (16S rRNA and amoA) increased at inshore and nearshore stations starting in July and peaked in August at >107 copies L−1. The bloom did not extend onto the mid-shelf, where Thaumarchaeota genes ranged from 103 to 105 copies L−1. Ammonia oxidation rates (AO) were highest at inshore stations during summer (to 840 nmol L−1 d−1) and were always at the limit of detection at mid-shelf stations. Nitrite concentrations were correlated with AO (R = 0.94) and were never elevated at mid-shelf stations. Gene sequences from samples collected at mid-shelf stations generated using Archaea 16S rRNA primers were dominated by Euryarchaeota; sequences from inshore and nearshore stations were dominated by Thaumarchaeota. Thaumarchaeota were also abundant at depth at the shelf-break; however, this population was phylogenetically distinct from the inshore/nearshore population. Our analysis shows that the bloom is confined to inshore waters during summer and suggests that Thaumarchaeota distributions in the SAB are controlled primarily by photoinhibition and secondarily by water temperature.
In coastal marsh ecosystems, porewater salinity strongly affects vegetation distribution and productivity. To simulate marsh porewater salinity, an integrated, spatially explicit model was developed, accounting for tidal inundation, evaporation, and precipitation, as well as lateral and vertical exchanges in both surface waters and the subsurface. It was applied to the Duplin River marsh, Sapelo Island, USA, over a 3-year period, which covered both drought and wet conditions. Simulated porewater salinity in the low and high marsh correlated with Duplin River salinity, with evapotranspiration and precipitation leading to substantial variations in porewater salinities across seasons, in particular in the high marsh. The model revealed substantial interannual variability in marsh soil conditions, and—due to its process-based approach linked to external forcings—can be used to explore effects of sea level rise and changes in hydrological forcings on marsh soil conditions.
We measured plant community composition and productivity, soil accretion, and C, N, and P burial in a tidal freshwater forest of the Altamaha River, Georgia to gain a better understanding of the ecosystem services they deliver and their ability to keep pace with current and future rates of sea level rise. Ten species were identified in two 0.1 ha plots. Nyssa aquatica (Tupelo Gum) made up 50% of the density and 57% of the total basal area. Nyssa biflora, Liquidambar styraciflua, and Fraxinus pennsylvanica were the next dominant species, collectively accounting for 37% of the density and 26% of the total basal area. Taxodium distichum only accounted for 3% of the density, but 12% of the total basal area. Aboveground productivity, measured as litterfall and stem wood growth, averaged 927 and 1030 g/m2 in 2015 and 2016, respectively, with litterfall accounting for 60% of the total. Tidal forest soils in the streamside and the interior (0–60 cm) contained 3–6% organic C, 0.20–0.40% N, and 270–540 µg/g P. Soil accretion based on 137Cs was 4.0 mm/year on the streamside and 0.2 mm/year in the forest interior. The rate of accretion in the interior is considerably less than the current rate of sea level rise (3.1 mm/year) along the Georgia coast. Because the accretion rate was much higher on the streamside, rates of C sequestration, N and P accumulation, and mineral sediment deposition also were much greater. Low accretion rates in the interior of the forest that accounts for most of the acreage suggests that accelerated sea level rise is likely to lead to foreseeable death of tidal forests from saltwater intrusion and submergence.
Despite the importance of tidal ecosystems in the global carbon budget, the relationships between environmental drivers and carbon dynamics in these wetlands remain poorly understood. This limited understanding results from the challenges associated with in situ flux studies and their correlation with satellite imagery which can be affected by periodic tidal flooding. Carbon dioxide eddy covariance (EC) towers are installed in only a few wetlands worldwide, and the longest eddy-covariance record from Georgia (GA) wetlands contains only two continuous years of observations. The goals of the present study were to evaluate the performance of existing MODIS Gross Primary Production (GPP) products (MOD17A2) against EC derived GPP and develop a tide-robust Normalized Difference Moisture Index (NDMI) based model to predict GPP within a Spartina alterniflora salt marsh on Sapelo Island, GA. These EC tower-based observations represent a basis to associate CO2 fluxes with canopy reflectance and thus provide the means to use satellite-based reflectance data for broader scale investigations. We demonstrate that Light Use Efficiency (LUE)-based MOD17A2 does not accurately reflect tidal wetland GPP compared to a simple empirical vegetation index-based model where tidal influence was accounted for. The NDMI-based GPP model was capable of predicting changes in wetland CO2 fluxes and explained 46% of the variation in flux-estimated GPP within the training data, and a root mean square error of 6.96 g C m−2 in the validation data. Our investigation is the first to create a MODIS-based wetland GPP estimation procedure that demonstrates the importance of filtering tidal observations from satellite surface reflectance data.
Vorobev, A., Sharma, S., Yu, M., Lee, J., Washington, B., Whitman, W., Ballantyne, F., Medeiros, P.M. and Moran, M.A. 2018. Identifying labile DOM components in a coastal ocean through depleted bacterial transcripts and chemical signals. Environmental Microbiology. 20:3012-3030. (DOI: 10.1111/1462-2920.14344)
Understanding which compounds comprising the complex and dynamic marine dissolved organic matter (DOM) pool are important in supporting heterotrophic bacterial production remains a major challenge. We eliminated sources of labile phytoplankton products, advected terrestrial material and photodegradation products to coastal microbial communities by enclosing water samples in situ for 24 h in the dark. Bacterial genes for which expression decreased between the beginning and end of the incubation and chemical formulae that were depleted over this same time frame were used as indicators of bioavailable compounds, an approach that avoids augmenting or modifying the natural DOM pool. Transport‐ and metabolism‐related genes whose relative expression decreased implicated osmolytes, carboxylic acids, fatty acids, sugars and organic sulfur compounds as candidate bioreactive molecules. FT‐ICR MS analysis of depleted molecular formulae implicated functional groups ~ 30–40 Da in size cleaved from semi‐polar components of DOM as bioreactive components. Both gene expression and FT‐ICR MS analyses indicated higher lability of compounds with sulfur and nitrogen heteroatoms. Untargeted methodologies able to integrate biological and chemical perspectives can be effective strategies for characterizing the labile microbial metabolites participating in carbon flux.
Predators can significantly affect prey by removing prey individuals and by changing prey behavior. The tradeoff between foraging behavior and predation risk may result in a trophic cascade that can have important effects on ecosystem processes. For herbivores that can feed both above‐ and belowground, it is likely that predation risk affects the location of feeding. We tested whether two species of predatory marsh crabs affected feeding behavior of the herbivorous crab, Sesarma reticulatum. We found that predatory crabs could kill or injure Sesarma and that Sesarma did less damage to its food plant Spartina alterniflora in the presence of the more dangerous predator. Sesarma prefers to feed on and grows better on belowground rhizomes than aboveground leaves; however, the costs of digging burrows to access rhizomes lead to higher mortality if it is the only diet option. The location of feeding did not affect total biomass of S. alterniflora. For Sesarma, a choice in feeding location allows the crabs the behavioral flexibility to balance the risks of predation, the nutritional benefit of feeding belowground, and the survival costs of belowground feeding. Similar tradeoffs are likely to increase the success of other herbivores that can feed both above‐ and belowground.