Decades of environmental justice research has focused on identifying existing patterns of disproportionate burdens to environmental harms across social difference. However, relatively few studies examine the “legacy effect” of historical patterns. In flood risk studies specifically, several scholars have highlighted the role of systemic processes in historically shaping and producing observed disparities in flood risk patterns. These studies reveal that such relations are tied to histories of racialized land struggles and territorial dispossessions. In this paper, I argue that scholars need to do more than quantify today’s disproportionate burdens across social difference or explain the systemic processes causing those disparities. I suggest that “legacy vulnerability” helps identify how the potential for harm from flood risk to marginalized groups may reside in events of the past that have imprinted a spatially hidden, but spatiotemporally revealed unjust pattern upon today’s landscape. In a flood risk assessment of Sapelo Island, the initial results suggest that when comparing contemporary flood risk of Sapelo’s Geechee descendant (Black and mostly low-to-middle income) to non-descendant newcomer owners (mostly white and affluent) an environmental justice disparity in proportional flood risk burden does not exist. However, results of a counterfactual flood risk assessment show that approximately one-third of historically owned, Geechee property is located outside the contemporary 100-year flood zone compared to zero percent outside of it today. In other words, roughly one-third of Geechee property’s flood risk today is a legacy vulnerability directly tied to racialized land dispossessions that unfolded in the middle twentieth century.
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.