At first glance, the Sapelo saltmarsh appears to be composed
only of smooth cordgrass (Spartina alterniflora). There are numerous
other plants present also, but Spartina is dominant in most of the lower
lying, tide dominated areas. Closer inspection will show that
the Spartina in the vicinity of the creeks and levees is much taller
and more luxuriant than that on the marginally higher ground, where growth
is shorter and more sparse.
A number of factors may be influential
in determining the height and luxuriance of Spartina. The pattern
of water flow through the creek bank is active and the mud is “flushed”
by tidal action even though it remains largely anaerobic (all oxygen
is used up). On the other hand, the water trapped in the mud of the
high marsh behind the levees is not flushed out on neap tides and becomes
more saline and stagnant with time. Nutrients are used up, and
toxic substances such as sulphide build up. Differences between
the creek banks and the high marsh are evident not only in the above
ground growth, but also in the root growth and the bacterial interactions
occurring within the soil itself. All marsh soils are anaerobic except
for the surface few millimeters and around the roots and burrows.
The soil bacteria responsible for the breakdown of accumulated organic matter
require just such an anaerobic environment. However the rate at which
such breakdown occurs and hence the rate at which plant nutrients are
made available for new growth appears to be related to the water flow characteristics
and the rate of removal of waste products.
Nitrogen and phosphorus are two major
nutrients required by plants. Phosphorus is readily available
both in the soil and in the tidal waters. Nitrogen is not so freely
available. Despite the fact that nitrogen gas is the most abundant
element in the air, it is not usable in that form. Higher plants require
it to be converted to ammonia, nitrate or nitrite before they can use it.
Blue green algae on the marsh surface, and nitrogen transforming bacteria
within the soil, convert nitrogen into conveniently utilizable forms.
Nitrogen is ultimately exported from the saltmarsh in the form of
dissolved and particulate nitrogen-containing organic matter. These
are important nutrients for the algae and simple animals of the water
Spartina is predominant largely because
it is so well adapted to its environment, having developed with time
the physiological mechanisms which enable it to cope with the estuarine
salinity and temperature ranges. The only other plants in the
lower marsh areas are the microalgae already referred to, such as blue
greens and diatoms, and the occasional occurrence of such simple macroalgae
as sea lettuce (Ulva lactuca).
Generally, a little higher on the marsh,
glasswort (Salicornia sp.) plants become more obvious and saltgrass
(Distichlis sp.) also appears as patches amongst the stands of Spartina.
The black needle rush (Juncus) forms thick stands and is readily distinguishable
by its fine stalks, sharp points and greyish appearance. Sea-purslane
(Sesuvium sp.), saltmarsh bulrush (Scirpus sp.), sea ox-eye (Borrichia
sp.) and the saltmarsh cordgrass (S. patens) are all present in the
“high marsh”, an area of higher ground, but shorter Spartina growth.
Another feature of this area is the presence of salt barrens, a highly
saline area where nothing but diatoms grow.
The hammocks are areas of high ground
above all normal tides and support tall vegetation such as redcedars,
wax myrtles and yaupon holly (Ilex vomitoria). The hammocks stand
out conspicuously from the low lying marsh, both in terms of height and
difference in the color of the vegetation. Pumpkin and Fishing
Hammocks on the Duplin River are Pleistocene bases surrounded on three
sides by Holocene marshes.