THE CONTINENTAL SHELF/NEARSHORE:
The continental shelf off the coastline
is shallow and about 70-75 miles wide. The inner boundary of
the Gulf Stream coincides approximately with the outer edge of the shelf.
This major oceanic current flows northeast, contrasting with the coastal
water flow, which is generally to the south. Because of the wide
shelf and the containing influence of the Gulf Stream, the shelf water
is discretely different from that of the open ocean. It is influenced
by input from the Savannah and the Altamaha rivers, together with
that of several minor rivers, which discharge into the coastal water
via the saltmarsh distributary systems. The water is mixed via the
combined agencies of tides, wind and waves.
The inshore water near Sapelo Island
is therefore less salt than true seawater and turbid because of the
suspension of clays, phytoplankton (microscopic plant life), and fine
particles of organic matter. This turbidity prevents the light penetration
necessary for photosynthesis. Consequently, there are few plants
growing on the nearshore bottom. However, it appears that the phytoplankton
suspended in the water has adapted to low light levels and that the
turbulence created by wave action exposes these organisms to some light
at sufficiently frequent intervals for them to be very productive.
Much of the activity here is unseen. Within the bottom sand, bacteria
grow on an accumulation of organic matter. They, in turn, become
part of the food supply of larger organisms and also produce nutrients
which may be used by phytoplankton.
Just offshore there is a system of shoals,
which can be seen as a line of breakers, or occasionally exposed
as sandbars at low tide. These shoals influence the pattern of
sedimentation, which is also affected by wave action and longshore
currents. On Nannygoat Beach (E) the shoals are themselves strongly
influenced and determined by the ebb flow delta pattern of Doboy Sound.
Cabretta’s wave-formed spits and shoals are commonly exposed at low tide.
This is a very interesting area in which to see evidence of a large range
of intertidal and displaced subtidal animals.
The animal community in the nearshore must contend with
poor visibility and make use of other sensory capacities. The
dominant animals found here are shrimp; horseshoe crabs (a living fossil,
not a crab at all); a variety of worms, both free-living and tube worms;
molluscs, especially bivalves and whelks; sea pansies, at the low water
mark; and an assortment of fish which tends to vary from season to season.
Mullet and channel bass are found in summer, and sharks and dolphins feed
fairly close inshore.