All Georgia estuaries are the sites of
tremendous water exchange, as there are two tidal cycles daily with
an average difference between low and high tides of about 7 feet.
During each tidal cycle about 600 square miles of saltmarsh and tidal
creeks are drained and refilled. This often means that most of the
water returning in the next tidal cycle is the same water, which has
recently drained off the marshes. Mixing depends on a number of
factors and varies from estuary to estuary.
Doboy Sound is considered an estuary,
not in the more classical category of a river mouth entering the sea,
but because it drains a substantial area of saltmarsh through a break
in the barrier island chain. Also, the salinity is always less than
that of oceanic water. This lower salinity is caused by a combination
of local runoff, and freshwater input from the major rivers to the north.
The Altamaha River to the south sometimes has a major effect when heavy
outflow backs up through the network of tidal creeks and the Intracoastal
Waterway. This input may then be seen as a front of different colored
water running NW-SE along Doboy Sound. Tidal fronts are usually marked
by a narrow line of foam and run crossways to the water flow.
The combination of wind strength and
direction and tidal flow can produce local boating problems in Doboy
Sound. At times the current flow may be substantial, and when there
is an ebb tide with an opposing southeast wind, steep waves can make small
boat travel uncomfortably rough. This is the summer wind pattern.
Conversely a northwest wind opposing and incoming tide may also be rough.
Northerlies and Nor-easterlies are not usually a problem. Spring
and early summer bring their share of thunderstorms, with the concomitant
hazards of lightning, heavy rain and local violent wind squalls, which
may also pose problems for small boats.
While the bottom of Doboy Sound is largely
composed of clays and silts, sand is present in the outer sound as
laminated beds. Near the mouth of the sound, the influence of the
adjacent shore is evident in an increased grain size and higher energy
bottom structure. High-energy areas may contain deposits of shells.
Brittle stars burrow at the sound margins, and the bottom muds are known
to contain polychaete worms and the clam Mulinia lateralis. Spring
and summer trawls for student groups have produced a wide variety of
animals, including crabs; white, brown and mantis shrimps; squid; jellyfish;
catfish; star drum, tongue sole and southern flounder. When trawls
have been conducted in the winter the numbers and variety of these larger
animals are less.
Many microscopic animals may be found
in the water itself, larval forms of crabs, fish, polychaete worms,
molluscs; copepods in large numbers and of many varieties; ciliated
and flagellated protozoans and amebae. Microscopic algae are also
present, especially microflagellates, which are extremely important
in the productivity of the water column. The seasonal variations
in numbers and types of organisms found is considerable, as are the differences
between these organisms normally found in the creeks and those of the
sound or the open sea.