THE DUPLIN RIVER:

The Duplin River is not a true river. It is a large tidal creek about 11 km in length separating Sapelo Island from Little Sapelo Island and entering Doboy Sound a little south of Marsh Landing.

The water in the creeks is driven by tidal influence. Water is pushed out onto the marsh by the pressure of high tide, and drains from the marsh into the creeks on low tide. This does not mean that the water that returns the next tidal cycle is “new”. On the contrary, the water of the Duplin oscillates backward and forward. The effective exchange of nutrients and organisms is dependent on such factors as water volume and velocity (influencing mixing with the sound and nearshore waters), local fresh water runoff, and the active entry and departure of motile feeding organisms.

At ebb tide the current velocity is greater than during the flood tide, for reasons relating to the hydrology of the Duplin basin. Ebb currents have been measured at 3 ft./second. The amount of water flowing into the river and onto the marshes depends on the height of the tide, substantially increasing on spring tides. Current flows are responsible for the deposition of several bar shoals, where the river makes closely spaced bends in opposite directions.

Despite the murky appearance of its water, the sediments of the bed of the Duplin are low in mud content and contain large accumulations of shell material, much of which is derived from the oyster banks along the riverbanks. At Pumpkin Hammock and Little Sapelo Island the river is eroding Pleistocene sandy deposits. Geological studies have shown the Duplin to be sediment source area (that is, an area losing sediments), rather than an area of sediment accumulation.

The river has been the site of much research since the founding of the Marine Institute. It’s nutrient flows and the microflora and fauna have all been intensively studied. Part of the attraction of the Duplin as a site is its accessibility, its oscillating water flow patterns (up and downstream, on and off the marsh), and the fact that its use as a research site for so many years has provided a very extensive data base.

Within the water column itself is to be found a range of organisms. Despite the poor light penetration, microflagellates and other microalgae are remarkable productive. Tintinnids, other ciliates, various amebae and rotifers, predatory and omnivorous copepods, and the larval forms of polychaete worms, gastropods, tunicates and fish are all to be found, varying in numbers and composition with the season. These organisms provide a food source for such fish as juvenile menhaden, a plankton feeder. Menhaden, in turn, are preyed upon by larger fish and birds. Such interaction are all part of the natural checks and balances within any healthy system. Not all of the fish normally found in the Duplin will feed on the same food source or in the same area. For example: flounder are predators who lie on the bottom awaiting an unsuspecting prey; bluefish and yellowtail are also predatory and chase their prey; mullet are deposit feeders; and mummichog (Fundulus sp.) live in the shallower creeks and headwaters where they are rarely threatened by the larger fish. Shrimp utilize the tidal creeks throughout the year, but are especially obvious in summer. Channel Bass will feed on the saltmarsh, but only on the higher spring tides, needing a certain critical depth of water before they will leave the comparative safety of the tidal creek.

Then come the larger predators -- dolphin are often seen at Marsh Landing Dock, but if one is especially lucky, they may be seen fishing close inshore in the tidal creeks, actually running mullet into the shallows. Mink hunt by the marsh edge and may sometimes be seen swimming well out in the river.

The birds fall into two groups along the course of the Duplin. Towards the Sound is pelican, gull and tern territory, while further up the river herons and osprey are to be found. Herons come in a variety of sizes, Great Blues with their long legs and necks fish deeper waters than the smaller Snowy Egrets etc., so there is a partitioning of territory between them to their mutual benefit.