In contrast to the winter months, when offshore Gulf Stream waters are comparatively warmer and richer in nutrients, estuaries offer juvenile marine life better sanctuaries and more plentiful food supplies in the spring and summer, Crowder said.

"They make the transition into an estuary at about the time when it becomes warmer and more productive than the continental shelf," he said. "They evolved that lifestyle for a number of reasons. Basically, their life history tracks productivity cycles."

Some scientists also have proposed an "out-welling hypothesis," which suggests that organic material emanating from estuarine salt marshes also finds its way to the offshore waters, where it provides nutrients to animals that live there, Crowder added.

"The out-welling hypothesis provides a lot of impetus for the protection of salt marsh habitats because those not only support the nursery phase of food webs, but their continental shelf phase as well," he said.

A second hypothesis posits that such nutrients do not diffuse directly from estuaries to the open continental shelf ocean, but rather get there indirectly within the bodies of the fish that move back out there after their juvenile phase. Crowder said that second hypothesis might be especially relevant in North Carolina estuaries, because outward water movement there is restricted by the barrier island chain known as the Outer Banks.

Both scenarios would make estuaries another kind of nutrient source for ocean life in addition to the well-known upwelling zones off California, Africa and Chile, he said.

Crowder's bottom line is that "if we want to keep these systems working to our benefit in terms of fisheries production, we need to protect the nursery area habitats so that fish can feed and grow successfully in there.

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Contact: Monte Basgall
Monte.basgall@duke.edu
919-681-8057
Duke University
24-Jan-1999