Contrasting food-web support bases for adjoining river-influenced and non-river influenced continental shelf ecosystems

Nutrient and chlorophyll a concentrations and distributions in two adjoining regions of the South Atlantic Bight (SAB), Onslow Bay and nearshore Long Bay, were investigated over a 3-year period. Onslow Bay represents the northernmost region of the SAB, and receives very limited riverine influx. In contrast, Long Bay, just to the south, receives discharge from the Cape Fear River, draining the largest watershed within the State of North Carolina, USA. Northern Long Bay is a continental shelf ecosystem that has a nearshore area dominated by nutrient, turbidity and water-color loading from inputs from the river's plume. Average planktonic chlorophyll a concentrations ranged from 4.2 μg l−1 near the estuary mouth, to 3.1 μg l−1 7 km offshore in the plume's influence, to 1.9 μg l−1 at a non-plume station 7 km offshore to the northeast. Average areal planktonic chlorophyll a was approximately 3X that of benthic chlorophyll a at plume-influenced stations in Long Bay. In contrast, planktonic chlorophyll a concentrations in Onslow Bay were normally <0.50 μg l−1 at a nearshore (8 km) site, and <0.15 μg l−1 at sites located 45 and 100 km offshore. However, high water clarity (KPAR 0.10-0.25 m−1) provides a favorable environment for benthic microalgae, which were abundant both nearshore (average 58.3 mg m−2) and to at least 45 km offshore in Onslow Bay (average 70.0 mg m−2) versus average concentrations of 10-12 mg m−2 for river-influenced areas of Long Bay. This provides evidence that much of the inner shelf food web in Onslow Bay is based on benthic microalgal production, in contrast to a plankton-based food web in northern Long Bay and more southerly areas of the SAB.