The Contribution of Benthic Nutrient Regeneration to Primary Production in a Shallow Eutrophic Estuary, Weeks Bay, Alabama

Benthic oxygen, dinitrogen, and nutrient fluxes (NH (4) (+) , NO (3) (-) , and PO (4) (3-) ) were measured monthly during a 1-year period at two locations in Weeks Bay, a shallow (1.4 m) and eutrophic estuary in Alabama. Gross primary productivity (GPP), ecosystem respiration (R), and net ecosystem metabolism were determined from high-frequency dissolved oxygen measurements. Peak water column NO (3) (-) (55 mu M) and chlorophyll a (138 mu g/l) concentrations were measured during spring and fall, respectively. Sediments were a net source of NH (4) (+) (102 mu mol m(-2) h(-1)) and PO (4) (3-) (0.9 mu mol m(-2) h(-1)) but a sink for NO (3) (-) (-30 mu mol m(-2) h(-1)). Benthic N-2 fluxes indicated net N fixation (12 mu mol N m(-2) h(-1)). Sediment oxygen demand (0.55 g O-2 m(-2) day(-1)) accounted for < 10% of R (7.3 g O-2 m(-2) day(-1)). Despite high GPP rates (4.7 g O-2 m(-2) day(-1)), the estuary was net heterotrophic. Benthic regeneration supplied, on average, 7.5% and 4% of primary productivity N and P demands, respectively. These results contrast with the conventional view that benthic regeneration accounts for a large fraction of phytoplankton nutrient demand in shallow estuaries.