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

Abstract

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 μM) and chlorophyll a (138 μg/l) concentrations were measured during spring and fall, respectively. Sediments were a net source of NH 4 + (102 μmol m−2 h−1) and PO 4 3− (0.9 μmol m−2 h−1) but a sink for NO 3 − (−30 μmol m−2 h−1). Benthic N2 fluxes indicated net N fixation (12 μmol N m−2 h−1). Sediment oxygen demand (0.55 g O2 m−2 day−1) accounted for <10% of R (7.3 g O2 m−2 day−1). Despite high GPP rates (4.7 g O2 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.