Spatial and Temporal Patterns of Benthic Respiration and Net Nutrient Fluxes in the Atchafalaya River Delta Estuary
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Benthic respiration and nutrient flux rates were quantified during four distinct periods (spring peak in river discharge, summer period of offshore hypoxia, autumn period after breakdown of offshore hypoxia, and the non-hypoxic winter period) over an annual cycle in the Atchafalaya River Delta Estuary (ARDE). The overall mean benthic respiration rate for the ARDE was 12.9 mmol O2 m−2 day−1 with the highest and lowest rates occurring in summer and winter (22.1 and 5.4 mmol O2 m−2 day−1), respectively, and surprisingly little variance in relation to distance offshore of the deltas. Bottom water temperature explained 94 % of the variance in the natural log of benthic respiration. The mean annual fluxes for ammonium, dissolved organic nitrogen (DON), phosphate, and silicate were from the sediments to the water column (0.91, 0.44, 0.03, and 1.04 mmol m−2 day−1, respectively), but nitrate had a mean annual flux (−0.90 mmol N m−2 day−1) from the water column into the sediments. Net nitrate fluxes were greatest in summer, were generally higher at delta than at offshore sites, and were most strongly correlated with benthic respiration. Net NH4 + fluxes from the sediments to the overlying water column were greater at offshore sites than at delta sites resulting in the delta and offshore sites behaving as net dissolved inorganic nitrogen (DIN) sinks and sources, respectively. The results from this study suggest that benthic processes in the ARDE greatly alter bottom water oxygen availability as well as the size and form of nutrient fluxes exported to the northern Gulf of Mexico.
KeywordsSediment oxygen demand Benthic nutrient fluxes Benthic-pelagic coupling Metabolic theory Climate change Atchafalaya Bay
We would like to thank Carrie Semmler for help with sample collection and processing and completion of the incubation experiments, Nancy Rabalais for assistance in the original study design, and Melissa Baustian for help with the design of the core incubation setup. Sediment grain size analyses were performed in the laboratory of Alex Kolker. Carl Sevin and Max Wike served as captains of the R/V Whiskey Pass and helped with the collection of water samples and benthic cores. Comments from two anonymous reviewers greatly improved an earlier version of this manuscript. Funding for this project was provided by the Louisiana Board of Regents Support Fund (LEQSF(2008–11)-RD-A-22) and National Oceanographic and Atmospheric Administration, Center for Sponsored Coastal Ocean Research program, NGOMEX09.
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