RESPIRATION RATES AND HYPOXIA ON THE LOUISIANA SHELF

The spatial and temporal variation in water-column respiration, estima ted from enzymatic respiratory electron-transport-system activity, was measured monthly on a cross-shelf transect on the Louisiana shelf fro m May through October 1991. In July 1991, water-column respiration was also determined on an alongshore transect, and in situ benthic respir ation and photosynthesis rates were determined at four stations on the cross-shelf transect. Bottom waters were persistently hypoxic (O-2 < 2 mg l(-1)) at most stations in July and August and sporadically hypox ic at other times. Water-column respiration rates were in the same ran ge as earlier, less extensive studies and not unusually high for coast al and estuarine waters. They were highest in summer, decreased with d istance offshore and depth, and increased with temperature. Their vari ation with pigment and oxygen concentrations were complex functions of season and depth. Oxygen depletion below the oxycline could occur wit hin days to months, depending on the season and location. In July, ben thic respiration rates were also not unusually high in comparison with other shallow sediments, although the ratio of benthic:total (water c olumn + benthic) respiration was high. Combined water-column and benth ic respiration could deplete the bottom water oxygen in approximately 1 mo. Because the system rarely goes anoxic (defined as observing sulf ide), some mechanism(s) must exist to reaerate bottom waters. Most phy sical mechanisms are unlikely to provide significant reaeration at thi s time of year. Measured benthic and conservatively estimated bottom-w ater photosynthesis could resupply 23% of the oxygen lost daily by res piration. Although this is too limited a dataset from which to draw co nclusions about the relative importance of bottom-water and benthic re spiration and photosynthesis in determining bottom-water oxygen concen trations, it does suggest that all these processes must be considered.