Respiration rates and hypoxia on the Louisiana shelf

Dortch, Q., N.N. Rabalais, R.E. Turner, and G.T. Rowe

ABSTRACT

The spatial and temporal variation in water column respiration, estimated from enzymatic respiratory electron-transport-system activity, was measured monthly on a cross-shelf transect on the Louisiana shelf from May through October 1991. In July 1991, water-column respiration was also determined at four stations on the cross-shelf transect. Bottom waters were persistently hypoxic (O2 < 2 mg l-1) at most stations in July and August and sporadically hypoxic at other times. Water-column respiration rates wee in the same range as earlier, less extensive studies and not unusually high for coastal and estuarine waters. They were higher in summer, decreased with distance offshore and depth, and increased with temperature. Their variation with pigment and oxygen concentrations were complex functions of season and depth. Oxygen depletion below the oxycline could occur within days to months, depending on the season and location. In July, benthic respiration rates were also not unusually high in comparison with other shallow sediments, although the ratio of benthic:total (water column + benthic) respiration was high. Combined water-column and benthic respiration could deplete the bottom water oxygen in approximately 1 mo. Because the system rarely goes anoxic (defined as observing sulfide), some mechanism(s) must exist to reaerate bottom waters. Most physical mechanisms are unlikely to provide significant reaeration at this time of year. Measured benthic and conservatively estimated bottom-water photosynthesis could resupply 23% of the oxygen lost daily by respiration. Although this is too limited a dataser from which to draw conclusions about the relative importance of bottom-water and benthic respiration and photosynthesis in determining bottom-water oxygen concentrations, it does suggest that all these processes must be considered.