Microphytobenthos of the northern Gulf of Mexico hypoxic area and their role in oxygen dynamics

Abstract

The presence or absence of microphytobenthos on the seafloor provides clues about whether benthic oxygen evolution contributes significantly to the oxygen budget of the hypoxic area in the northern Gulf of Mexico. Hypoxia (oxygen < 2 mg l-1) creates inadequate concentrations of dissolved oxygen to support most organisms, such as fish, shrimp and crabs, and occurs over large areas of the Louisiana continental shelf from spring through summer in most years. Oxygen production by benthic autotrophs may offset a decline in oxygen concentrations if there is a functioning community and sufficient light. I sampled three stations (14, 20 and 23 m depths) ~ 100 km west of the Mississippi River over three hypoxic annual cycles (2006 – 2008), and 11 stations along a 14 - 20 m contour on the shelf in late-July in 2006, 2007 and 2008. I used microscopy and high-performance liquid chromatography to estimate the biomass and composition of phytoplankton and microphytobenthos. The potential seasonal oxygen production was estimated in 2007 and 2008 by incubating coupled light/dark sediment cores and bottom water from two stations. The sediment community (cells > 3 um) differed from those in the water column and were frequently benthic pennate diatoms and filamentous cyanobacteria (58-88% seasonally and 1-99% in mid-summer). The concentration of microphytobenthic biomass was usually < 2.0 ug g dry sed-1, and various biotic parameters were influenced by light at the seafloor. Declines in dissolved oxygen over a seasonal cycle in 2007 and 2008 were affected more by the initial dissolved oxygen concentration than by the presence of microphytobenthos that could generate oxygen. The sediment (1.2 - 27.3 mmol O2 m-2 d-1, n = 97) and bottom-water (1.1 - 17.5 mmol m-2 d-1, n = 23) oxygen consumption rates were within the range of the few previously-reported data. This work adds to these data and also provides the only sediment oxygen consumption rates at fixed sites over seasonal time scales. These results provide critical input to three-dimensional, physical-biological models of oxygen dynamics for this hypoxic area.