Seasonal microphytobenthos on the hypoxic northern Gulf of Mexico continental shelf

The presence of photosynthetic organisms on the seafloor may indicate whether oxygen evolution contributes to the bottom water oxygen pool in the hypoxic area of the northern Gulf of Mexico. We sampled 3 stations (depth: 14, 20 and 23 m) 100 km west of the mouth of the Mississippi River over 3 hypoxic annual cycles to determine whether microphytobenthos or settled phytoplankton existed on the sediment surface. Microscopy and high-performance liquid chromatography were used to determine the presence and composition, and to estimate the biomass of microphytobenthos and phytoplankton in surface and bottom waters and sediments. The sediment community (cells > 3 mu m) found during hypoxia differed from those in the water column and were primarily benthic (58 to 88%). Settled pelagic phytoplankton (1 to 36%) and tychopelagic phytoplankton (5 to 10%) were also present. The settled phytoplankton were mostly present on the sediment during fall and winter. The abundance of benthic cells was directly correlated with light levels on the seafloor and sediment chlorophyll a values. Picocyanobacteria, pennate diatoms and filamentous cyanobacteria dominated the sediment community (by density for all cells 0.2 to 8.0 mu m in diameter) during summer. The presence of a viable community of microphytobenthos during hypoxia indicates that the potential for photosynthetic oxygen production exists and may influence the oxygen dynamics in the hypoxic zone.