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 µm) 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 µm 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.KEY WORDS: Microphytobenthos · Benthic microalgae · Cyanobacteria · Phytoplankton · Hypoxia · Northern Gulf of Mexico · Mississippi River Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 436: 51-66, 2011 52 the primary productivity (~50%) from spring through fall sinks to the bottom primarily as fecal pellets (which average 55% of particulate organic flux) (Qureshi 1995), diatom chains, and aggregates of diatoms and/or picocyanobacteria (Dortch et al. 2001). This high flux of organic matter increases the respiratory demand in the bottom water and sediments, and leads to oxygen depletion (Turner & Allen 1982). The stratification between the surface and bottom water acts as a barrier to re-aeration of the bottom water, thus increasing the likelihood of hypoxic water formation.The bottom waters have the potential to become anoxic within 4 wk or less if the organic matter supply is sufficient and bottom water temperatures are warm enough (Turner et al. 1998). If there is no mixing of the stratified layers, the time to reduce the bottom water oxygen concentration from about 6 to less than 2 mg l -1 (based on decline of continuous oxygen concentration data) is 18, 11 and 9 d in April, May and July, respectively (Rabalais et al. 2007a). Murrell & Lehrter (2011), using below-pycnocline total respiration, estimated that 22 d were required for the lower water column to go from saturation to hypoxia.Benthic photosynthesis is one process that could affect oxygen concentrations in the bottom water and could explain why severely depleted botto...