Effects of bottom-layer hypoxia on abundances and depth distributions of organisms in Patuxent River, Chesapeake Bay

Oxygen depletion, seasonally common in bottom waters of many stratified aqu atic systems, may have strong effects on abundances, distributions, and int eractions among organisms, and therefore community dynamics. To examine eff ects of bottom-layer hypoxia on densities and vertical distributions in a s tratified subestuary, fish larvae, their gelatinous predators, and copepod prey were surveyed near-surface, within the pycnocline, and near-bottom in the Patuxent River (Chesapeake Bay) under a range of near-bottom dissolved oxygen (DO) conditions. Overall abundances of fish larvae and copepod naupl ii were lower throughout the water column when bottom-layer DO was low (les s than or equal to2 mg DO l(-1)). When bottom-layer DO was low (less than o r equal to2 mg l(-1)), densities of naked goby larvae were less than one-th ird of those observed during high (>2 mg l(-1)) DO conditions, and overall density of copepod nauplii declined by >50%. Depth-distributions of several organisms also were affected by bottom-oxygen depletion: fish larvae, scyp homedusae, and copepods were much less common near the bottom when bottom-l ayer DO was low than when it was >2 mg l(-1). The ctenophore Mnemiopsis lei dyi occurred in high densities at DO as low as 1.3 mg l(-1), but was nearly absent at <1 mg l(-1). The results indicate the potential for substantial differences in organism interactions, especially predator-prey relationship s, between times of high and low bottom-layer DO.