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 aquatic systems, may have strong effects on abundances, distributions, and interactions among organisms, and therefore community dynamics. To examine effects of bottom-layer hypoxia on densities and vertical distributions in a stratified 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 nauplii were lower throughout the water column when bottom-layer DO was low ( 2 mg DO l -1 ). When bottom-layer DO was low ( 2 mg l -1 ), densities of naked goby larvae were less than one-third 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, scyphomedusae, and copepods were much less common near the bottom when bottom-layer DO was low than when it was > 2 mg l -1 . The ctenophore Mnemiopsis leidyi 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 predatorprey relationships, between times of high and low bottom-layer DO.