Hypoxia affects spatial distributions and overlap of pelagic fish, zooplankton, and phytoplankton in Lake Erie

Bottom hypoxia has reemerged as a prominent feature of Lake Erie's central basin during late summer. Similar to coastal and marine systems, the influence of hypoxia on pelagic organisms remains largely enigmatic in Lake Erie. During 2005, we used a plankton survey system (a sensor package consisting of an optical plankton counter, fluorometer, dissolved oxygen sensor, light sensor, and conductivity–temperature–depth sensor), coupled with a fish hydroacoustics system, to explore how the distribution of phytoplankton (chlorophyll), mesozooplankton, and fish varied vertically and horizontally in relation to oxygen concentrations. To do so, we conducted surveys of the entire water column on a continuous basis during mild (August) and severe (September) hypoxia. Our surveys included two sampling designs: 1) basin-wide transects sampled during day and night to define broad-scale patterns of spatial overlap among pelagic organisms; and 2) shorter (5 km) transects sampled every 4 h over a 24-h period to explore how diel vertical migration and hypoxia interact to affect time-specific spatial overlap among fishes, mesozooplankton, and phytoplankton. Our findings indicated that fish avoided regions of the hypolimnion with dissolved oxygen concentrations < 3 mg l− 1 by 1) moving horizontally into areas with higher oxygen or 2) moving vertically into the metalimnion, where a sharp thermocline and oxycline existed. A portion of the mesozooplankton continued to use the hypoxic hypolimnion as a refuge from fish predation during daytime at oxygen concentrations between 1 and 3 mg l− 1; however, there was usually a mesozooplankton maximum in the metalimnion, even when fish were compressed into this region. Prior to development of hypoxia, the metalimnion in some areas may have served as thermal refuge from predation from the epilimnetic planktivore, the emerald shiner, and the hypolimnetic planktivore–benthivore, the rainbow smelt. Overall, the horizontal compression of fish into less-hypoxic regions in the deep area of the central basin followed by vertical compression into the metalimnion as hypoxia developed further may have led to local reduction of mesozooplankton prey in these regions. Herein, we discuss the potential implications of these hypoxia-induced impacts for understanding food web interactions and fisheries management.