Joint Effects of Physical Processes and Multiple Invasive Species on Great Lakes Zooplankton Production, with Implications for Fish Recruitment
RATIONALE: Declines in food availability can limit fish recruitment. For instance, recent dramatic declines in prey fish in four Great Lakes, including key forage fish (e.g., alewives) and native deepwater species (e.g., bloater), have been attributed in part to changes in zooplankton community composition and biomass. The changes in zooplankton have likely been influenced by invasive species, especially the spiny water flea (Bythotrephes longimanus) and dreissenid mussels. Although there is research to support the claim that these invasive species affect zooplankton, major gaps remain in our understanding because it is difficult to discern if and which zooplankton species are being affected and to what degree. Further, physical factors (i.e., water clarity, temperature, and stratification) may influence invader effects. We hypothesize that Bythotrephes and Dreissena jointly decrease zooplankton production to a degree that influences growth and survival of larval and juvenile prey fishes, and that these effects are mediated through physical attributes of the water column. First, by increasing water clarity, Dreissena may enhance Bythotrephes foraging on zooplankton in space (potential to forage deeper) and time. Second, Bythotrephes can increase zooplankton vertical migration to deeper, darker (safer) and colder waters, causing reduced zooplankton production – an effect mediated by stratification. Third, stratification also influences Dreissena access to phytoplankton, leading to potential bottom-up effects on zooplankton production. Such effects may alter zooplankton production in multiple habitats, with consequences for energy dynamics and fish production. A more comprehensive understanding of the direct effect of invasive species on zooplankton, and these more complex interactions, could inform recent fisheries declines and improve forecasting of prey fish recruitment.
OBJECTIVES: We will: 1) Evaluate how Bythotrephes, Dreissena (through effects on phytoplankton resources), and interactive effects (e.g. mussel effects on water clarity and thereby Bythotrephes predation rates) affect zooplankton production and vertical position. 2) Examine how additional physical factors (stratification and temperature) influence the effects of Bythotrephes and Dreissena on zooplankton production across habitats. 3) Forecast how changes in (a) invader densities, (b) oligotrophication, and (c) climate change-induced shifts in temperature and stratification influence zooplankton production, community structure, and vertical position. 4) Explore whether the predicted changes in the zooplankton community (biomass, composition, and location) will limit prey fish (bloater and alewife) recruitment through reduced first-year growth and survival.