Did Mysis have a role in the decline of Diporeia

RATIONALE: Diporeia declines in Lakes Huron, Michigan, Erie, and Ontario have been linked to dreissenid invasions. However, despite considerable research effort, we still do not know the causal mechanism. Continued co-existence of healthy population of Diporeia and dreissenids in smaller lakes like the New York Finger Lakes indicate that possible mechanisms linking Diporiea declines with dreissenid mussels may not operate outside the Great Lakes and dreissenids may simply not be the sole causal agent of the decline. An alternative that has not has not been proposed to date is the connection between mysids and Diporeia. Mysis and Diporeia are coevolved marine relics occupying similar habitats and would be expected to have strong interactions. In Lake Ontario and Lake Michigan, Mysis prey on Diporeia. In Lake Ontario, coincident with the colonization by dreissenids and other food web changes, Mysis may have become the dominant consumer of Diporeia. We hypothesize that oligiotrophication and associated increases in water clarity resulted in shifts in the location of the preferred habit of Mysis increasing their habitat overlap with Diporeia. At the same time shifts in food web structure favoured Mysis, such that Mysis production was largely maintained. The combination of a shift in Mysis habitat, sustained Mysis production, and perhaps declines in alternative Mysis food supplies, increased predation pressure on Diporeia. Increased predation by Mysis contributed in the observed declines in Diporeia populations, most evident in areas of increasing habitat overlap with Mysis. We propose to do laboratory experiments to determine the potential magnitude of Mysis predation on Diporeia, and whether the presence of dreissenids or availability of alternative zooplankton food changes the predation rate. We further propose to evaluate the hypotheses that increased water clarity in Lake Ontario following dreissenid invasion increased the probability of Mysis occurrence near bottom at bathymetric depths from 30 to 90-m, coincident with the decline of Diporeia at the same depth interval. Finally, we will evaluate data sources and modeling approaches to determine if we can extend the analysis to additional years and other Great Lakes.
OBJECTIVES:
1. Test the hypothesis that Mysis feed on Diporeia under laboratory conditions, with and without quagga mussels present, at rates sufficient to potentially suppress Diporeia populations in the wild.
2. Test the hypothesis that predation rates of Mysis on Diporeia decline when alternative zooplankton food is made available.
3. Test the hypotheses that the probability of occurrence of Mysis near bottom at 30-90-m increased in Lake Ontario coincident with the decline in Diporeia observed at the same depth interval.
4. Investigate data sources and approaches to extend the Lake Ontario prediction of Mysis probability of occurrence at depth to more years and other Great Lakes.
METHODS: Experimental populations will be collected from Cayuga Lake, NY and experiments will be done at the Cornell Biological Station laboratories. Based on measures of light and temperature in Lake Ontario and Mysis temperature and light preference functions, we will compare changes in Mysis preferred depths over time with changes in depth distributions of Diporeia. We will assemble and evaluate additional data sources and modeling approaches to determine if we can extend the analysis to additional years and other Great Lakes.