**ABSTRACT NOT FOR CITATION WITHOUT AUTHOR PERMISSION. The title, authors, and abstract for this completion report are provided below. For a copy of the full completion report, please contact the author via e-mail at rudstam@cornell.edu or via telephone at (316) 633-9243. Questions? Contact the GLFC via email at frp@glfc.org or via telephone at 734-662-3209.**

Did Mysis have a role in the decline of Diporeia?

Thomas J. Stewart¹ and Lars Rudstam²

¹ Lake Ontario Management Unit, Ontario Ministry of Natural Resources, RR # 4, Picton, Ontario, K0K 2T0

² Department of Natural Resources, Cornell Biological Field Station, 900 Shackelton Point Road Bridgeport, NY 13030

August 2012

ABSTRACT:

Predation experiments indicate that adult Mysis feed on juvenile but not on adult Diporeia. The level of predation was not influenced by the presence of quagga mussels. Access to alternative zooplankton food reduced the predation rate. Feeding rates of adult mysids on juvenile Diporeia were in the order of 1 prey per week. A simple model combining Mysis consumption rate, diet composition, and ratios of Mysis to Diporeia production was applied to calculate the potential for Mysis predation to impact Diporeia populations. The model showed that Mysis consumption could equal or exceed Diporeia production at very modest and realistic diet proportions and production ratios, well within the range of values reported in the literature and observed experimentally. Application of Mysis light-preference functions to Lake Ontario water transparency from the 1970s, 1980s, and 2000s indicates that a larger proportion of the Mysis population should reside on the bottom during the day in the 2000s than in the earlier decades, increasing the habitat overlap between Mysis and Diporeia in recent decades. The chronology of ecosystem changes and the findings of this study point to an indirect mechanistic link between Diporeia declines and quagga mussel colonization. Mysis predation on Diporeia may have increased and suppressed Diporeia production as a consequence of increased water transparency and declines in alternative Mysis food associated with quagga mussel colonization. Specific recommendations are made to further investigate Mysis and Diporeia interactions and extend this analysis to the other Great Lakes.