Invasive ecosystem engineers on soft sediment change the habitat preferences of native mayflies and their availability to predators

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SUMMARY 1. Dreissenid mussels (quagga mussels, Dreissena bugensis, and zebra mussels, D. polymorpha) are invasive species that function as ecosystem engineers in the Laurentian Great Lakes. Dreissena are increasingly abundant on silt, sand and other soft substrates; by altering benthic habitat, these mussels can alter benthic community structure. 2. We used laboratory mesocosm experiments to examine the effects of soft-sediment Dreissena clusters on the habitat preference of Hexagenia, a native burrowing mayfly that is an important food source to fish. We conducted three experiments to test whether Hexagenia: (1) select for bare sediment, soft sediment covered with live Dreissena (added structure and food resources) or soft sediment with clusters made of empty Dreissena shells (added structure only), (2) prefer a specific density of live Dreissena on soft sediment and (3) select for or avoid sediment with an accumulation of empty Dreissena shells. 3. Contrary to initial expectations, we found that Hexagenia selected for sediment covered with live Dreissena clusters, followed by empty Dreissena shells clusters, and lastly what was previously thought to be the preferred habitat, bare sediment. Not only did Hexagenia prefer Dreissena-covered sediment, but they also preferred high densities of Dreissena. 4. We also experimentally tested the effects of Dreissena-covered soft sediment on the availability of Hexagenia to fish. We had three treatment levels representing three distinct habitat types: (1) bare sediment (no Dreissena) treatment in which water was turbid because of mayfly activity, (2) Dreissena-covered sediment treatment in which water was clear because of Dreissena filtration and (3) Dreissena-covered sediment with added turbidity. We found that in low light conditions, similar to many locations where both organisms are found to co-occur, both yellow perch and round goby consumption of Hexagenia significantly decreased when Dreissena covered the bottom sediment. 5. These results suggest that by choosing Dreissena-covered habitat, Hexagenia receive protection from fish predation in turbid ⁄low light systems. However, protection from predation cannot be the only reason Hexagenia select Dreissena-covered sediments, as Hexagenia selected for live clusters more often than empty clusters and may be a result of additional food resources.