Turbidity‐mediated interactions between invasive filter‐feeding mussels and native bioturbating mayflies

Summary 1. Invasive dreissenid mussels are known to cause large ecosystem changes because of their high filter‐feeding capacity, while native bioturbators may interfere with the mussels filter feeding. In this experiment, we investigated indirect environmental interactions between invasive filter‐feeding dreissenid mussels (zebra and quagga mussels) and native recolonizing bioturbating hexagenid mayflies ( Hexagenia ) at two mussel densities and two Hexagenia densities in a 2‐month long laboratory experiment. 2. Mean turbidity increased with increasing density of Hexagenia and decreased with increasing density of mussels. Turbidity showed the fastest decline at the highest mussel density, and no decline or a lower rate of decline at the low mussel density, dependent on Hexagenia density. 3. Mussel growth decreased with increasing Hexagenia density at low but not at high mussel density. Moreover, growth of mussels decreased as a function of increased mean turbidity at low mussel density but not at high mussel density. Filtering activity at the highest mussel density increased after introduction of food at the lower two densities of Hexagenia , but was constantly high at the highest Hexagenia density. 4. There was no difference in emergence of Hexagenia among the treatments, but mortality of Hexagenia was higher in the presence of mussels than in their absence. 5. Our results indicate that interactions between dreissenids and hexagenids are mediated through the sediment, and depend on density of both dreissenids and hexagenids. As the natural densities of these animals vary considerably within lakes, their growth and survival because of indirect environmental interactions is expected to vary spatially.