Do rusty crayfish (Faxonius rusticus) invasions affect water clarity in north temperate lakes?

Invasive crayfish can cause shifts in lakes from clear, macrophyte-dominated states to eutrophic, phytoplankton-dominated states because of their burrowing and foraging behavior. While invasive crayfish populations have been linked to declines in water clarity of shallow lakes and wetlands in Asia and Europe, little research has been done on the potential for similar effects of invasive rusty crayfish (Faxonius rusticus) in large temperate lakes of the Midwestern USA. We related F. rusticus abundance in 17 lakes of northern Wisconsin, USA over time (1984‒2018) to measures of lake clarity (chlorophyll a concentration and Secchi disc depth) estimated from remote sensing (Landsat imagery). Contrary to the effects of invasive crayfish in other study systems, we found a weak, positive association between F. rusticus abundance and water clarity. We propose that lake clarity may increase if declines in small fishes caused by F. rusticus lead to population growth of zooplankton and consequent decreases in phytoplankton through a trophic cascade. Alternatively, F. rusticus could be passengers to, rather than drivers of, lake clarity trends, responding positively to increased littoral benthic productivity when lakes are clearer. Future research should aim to determine if F. rusticus causes or responds to changes in water clarity, but should also investigate the impacts of crayfish invasions on water clarity across a greater variety of lentic ecosystems.