Effects of Winter Hypoxia on Fish Communities in Northern Wisconsin

Abstract

Hypoxia is a significant source of winter mortality for freshwater fish in north-temperate lakes and has the potential to alter fish communities. A multi-year dataset on Buckskin Lake, a shallow, productive drainage lake in northern Wisconsin with a history of winterkill, allowed the investigation of the effects of periodic winter hypoxia on fish communities. An aeration system was installed in 1984, which raised winter oxygen levels to levels sufficient for higher fish survival. In the winter of 2007-2008, the aerator system failed, causing an extensive winterkill. The lake was sampled from 2002-2005, before the aerator failure, and in 2008-2009, after the winterkill event. In addition, 16 similar lakes in the area with no winterkill history were sampled using similar methods. Using the combined datasets, two hypotheses were tested with non-metric multidimensional scaling (NMDS) and PERMANOVA analyses. We hypothesized that (1): after 18 years of aeration, the Buckskin Lake fish community from 2002-2005 would be similar to the non-winterkill lakes due to the recolonization of species from connected lakes, and (2): the fish community changed significantly in the lake due to the 2007-2008 winterkill event. The first hypothesis was not supported, as NMDS and PERMANOVA analyses showed that Buckskin Lake retained a fish community distinct from the non-winterkill lakes. Our findings were consistent with the second hypothesis: an incomplete winterkill occurred in 2007-2008 due to lack of aeration, causing changes in fish abundances, including the reduction of game fish species and centrarchids, with no notable extinctions. Together, our findings indicate that winter aeration may improve gamefish survival and allow lakes with winterkill tendency to support sport fish communities with healthy piscivore populations such as walleye and largemouth bass, which would be otherwise greatly reduced.