Abundance and Biomass of Benthic Macroinvertebrates in Lake Michigan in 2015, with a Summary of Temporal Trends

This technical report gives results of a lake-wide benthic survey conducted in Lake Michigan in 2015. The purpose of the survey was to assess the status of the benthic macroinvertebrate community, with a primary focus on the invasive mussels Dreissena polymorpha and Dreissena rostriformis bugensis, and the native amphipod Diporeia. Similar lake-wide surveys were conducted to assess the status of these three taxa beginning in 1994/1995 and repeated every five years through 2010 (Nalepa et al. 2014). Based on these previous surveys, major changes in population abundances of all three taxa were observed over this 15-year period. D. polymorpha was first reported in Lake Michigan in 1989 (Marsden et al. 1993). Based on the surveys, densities subsequently increased to reach a peak in 2000. Thereafter, densities declined to such an extent that by 2010 it was rarely found. Over the entire period, D. polymorpha was mainly found at depths < 50 m. D. r. bugensis was first reported in the lake in 1997 (Nalepa et al. 2001), and densities have mostly continued to increase at all depths through 2010, attaining densities that exceeded those of D. polymorpha even at depths < 50 m. Lastly, the native amphipod Diporeia has been in a steady state of decline ever since Dreissena became established. Lower densities relative to those in pre-Dreissena years were first observed in the early 1990s (Nalepa et al. 1998), and declines continued at all depths from 1994/1995 through 2010. In 2010, it had mostly disappeared at depths < 50 m and had declined by 95% at depths > 50 m. GLERL Contribution No. 1937