Trends in Diporeia populations across the Laurentian Great Lakes, 1997–2009

Abstract Benthic communities in the Laurentian Great Lakes have been in a state of flux since the arrival of dreissenid mussels, with the most dramatic changes occurring in population densities of the amphipod Diporeia . In response, the US EPA initiated an annual benthic macroinvertebrate monitoring program on all five Great Lakes in 1997. Although historically the dominant benthic invertebrate in all the lakes, no Diporeia have been found in Lake Erie during the first 13 years of our study, confirming that Diporeia is now effectively absent from that lake. Populations have almost entirely disappeared from our shallow (  90 m) sites still supported Diporeia populations in 2009, with densities at those sites ranging between 96 and 198/m 2 . In Lake Michigan, populations were still found at six of our seven deep sites in 2009, with densities ranging from 57 to 1409/m 2 . Densities of Diporeia in 2009 at the four deep sites in Lake Huron were somewhat lower than those in Lake Michigan, ranging from 191 to 720/m 2 . Interannual changes in population size in Lake Huron and Lake Michigan have shown a degree of synchrony across most sites, with periods of rapid decline (1997–2000, 2003–2004) alternating with periods of little change or even increase (2001–2002, 2005–2009). There has been no evidence of directional trends at any sites in Lake Superior, although substantial interannual variability was seen.