Spatio-temporal dynamics of parasites infecting Diporeia spp. (Amphipoda, Gammaridae) in southern Lake Michigan (USA).

Since the 1990s, populations of the benthic amphipod Diporeia spp. (Diporeia) have sharply declined across much of the Laurentian Great Lakes. This study was undertaken to identify contemporary and historical community composition, structure, and dynamics of parasites infecting Diporeia collected from nine sites in the southern basin of Lake Michigan, where declines of the amphipod have been well documented over the past 20years. An additional aim of this study was to assess whether infection dynamics and dreissenid densities could explain the declines in Diporeia densities that have occurred. We found that Diporeia were host to eight groups of uni- and multicellular pathogens. Of the 3082 amphipods analyzed, 1624 individuals (52.7%) were infected with at least one type of parasite. Ciliophora was the most prevalent parasite (50.08% prevalence of infection), followed by Gregarinasina (2.79%), Microsporidia (0.68%), Cestoda (0.45%), Acanthocephala (0.36%), Haplosporidia (0.23%), Yeast (0.32%), and filamentous Fungi (0.10%). Considerable spatial and temporal variability were observed in parasite prevalences, with prevalences frequently appearing to cycle between low and high values. Parasite species belonging to Microsporidia and Haplosporidia were associated with tissue alteration and host inflammatory response; however, parasite prevalences explained very little in terms of Diporeia density declines at assessed sites. Despite these findings, we do not discount the possibility that parasitic infections may have played a role in declining Diporeia densities in the Great Lakes, as the cyclical prevalences that were observed are possibly suggestive of parasitic outbreaks that are followed by die-offs at affected sites. This study suggests that if parasites have affected Diporeia densities in the Laurentian Great Lakes, then the relationship may be a complicated one.