Pre- and Post-Dreissenid Diets of Diporeia in Southern Lake Michigan and Their Relationship to Abundance of Diatoms in Sediment Cores

Diporeia spp. is a key component of the benthos of the Great Lakes, converting the pelagic algal rain to secondary production, which is critical for most fish species in the Great Lakes.  However, declines in Diporeia populations in the Great Lakes since the 1980s have been rapid and widespread.  There is correspondence between the decline of Diporeia and spread of zebra Dreissena polymorpha and quagga D. rostriformis bugensis mussels in the Great Lakes, but establishing a mechanistic link has been a research challenge.  The Diporeia decline is thought to result from competition for food resources with zebra/quagga mussels, but conflicting evidence suggests food limitation may not be the direct link.  To test the food - limitation hypothesis, we analyzed gut contents of Diporeia collected between the 1980s and 2009 from two offshore (>100 - m depth) and one near shore sampling station (57 - m depth) in southern Lake Michigan. We used paleolimnological analysis of sediment cores from southern Lake Michigan to resolve historical relationships among food resources, Diporeia diet shifts, and diet selectivity during the pre- and post-dreissenid invasion.  Results showed that in spring Diporeia feeds selectively and almost exclusively on large centric (Stephanodiscus spp.) and filamentous centric diatoms (Aulacoseira spp.).  Diets differed among Diporeia size classes, sampling stations, and years.  Springtime diets in offshore Diporeia populations showed significant shifts during the 2000s, which included greater proportions of small Cyclotella spp., small Stephanodiscus spp., and araphid planktonic diatoms, coincident with widespread Diporeia declines and rapid expansion of quagga mussels into the profundal sediments of Lake Michigan. Sediment cores recorded changes in diatom communities from 1960 to 2009 including declines in Aulacoseira and large Stephanodiscus, and increases in small centrics after dreissenid introduction.  Community changes in the sediment record are consistent with changes observed in the diets of Diporeia. Hence, part of the explanation for the decline in Diporeia populations may be the shift in diatoms from more nutritious and preferred species to less preferred and less nutritious species.