Spatial variation in RNA:DNA ratios of Diporeia spp. in the Great Lakes region

Over the past two decades, Diporeia in all of the Laurentian Great Lakes, except Superior, have declined dramatically. These declines have seemingly coincided with expansion of invasive Dreissena polymorpha and D. bugensis, however the exact mechanisms underlying decreasing Diporeia densities are obscure. We explored the use of RNA:DNA (R/D) ratios as a conditional index for Diporeia by experimentally demonstrating that Diporeia R/D responds to periods of starvation. Moreover, during 2008–2009 we collected Diporeia from throughout the Great Lakes and Cayuga Lake (New York, USA), and used R/D ratios to index condition of these in situ collected animals. We evaluated spatial and temporal variation of nucleic acid indices using classification and regression tree (CART) analysis with a suite of environmental variables included as potential predictors. Diporeia R/D of in situ collected specimens exhibited pronounced spatial and temporal variation, but multiple CART models described only a small amount of this variation. While we observed some variation in Diporeia R/D among lakes, nucleic acid ratios appeared to respond weakly to Diporeia population characteristics and local environmental conditions. Specifically, CART analyses revealed that Diporeia R/D was particularly low at extreme depths, and interestingly, Diporeia nucleic acids were not strongly associated with the presence of dreissenids. In summary, while a limited amount of variation in Diporeia R/D was attributable to environmental conditions, the majority of Diporeia R/D variation was unaccounted for. Hence, the causative factors underlying spatio-temporal variation of Diporeia R/D and the mechanistic reasons for Diporeia declines in the Great Lakes remain largely unknown.