Historical biomass of Limnocalanus in Lake Michigan

The magnitude of a reported recent increase in the biomass of Limnocalanus macrurus Sars in Lake Michigan was based on a length-weight regression model developed for a different species, Diaptomus siciloides. We created a length-weight model specifically for Limnocalanus in lake Michigan and discovered that the D. siciloides model gives satisfactory estimates for animals in May but not in subsequent months. After Limnocalanus mature to C6 copepodid instars, animals progressively add mass through June. July, and August while maintaining near-constant metasome lengths and widths. Adults begin to lose mass in September as mating and reproduction begins. By August, mass estimates based on D. siciloides can underestimate Limnocalanus biomass by more than four-fold. We inspected archival Lake Michigan collections of Limnocalanus from 1985 to 1992 and calculated a mean summer biomass of 16.2 mg DW/m(3), which is approximately 3.5 times greater than previously reported. The pattern displayed by Limnocalanus is consistent with its univoltine life history and synchronized cohort structure. Biomass may have genuinely increased since 2004, but the magnitude of the increase is likely much greater than reported. Increased numerical abundance of Limnocalanus cohorts surviving to reproductive age appears to be the main reason for their increased biomass. This has important consequences for trophic interactions, bioenergetic considerations, and relative dominance with respect to Cladocera and Diaptomidae in Lake Michigan. (C) 2010 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.