Correspondence among methods of zooplankton biomass measurement in lakes: effect of community composition on optical plankton counter and size-fractionated seston data

摘要

The effectiveness of the optical particle counter (OPC) to estimate zooplankton biomass depends on the variability in zooplankton shape and the presence of interfering particles. In marine environments where zooplankton are composed of similarly shaped copepods, an average shape is relatively easily obtained. However, in freshwater environments, spheroid cladocerans mix with ellipsoid copepods and make the application of a single morphometric model difficult. To expand the use of the OPC to freshwater environments, we developed new ellipsoid models for three common lake types (eutrophic, mesotrophic, and oligotrophic). In addition, we assessed how closely different size fractions of seston corresponded to zooplankton biomass. When expressed in common dry mass units, OPC- and seston-derived zooplankton biomass estimates showed a 1:1 correspondence with taxonomically derived estimates in productive lakes (r > +0.70, P < 0.001) but not in oligotrophic systems. OPC ellipse models differed among lake sets (major-to-minor axis ratio: 1.5 to 2.7) but were not a simple function of the cladoceran-to-copepod ratio. The seston size fraction that provided the best estimates of zooplankton biomass was smaller in mesotrophic lakes (>200 μm) than in eutrophic or oligotrophic lakes (>500 μm). The presence of algae and rotifers had no detectable influence on OPC and size-fractionated seston estimates. Overall, these analyses suggest that OPC and seston provide reliable estimates of lacustrine zooplankton biomass as long as region-specific ellipse models and size fractions, respectively, are used.