The use of the Laser Optical Plankton Counter to measure zooplankton size, abundance, and biomass in small freshwater lakes

Abstract

The Optical Plankton Counter (OPC) has been used in a variety of environments since its introduction over decade ago, but its use in freshwater lakes has been limited by high densities of zooplankton and detritus. The newer Laser Optical Plankton Counter (LOPC) has several modifications from its predecessor, and the goal of this study was to examine whether it could be used to measure average size (µm equivalent spherical diameter, ESD), abundance (particles L−1), and biomass (µg dry weight L−1) of zooplankton in samples from 18 lakes in the Eastern Townships region of Quebec, Canada. The LOPC slightly overestimated the size of copepods, and consistently underestimated Daphnia by approximately 25% ESD. Densities and biomass of net samples were very similar between the LOPC lab version and traditional microscope analyses suggesting that the LOPC can be reliably used to process preserved net samples. When the LOPC was towed in situ vertically in Lake Memphremagog, QC, Canada, estimated zooplankton abundances were ten times net sample values from the same water column, but similar abundances were found between the LOPC and pumped zooplankton samples at 2 m depth. These results indicate that the LOPC may be well suited for analyses of zooplankton abundance and biomass in productive freshwater lakes.