Calibration of an optical plankton counter for use in fresh water

A towed optical plankton counter (OPC-1T) developed for automated counting and sizing of marine zooplankton was calibrated for use in fresh water. Zooplankton in lakes are generally smaller in size, more variable in shape, and more abundant than in oceans. Our objectives were to study the conditions and parameters for utilizing the OPC-1T in the relatively high zooplankton concentrations typical of fresh water while maintaining its full capability for measuring large and small organisms. By passing low concentrations of monospecific and mixed zooplankton assemblages through a laboratory OPC-1L, we first verified that the OPC can measure freshwater zooplankton size very accurately. We then compared simultaneously collected plankton net and OPC-1T field samples from Lakes Erie and Ontario in order to quantify the known measurement bias of the OPC caused by coincident effects in the high zooplankton concentrations (up to 212 liter(-1)) typical of these lakes. The correlation between plankton net ansi OPC-1T biomass estimates was greatest when each organism detected by the OPC was treated as an ellipsoid with major axis (a) equal to the recorded OPC diameter and minor axis equal to a/1.33. The effect of increasing zooplankton abundance, and hence coincident counts, on zooplankton size measurements of the OPC-1T was also quantified. Results suggest that with proper choice of beam length and calibration algorithms, lake zooplankton biomass, numerical concentrations, and size distributions can be accurately measured at concentrations up to 100 organisms liter-1.