CALIBRATION OF AN OPTICAL PLANKTON COUNTER FOR USE IN FRESH-WATER

A towed optical plankton counter (OPC-1T) developed for automated coun ting and sizing of marine zooplankton was calibrated for use in fresh water. Zooplankton in lakes are generally smaller in size, more variab le in shape, and more abundant than in oceans. Our objectives were to study the conditions and parameters for utilizing the OPC-1T in the re latively high zooplankton concentrations typical of fresh water while maintaining its full capability for measuring large and small organism s. By passing low concentrations of monospecific and mixed zooplankton assemblages through a laboratory OPC-1L, we first verified that the O PC can measure freshwater zooplankton size very accurately. We then co mpared 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 corre lation between plankton net ansi OPC-1T biomass estimates was greatest when each organism detected by the OPC was treated as an ellipsoid wi th major axis (a) equal to the recorded OPC diameter and minor axis eq ual to a/1.33. The effect of increasing zooplankton abundance, and hen ce coincident counts, on zooplankton size measurements of the OPC-1T w as also quantified. Results suggest that with proper choice of beam le ngth and calibration algorithms, lake zooplankton biomass, numerical c oncentrations, and size distributions can be accurately measured at co ncentrations up to 100 organisms liter-1.