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.