Using an optical plankton counter to determine the size distributions of preserved zooplankton samples

Long time series and/or spatially extensive observations of zooplankton are needed in order to examine fluctuations in community composition and biomass. We tested the utility of a laboratory optical plankton counter (OPC) in determining the size spectra and biomasses of a large number of preserved zooplankton samples. OPC size measurements of a variety of taxa from the California Current region were well correlated with visual measurements (r(2) = 0.80). Although organisms preserved for long time periods may fragment, trends in total biovolumes of similar to 800 samples that were > 10 years old were consistent with displacement volumes measured just weeks after collection. A laboratory test in which euphausiids were damaged manually indicated that OPC counts of large zooplankton are robust to moderate fragmentation. The greatest effect of formalin preservation on crustacean zooplankton was increased opacity, resulting in an increase of 3-25% over the digital sizes measured when alive. Formalin preservation significantly reduced the biovolume of gelatinous zooplankton measured visually, but did not change the size measured by the OPC due to the compensating increase in opacity. As examples of the advantage of using the OPC to analyze a large number of preserved samples, we examined (i) the positive relationship of biomass to chlorophyll a in three size classes at >100 stations occupied during a 1984 cruise and (ii) the longshore pattern of size-fractionated zooplankton biomass in similar to 800 samples from the California Current region in 1981 and 1984.