Can an optical plankton counter produce reasonable estimates of zooplankton abundance and biovolume in water with high detritus?

The Optical Plankton Counter (OPC) has been used in oceanic and fresh water s to estimate zooplankton abundance and biovolume. However, it is not clear whether the OPC can produce accurate estimates of zooplankton abundance an d biovolume in waters with high detritus. In order to test the capability o f the OPC to estimate zooplankton abundance and biovolume in Chesapeake Bay , two sets of laboratory experiments were conducted using water with high d etritus concentrations collected from the upper Choptank estuary of Chesape ake Bay and laboratory cultured Artemia. Our results suggest that the OPC i s able to produce accurate estimates of zooplankton biovolume after correct ing for the influence of background detritus in all the detritus concentrat ions used, but accurate estimates of zooplankton abundance only in water wi th background detritus <100 particles l(-1). The relationship between light attenuation and OPC background particle concentrations provides a useful w ay to estimate OPC background particle concentrations when direct OPC backg round particle measurements are not available. Light attenuation corrected OPC particle abundance and particle volume gave accurate estimates of zoopl ankton abundance and biovolume. However, the accuracy of the corrected OPC measurements by the estimated background particle concentrations was not as high as the corrected OPC measurements by the direct background particle m easurements.