Causes of under- or overestimation of zooplankton biomass using Optical Plankton Counter (OPC) : effect of size and taxa

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Size-fractionated (0.25, 0.5, 1, 2, and 4 mm mesh size) wet mass (WM) and dry mass (DM) determinations and optical plankton counter (OPC) measurements were carried out on zooplankton samples collected at 15 stations in the northern North Pacific Ocean, Bering Sea and Chukchi Sea during July–August 2007. The total sample WM and DM estimated from OPC data corresponded closely to those of measured values by a factor of 0.970–1.098. However when the sample was portioned into different size groups, estimates of size-fractionated WM and DM by OPC data varied from measured masses by a factor of between 0.202 and 1.768. The high variability was caused by an underestimation of sizes of the large sized (>4 mm) fraction, or an overestimation of the number of the small size fraction (2–4 mm). The underestimation in the >4 mm and overestimation in the 2–4 mm respectively were caused by the dominance of transparent hydromedusae, and slender-shaped euphausiids in the >4 mm fraction. This study suggests that OPC analysis could be susceptible to errors in zooplankton biomass estimates in the large size fraction (>4 mm) especially when euphausiids and hydromedusae dominate the population. On the other hand, OPC based estimates of DM within 0.25–4 mm size fraction are more robust, which may be due to the dominance of large copepods, and low detritus content in the samples from the oceanic subarctic Pacific, in summer 2007.
2009-11-01