Field calibration of the Optical Plankton Counter with respect to Calanus finmarchicus

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Zooplankton in the Faroe–Shetland Channel and north‐western North Sea were sampled concurrently with a profiling serial net system and an Optical Plankton Counter (OPC) during January and March 1995. Calanus finmarchicus, together with a small proportion of Calanus helgolandicus (< 3% overall), made up > 75% of the zooplankton in the region, with C. finmarchicus being predominant below 500 m depth. Sensitivity analysis of the OPC data was carried out to determine the particle size range providing the highest correlation between the concentrations of stage CIV–CV and stage CVI C. finmarchicus estimated from net catches, and particle concentrations from the OPC. Significant correlations were obtained for particle size ranges that corresponded closely with microscopic measurements of the dimensions of copepodites. The highest correlations were obtained for stages CIV–CV in January.According to the net sampling, the concentration of C. finmarchicus in January varied by at least four orders of magnitude over the survey region (< 0.05 to > 500 m−3), although only 4% of the samples contained > 100 m−3. When C. finmarchicus was present at concentrations of around 120 m−3, similar estimates of its abundance were obtained from the OPC data as from the net samples. At lower concentrations, the OPC increasingly overestimated the net catches, and the overestimate reached 40:1 at the detection limit of the nets (0.05 m−3). The average overestimate, taking into account the distribution of concentrations, was 2.5:1. This over‐estimate could not be wholly accounted for by the occurrence of other zooplankton species or potential errors associated with the OPC or net system, and the conclusion was that it arose mainly from the detection of detrital aggregates by the OPC. Thus, the OPC was effective at resolving the peak concentrations (> 100 m−3) of C. finmarchicus in the survey region, but was less effective at delineating the marginal areas of the distribution where C. finmarchicus was outnumbered by other similarly sized particles. Nevertheless, the results indicated that, with the application of calibration data such as described here, the OPC is a valuable tool for broad scale surveys of the spatial distribution of C. finmarchicus during the winter, when the animals are concentrated at depths > 500 m in areas such as the Faroe–Shetland Channel.