Field comparison of an LHPR net sampling system and an Optical Plankton Counter (OPC) in the Southern Ocean

A field comparison of an optical plankton counter (OPC) and a Longhurst-Hardy Plankton Recorder (LHPR) was undertaken around the sub-Antarctic island of South Georgia during austral spring, 1997, to investigate the capacity of the OPC to accurately describe fine-scale plankton distributions. Initially, formalin-preserved developmental stages of representative zooplankton were passed through a benchtop OPC and a relationship established between maximum cross sectional area (CSA) of the plankton and the digital output value (DOV) of the OPC. The LHPR with an OPC attached to the net frame was then deployed at two stations representing contrasting plankton abundances. Outputs from two oblique hauls taken at a shelf station, and one at an oceanic station, were compared. Following corrections for volume swept by both instruments, and standardization of OPC particle and net plankton counts on the basis of sampler mouth area, the OPC was found to ‘overestimate' relative to the net by up to several orders of magnitude. However, following the correction for rate of flow though the OPC, and the application of a size threshold to the OPC data excluding the proportion of particles <0.25 mm2 CSA, based on the expected retention of plankton by the net there was a greatly improved and significant correlation. Overall there was no consistent over- or undersample by the OPC relative to the net, with mean OPC:net ratios being 1–1.8 for net plankton densities of up to 6000 ind. m–3. There was also a generally good agreement between the abundances of the rarer large biomass dominant calanoid copepods estimated by both samplers. An OPC overestimate of 2–3× relative to the net was seen on the ascent portion of the haul at the oceanic station, attributable to the presence of high abundances (estimated density ≥1600–4000 ind. l–1) of large centric diatoms which appeared to cause coincident counts. We conclude that with careful interpretation, OPC data can accurately describe small-scale plankton distributions in the Southern Ocean as well as order of magnitude changes.

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