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-Har dy Plankton Recorder (LHPR) was undertaken around the sub-Antarctic island of South Georgia during austral spring, 1997, to investigate the capacity o f the OPC to accurately describe fine-scale plankton distributions. Initial ly, formalin-preserved developmental stages of representative zooplankton w ere passed through a benchtop OPC and a relationship established between ma ximum cross sectional area (CSA) of the plankton and the digital output val ue (DOV) of the OPC. The LHPR with an OPC attached to the net frame was the n deployed at two stations representing contrasting plankton abundances. Ou tputs from two oblique hauls taken at a shelf station, and one at an oceani c station, were compared. Following corrections for volume swept by both in struments, and standardization of OPC particle and net plankton counts on t he basis of sampler mouth area, the OPC was found to 'over-estimate' relati ve 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 t hreshold to the OPC data excluding the proportion of particles <0.25 mm(2) CSA, based on the expected retention of plankton by the net there was a gre atly improved and significant correlation;Overall there was no consistent o ver- or undersample by the OPC relative to the net, with mean OPC:net ratio s being 1-1.8 for net plankton densities of up to 6000 ind. m(-3). There wa s also a generally good agreement between the abundances of the rarer large biomass dominant calanoid copepods estimated by both samplers. An OPC over estimate of 2-3x relative to the net was seen on the ascent portion of the haul at the oceanic station, attributable to the presence of high abundance s (estimated density greater than or equal to 1600-4000 ind. l(-1)) of larg e centric diatoms which appeared to cause coincident counts. We conclude th at with careful. interpretation, OPC data can accurately describe small-sca le plankton distributions in the Southern Ocean as well as order of magnitu de changes.