Evaluation and correction of subresolved particles by the optical plankton counter in three Australian estuaries with pristine to highly modified catchments

Abstract

High concentrations of subresolved particles smaller than the 250 μm equivalent spherical diameter (ESD) detection limit of the optical plankton counter (OPC) have hampered its use in turbid estuarine waters. Coincidence of these subresolved particles produced erroneous counts of up to 58 L−1 for 100 μm mesh filtered water samples from three subtropical east Australian estuaries using the laboratory OPC‐1L. The influence of these erroneous counts on in situ OPC‐2T measurements was assessed by comparison with measurements of simultaneously collected net zooplankton using the laboratory OPC‐1L. The total zooplankton abundance from the in situ OPC‐2T measurements could be corrected for erroneous counts of subresolved particles using OPC‐1L measurements of 100 μm mesh filtered water sampled from the same site but with large error. No such corrections were possible for OPC‐2T measurements of total zooplankton biomass or normalized biomass size spectra (NBSS). No meaningful or significant correlations were found between the abundance or biomass of subresolved particles and in situ light attenuance, probably due to tannin‐rich waters with low subresolved particle concentrations. NBSS of simultaneously collected net zooplankton from OPC‐1L measurements indicated higher biomass in the disturbed Manning and Wallamba rivers, whose catchments support intensive livestock agriculture and some residential development, compared to the forested Wallingat River. NBSS may therefore be a useful indicator of nutrient enrichment in estuaries. The slope of NBSS may respond to both production of small particles and the predation and loss of large particles.