The utility of body size indices derived from optical plankton counter data for the characterization of marine zooplankton assemblages

To evaluate the utility of size-based indices derived from an Optical Plankton Counter (OPC) through detection of spatial and temporal trends in zooplankton biomass, zooplankton size data were collected with an OPC across the Scotian Shelf region of the northwest Atlantic Ocean in April and October of 1997, 1998, and 1999. Eight size-based indices were computed – three simple size metrics (arithmetic mean, geometric mean, coefficient of variation) and metrics derived from the Normalized Biomass Size Spectrum (NBSS; X- and Y-coordinates and curvature of a fitted quadratic function) and the Pareto distribution (Y-intercept and slope). Results indicate that the simple size indices and those derived from the Pareto distribution consistently accounted for the greatest portion of annual variation in zooplankton biomass whereas indices derived from the NBSS accounted only for some secondary patterns. Simple indices also accounted for the greatest portion of spatial variance in zooplankton biomass whereas the NBSS and Pareto accounted for secondary patterns. Patterns in zooplankton communities based on these indices reflected broad taxonomic trends and were related to independent observations on atmospheric and hydrographic conditions in the study area. Size-based zooplankton data from continuous survey instruments can provide powerful adjuncts to both freshwater and marine aquatic monitoring.

► We collect optical plankton counter (OPC) survey data on the Scotian Shelf, Canada.
► We seek the best OPC size metrics to describe spatio-temporal trends in zooplankton.
► Simple (mean, CV) and Type I Pareto indices best described temporal variation.
► Mean and CV outperformed Pareto and Normalized Biomass metrics for spatial variation.
► We suggest different OPC size metrics apply to marine versus freshwater zooplankton.