Reliability of rapid, semi‐automated assessment of plankton abundance, biomass, and growth rate estimates: Coulter Counter versus light microscope measurements

Abstract

Here we determined (1) how accurately the semi‐automated Coulter Counter (CC) measured the abundance, volume, and growth rates of mono‐specific plankton cultures compared to light microscopy and (2) whether CC‐derived particle size‐abundance spectra of whole water samples provided a near‐real time assessment of plankton community composition. A significant, positive relationship was found between CC particle and microscope cell abundance and volume estimates for 16 phylogenetically diverse plankton species. However, abundance estimates of the 2 methods differed significantly from a 1:1 relationship, with CC counts on average 15% greater than microscopy counts. Growth rate estimates showed excellent agreement for both methods for all species and were consistent over time. No significant differences in abundance, growth rate, or size estimates were observed due to light levels (Scrippsiella trochoidea) or strain variation (Heterosigma akashiwo). CC‐based, ship‐board characterization of whole plankton communities, from Georges Bank, captured changes in size‐abundance spectra and thus provided a coarse picture of in‐situ community composition. In the laboratory, the CC appears particularly well‐suited to measuring relative changes in cell abundance, such as for growth and grazing rate measurements of mono‐specific laboratory cultures, or when predator and prey species are well separated by size differences. Although microscopy remains essential for plankton analyses, these results suggest that the CC can increase sample size, improving accuracy, and precision of studies requiring frequent sampling or high degrees of replication. Deployment of these automated techniques in the field may aid in quantifying spatial and temporal variability in plankton community structure by forthcoming ocean‐observing systems.