Effect of phytoplankton morphology on the measurement of biovolume

Algal biovolume is a standard measurement for limnological and oceanographic studies, which is used to proxy biomass and productivity. Manual and automatic methods can be applied to estimate biovolume from phytoplankton images. We examined the effects of algal shape and taxon (largely genus) on the accuracy of automatic image-based biovolume estimates for samples from a large oligotrophic embayment (Georgian Bay, Lake Huron). Specifically, we compared biovolume estimates based on equivalent spherical diameter, Feret-based diameter and two shape-based estimates (cylinder and prolate spheroid) against the manual biovolume measurements using correlation, linear regression and analysis of variance. The automatic biovolume estimates were all moderately to strongly correlated with the manual measurements, but the automatic cylinder method was the most accurate, overall. The automatic cylinder method overpredicted the manual total biovolume measurement of the community by 59%, but it had the strongest linear relationship with manual total biovolume, and the strongest correlation based on the biovolumes of individual genera. When algal taxa were assigned to 11 general shape categories, we observed significant differences in the accuracy with which they were measured. Results suggest that the cylinder biovolume method provides a reasonable estimate of total biovolume in algal communities that have a diversity of elongated shapes.