Analyzing semiquantitative phytoplankton counts

Semi-quantitative microscope counts of phytoplankton are often a compromise between time-consuming cell biomass analyses and no phytoplankton data. We demonstrate how semiquantitative data from a monitoring program can be used to study phytoplankton community composition, its annual cycle, and aspects of the ecosystem it inhabits. Semi-quantitative counts from Agmon Wetlands, Israel, collected monthly from 2008 to 2021, were generated by allocating a score from 1 (rare) to 6 (extremely abundant) to each taxon observed in a sample. Five samples could be analyzed at the time it takes to count one sample by the conventional Utermöhl method. Using an exponential regression equation, the scores were transformed to estimated concentrations (algal units/ml), then summed into taxonomic or other groups of species. A strong annual pattern of the sum of scores for each taxonomic group was observed. The method was useful for assessing ecosystem features based on indicator species, and for presenting the contribution of morpho-functional groups to the phytoplankton community. If making a species list is planned, we recommend assigning scores, creating calibration curves, converting the scores to concentration estimates, and using those estimates to achieve higher resolution and better conclusions than possible with a species list alone.