Comparing microscopic counts and pigment analyses in 46 phytoplankton communities from lakes of different trophic state

Summary Comparison between algal pigment analyses using HPLC and subsequent CHEMTAX analysis with microscopic counts from different lakes ranging from ultra‐oligotrophic to eutrophic allowed testing of pigment:chlorophyll a ratios for lakes of different trophic conditions. Microscopic counts and pigment analyses were positively correlated for all groups except for the chrysophytes. When examining the oligotrophic lakes only, microscopic analyses and pigment analyses of chrysophytes were positively correlated, which showed that the oligotrophic pigment ratios were well suited for determining the abundance of the chrysophytes in oligotrophic lakes. In the eutrophic lakes, where chrysophytes constituted less than 10% of the biomass, the weak correlation was most likely related to counting inaccuracies. Chrysophytes, generally indicative of oligotrophic conditions, were three times more abundant in the oligotrophic lakes. They constituted 28% of the total phytoplankton biomass as chlorophyll a measured by the pigment method and 26% of the total phytoplankton biomass measured as biovolume by microscopy. In mesotrophic/eutrophic lakes, chrysophytes constituted only 9% (pigment method) and 8% (microscopy). Similarity analyses comparing the results of these two methods on functional group level documented the high accuracy of the pigment method in detecting all the functional groups present. The results of the pigment analyses showed a more even composition of phytoplankton groups than found by microscopy, where often one or two of the less abundant groups were absent or only sporadically present. The Shannon's diversity index, H’, for functional groups determined from results of the pigment method were higher than the H’ values based on microscopic counts, confirming a more continuous composition of the phytoplankton groups when detected by the pigment method. The pigment method was found to be a useful alternative, which supplemented microscopic counts.