Which metric to choose? Differences between abundance and biomass responses to environmental conditions in a planktonic diatom community

Abstract

Abundance and biomass estimates are metrics usually taken for phytoplankton assays. Biomass is a proxy measure today in phytoplankton assays, while relative abundance is broadly used in diatoms investigations and application of ecological indexes. The question is how the two metrics respond to environmental conditions, based on diatoms species composition, in the plankton of lotic system, using the same statistical approach. The investigation was carried out based on monthly plankton sampling in four rivers during an annual cycle in southern Brazil. Relative abundance and relative biomass of diatoms showed distinct responses for each river, and the dominant species according to each metric were different. Relative abundance significantly correlated with seven of the eight physical and chemical variables, while relative biomass did not. Both metrics were appointed to the major diatom species of each river, but biomass showed low statistic robustness. The analysis with higher taxonomic rank (families) yielded more coherent outcomes between the metrics. The results showed that diatom relative abundance is effective to relate the composition to environmental conditions; therefore, the abundance should not be neglected in studies of phytoplankton structure and dynamics, mainly when the diatoms are dominant in the community, and the low taxonomic rank ecological indication is regarded.