Bacterial and phytoplankton responses to nutrient amendments in a boreal lake at different taxonomic resolutions

Nutrient limitation and resource competition in bacterial and phytoplankton communities may appear different when considering different levels of taxonomic resolution. Nutrient amendment experiments conducted in a boreal lake on three occasions during one open water season revealed complex responses in overall bacterioplankton and phytoplankton abundance and biovolume. At kingdom level bacteria were dominant in spring, while phytoplankton was clearly the predominant group in autumn. Of the main bacterioplankton phyla, only Actinobacteria had a treatment response that was visible even at the phylum level throughout the season. With increasing resolution (from 75 to 99% sequence similarity) major responses to nutrient amendments appeared using 454 pyrosequencing data of 16S rRNA amplicons. This further revealed that OTUs (defined by 97% sequence similarity) within previously defined freshwater tribes appeared to have clear temporal and treatment-dependent differentiation and thus were not ecologically coherent. Similarly, phytoplankton species from the same genera responded to different nutrient amendments even though biovolumes of the majority of taxa increased with the additions of both nutrients. The bacterioplankton and phytoplankton community compositions showed parallel trajectories that could be seen in parallel succession patterns over the season and parallel responses to nutrient additions. Overall, our data revealed that the response of both communities to nutrient changes was highly dependent on season and that conflicting results may be obtained when using different taxonomic resolutions.