Different phytoplankton descriptors show asynchronous changes in a shallow urban lake (L. Kastoria, Greece) after sewage diversion

Abstract

Phytoplankton long-term changes were examined in a shallow urban lake (L. Kastoria, Greece) after sewage diversion in nine distinct years from 1994-2010. Using different levels of phytoplankton descriptors (diversity, taxonomic and functional composition, dominant species, total phytoplankton and cyanobacterial biomass) asynchronous changes were observed in the lake's phytoplankton community. The first dynamic response was observed five years after sewage diversion by a sharp decrease in total phytoplankton and cyanobacterial biomass. However, cyanobacteria remained the predominant taxonomic group. Only in 2005, a shift from cyanobacteria to chlorophytes-diatoms was observed, coinciding with a considerable decrease in the lake's water level due to flushing. A depletion of the long-lasting dominant filamentous cyanobacteria Limnothrix redekei and Cylindrospermopsis raciborskii was observed from 2005-2010. In contrast, a persistent dominance of toxic Microcystis species was recorded, indicating insufficient water quality. Fifteen years after sewage diversion, phytoplankton species richness doubled and functional diversity increased to include newly established species. Overall, the presented data provide insights into how the phytoplankton community assembles during restoration, with implications both for scientific analysis and ecological water quality management in the Mediterranean region.