Lake phytoplankton status and trends: a case study from Greek lakes, Eastern Mediterranean

Until now, little information was available regarding phytoplankton communities of Greek lakes. In this study, we present the first comprehensive analysis of phytoplankton composition and biovolume, as well as their inter-annual variations, across 15 natural lakes in Greece from 2016 to 2021. The ecological status and trends of these lakes were assessed using HeLPhy, a phytoplankton index of the Water Framework Directive (WFD). Additionally, this study examined potential similarities among lakes based on the number of phytoplankton taxa identified, the composition of phytoplankton communities, and the ecological classification. A total of 462 phytoplankton taxa from 10 taxonomic groups were recorded in 287 phytoplankton samples collected between 2016 and 2021. The phytoplankton communities in all lakes were dominated mainly by Cyanobacteria, Bacillariophyta, and Chlorophyta. The highest number of phytoplankton taxa was recorded in Chlorophyta and Cyanobacteria taxonomic groups. Based on the HeLPhy index, the lakes were classified into four ecological status classes: high, good, moderate, and poor. Inter-annual variations in ecological classification were observed. Eight of the 15 lakes were classified as having good or better ecological status according to WFD criteria, with deep lakes generally exhibiting a better status than shallow lakes. Bacillariophyta were more prevalent in lakes with high and good ecological status, while Cyanobacteria were more abundant in poor status lakes and dominated in bad status lake-years. Lakes in moderate and poor status exhibited higher total phytoplankton and Cyanobacteria biovolume values. This research improves our understanding of the status and temporal variations of phytoplankton communities in Greek lakes.