Eutrophication caused by nutrient enrichment is a predominant stressor leading to lake degradation and, thus, the set-up of boundaries that support good ecological status, the Water Framework Directive’s main target, is a necessity. Greece is one of the Member States that have recorded delays in complying with the coherent management goals of European legislation. A wide range of different statistical approaches has been proposed in the Best Practice Guide for determining appropriate nutrient thresholds. To determine the nutrient thresholds supporting the good status of natural Greek lakes, the phytoplankton dataset gathered from the national monitoring programme (2015–2020) was used for shallow and deep natural lakes. The regression analyses were sufficient and robust in order to derive total phosphorus thresholds that ranged from 20 to 41 μg/L in shallow and 15–32 μg/L in deep natural lake types. Nutrient boundaries that encompass the stressors these lakes are subject to, are essential in proper lake management design.

This study investigates the performance of published bio-optical remote sensing indices/algorithms for monitoring water quality changes in Lake Karla, Greece, caused by Storm Daniel after the September 2023 flooding event. Commonly applied indices were utilised to estimate chlorophyll-a (Chl-a) and total suspended solids (TSS) using Sentinel-2 high-resolution optical imagery. In situ measurements were undertaken and water samples were collected during the pre-flooding period, post-flooding, and one-year post-flood, providing a basis for validating the remote sensing models. Monitoring results showed that most physicochemical parameters changed considerably. Chl-a and TSS were estimated by testing five and seven indices, respectively. Regarding the Chl-a estimation, the NDCI and 2-BDA indices outperformed other models, having high correlations with in situ Chl-a measurements and effectively following the in situ Chl-a temporal trends. Among the TSS indices, NDWI and TUR-IND demonstrated better performances, effectively capturing the variations in suspended solids. Overall, this study highlights the potential of Sentinel-2 imagery in assessing water quality changes, particularly in response to flooding events. It is an exploratory approach to assess the feasibility of utilising optical satellite data for evaluating the environmental impacts of natural disasters on lake water quality and supports decision-making in environmental management. Additionally, it identifies potential challenges and considerations that must be addressed to ensure effective application.

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.

Eutrophication, driven by nutrient enrichment, represents substantial anthropogenic pressure with harmful consequences for aquatic ecosystems. The Water Framework Directive provides a structured approach to addressing this challenge as it requires European Union Member States to achieve at least good ecological status for their surface waters. The establishment of realistic nutrient boundaries, above which negative effects become pronounced, is essential to guide regulatory intervention aimed at securing long-term water sustainability in Europe. Greece is one of the Member States which should determine nutrient boundaries supporting the good ecological status of lakes. Two statistical approaches, ranged major axis regression and binomial logistic regression, were applied for setting appropriate nutrient boundaries for Greek natural lakes and reservoirs, using datasets of phytoplankton and total phosphorus concentrations, retrieved from the national monitoring program (2016–2023). The predicted boundary values for total phosphorus supporting good ecological status ranged from 32 to 76 μg/L, with stricter boundaries corresponding to deep lakes. Nutrient boundaries that reflect the environmental pressures on Greek natural lakes and reservoirs are fundamental to ensure proper design of lake management strategies.