Phytoplankton metrics for trophic and ecological status assessment of a natural karstic lake

According to the Water Framework Directive (WFD), biological quality elements constitute the basis for assessing the ecological status of surface waters. Biological communities are good indicators of water quality because they reflect environmental conditions over time and do not require frequent sampling. The main aims of this study were to assess the trophic and ecological status of the Lake Visovac (Krka River hydrosystem, South Croatia) using phytoplankton together with supportive physico-chemical parameters, eutrophication status and impact indicators. We also tested the viability of chlorophyll-a (Chl-a) and Hungarian Lake Phytoplankton Index (HLPI) as proposed metrics in the standard regulatory monitoring procedure. The phytoplankton samples were taken and Chl-a and physical- chemical parameters measured on a monthly basis from April to September in 2016 and 2019. In 2016, the dominant species was Pantocsekiella ocellata (Pantocsek) K.T.Kiss & Acs, while in 2019 it was accompanied by Ceratium hirundinella (O.F.Muller) Dujardin. According to Chl-a, Lake Visovac was within the limits of oligo- to mesotrophic status. Based on the HLPI index, the Lake ecological status was assessed as Good. Chl-a showed a statistically significant positive correlation with temperature, while HLPI was positively correlated with oxygen and Secchi depth. We propose the use of Chl-a for rapid bioassessment on a weekly basis, whilst the more complex HLPI index should be applied monthly. Further improvement of the confidence level of the metrics used to assess the ecological status and a comprehensive revision of boundaries for included indicators is of fundamental importance.