Sensitivity of plankton indices to lake trophic conditions.

Herein, we report the response of indices based on phytoplankton and zooplankton and their combination to different nutrient concentrations in lakes. The study was carried out in ten lakes in northeastern Poland. Integrated samples were collected from the epilimnion during the summer of 2012-2013. Secchi disk visibility (SD), total phosphorus (TP), total nitrogen (TN), and chlorophyll a were used as proxies for eutrophication. We calculated 16 plankton indices: two phytoplankton indices, six crustacean indices, five rotiferan indices, two zooplankton diversity indices, and one combined phytoplankton and zooplankton index. Among them, nine indices with the strongest correlations with TP were selected: percentage share of Crustacean species indicative of high trophy in the indicative group's numbers (IHT), percentage share of Rotifera species indicative of high trophy in the indicative group's numbers IHT, Crustacean ratio of biomass to numbers B/N, phytoplankton trophic index (TI), Margalef's index, percentage share of cyclopoid biomass in total biomass of Crustacea (CB), Rotifera numbers (N), biomass of Cyclopoida (B), and ratio of the cyclopoid biomass to the biomass of Cladocera (CY/CL). The sensitivity of the normalized values of these indices to proxies of eutrophication was tested. IHT, IHT, and B/N were the most sensitive and gave the strongest responses at lower TP concentrations (<35 μg/L). The phytoplankton trophic index, TI, together with the zooplankton-based Margalef's index and CB were very sensitive in both low (<35 μg/L) and high (>60 μg/L) TP conditions. On the other hand, N, B, and CY/CL were slightly sensitive at low TP concentrations while their reaction was notable at high TP concentrations. A similar pattern of response was observed for TN concentration and SD visibility.