Abstract
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 (IHTCRU), percentage share of Rotifera species indicative of high trophy in the indicative group’s numbers IHTROT, Crustacean ratio of biomass to numbers B/NCRU, phytoplankton trophic index (TITP+TN), Margalef’s index, percentage share of cyclopoid biomass in total biomass of Crustacea (CB), Rotifera numbers (NROT), biomass of Cyclopoida (BCY), 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. IHTCRU, IHTROT, and B/NCRU were the most sensitive and gave the strongest responses at lower TP concentrations (<35 μg/L). The phytoplankton trophic index, TITP+TN, 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, NROT, BCY, 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.
Highlights
Plankton is a key component of pelagic ecosystems, forming the basis for most trophic webs
We demonstrated the response to eutrophication and the consistency of selected planktonic indices, calculated on the summer data from lakes characterized by different trophy
The maximum value of chlorophyll a concentration was only 4.3 μg/L, which was reflected in its high water transparency (6.7 m) and low phytoplankton biomass (0.7 mg/L)
Summary
Plankton is a key component of pelagic ecosystems, forming the basis for most trophic webs. A strong relationship exists between the two main components of plankton communities—phytoplankton and zooplankton. Phytoplankton, which are the main producers of organic matter in the pelagic zone (Kawecka and Eloranta 1994), are an essential source of food for zooplankton—directly for herbivorous animals and indirectly for detritus feeders. Eutrophication has a considerable impact on both plankton components, causing many changes in their abundance and species composition and affecting the relationships between them. Changes in the plankton community structure in relation to physicochemical parameters may be a first sign of a deterioration in the water quality.
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