Industrial processes and the use of fertilizers are the main causes for the rapid eutrophication of lakes. Different indices, both chemical and biological, may be used to assess a level and a rate of the eutrophication process. Zooplankton indices can be among them, as zooplankton community structure is determined primarily by the physical and chemical environment and modified by biological interactions, i.e. predation and interspecific competition for food resources. Among biological indices of trophic state of lake, those based on densities and structure of crustacean communities seem to respond weaker. There are, however, patterns of crustacean communities connected with trophic state of lakes. Thus, an increase in trophic state causes: (1) an increase in the total numbers of crustaceans; (2) an increase in the total biomass of Cyclopoida; (3) an increase in the contribution of the biomass of Cyclopidae to the total crustacean biomass; (4) an increase in the ratio of the biomass of Cyclopoida to the biomass of Cladocera; (5) a decrease in the average body weight of Crustacea; (6) an increase in the ratio of Cladocera to Calanoida numbers; (7) an increase in the ratio of Cyclopoida to Calanoida numbers; (8) an increase in the dominance of species indicative of high trophy (Mesocyclops leuckartii, Thermocyclops oithonoides, Diaphanosoma brachyurum, Chydorus sphaericus, Bosmina (Eubosmina) coregoni thersites) in the numbers of all indicative species. Crustacean zooplankton was sampled at the deepest place in a lake at 1 m intervals from the surface to the bottom of epilimnion layer, and then samples were pooled together for the layer. Samples were taken once a year, during the summer stagnation. The material was collected from a total of 41 dimictic and 33 polymictic lakes within Masurian Lake District, Iława Lake District and Lubawa Upland. Among above-mentioned indices, six were the best correlated with trophic state of lakes. Below are formulas which enable to assess trophic state of lakes regardless of their mixis type (TSICR) from parameters of abundance and structure of crustacean communities: (1) TSICR1 = 25.5 N⁰‧¹⁴² (R² = 0.32), where TSI = trophic state index; N = numbers (ind. l⁻¹); (2) TSICR2 = 57.6 B⁰‧⁰⁸¹ (R² = 0.37), where B = biomass (mg w.wt. l⁻¹); (3) TSICR3 = 40.9 CB⁰‧⁰⁹⁷ (R² = 0.35), where CB = percentage of biomass of Cyclopoida in the total biomass of Crustacea (%); (4) TSICR4 = 58.3 (CY/CL)⁰‧⁰⁷¹ (R² = 0.30), where CY/ CL = ratio of the Cyclopoida biomass to the biomass of Cladocera (%); (5) TSICR5 = 5.08 Ln (CY/CA) + 46.6 (R² = 0.37), where CY/CA = ratio of Cyclopoida numbers to the numbers of Calanoida; (the relationship covering exclusively dimictic lakes); (6) TSICR6 = 43.8 e⁰‧⁰⁰⁴ (IHT) (R² = 0.30), where IHT = percentage of species indicative of high trophy in the indicative group’s numbers. It was assumed that the lakes with a TSICR under 45 are mesotrophic, those with a TSICR value of 45–55 are meso-eutrophic, those with a TSICR value of 55–65 – eutrophic and those with a TSICR above 65 – hypertrophic. Although crustacean indices of trophic state of lakes seem to be less useful than other biological indices, they may be recommended in assessing the quality of lake waters.
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