On characteristics reflecting the trophic state of large and shallow Estonian lakes (L. Peipsi, L. Võrtsjärv)

Mean zooplankter weight (W_Zp) and the ratio of zooplankton to phytoplankton biomass (B_Zp/B_Phyt) reflect adequately the trophic state of a body of water; they decrease with increasing trophy. In the moderately trophic Lake Peipsi, W_Zp is 4.4 μg and in the strongly eutrophic Lake Võrtsjärv, 2.7 μg, while B_Zp/B_Phyt is 0.49 and 0.13, respectively. Mean cladoceran weight is 28 and 10 μg, mean copepod weight 10 and 6.7 μg, mean rotifer weight 0.9 and 0.6 μg, respectively. Interestingly, in winter, W_Zp and B_Zp/B_Phyt are also significantly larger for L. Peipsi compared with L. Võrtsjärv: W_Zp 1.2 and 0.7 μg, B_Zp/B_Phyt 0.4 and 0.1, respectively. This difference is caused by E. gracilis (Sars) and Cyclops kolensis Lilljeborg which are still living in L. Peipsi, but have disappeared from L. Võrtsjärv. From the 1960s up to the present W_Zp has decreased appreciably. Correlation analysis showed the weak relationship between the studied characteristics and nutrient content, while the relationship between mean weight and water nutrient content is more pronounced in the strongly eutrophic L. Võrtsjärv than in L. Peipsi. There was a positive correlation between W_Zp and the biomass of the genus Daphnia in L. Peipsi, while in L. Võrtsjärv W_Zp was positively correlated with the biomass of Chydorus sphaericus. In L. Võrtsjärv, W_Zp was negatively correlated with the abundance and biomass of rotifers, as well as with the abundance of zooplankton comprised primarily of rotifers. A clear difference in both characteristics was revealed in the comparison of Lakes Peipsi and Võrtsjärv with ANOVA (p-level 0.002–<0.0001 for the years 1997–2001). W_Zp and B_Zp/B_Phyt characterize both the trophy and the whole ecosystem of a body of water. Indirectly, they characterize dominating groups in phytoplankton and zooplankton, feeding relationships between phyto- and zooplankton and between zooplankton and fish, as well as the pressure of fish on zooplankton. The W_Zp and B_Zp/B_Phyt can be used as marker characteristics in the evaluation of the trophy of a body of water and its ecosystem and, particularly, in permanent monitoring of bodies of water.