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

Abstract

Mean zooplankter weight (WZp) and the ratio of zooplankton to phytoplankton biomass (BZp/BPhyt) reflect adequately the trophic state of a body of water; they decrease with increasing trophy. In the moderately trophic Lake Peipsi, WZp is 4.4 µg and in the strongly eutrophic Lake Vortsjarv, 2.7 µg, while BZp/BPhyt 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, WZp and BZp/BPhyt are also significantly larger for L. Peipsi compared with L. Vortsjarv: WZp 1.2 and 0.7 µg, BZp/BPhyt 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. Vortsjarv. From the 1960s up to the present WZp 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. Vortsjarv than in L. Peipsi. There was a positive correlation between WZp and the biomass of the genus Daphnia in L. Peipsi, while in L. Vortsjarv WZp was positively correlated with the biomass of Chydorus sphaericus. In L. Vortsjarv, WZp 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 Vortsjarv with ANOVA (p-level 0.002–<0.0001 for the years 1997–2001). WZp and BZp/BPhyt 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 WZp and BZp/BPhyt 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.