Biological Interpretation of the Third Main Factor Influencing the Long-Term Dynamics of Hydrological and Ecological Parameters of the Three Naroch Lakes under Changing Nutrient Load in 1978–2015

The data of continuous monitoring of the Naroch Lakes system, which includes eutrophic Lake Batorino, mesotrophic Lake Myastro, and oligo-mesotrophic Lake Naroch, are used to search for the factors most affecting the long-term dynamics of the state of a lake ecosystem under a changing nutrient load. The set of continuous series of seasonal mean values of eight parameters of each ecosystem obtained in 1978–2015 is analyzed by singular spectrum analysis (SSA) and principal component analysis (PCA). The third principal component (F3), which contributed 8.2, 9.8, and 13.3% to the dynamics of ecosystems of Lake Batorino, Lake Myastro, and Lake Naroch, respectively, turns out to be connected with phyto- and zooplankton biomass, as well as with the parameters characterizing the processes of destruction and biochemical decomposition of organic matter. A comparison of statistical relationships and periods of the dominant cyclic components of principal components F3 and the associated parameters of ecosystems allow us to conclude that the interpretation of the third main factor under conditions of changing nutrient load depend on the characteristics of a particular ecosystem. In nutrient-rich ecosystems, it can be interpreted as the transfer of produced primary organic matter to consumers of the next trophic level, related to the structure of the food chain in the ecosystem. In the ecosystems where the process of primary production is limited by nutrient(s), this factor is represented by the available amount of nutrients, which is the third main factor affecting its long-term dynamics.