The Role of Thermohydrophysical Processes in the Distribution of Chlorophyll “a” in a Water Column of a Small Mesotrophic Lake
Based on the results of long-term field measurements (2009–2019), the variability of the concentrations of chlorophyll “a” in the water column of a small lake Vendyurskoe (Karelia) during the open water period was investigated. The depth of the euphotic zone was estimated based on the solar radiation fluxes, and the stability of the lake’s water column (Brent-Väisälä frequency) was calculated based on water temperature. When water mass was in a state of homothermy, the concentration of chlorophyll “a” was evenly distributed over the water column, with some increase of cryptophytes and cyanobacteria within the euphotic zone. When thermal stratification developed and gravitational stability in the water column increased (N ≥ 0.025 rad/s), the heterogeneity of chlorophyll “a” distribution within a water column appeared, the maximum concentrations of chlorophyll “a” were fixed within a surface mixed layer, and below the thermocline, the concentrations sharply decreased. Green algae (30–50% in May, 35–55% in June) and diatoms (30–50% in May, 17–47% in June) prevailed in the composition of the plankton community (in terms of chlorophyll “a” concentration) in spring and summer. In autumn, the proportion of green algae sharply decreased to 5–20% and the proportion of diatoms increased to 40–80%. In some years, in autumn, there was a massive development of cyanobacteria (up to 30–55%) and cryptophytes (up to 40%). The functioning of the trout farm on the lake Vendyurskoe for 12 years did not lead to significant changes in the total concentration of chlorophyll ”a”. During the study period in autumn (except for two years of observations), the proportion of diatoms chlorophyll “a” concentration gradually increased from 40–50 to 80%, the proportion of green algae, cryptophytes and cyanobacteria decreased.