Biogeochemical Changes in Lake Ladoga: Insights from Satellite Data

The trophic system of the largest freshwater lake in Europe, Ladoga, is in a state of transformation. Initially oligotrophic, the lake became mesotrophic as a result of anthropogenic eutrophication in the late 1950s. Anthropogenic nutrient loads have been sharply reduced since the late 1990s and the status of the lake is changing again, this time against a background of climate warming. Based on MODIS Aqua data, a numerical evaluation of these changes over the past 18 years (2003–2020) was carried out through assessment of the dynamics of the concentration of chlorophyll (CHL) of phytoplankton, mineral suspended matter (SM) and colored dissolved organic matter (CDOM). The observed decrease in CDOM concentration concurringly with the decline of the CHL concentration might speak in favor of continuing supply of digestible phosphorus into the lake due to the lysis of dissolved allochthonous organic matter. This is able to slow down the return of the lake to the initially oligotrophic state regardless of the reported continuing decline of the lake average inorganic phosphorus concentration. It is shown that the climatic impact is manifested, in particular, in the expansion of the time period of biological activity at the primary production level: the growth of phytoplankton becomes appreciable in March, while its end is shifted to between October and November. At the same time, the intra-annual dynamics of CHL has increased presumably due to high occurrence of extreme meteorological conditions, which is a feature of ongoing climate change.