Abstract
The study continues a series of observations started in the late 1950s, aimed at inferring changes in the Lake Ladoga ecosystem state recorded in the surface-sediment diatom assemblages. At the pre-anthropogenic stage (prior to the 1960s), the composition of the surface-sediment diatom assemblages indicated an oligotrophic state of Lake Ladoga. With the increased P load to the lake (late 1960s–1980s), the transition to a mesotrophic state was recorded via increased proportions of eutrophic species and decreased abundances of the taxa typical of the pre-anthropogenic stage. In the early 1990s, the composition of the surface-sediment diatom assemblages still indicated a mesotrophic state despite a decreased external P load. At the present de-eutrophication stage of Lake Ladoga (the 2000s), the abundances of eutrophic taxa steadily decrease while some taxa typical of the pre-anthropogenic period return to their dominating position in the surface-sediment diatom assemblages. However, despite the decreased P concentrations, the Lake Ladoga ecosystem has not returned to its pre-anthropogenic state as indicated by the present-day composition of the surface-sediment diatom assemblages. This suggests a delayed ecosystem response to the decreased anthropogenic pressure, and possibly some irreversible changes resulting from the eutrophication. At present, de-eutrophication processes and ecosystem recovery are superimposed upon the recent climatic changes that govern the onset and duration of the vegetative seasons for the phytoplankton communities in Lake Ladoga. The diatom-inferred changes in the ecological state of Lake Ladoga are in agreement with the results of longterm hydrochemical and hydrobiological studies.
Highlights
Being indispensable for all living things on Earth, freshwater is one of the most limited resources
C. dubius mainly occurred in diatom assemblages in the archipelago part, while S. minutulus was observed at the majority of the sampling sites except for the Volkhov Bay in the south
Studies of the surface-sediment diatom assemblages enabled enabled the changes that have occurred in Lake Ladoga ecosystem, starting from the late 1950s as a result of human impact and recent climate changes
Summary
Being indispensable for all living things on Earth, freshwater is one of the most limited resources. It only accounts 2,5% of all freely available water in the world, only 0,3% of which is readily accessible in lakes, reservoirs and rivers (Kalff 2001). Population growth and enhanced industrial and agricultural activities have played important role in water pollution and rapid deterioration of aquatic ecosystems starting from the mid-20th century. I.e. anthropogenic nutrient (P and N) loading to aquatic ecosystems, is acknowledged as the most widespread form of lake pollution on a global scale (Smol and Stoermer 2010). Major advances in the scientific understanding and management of eutrophication have been made since the late 1960s, eutrophication continues to be ranked as the most common water-quality problem in the world, and remains an active area of scientific research (Schindler 2006; Smol 2008)
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