Although many researchers assume the temperature regime to be a sensitive marker for the testing of climate changes, other characteristics Pexidartinib mw such as the duration of the ‘biological summer’

(the period with temperatures > 10°C, Efremova & Palshin 2012) can be used as an important marker of climate change, because it determines the initial biomass growth rate and the reproduction rate (abundance) of aquatic organisms. The example of six lakes in Karelia from 1953 to 2009 shows that the duration of the ‘biological summer’ has increased by 12–23 days and that the trend of the prolongation of the ‘biological summer’ is positive (p < 0.05) ( Efremova & Palshin 2012). The majority of the lakes in East Fennoscandia are characterised by an increase in the ice-free period (Filatov MDV3100 research buy et al. 2012). Earlier ice-melting in Lake Onega can result in a shift of the spring bloom period of diatoms. The negative correlation between the ice-free period and plankton characteristics (Chl a and N phytoplankton) may be explained by the predominance of large-sized diatoms (Tabellaria fenestrata and Aulacoseira islandica) in the summer phytoplankton. Chl a in these species is lower than in other algae (diatoms). The negative correlations between NAO, AO, precipitation

and zoobenthos abundance and biomass testify that nutrient and organic matter loads from the catchment area can increase together with the increase of precipitation in years with a positive NAO. In turn, eutrophication next phenomena (hypoxia, H2S production etc.) can reduce the numbers of sensitive species (relict amphipods) and, conversely, favour eurybiotic taxa (oligochaetes). Oxygen depletion and higher temperatures accelerate nutrient release processes at the sediment-water

interface (Søndergaard et al. 2003) and increase the stress on aquatic organisms (Weider and Lampert, 1985, Saeger et al., 2000 and Wilhelm and Adrian, 2007), resulting in a decrease in their abundance. Significant correlations between climate indices, physical parameters in Petrozavodsk Bay, Lake Onega, and some characteristics of its biota (phytoplankton, zoobenthos) were found in this research. We conclude that global climate primarily determines the regional hydrological variables of a lacustrine ecosystem and its productivity level, whereas biotic characteristics are a reflection principally of the variability in the water temperature and the ice-free period, both of which determine the duration of the ‘biological summer’ (WT > 10°C). At the same time, the responses of biological communities and whole ecosystems to climate variability are complex and often difficult to recognise, especially in the case of large ecosystems with a long period of water exchange. We cordially thank Professor Nikolai N. Filatov, Dr Natalia M. Kalinkina for the valuable discussion and also Mrs Y.