The paper gives the owerview of methodological problems, methods and indicators for assessing the reactions of algae communities to climatic factors. We discuss the resistance of algal communities to the fluctuations and anomalous impact of climatic factors (solar energy flow, water temperature, precipitation, river runoff) with reference to various life forms of the Black Sea algae. To assess the reaction of algal communities to the impact of climatic factors a number of methodological approaches are proposed. It is necessary to carry out the comparison of the variability of both climatic factors and plant communities only at the appropriate levels of organization of natural systems (local, regional, global). The changeability of climatic factors and algal communities must be measured using the unified methods and indicators (variability, anomalies etc.), and within the same time intervals (monthly, seasonal, annual). Authors propose to use the ratio of the oscillation coefficients (VR, %) for the structural-functional parameters of algal communities, and the climatic factor indices: (VR) algae communities/(VR) climatic factors, as a measure of “stability” of different life forms of algae. For a sound conclusion about the significant impact of climatic factors on plant communities, we proposed to use the criterion of a significant (more than 30%) deviation of structural-functional parameters of algae communities from the multi-year’s regional level. Analysis of the original results of both long-term observations (1997–2016) and the model period (2008–2009) made it possible to assess the stability of macrophytobenthos, phytoplankton and microepiphyton to fluctuations in climatic factors, as well as their anomalous manifestations. High natural stability of algal communities of the northwestern part of the Black Sea (NWBS) ecosystems to variability of climatic factors, as well as synchronism of anomalous reactions of algae with anomalous climatic conditions is noted. It includes intra- and interannual synchronicity of the oscillation coefficient (VR, %) of climatic factors (water temperature, solar energy flow) and morphofunctional parameters of algae (phytobenthos, phytoplankton, epiphyton). Cases of violation of such synchronism may indicate the effect of a group of third factors not related with climatic conditions, for example, changes in nutrient concentrations, biotope disturbance, etc. For the period of 1978–2016, synchronicity of dynamics between climatic anomalies and structural-functional parameters of the algal communities of the NWBS was recorded. Annual anomalies of climatic factors, exceeding 20% of the regional norm, cause evident restructuring of algal communities. Particularly strong anomalous reactions of plant communities (35–60% from the regional norm) were recorded in 2002–2003 under the long-term anomaly of water temperature and in 2010 under combination of high anomalies of two factors: the Danube runoff (45%) and water temperature (22%).
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