Marine indicators of climate change in the Azov Sea ecosystem

Abstract

The study is a contemporary overview of accumulated data and knowledge about the state of water resources in the Sea of Azov basin. The focused area has been rather thoroughly studied since the 1970s, however the information presented in the literature sources is mainly in Russian. The main research objective is to consider the changes that occurred in the second half of the 20th – early 21st centuries in the lower reaches of the Don and Kuban rivers as a result of economic development of the territory (construction of water-storage reservoirs, dams, irrigation canals, etc.). We provide the most complete summary of data on the inter-annual and seasonal variability of water and solid river runoff, the nutrient load on the ecosystem of the Sea of Azov, as well as new data on the dynamics of organic carbon in the Don runoff in the recent low-water period of 2007–2020. The most significant changes include redistribution of river runoff during the year, a catastrophic reduction in solid runoff, and a change in the structure of terrigenous matter entering the sea. In conclusion, we indicate the chains of consequences of long-term changes in water, solid, and nutrient river runoff for the ecosystem of the Sea of Azov. These include changes in seawater salinity and temperature, terrigenous sedimentation, chlorophyll-a concentration, primary production values, the relative amount of organic matter in bottom sediments (one of the causes of oxygen deficiency at the bottom), structure of planktonic and benthic organisms, as well as in dynamics of valuable commercial fish populations. In particular, we discuss recent observations that confirm the earlier long-term probabilistic forecast of the collapse of the Azov pikeperch population in the beginning of the 2000s. We consider the salinity and temperature of sea waters, data on the content of diatom valves, and the appearance of the Black Sea benthic species in the ecosystem of the Sea of Azov as reliable indicators of climate change. Some of the observed phenomena are still poorly explained and require further research. Using the example of the Sea of Azov and the rivers flowing into it, we indicate how climate variability and economic activity affect the state and functioning of marine ecosystems. Some identified regularities of the processes occurring in the Azov Sea ecosystem may be applied to other water bodies (marine areas).