No abstract

In the world's largest hypersaline lagoon Bay Sivash, its ecosystem twice transformed from a previous state to a new one due to human intervention. Before the North Crimean Canal construction, it was hypersaline (average salinity of 140 g l−1). The canal was built between 1963 and 1975, which resulted in intensive development of irrigated agriculture discharging drainage water into the bay. Between 1988 and 2013, salinity gradually dropped to average of 18–23 g l−1; a new ecosystem with a different biotic composition formed. In April 2014, the supply of Dnieper water into the North Crimean Canal ceased. This resulted in a gradual salinity increase in the bay to an average of 52 g l−1 in 2015. The start of second ecosystem shift was observed in 2015. In 2018, TSS, DOM and meiobenthos were studied in a salinity gradient from 30 to 88 g l−1. There was an increase in TSS and DOM with increasing salinity. The meiobenthos structure was significantly different in the bottom community and floating mats of filamentous green algae. No correlation was recorded between the number of meiobenthic macrotaxa in the sample and salinity. From 2013 to 2018, changes were seen in the taxocene structure of Nematoda and Harpacticoida. Changes in Nematoda were more dramatic and prolonged than in Harpacticoida. The structure of the Harpacticoida and Ostracoda taxocenes are less variable and more stable than that of the Nematoda taxocene. One of the reasons may be more mobility of Harpacticoida/Ostracoda than Nematoda and/or better osmoadaptation mechanisms.

Data on the presence and characteristics of smooth scallop Flexopecten glaber (Linnaeus, 1758) aggregations in various parts of the Black Sea shelf (BSS) have been subjected to a retrospective analysis. The waters of the northern (Crimean coast), eastern (Central and North Caucasian), and Anatolian sectors of BSS can be considered the most favorable habitats for smooth scallop, where massive beds of this species were found in the first half of the 20th century. However, no such data are available for the northwestern and western BSS, where freshwater discharges from the Danube, Dniester, and Dnieper form a permanent zone of lower water salinity. The period of decline in the Black Sea scallop population recorded in the 1970s–1990s generally coincided with the well known ecological crisis due to the anthropogenic eutrophication of the Black Sea in the second half of the 20th century. In the late 2000s and the 2010s, the scallop beds in the Crimean BSS showed a tendency to recover, which may be associated with the general improvement of the ecological condition of the Black Sea basin. A similar recovery of scallop beds is expected to occur in the eastern (North and Central Caucasian) and southern BSS, but has not yet been recorded. The results obtained suggest a possibility of downgrading the conservation status of this species in the Red Data Books of the City of Sevastopol and the Republic of Crimea from the `species declining in abundance` to the `species recovering in abundance and distribution`.

Lakes and lagoons play an important role worldwide, and salinity fluctuations significantly affect their ecosystems. Bay Sivash, the world’s largest hypersaline water body, underwent a sharp change in salinity, induced by the closing of the North Crimean Canal. To monitor a shift in the ecosystem, a study was carried out from 2014 to 2020 at 15 sites of the lagoon. Since the closure of the canal, the average salinity increased from 22 g L−1 (2013) to 94 g L−1 (2020). Suspended solids and dissolved organic matter also increased. When salinity increased above 50 g L−1, the number of taxa significantly decreased; this was a negative linear relation. The increase in salinity significantly changed the structure of zooplankton and benthos. The most dramatic changes occurred with the salinity increase from 25 to 70 g L−1. Chironomidae larvae numbers began to increase greatly in the ecosystem of the bay, and since 2014, they have rapidly increased their contribution to the abundance of benthos and plankton. The concentration of benthic–planktonic species increased in plankton, in particular, in Harpacticoida and Chironomidae. At salinity above 80–90 g L−1, nauplii and adult brine shrimp appeared to become abundant in plankton and benthos. The transit of the ecosystem to a new alternative state occurred.