Macrostructure of benthos along a salinity gradient: The case of Sivash Bay (the Sea of Azov), the largest hypersaline lagoon worldwide

Abstract

In 2018, total suspended solids (TSS) and dissolved organic matter (DOM) were studied as well as macrostructure of the macro- and meiobenthos (by sampling soft-sediments and algal mats) along a spatial salinity gradient from 30 to 88 g l−1 in Sivash Bay. The main goals of the study were to evaluate current ecosystem state, analyze long-term trends and verify the suggestions made previously. There was a significant increase in TSS and DOM with increasing salinity. No correlation between the number of taxa in the sample and salinity was found for meiobenthos and total benthos, but a negative reliable relationship was obtained for macrobenthos of the soft bottom. Salinity had significant relationships with the total number of taxa in a sample and with the number of macrobenthic taxa associated with floating filamentous green alga mats. There was a positive relationship between the total abundance of soft-bottom benthos and the total number of macrotaxa. In 2013, before the closure of the channel, the number of soft-bottom macrobenthos taxa was 11, in 2015–8, and in 2018 – only 5. These changes negatively correlated with changes of salinity and TSS in 2013–2018. There was an exponential increase in abundance and the percentage of chironomid larvae in the total number of benthos from 2013 to 2018, which was significantly positively correlated with changes in salinity and TSS. Macro- and meiobenthos showed different responses to the salinity increase. Currently, hypoxic and anoxic events often occur in Sivash Bay under floating algal mats, and this also may suppress taxon diversity in macrobenthos more than in meiobenthos.