Hydrogen Sulfide Contamination Studies in the Deep-Water Basin of the Middle Caspian Sea during a Cruise of the R/V Issledovatel Kaspiya in September 2022

The article presents the results of comprehensive studies of the hydrological and chemical structure of the Middle and Northern Caspian Sea carried out aboard R/V Issledovatel Kaspiya in September 2022. It is shown, despite the continuing decrease in sea level, that there is no aeration of deep layers and a hydrogen sulfide layer persists with a tendency towards an increasing hydrogen sulfide concentration in the bottom layer. There is an enlargement in the oxygen minimum zone. The changes of the Caspian Sea ecosystem have led to weakening of nutrient transfer from the photic layer into the bottom layers and, as a consequence, to a decrease in the silicon concentration in the bottom layer. The results will make it possible to evaluate trends of ongoing changes.