Distribution of hydrogen sulfide, nitrogen and phosphorous species in inshore and offshore sediments of the south Caspian Sea.

This study aimed to geochemically investigate the sediments of the south Caspian Sea at different depths in summer and winter 2020. Sampling was conducted in 5 transects along the south coastline of the Caspian Sea and sediment grain size, hydrogen sulfide, Oxidation-reduction potential (Eh), total nitrogen, nitrite, nitrate, ammonium, total phosphorus, organic and inorganic phosphorous were measured. Eh values showed significant differences between seasons and between different transects (p < 0.05). Hydrogen sulfide ranged from 1.87 to 307.00 ppm. No significant difference was observed in hydrogen sulfide between seasons and among depths (p > 0.05). Total, inorganic and organic phosphorus contents were 782.96-1335.79 ppm, 639.66-1183.60 ppm, and 42.58-205.46 ppm, respectively. Total nitrogen revealed significant differences among transects (p < 0.05). Based on sediment quality guidelines, most sampling sites had alerting conditions for organic matter, and phosphorous contamination was detected at all stations. Anoxic condition was seen at most sites according to sedimentary Eh.