In the face of decline: The dominant role of sediment composition over hypersalinity and hypoxia in shaping Caspian Sea nematode assemblages

This study examined free-living nematode communities at 16 stations along the southern coast of the Caspian Sea and their relationship with environmental drivers. Strong environmental gradients were observed along this coastline, including hypersaline conditions in the east (up to 45 ppt) and critically low dissolved oxygen (2.0 mg/L) at Bandargaz. Sediment composition shifted from sandy (>88 % sand) in the west to silt-clay-dominated (>80 %) in the east. Sixteen nematode species were identified, with densities and diversity varying significantly. The highest densities were found at the Gomishan (574.67 ind./10 cm²) and Gorganrod (396.33 ind./10 cm²) stations in the southern Caspian Sea, and the lowest at Lisar and Sorkhrod (2.67 ind./10 cm²). Species richness peaked at Sisangan (8.33) and Anzali (4.33). Multivariate analyses showed distinct community structures, ranging from assemblages dominated by the deposit-feeder Daptonema karabugasensis to those with predators or diverse feeding guilds. The DISTLM analysis identified four key environmental variables shaping the nematode community structure. Sand content was the most influential factor (11.2 %), followed by organic matter (14.4 %), salinity (8.6 %), and temperature (6.0 %). Together, these variables cumulatively explained 40.2 % of the total observed variation. This study provides the first report of coastal hypoxia in the southern Caspian Sea, based on measurements taken during sampling. The findings indicate that local sediment characteristics, exacerbated by extreme gradients in salinity (11.07-45 ppt) and dissolved oxygen (2-9.26 mg/L), are the primary drivers shaping the structure, diversity, and trophic functioning of the nematode community.