Systemic Restructuring of the Caspian Sea Benthic Ecosystem: Long‐Term Impacts of Invasive Species and Environmental Drivers

ABSTRACT Over the past three decades, the ecosystem of the Caspian Sea (CS) has undergone serious changes, which are consequences of human activity, climate change, changes in sea level and the invasion of the ctenophore Mnemiopsis leidyi , which has disrupted trophic interactions and fisheries productivity. This study investigates long‐term changes in the macrobenthic community structure of the CS for 25 years, looking especially at the impacts of exotic species and environmental variability. Fifty‐three macrobenthic taxa belonging to 18 families and Oligochaeta were recorded in the following three sampling periods: 1996–1997, 2010–2011 and 2018–2019. Species richness and abundance declined markedly, with both indices reduced to less than one‐third in the final period. The exotic polychaete Streblospio gynobranchiata became the dominant species in the second and third periods, replacing Hypaniola kowalewskii , which prevailed in the first. Diversity indices fluctuated strongly, with the highest values observed in 1996–1997. Multivariate analyses indicated that depth and environmental shifts (salinity, dissolved oxygen, pH, nutrients) contributed substantially to community changes, highlighting the multifactorial nature of ecosystem dynamics. PCA and dbRDA identified salinity as the primary driver in the community, followed by DO, pH and depth. The DisTLM model explained 76.9% of species variation, although changing the sampling design may introduce uncertainty. Findings suggest invasive species, combined with environmental stressors, have caused dramatic ecological reorganization of the benthic ecosystem. The study highlights the need for adaptive management strategies to mitigate biodiversity loss and preserve ecosystem function in the face of ongoing anthropogenic and climatic pressures.