Spatial distribution and structure of benthic polychaete communities in shallow waters of the south Caspian Sea, Iran

Polychaetes are the most abundant taxon in benthic communities. The polychaete assemblages on the Iranian Caspian Sea coast were studied during the summer of 2016 and winter of 2017 to examine the spatial-temporal variability in diversity and community structure, as well as their relationships with the main environmental variables. As a representative of the study area, fifteen stations at 1, 3, and 5 m depths were chosen. Capitella capitata was reported for the first time in the Caspian Sea. Four species were found: Streblospio gynobranchiata, Alitta succinea, Hypania invalida, and Capitella capitata.  The eastern transect (Gorgan Bay) had the highest total abundance of polychaetes (15146 ind.m2) with increasing in the abundance of S. gynobranchiata. Furthermore, S. gynobranchiata appears to be well-distributed in this sea and plays the most prominent role in the community structure. A. succinea, S. gynobranchiata, and C. capitata showed an increasing trend from the western to the eastern parts of the shoreline. Changes in polychaetes abundance can be attributed to changes in the sediment type, with western transects containing more sand and eastern transects comprising silt-loam. Total polychaete abundance was correlated with salinity, water temperature, depth, dissolved oxygen, Total Organic Matter and sediment characteristics in the summer and winter seasons, according to the Canonical correlation analysis. S. gynobranchiata and C. capitata, as invasive species, may have altered the current polychaete community distribution, structure, and diversity in the southeast Caspian Sea. The disappearance of some species and the decrease in species richness could be due to the success of S. gynobranchiata in the competition for resources and space. Other conditions such as environmental factors and pollutants can also affect the presence of sensitive and tolerant organisms.