Seasonal variations of diversity and abundance of the benthic Annelida in coastal sediments of the southern Caspian Sea (Mazandaran-Sari)

In order to investigate seasonal variation of diversity and abundance of Annelida, seasonal sediment sampling was performed on 10 stations with depths of 1, 5, 10 and 20 meters in the coastal area of Sari, Iran in 2015-2016. The results led to the identification of 8 genera and species of Annelida belonging to four families of Polychaeta and two families of Oligochaeta. The highest average annual abundance belonged to Streblospio gynobranchiata and the lowest average belonged to Prenereis sp. The average frequency of the Annelida was significantly and positively correlated with the percentage of the sand and the total amount of organic matter in sediments. The Shannon-Wiener and Margalef Indices showed that, the Annelida had the highest diversity in summer and the lowest species diversity in winter. The Simpson Heterogeneity Index showed the highest amount in the estuary on winter, and heterogeneity showed reduction with increasing the depth. The value of the brillouin index was increased along with the increase of the depth of the stations. The results of this research indicated that the diversity of Polychaeta and Oligochaeta in the study area is reduced compared to previous studies, while the prevalence of dominant species showed a significant increase.