The effect of environmental factors on body size indices and the diversity of macrobenthic communities in Gomishan Lagoon and Miankale Bay

Gashtrudkhan River is located in the western drainage basin of Guilan Province. Monthly sampling of macroinvertebrates was carried out in five stations by using a surber sampler with Dimensions of 30.5 cm ×30.5 cm and 360 micron mesh size for eight months in different seasons from autumn 2019 to spring 2020. Moreover, some physicochemical parameters of water, i.e., pH, EC, TDS, DO, temperature and salinity were measured, so that these parameters showed significant differences in different seasons. In addition, diversity indices (Simpson and Shannon), species richness, EPT, EPT/Chir, biological indices (Hilsenhoff and BMWP) and the WQI index were studied and calculated. During the sampling period, a total of 1017 macroinvertebrates were isolated and identified from different stations, belonging to one Phylum, one Kingdom, four orders, 19 families and 22 genera in this river. EPT and EPT/CHIR indices were the highest in spring and the lowest in winter. Also, the highest and lowest values of Shannon's diversity and species richness indices were observed in autumn and spring, respectively. The water quality status in the studied period was classified as good to excellent based on the Hilsenhof biological index. Furthermore, the water quality status was evaluated based on the BMWP and WQI indicators as clean and average, respectively. According to the investigated area in Gashtroudkhan River, located in a forest area with gravel and stone beds, no significant differences were observed in many of the studied indicators.