Temporal-spatial changes of the trophic index (TRIXcs), the risk of eutrophication (UNTRIX) and the determination of affected areas using a spatial salinity pattern in the southern of Caspian Sea

The spatial salinity patterns which give a picture of the area affected by eutrophication and trophic status changes at the ecosystem. The aim of this study was to determine the area affected by freshwater eutrophication using trophic level (TRIXcs), the risk of eutrophication (UNTRIX) and the nutrient-salinity scatter diagram. Also, the data of the present study are compared with previous year of studies. This research was conducted during 2018 to 2019 in eight transects and at 5, 10 and 30 meter depths of Iranian coasts of southern Caspian Sea. The results showed that mean seasonal changes of TRIXcs and UNTRIXcs were 4.94-5.59 and 3.57-4.17, respectively, which the highest trophic level and risk of eutrophication was observed in summer season. In addition to, trophic level (TRIXcs) risk of eutrophication (NUTRIX=4.30) of near the coast (5m depth) were high. Mean of TRIXcs and UNTRIX between different regions (western, middle and eastern) were not significant. In the present study, the maximum amounts of ammonium and soluble silica near the west coast were recorded in Anzali transect and also under the influence of the largest river Sefidrud in the southern shores of the Caspian Sea, but the points with maximum NOx and phosphate values were observed in the east (Amirabad and Bandar-e-Turkmen transects). The nonlinear relationship between NOx, NH4, DIP and DSi with salinity indicates that river flow and water mixing were affected nutrient concentration and changes of trophic levels in the study area.