Spatiotemporal dynamics of chlorophyll-a in the Gorgan Bay and Miankaleh Peninsula biosphere reserve: Call for action

Gorgan Bay (GB) and Miankaleh Peninsula biosphere reserve has faced an endangering condition because of a significant water loss and quality degradation due to intensive eutrophication. How-ever, a synoptic view of the spatiotemporal dynamic of eutrophication is missing. In this study, we first used MODIS data to trace the long-term trend of eutrophication in GB between 2003 and 2021. To investigate the intra-annual variability of Chl-a in GB at a much higher spatial resolu-tion, we developed an Artificial Neural Network (ANN) algorithm that uses Sentinel-2 imagery to map monthly Chl-a in the water year 2020-2021. Finally, we explored the major physical con-trols on eutrophication in GB. Long-term status of trophic index in GB revealed that 4%, 18%, 15%, 24%, and 39% of the months between 2003 and 2021 were under the oligotrophic, intense oligotrophic, mesotrophic, eutrophic, and intense eutrophic condition, respectively. The results also showed a very good agreement between the observed Chl-a concentration and that estimated from ANN, as evidenced by R-2 = 0.88 and RMSE = 1.51 mg/m(3). We found that in eight months of the water year 2020-2021, there was a region of high Chl-a concentration in the southeast of GB near the outlet of the Qarasu River. We also observed that nutrient entrance to GB from the Qarasu River imposes a greater impact on Chl-a concentration in GB than that from the Gorgan-rud River. Our findings suggest that under the current GB's water circulation, the critical eutroph-ication status in GB has a slight impact on the pollution of the Caspian Sea, except along the southeastern shoreline.

Automated summary

The Gorgan Bay and Miankaleh Peninsula biosphere reserve is facing endangerment due to water loss and degradation caused by eutrophication. A study reveals the long-term trend and intra-annual variability of eutrophication in the region. It is found that the impact on the Caspian Sea is minimal except for the southeastern shoreline.