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

Abstract

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. However, 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 resolution, 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 controls 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 R2 = 0.88 and RMSE = 1.51 mg/m3. 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 Gorganrud River. Our findings suggest that under the current GB's water circulation, the critical eutrophication status in GB has a slight impact on the pollution of the Caspian Sea, except along the southeastern shoreline.