Investigation of sediment‐induced eutrophication in two coastal lagoons of the southern Caspian Sea

This study provides a multidisciplinary analysis of sediment-induced eutrophication in two coastal lagoons of the southern Caspian Sea, integrating hydrochemical assessments, remote sensing techniques, and sediment core investigations. Hydrochemical analysis confirmed that sediment resuspension during flushing operations led to elevated turbidity, which facilitated the transport of phosphorus-rich particles into the lagoons and resulted in a 127.6% increase in total phosphorus levels. Remote sensing data further validated these findings, demonstrating a marked increase in turbidity following flushing events, directly correlating with the influx of nutrient-laden sediments. The strong positive relationship between turbidity and chlorophyll-a (Chl-a) underscores the role of these suspended sediments in fueling eutrophication. Sediment core analyses revealed critical historical evidence of environmental shifts, the disappearance of benthic foraminifera and charophytes, along with the formation of an azoic layer, signaling severe ecological degradation driven by sediment deposition. Principal component analysis revealed the relationships between river discharge, turbidity, Caspian Sea level fluctuations, total phosphorus, and Chl-a concentration on both coastal lagoons, showing that eutrophication was primarily driven by phosphorus-rich sediments delivered during dam flushing events. Sediment core analysis revealed the formation of an orange-shiny silty layer within the azoic deposits of Zibakenar Lagoon, indicating a severe anoxic environment. This evidence suggests that Zibakenar Lagoon developed hyper-eutrophic conditions, whereas Kiashahr Lagoon experienced comparatively milder eutrophication, influenced by variations in Caspian Sea level. This study highlights that uncontrolled sediment release from reservoirs can drive downstream eutrophication, emphasizing the urgent need for revised, sustainable sediment and nutrient management strategies.