The Northern Caspian ecosystem’s biological productivity largely depends on nutrient supply from the Volga River runoff. Since 1995, the Caspian Sea level has been declining, reaching –29.4 m in winter 2024/2025. This regression reshapes the delta’s shallow-water zones, potentially altering nutrient transformation before river waters reach the sea. This study assesses the transformation of biogenic river runoff under falling sea level conditions, using the Gandurinsky fish-pass channel as a model area. We analysed field data from four spring expeditions (2021, 2022, 2024, 2025) in the Volga delta and the Chistaya Bank polygon. Temperature, salinity, dissolved oxygen, and nutrients (silicate, nitrate, nitrite, ammonium, phosphate, total N and P) were measured in surface waters. A sharp drop in nutrient concentrations occurs at ~45.28°N, where water depth decreases to 0.2–1 m — well before the freshwater-seawater mixing zone. 

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