Impact of summer surface hydrodynamics on the advection of external phosphorus loads to the Archipelago Sea, northern Baltic Sea

The Archipelago Sea receives substantial external nutrient input via advection of phosphorus-rich surface waters from the Baltic Proper. We quantified net summer (15 May–15 October) phosphorus fluxes for 1993–2025 by combining simulated volume transport from a three-dimensional NEMO circulation model with observed total phosphorus concentrations. In addition, historical variability in surface-layer throughflow was reconstructed back to 1940 using a wind-based regression model. Summer throughflow exhibited a statistically significant but modest negative relationship with the summer NAO index, indicating that large-scale atmospheric variability modulates water exchange during the productive season. A structural shift occurred around 2008–2010, when reduced inflow and associated declines in phosphorus influx coincided with a transition from increasing to decreasing chlorophyll- a concentrations. Background phosphorus loading explained long-term variability in chlorophyll- a in the central Archipelago Sea (R 2 = 0.86), demonstrating strong coupling between external phosphorus supply and phytoplankton biomass. Interannual variability in summer water exchange strongly influences phosphorus availability. During summers with weaker inflow or stronger southward export, external phosphorus supply is reduced, potentially delaying or modulating ecological responses to basin-scale nutrient reduction measures. These findings highlight the importance of accounting for circulation-driven variability when evaluating eutrophication status and management outcomes.