Impact of Summer Surface Hydrodynamics on the Advection of External Phosphorus Loads to the Archipelago Sea, Northern Baltic Sea

Abstract The Archipelago Sea receives substantial nutrient inputs from surrounding marine areas, primarily via currents from the Baltic Proper. We used the three-dimensional NEMO hydrodynamic model to update estimates of net phosphorus fluxes from the Baltic Sea and the Gulf of Bothnia into the Archipelago Sea for the summer seasons of 1993–2025 and to calculate surface-current throughflow for 1940–2025. July–August mean flow showed a persistent negative relationship with the summer NAO index over the full study period. Pronounced structural shifts were identified in both physicochemical and biological components, with a distinct breakpoint around 2010 reflected in changes in flow and chlorophyll-a concentrations. Background phosphorus loading explained nearly 90% of the variability in chlorophyll- a in the central Archipelago Sea (R² = 0.863). These results indicate that the ecological status of the central Archipelago Sea is largely controlled by summer water-exchange dynamics and dominant phosphorus transport pathways.