Radium isotopes trace cross-shelf nutrient transport in a marginal sea

Nearshore coastal zones play a critical role modulating terrestrial nutrient transport into the ocean. While nutrient dynamics of marginal seas like the Baltic Sea are well documented, they are predominantly based on riverine inputs, overlooking the biogeochemical transformation and exchange of nutrients beyond the estuarine filter. Here, we use short-lived radium isotopes to investigate cross shelf nutrient transport along 14 transects, spatially distributed across 12 locations along the Baltic Sea coastline. The radium tracer approach revealed a horizontal eddy diffusivity rate of (3.9 ± 1.1) x 10 6 m 2 day −1 and nearshore water residence time of 7.6 ± 4.5 days. Cross-shelf nutrient loads extrapolated along the 8000 km coastline were 8.2 ± 2.9 kt Si day −1 for silicate, 0.9 ± 0.5 kt N day −1 for dissolved inorganic nitrogen (DIN), and 0.1 ± 0.02 kt P day −1 for phosphate. DIN loads were 4.5 times higher than average historical river-derived loads for September. Phosphate inputs were 20 % higher, while silicate transport rates were 57 % lower than the corresponding historical river loads. While comparisons between our radium-derived cross-shelf fluxes with river loads create perspective, they should be seen with care due to potential spatial and temporal biases. Overall, these findings illustrate the use of radium as a tracer of terrestrial inputs and underscore the need to integrate shelf-scale processes beyond the estuarine filter into coastal nutrient assessments. • Eddy diffusion in nearshore waters drives lateral dispersal of terrestrial nutrient inputs. • ·Inner-shelf biogeochemical transformations cause phosphorus limitation and silicate enrichment. • ·Elevated radium-derived phosphate and inorganic nitrogen loads suggest additional lateral subsurface pathways.