Temperature driven coastal processes and their far reaching effects on deep Baltic Sea biogeochemical dynamics

Abstract. The coastal zone of the Baltic Sea plays a critical role in shaping the biogeochemistry of the deep ocean, mainly through the coastal filter. In this study, we investigated the role of temperature-driven biogeochemical processes in the sediment and water column in defining the biogeochemistry of the nine distinct basins of the Baltic Sea using a coupled physical biogeochemical model, MOM-ERGOM. In ERGOM, the temperature-driven biogeochemical processes are represented by the q₁₀ parameterisation, which is uniform in space and time and neglects that temperature sensitivities may differ with depth. We conducted two sets of sensitivity experiments to examine the effect of enhancing the temperature-driven biogeochemical processes by increasing the q₁₀ parameter both basin-wide and selectively in the coastal zone of the Baltic Sea. We found that detritus recycling in both sediment and the water column is the key process regulating basin-scale biogeochemistry. A modest 10 % enhancement in the q₁₀ parameter for these processes caused disproportionately big changes in nitrogen and phosphorus cycles of the Baltic Sea, demonstrating a nonlinear system response. The results reveal significant spatial heterogeneity in system-wide responses, with strong accumulation of ammonium and depletion of nitrate in the anoxic basins, indicating stronger denitrification over nitrification in warmer conditions. The rising nutrients from enhanced temperature-driven remineralisation indicate potential for higher primary productivity under a future warmer climate. Basin-wide enhanced water column remineralisation also caused increased phosphate concentrations in the Bothnian Bay, suggesting that the basin could potentially shift away from phosphate limitation under warming, with consequences for future productivity regimes. We introduced a non-dimensional metric of relative coastal sensitivity to assess the disproportional role of the coast in defining the biogeochemistry of the deep Baltic basins. The analysis shows that the nitrate cycle is disproportionately sensitive to coastal sediment recycling, with the Bothnian Bay displaying two to fourfold stronger responses in nutrient cycles than basin-wide perturbations, underscoring the disproportionate influence of coastal processes on basin biogeochemistry. In the Bothnian Bay, phosphate dynamics depend on the spatial scope of sediment recycling. When enhanced only in coastal sediments, oxic conditions trap much of the released phosphate as iron–phosphate, strengthening the coastal filter and limiting export to the deep basin. In contrast, basin-wide enhancement releases phosphate from adjacent anoxic basins, which is transported northward, increasing phosphate availability in the Bothnian Bay. Accurately resolving coastal processes is therefore essential to capture the coastal filter and avoid misrepresenting nutrient transport and ecosystem responses under climate change.