| 19 Nov 2025
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.
This study uses the coupled physical–biogeochemical model MOM-ERGOM to investigate how temperature-driven processes influence nutrient cycling across the nine basins of the Baltic Sea. By increasing the temperature sensitivity of biogeochemical processes, both basin-wide and specifically in coastal zones, the authors show that a small enhancement in temperature-dependent recycling lead to large and nonlinear changes in nitrogen and phosphorus cycles.
The topic addressed in this manuscript is timely and highly relevant, and the work presented, including the figures and results, is interesting and clearly based on substantial effort. The study has strong potential to make a valuable contribution to the field.
However, the manuscript would benefit from further development before it is ready for publication. In particular, the discussion section currently reads largely as an extension of the results rather than a fully developed discussion. Expanding this section to include engagement with relevant literature, comparison with previous findings and existing models, and clearer positioning within the broader scientific context would significantly strengthen the manuscript.
Providing appropriate citations and benchmarking the findings against published studies would allow readers to better assess the robustness, novelty, and broader applicability of the results. Additionally, a more explicit evaluation of the magnitude and significance of the reported differences would enhance clarity and impact. In some sections, the text would also benefit from greater precision to avoid ambiguity.
In addition, while the authors state that a relatively small change in temperature sensitivity leads to a disproportionately large basin-scale response, it would be helpful to more clearly explain what this disproportion is relative to (e.g., in comparison to baseline variability?). Providing this context would make the nonlinearity of the response more transparent and convincing.
I believe this manuscript has strong potential for publication following major revision. Strengthening the discussion and conclusion sections, clarifying less precise passages, and clearly articulating the novel contributions of the study will substantially enhance its scientific value and overall impact. With these improvements, the work could represent a meaningful addition to the scientific knowledge of the Baltic Sea.
Further comments for each section and specific comments are given in pdf below.
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