Fosferrox: A biogeochemical model extension for coupled iron, phosphorus and sulphur dynamics in response to changes in bottom water oxygen in BALTSEM

Marginal marine systems, such as the Baltic Sea, are naturally susceptible to bottom water oxygen (O₂) depletion due to strong stratification and restricted horizontal water exchange. In recent decades, bottom water hypoxia (O₂ < 63 μM) and anoxia (O₂ = 0 μM) have been further exacerbated in coastal areas due to excessive anthropogenic nitrogen (N) and phosphorus (P) inputs. Feedback mechanisms in the coupled biogeochemical cycling of P, iron (Fe) and sulphur (S) play a major role in controlling bottom water O₂ conditions. Phosphorus release from the seafloor amplifies when bottom water O₂ availability is low due to reductive dissolution of iron (Fe) oxide-bound P and preferential P regeneration from organic matter. This Fe oxide-bound P feedback mechanism has been suggested to play a key role in the rapid transitions at the onset and end of multidecadal hypoxic events in the Baltic Sea. Currently, the coupled biogeochemical cycling of Fe, P and S is not explicitly described in Baltic Sea models. For example, BALTSEM, the principal model used in decision making under the Baltic Sea Action Plan, does not include a representation of coupled Fe, P and S cycling and therefore utilises simplified parameterisations to mimic feedback mechanisms. A critical deficiency is that such parameterisations are calibrated for present-day state only, and do not take into account large-scale changes in the spatial distribution of Fe, P and S over long time-scales. Therefore, it can become difficult to predict possible future changes or to reproduce past events. Here, we introduce a new model extension for BALTSEM, so-called Fosferrox, that simulates the coupled dynamics between Fe, P and S in response to changes in bottom water oxygen for present day (1850-2100 A.D.). The implementation of such a mechanistic coupled biogeochemical cycling between Fe, P and S, and its associated feedback mechanisms in Baltic Sea models is fundamental to better understand how changes in, for example, P loading might impact water column redox conditions, as well as to improve hypoxia abatement strategies. The main impetus is to extend the functionality of Fosferrox to gain a better mechanistic understanding of how the coupled Fe, P and S feedback mechanisms drive the multidecadal oscillations in Baltic Sea hypoxia.