Modeling the Oslofjord bottom oxygen regime interdecadal changes affected by increased waste water discharge in 1970s

Oxygen regime in the vicinity of the sediment-water interface (SWI) was studied with a coupled benthic-pelagic model 2DBP (Yakushev et al., 2020), that considers biogeochemical processes occurring in the water column and the sediments in the same model domain. C-N-P-Si-O-S-Mn-Fe biogeochemical model BROM describes in detail the processes of organic matter mineralization in oxygen-depleted conditions that are vitally important for assessing biogeochemical impacts (i.e., denitrification, metal reduction, sulfate reduction). This allows us to predict changes in redox conditions at the bottom as a function of seasonality (organic matter production and destruction, vertical mixing), anthropogenic (eutrophication), and climatic (temperature and mixing ) factors.

This model was applied to the Western Oslo Fjord subjected to an intensive organic waste discharge in the 1960s-1970s. In this period, it was documented the disappearance of shrimps in the bottom layer and increased primary production in the surface layer.

This work aimed to reconstruct the Oslo Fjord SWI redox conditions in the past decadal period (1930-2023). It was numerically demonstrated the interconnection between an amount of supplying with waste water organic matter and oxygen depletion processes in the bottom layer. It was shown that in the 1970s there was a shift from normoxic to anoxic conditions. It was also estimated the period of the system restoration to normoxic conditions after the improvement of the wastewater plant in the 1980s. This model can be used for analyzing the consequences of future changes under different scenarios of nutrient discharges to the Fjord.