Applicability of Copernicus marine service products for the eutrophication status assessment of the Baltic Sea

Eutrophication status assessments of the Baltic Sea rely on regionally agreed monitoring and assessment methodologies based on indicators with thresholds of achieving or not achieving a good environmental status. Model outputs can be used to increase the data availability and assessment confidence. We evaluated the applicability of Copernicus Marine Service (CMEMS) reanalysis products for four eutrophication indicators - dissolved inorganic nitrogen (DIN) and phosphorus (DIP), chlorophyll-a and oxygen debt. The most significant discrepancies between the model-based and monitoring-based indicator results were found for DIN. Compared to observations, significantly lower winter DIN concentrations were also reflected in modelled low summer chlorophyll-a levels. A general agreement between the modelled and observed DIP values was not the case in the northern and eastern basins with large freshwater discharges and either very low or high DIP concentrations. A poor agreement between the model and observations in these basins could be related to low data availability for the assessment and assimilation into the model. Significantly lower oxygen debt values in the deep basins derived using the reanalysis product than the observations point to model deficiencies in reproducing mixing and biogeochemical processes. More observations and biogeochemical data assimilation, which was skipped in the latest CMEMS reanalysis product, are obvious ways to improve the model outputs. However, biogeochemical models must be further developed to better match model outputs and observations before introducing more sophisticated eutrophication indicators.