Assessment of transparent exopolymer particles in the Arctic Ocean implemented into the coupled ocean–sea ice–biogeochemistry model FESOM2.1–REcoM3

Abstract. We present an assessment of the coupled ocean–sea ice–biogeochemistry model FESOM2.1–REcoM3, in which we integrated state equations for dissolved acidic polysaccharides (PCHO) and transparent exopolymer particles (TEP), as proposed by Engel et al. (2004), to explicitly describe these two organic carbon pools in the Arctic Ocean. PCHO is simulated as one fraction of the phytoplankton exudates, which can then aggregate to form larger particles, TEP. Since observational datasets on TEP are rare in time and space, we systematically assess the novel model implementation by stepwise discussing the essential components of the organic carbon cycle. Firstly, the simulated phytoplankton biomass yields good results when compared to in situ and remote-sensing products of total Chlorophyll a and particulate organic carbon. Secondly, we compare PCHO to observations in the Fram Strait, as an exemplary data-rich region, and to datasets in other regions of the Arctic Ocean. The model realistically reproduces a high phytoplankton exudation rate of PCHO under nutrient-depleted conditions. Thirdly, we assess simulated TEP concentrations by comparing them to in situ measurements from several campaigns to the Arctic Ocean. The simulation provides a first estimate of mean TEP concentrations of 200–400 μg C L⁻¹ on the continental shelves and 10–50 μg C L⁻¹ in the central basins (0–30 m depth range). Lastly, we put the model performance into a global context for TEP concentrations in the upper ocean layer. As such, the implementation of PCHO exudation, aggregation to TEP, and their remineralization processes into FESOM2.1–REcoM3 offers a reasonably good agreement with observations, on which further modeling work can build upon.