Implementation of Antarctic Krill into the Ocean-Ecosystem Model FESOM-REcoM-2 and its Effects on Southern Ocean Biogeochemistry.

In this study, a three-dimensional, coupled ocean ecosystem model (FESOM- REcoM2) is used to investigate the effect of krill on the biogeochemistry of the Southern Ocean. The implementation of Antarctic krill in the model was done in three steps. 1) A second zooplankton group was implemented, which grazes on diatoms, mesozooplankton and nanophytoplankton (in order of descending preference). 2) A new detritus group was added to the model, which represents faster-sinking krill faecal pellets. 3) The grazing impact of both zooplankton groups on detritus was described. Afterwards, four different simulations (control and three krill simulations for previously described steps) were conducted to evaluate, how the implementation of the new zooplankton group and additional features affected biogeochemical processes in the Southern Ocean. In our krill simulation, the spatial distribution of krill biomass in the Southern Ocean was reasonably reproduced. Preliminary results showed that the proportion of living compartments (phytoplankton and zooplankton groups) in the model changed, which led to different POC (particulate organic carbon) flux pathways to the deep ocean. Zooplankton biomass contribution to total carbon biomass increased from 2.4% to 10% in our model in Southern Ocean. The contribution of zooplankton to POC production doubled. The implementation of krill in the ecosystem model enhanced nutrient recycling in the upper ocean layer. Therefore, our novel krill implementation improved the bias between model and observations in surface spatial distributions of the macronutrients silicic acid and nitrate