Increasing Arctic River Discharge and Its Role for the Phytoplankton Responses in the Present-day and Future Climate Simulations

With the unprecedented rate of Arctic warming in recent decades, the hydrological cycle over high-latitude landmass began to accelerate, which would lead to increased river discharge into the Arctic Ocean. However, the recent climate models that participated in Coupled Model Intercomparison Project 6 (CMIP6) tend to underestimate Arctic river discharge. This study elucidates the role of overlooked Arctic river discharge for the phytoplankton responses in present-day and future climate simulations. In the present-day climate simulation, the run with additional river discharge simulates the decrease in the spring phytoplankton. Freshening of Arctic seawater leads to high freezing point that increases sea ice concentration in the spring, eventually decreasing phytoplankton due to the less light availability. On the other hand, in the summer, phytoplankton increases due to the surplus of surface nitrate and the increase in the vertical mixing induced by the reduced summer sea ice melting water. In the future climate, the role played by additional input of freshwater is similar to the present-day climate. However, the major phytoplankton responses are shifted from the Eurasian Basin to the Canadian Basin and the East-Siberian Sea. This is mainly due to the shift of the marginal sea ice zone from the Barents-Kara Sea to the East Siberian-Chukchi Sea in the future.