Projected Increase of Phytoplankton Carbon Exudation and Particle Formation in the Arctic Ocean until the End of the Century

The Arctic Ocean is projected to become ice-free by the middle of the century, accompanied by changes in freshwater input, stratification, and warming of the upper ocean. The marine ecosystem is predominantly influenced by the availability of light and nutrients for phytoplankton, which form the base of the food web. With the projected changes of the physical environment throughout the course of the century, CMIP6 models suggest a general increase in Arctic net primary production. It is anticipated that phytoplankton shift from a light-limited state to nutrient limitation across large areas of the Arctic Ocean, potentially leading to increased exudation of organic carbon into the water column.

We briefly discuss the mechanisms driving the dynamics of organic carbon in the upper Arctic Ocean before focusing on long-term trends in Arctic biogeochemistry projected until 2100. Using an ocean general circulation sea-ice biogeochemistry model based on the IPCC Shared Socio-economic Pathway high-emission scenario SSP3-7.0, we observe regionally varying increases in exuded organic carbon, alongside enhanced formation of particulate organic carbon in the upper water column. These particles can either be transferred from the ocean to the atmosphere, acting as precursors to primary marine organic aerosols, or sink in the water column, contributing to carbon export. Our findings align with other recent studies, showing a shift from light to nutrient limitation in phytoplankton growth, particularly in regions experiencing retreat of the marginal ice zone. Our simulation indicates that diatoms are the primary contributors to organic carbon exudation and subsequent particle aggregation. However, some regions do not exhibit an overall increase in particulate organic carbon due to elevated remineralization rates. Overall, our projection provides an assessment of the impact of changes in the physical environment on phytoplankton dynamics and, consequently, on organic carbon pools in the upper Arctic Ocean. This work is part of the DFG Transregional Collaborative Research Centre 172 on Arctic Amplification.