Changes in phytoplankton bloom dynamics in the future Arctic Ocean from a Regional Ecological Model

In the Arctic, where the effects of the changing climate are occurring faster than anywhere else on Earth, warming, sea-ice decline and changes in ocean circulation have already resulted in an overall increase of the marine primary productivity. According to global climate projections, the increased productivity is expected to continue in this region due to greater open-water habitats and larger growing seasons. Significant shifts in phytoplankton composition and an increasingly unstable community structure are also expected through the 21st century in response to climate change. Nevertheless, high uncertainties still exist in future net primary productivity (NPP) and the overall response of phytoplankton to climate change in the Arctic and subarctic regions.

This study assesses the effect of changing physical characteristics in the Nordic and Barents Seas on nutrient distribution and phytoplankton dynamics over the 21st century using the high-resolution NORWegian ECOlogical Model system (NORWECOM.E2E). The results show two distinct pathways of the phytoplankton response, differentiating Arctic conditions (i.e., Barents Sea) and Atlantic conditions (i.e., Nordic Seas). The Barents Sea, a shallow and well-mixed basin with persistent nutrient supply from the deep ocean to the surface, experiences a gradual intensification of the phytoplankton blooms towards the end of the century. This response is consistent with increasing temperatures, sunlight availability due to reduced sea-ice extent and the intensification of the vertical mixing.

In contrast, the Nordic Seas experience an abrupt change in the phytoplankton dynamics, with a sudden shift in the phytoplankton communities from a diatom-dominated to a flagellate-dominated bloom, according to the simulations. The rapid change in phytoplankton bloom dynamics is caused by an interplay between a shallowing mixed layer depth and changing nutrient consumption patterns by phytoplankton. These changes are consistent across climate scenarios SSP2-4.5, SSP3-7.0 and SSP5-8.5. However, the timing and magnitude of the changes vary significantly, with SSP3-7.0 showing the most abrupt changes.

As Arctic conditions continue at an accelerated pace, major implications for local and regional ecosystems are expected. These impacts will, most probably, not be limited to the Arctic region given its crucial role in the Earth’s system. Changes in phytoplankton bloom dynamics have the potential to impact the global carbon cycle by altering primary productivity and carbon export into the deep ocean, ultimately affecting the global climate.