The Influence of Meltwater on Phytoplankton Blooms Near the Sea-Ice Edge

Phytoplankton blooms occur annually at the sea-ice edge throughout the Arctic during the spring melt period. Our study considers how these spring blooms may depend on sea-ice meltwater, focusing on the role of horizontal mixing and advection. We extend the classic Fisher reaction-diffusion equation to consider a time- and space-varying death rate that represents the role of meltwater in the system. Our results indicate that blooms peak at a characteristic distance from the ice edge where (i) meltwater is concentrated enough to stratify the upper ocean such that the phytoplankton are confined near the surface and (ii) phytoplankton have been exposed to sufficient sunlight to allow for optimized growth. The results reproduce key characteristics of a large bloom observed in Fram Strait in May 2019. Our findings support the idea that sea-ice meltwater is of central importance in setting the spatial patterns of Arctic phytoplankton blooms. Plain Language Summary In the Arctic, each spring the appearance of the sun awakens the region's ecosystem. In particular, the blooming of phytoplankton-which form the base of the Arctic marine food web-is an early phenomenon that depends on the availability of sunlight. In this study, we present a model that supports the idea that sunlight alone is not enough to drive large plankton blooms in the open ocean: an influx from sea-ice meltwater is also needed. This meltwater (which is fresh and light) acts to maintain an ocean surface layer that is thin and separated from the ocean below. The plankton are confined to this surface layer where they can absorb plentiful sunlight and grow into large blooms. Our model sheds light on this central role of sea-ice meltwater for the growth of Arctic phytoplankton. Key Points . Observations show that sea-ice edge phytoplankton concentrations are spatially correlated with sea-ice meltwater We present an idealized model of phytoplankton dynamics where the influence of meltwater and sunlight is parameterized in phytoplankton growth and death rates Model output captures key characteristics of observed phytoplankton blooms in Fram Strait, highlighting the role of meltwater in bloom development

Automated summary

The study suggests that the spring blooms of phytoplankton in the Arctic Ocean are highly dependent on sea-ice meltwater for horizontal mixing and maintaining the stratified upper ocean. When meltwater concentration is sufficient, it allows phytoplankton to be confined near the surface, exposed to enough sunlight for optimal growth. The findings support the central role of sea-ice meltwater in shaping the spatial patterns of Arctic phytoplankton blooms.