Increase in the Frequency and Extent of Sub-Ice Phytoplankton Blooms in the Arctic Ocean

Phytoplankton are a fundamental component of the Arctic ecosystem and carbon cycle. Through their growth and decay, they form the foundation of the oceanic food web and are a sink for atmospheric CO2. Phytoplankton populations undergo periods of exponential growth, known as ``blooms", which occur seasonally in many of the world's oceans. In 2011, a phytoplankton bloom was observed underneath a region of the Arctic fully covered by sea ice, unexpected as sea ice is typically understood to transmit little solar radiation to the ocean below. To investigate the likelihood and location of sub-ice blooms, we develop a critical-depth model for regions of the ice-covered Arctic ocean that incorporates the transmission of solar radiation through regions of sea ice that are covered by melt ponds. We find that favorable conditions for sub-ice blooms exist over a large portion of the modern Arctic. The development of bloom-permitting regions of the ice-covered Arctic, which comprise greater than 30\% of the latitudes above 70N in July has occurred only in the recent two, coinciding with the thinning of Arctic sea ice and an increase in melt pond coverage. Our results demonstrate that these biological events may be both possible and likely in regions previously considered off-limits to photosynthetic activity. Projections of a thinner Arctic sea ice cover in a warming world suggests that the likelihood and extent of sub-ice phytoplankton blooms will increase in the future.