Alexandrium on the Alaskan Beaufort Sea shelf: Impact of upwelling in a warming Arctic.

The harmful algal genus Alexandrium has characteristically been found in temperate and subtropical regions; however recent evidence suggests global warming may be expanding its range into high latitude waters. Alexandrium cysts have previously been documented in the Chukchi Sea and we hypothesize that Alexandrium may be expanding further into the Arctic due to distribution by the Beaufort shelfbreak jet. Here we document the presence of Alexandrium catenella along the Alaskan Beaufort Sea shelf, marking an expansion of its known range. The observations of A. catenella were made using three different methods: FlowCAM imaging, 18S eukaryotic sequencing, and real-time quantitative PCR. Four occupations of a shelf/slope transect spanned the evolution of a strong wind-driven upwelling event over a 5-day period. A nearby mooring provided the physical context for the event, revealing that enhanced easterly winds reversed the Beaufort shelfbreak jet to the west and induced upwelling of colder, denser water onto the outer shelf. A. catenella sequences dominated the surface phytoplankton community at the onset of the upwelling event. This signal vanished during and after the event, likely due to a combination of alongstream advection, cross-stream advection, and wind mixing. These results suggest contrasting physical processes that are both subject to global warming amplification, delivery of warm waters via the Beaufort shelfbreak jet and upwelling, may control the proliferation of this potential harmful alga into the Arctic.

Massive phytoplankton blooms under Arctic sea ice.

Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.