In order to quantify pelagic-benthic coupling on high-latitude shelves
it is imperative to identify the different physical mechanisms by
which phytoplankton are exported to the sediments. In June-July 2023,
a field program documented the evolution of an under-ice phytoplankton
bloom on the northeast Chukchi shelf. Here we use in-situ data from
the cruise, a simple numerical model, historical water column data,
and ocean reanalysis fields to characterize the physical setting and
describe the dynamically-driven vertical export of chlorophyll
associated with the bloom. A water mass front separating cold,
high-nutrient winter water in the north and warmer summer waters to
the south – roughly coincident with the ice edge – supported a
baroclinic jet. A plume of high chlorophyll
fluorescence extending from the near-surface bloom in the winter water
downwards along the front was measured throughout the cruise. Using a
passive tracer to represent phytoplankton in the model, it was
demonstrated that the plume is the result of subduction due to
baroclinic instability of the frontal jet. This process, in concert
with the gravitational sinking, pumps the chlorophyll downwards an
order of magnitude faster than gravitational sinking alone. Particle
tracking using the ocean reanalysis fields reveals that a substantial
portion of the chlorophyll away from the front is advected off of the
northeast Chukchi shelf before reaching the bottom. This highlights
the importance of the frontal subduction process for delivering carbon
to the sea floor.

 

Chukchi Sea, phytoplankton sinking rates, baroclinic instability