Investigating Triggers of North Atlantic Spring Blooms Using Gliders as a Part of the OSMOSIS Project

We present a unique set of physical and bio-optical observations obtained by Seagliders during the NERC OSMOSIS (Ocean Surface Mixing Ocean Submesoscale Interactions Study) mission at the Porcupine Abyssal Plain Sustained Observatory time series site. The data set is used to test competing hypotheses for phytoplankton spring bloom initiation in the subpolar North Atlantic Ocean. In particular, we focus on the critical depth and critical turbulence hypotheses.

Satellite data describe the year of the glider mission as the year with no pronounced spring bloom event. However, the gliders detected that the bloom developed slowly in weakly stratified conditions over several months. We find that the critical turbulence mechanism is unlikely to explain observed phytoplankton accumulation. Estimated mixing time scales were significantly shorter compared to phytoplankton growth time scales in the suboptimal light conditions. The critical depth criterion was met during the growth period for the parametrized depth of active mixing. However over several months both integrated and surface values of chl-a fluorescence increased only by a factor of ~2 and ~3 respectively. Our results confirm that Sverdrup’s criterion is “necessary but not sufficient” for initiating rapid high-magnitude phytoplankton spring blooms in the North Atlantic Ocean. The criterion indicates if net phytoplankton growth can occur but not how rapidly the bloom will progress.