The Intraseasonal Dynamics of the Mixed Layer Pump in the Subpolar North Atlantic Ocean: A Biogeochemical‐Argo Float Approach

Abstract

The detrainment of organic matter from the mixed layer, a process known as the mixed layer pump (ML pump), has long been overlooked in carbon export budgets. Recently, the ML pump has been investigated at seasonal scale and appeared to contribute significantly to particulate organic carbon export to the mesopelagic zone, especially at high latitudes where seasonal variations of the mixed layer depth are large. However, the dynamics of the ML pump at intraseasonal scales remains poorly known, mainly because the lack of observational tools suited to studying such dynamics. In the present study, using a dense network of autonomous profiling floats equipped with bio‐optical sensors, we captured widespread episodic ML pump‐driven export events, during the winter and early spring period, in a large part of the subpolar North Atlantic Ocean. The intraseasonal dynamics of the ML pump exports fresh organic material to depth (basin‐scale average up to 55 mg C·m−2·day−1), providing a significant source of energy to the mesopelagic food web before the spring bloom period. This mechanism may sustain the seasonal development of overwintering organisms such as copepods with potential impact on the characteristics of the forthcoming spring phytoplankton bloom through predator‐prey interactions.

Phytoplankton Phenology in the North Atlantic: Insights From Profiling Float Measurements

“…Previous studies (e.g., Lacour et al, 2017Lacour et al, , 2019 have shown that vertical distributions of Chl and b bp did not always tightly track the density-based MLD. In this study, although the Chl and C phyto (derived from b bp ) profiles were mostly consistent with the MLD and/or Z(0.415) isolume depth, mismatches were also observed (especially during the winter-spring transition period, Supplementary Figures S5, S6).…”

Southern Ocean Phytoplankton Blooms Observed by Biogeochemical Floats

“…Our results show that vertically integrated biomass can be high even if concentration itself is low. Depending on the timing of blooms, wintertime enhancement in submesoscale vertical velocities can subduct this organic matter to depth (Erickson & Thompson, 2018;Lacour et al, 2017Lacour et al, , 2019Llort et al, 2018;Omand et al, 2015). Quantifying the timing and biomass of blooms, therefore, can have significant implications for estimating the biological carbon pump.…”

“…Their study is complementary to our work, as they report on the export of total organic carbon from the surface to interior, while we report on the growth and decay of the phytoplankton themselves. BGC Argo floats and gliders equipped with similar sensors have also been used to investigate bloom dynamics in the North Atlantic (Boss & Behrenfeld, 2010;Erickson & Thompson, 2018;Lacour et al, 2017Lacour et al, , 2019Mignot et al, 2016Mignot et al, , 2018 and Southern Ocean (Swart et al, 2015).…”

Factors Controlling the Lack of Phytoplankton Biomass in Naturally Iron Fertilized Waters Near Heard and McDonald Islands in the Southern Ocean

“…The best choice depends on the intended subsequent use of the MLD estimate. The chosen method used often reflects a timescale, and can also provide some separation between the mixing depth to which turbulence is currently stirring the water column and the mixed depth to which past events have homogenized its properties (e.g., Brainerd and Gregg, 1995;Lacour et al, 2017Lacour et al, , 2019.…”