Model projections of active transport by migrating zooplankton in the North Atlantic: consequences for the export of carbon and the biological pump.

Diel vertical migration (DVM) of zooplankton has been widely recognized as an important factor contributing to the biological carbon sink, releasing carbon and other nutrients at depth well beyond the surface layer of the ocean. There are however many uncertainties regarding the magnitude, occurrence and seasonality of active transport compared to the passive gravitational flux. Episodic measurements of such fluxes have shed light onto the role of DVM on a certain number of stations, but they seem insufficient to fully understand the mechanisms fueling the biological pump.

In this context, models are a useful tool to understand and quantify the magnitude of active transport. We present a model analysis of DVM and its impact on carbon export using the NEMO-PISCES ocean biogeochemical model in the North Atlantic basin, where different production regimes can be found. Active transport of organic matter is parameterized assuming a maximum transfer to depth of 30% of the total export production depending on the length of daylight. Results concerning changes in primary production (PP), carbon export (CEX), particulate carbon to PP ratio (pe-ratio), ratio of active to passive fluxes, respiration and excretion rates, and consequences for the nitrogen and oxygen biogeochemical cycles are presented.

Active transport accounts on average to 13% of the total particulate carbon export. While the maximum ratio is observed at subtropical regions with 20% -compared to 3% in subpolar regimes-, the absolute active transport is higher in these subpolar high productive regions. Compared to the control non-migrating model run, a -12% decrease in PP is observed followed by a -19% decrease in CEX in the subsurface, concomitant with an increase at 1000m deep where the biogeochemical imprint of active transport is more prominent. These results suggest that DVM could potentially explain part of the observed uncoupling between PP and its associated CEX, contributing in this way to unravel the biological pump mechanisms in the twilight zone.