Articles | Volume 14, issue 1
Research article
12 Jan 2017
Research article |  | 12 Jan 2017

Role of zooplankton in determining the efficiency of the biological carbon pump

Emma L. Cavan, Stephanie A. Henson, Anna Belcher, and Richard Sanders

Abstract. The efficiency of the ocean's biological carbon pump (BCPeff – here the product of particle export and transfer efficiencies) plays a key role in the air–sea partitioning of CO2. Despite its importance in the global carbon cycle, the biological processes that control BCPeff are poorly known. We investigate the potential role that zooplankton play in the biological carbon pump using both in situ observations and model output. Observed and modelled estimates of fast, slow, and total sinking fluxes are presented from three oceanic sites: the Atlantic sector of the Southern Ocean, the temperate North Atlantic, and the equatorial Pacific oxygen minimum zone (OMZ). We find that observed particle export efficiency is inversely related to primary production likely due to zooplankton grazing, in direct contrast to the model estimates. The model and observations show strongest agreement in remineralization coefficients and BCPeff at the OMZ site where zooplankton processing of particles in the mesopelagic zone is thought to be low. As the model has limited representation of zooplankton-mediated remineralization processes, we suggest that these results point to the importance of zooplankton in setting BCPeff, including particle grazing and fragmentation, and the effect of diel vertical migration. We suggest that improving parameterizations of zooplankton processes may increase the fidelity of biogeochemical model estimates of the biological carbon pump. Future changes in climate such as the expansion of OMZs may decrease the role of zooplankton in the biological carbon pump globally, hence increasing its efficiency.

Short summary
The biological carbon pump (BCP) plays a key role in regulating atmospheric CO2. Controls on the efficiency at which this occurs are poorly known. Here we combine in situ observations with an ecosystem model to show that zooplankton have an important role in regulating the efficiency of the BCP. Predicted future changes in ocean conditions, such as expansion of oxygen minimum zones, may decrease the role of zooplankton in the BCP globally, increasing its efficiency and altering atmospheric CO2.
Final-revised paper