Investigating zooplankton ecology and biogeochemistry in the Twilight Zone, one of the least studied ecosystems on Earth

Abstract

The biological carbon pump (BCP) is crucial to the regulation of Earth’s climate. Through a suite of processes that convert inorganic carbon dioxide (CO2) into particulate organic carbon (POC) and subsequent transfer to depth, the BCP is estimated to reduce atmospheric CO2 concentrations by 50 % of what they would otherwise be. Zooplankton are a key component of the BCP by grazing on phytoplankton, fragmenting and repackaging POC, and actively transporting carbon to depth through diel- and ontogenetic vertical migration. However, the role of zooplankton in POC transformation and vertical flux attenuation in the mesopelagic zone (100 – 1000m), where most POC attenuation occurs, remains poorly understood. This thesis investigates how vertical changes in zooplankton ecology and physiology shape carbon cycling within the mesopelagic. In the Scotia Sea, lipid-storing copepods were found to be emerging from overwintering, metabolically inactive, and not feeding on the spring diatom bloom. This highlights how the physiology of lipid-storing copepods causes a decoupling between primary production and when this organic matter is remineralised by these animals. In the northern Benguela Upwelling System (nBUS), zooplankton biomass peaked within the oxygen minimum zone (OMZ), whereas the larger animals were distributed above and below this region. These results suggest that the OMZ provides zooplankton with a refuge from predation. The main contribution of zooplankton to carbon sequestration was via their respiration at depth during diel vertical migration. This highlights the role of these animals in the spatial decoupling between carbon fixation in surface waters and remineralisation at depth. Lastly, analysis of zooplankton community size spectra in the Scotia Sea revealed that large, lipid-storing copepods, which integrate energy over an entire season, skew the shape of the size spectra, favouring larger size classes. This can mislead interpretations of energy flow unless taxonomy and physiology are integrated into size-based approaches. Collectively, results from this thesis highlight the role of zooplankton ecology and physiology in decoupling carbon cycling in both time and space in the mesopelagic.

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Identifiers

ORCID for Eloise Savineau: orcid.org/0000-0003-2038-2010

ORCID for Ben Ward: orcid.org/0000-0003-1290-8270

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