The role of zooplankton and mineral ballasting in the biological carbon pump

Abstract

The global ocean is an important carbon sink, and currently stores one-third of the total anthropogenically released CO2. One of the major processes affecting the oceans functioning as carbon sink is the ‘biological carbon pump’. The biological carbon pump is strongly affected by ballast minerals and zooplankton activity. In this thesis the quantitative importance of ballast minerals and zooplankton aggregate feeding on the biological carbon pump were studied. Ballast minerals added to a natural plankton community from the North African Upwelling region enhanced aggregate formation and sinking velocities, leading to a potential ten-fold increase in carbon export. The presence of ballast minerals could explain up to 91% of the observed variability in POC export in this upwelling region. Therefore, ballast minerals seemed to control the magnitude of the POC flux in the North African Upwelling region. Zooplankton can feed on sinking aggregates, but relatively little is known regarding the mechanisms and their quantitative importance for POC export. Two copepod genera Calanus and Pseudocalanus were observed to detect and feed on in situ collected aggregates in three subarctic fjords. We calculated that 60-67% of the observed flux attenuation could be explained by Calanus and Pseudocalanus aggregate feeding. This showed that zooplankton aggregate feeding can have a major impact on POC export fluxes in subarctic fjords. Ballast minerals and zooplankton are of varying importance for POC fluxes between geographical regions, seasons and years. Especially zooplankton-mediated degradation can cause intense but variable POC flux attenuation on temporal and geographical scales, and may even function as a buffer for POC fluxes at greater depth. In contrast, at some locations high deposition of ballast minerals may significantly enhance POC export. The understanding of these two contrasting processes is fundamental for improving our understanding of CO2 sequestration in both today’s and future oceans.