Meta-analysis of the role of zooplankton faecal pellets in ocean carbon export flux

Abstract Faecal pellets of marine zooplankton play a key role in the biological carbon pump, i.e. all biologically mediated processes by which organic carbon produced by photosynthesis is stored in the ocean’s interior. Numerous factors (size and biomass of faecal pellets, composition and abundance of zooplankton, etc.) can affect the sinking rate of zooplankton faecal pellets and thus the efficiency of their export at depth. A number of quantitative studies of the role of zooplankton faecal pellets in the biological carbon pump have been conducted, focusing either on a region or a type of faecal pellets. These studies highlighted the large variability in the contribution of faecal pellets to carbon fluxes, ranging from 0% to 100%. Here, we used a meta-analysis approach to extract quantitative data on the size, biomass, and role of marine zooplankton faecal pellets in ocean carbon export from 197 scientific articles. Our study focused on the six most studied faecal pellet types (mixed, cylindrical, ellipsoidal, tabular, spherical, and drop-shaped). We showed that abundance and biomass fluxes of faecal pellets, as well as their contribution to particulate organic carbon fluxes, increased with ecosystem productivity, here approximated by surface chlorophyll-a concentration. Furthermore, the fluxes of marine zooplankton faecal pellets (both by abundance and biomass) were positively correlated, and the sampling location, rather than the type of faecal pellet, better explained this correlation. Additionally, sinking rates were strongly correlated with volume, length, and width of faecal pellets, for all faecal pellet types. Sinking rates did not vary with depth, although measurements become scarcer with depth. Our literature review highlights the crucial role of faecal pellets in the biological carbon pump and the need to study less known types of faecal pellets, such as ellipsoidal faecal pellets, and to measure multiple variables on the same samples. Finally, we recommend that modellers wishing to represent faecal pellets in global biogeochemical models choose a constant sinking rate with depth within the range of the quantitative values reported here.