The Impact of Zooplankton Calcifiers on the Marine Carbon Cycle

Shelled pteropods and planktic foraminifers are calcifying zooplankton that contribute to the biological carbon pump via the sinking of their calcareous shells. However, their importance for regional and global plankton biomass and carbon fluxes is not well understood. Here, we modeled global annual patterns of pteropod and foraminifer total carbon (TC) biomass and total inorganic carbon (TIC) export fluxes over the top 200 m using five species distribution models (SDMs). An extended version of the MARine Ecosystem DATa (MAREDAT) of zooplankton abundance observations was used to estimate the biomass of both plankton groups. We found hotspots of mean annual pteropod biomass in the high Northern latitudes and the global upwelling systems, and in the high latitudes of both hemispheres and the tropics for foraminifers. This largely agrees with previously observed distributions. For both groups, temperature is the strongest environmental correlate, followed by chlorophyll-a. We found mean annual standing stocks of 52 Tg TC (48 to 57 Tg TC) and 0.9 Tg TC (0.6 to 1.1 Tg TC) for pteropods and foraminifers, respectively. This translates to mean annual TIC fluxes of 14 Tg TIC yr(-1) (9 to 22 Tg TIC yr(-1)) for pteropod shells and 11 Tg TIC yr(-1) (3 to 27 Tg TIC yr(-1)) for foraminifer tests. These results are similar to previous estimates for foraminifers, but approximately a factor of ten lower for pteropods. Pteropods contribute 0.2%-3.2% and foraminifers 0.1%-3.8% to global surface carbonate fluxes. Including global coccolithophore fluxes, this leaves 40%-60% of the global carbonate fluxes unaccounted for. Our figures are likely lower-bound estimates due to sampling data characteristics and uncertainty associated with organism growth rates.