Shelled pteropods and planktic foraminifers are calcifying zooplankton that contribute to the biological carbon pump, but their importance for regional and global plankton biomass and carbon fluxes is not well understood. Here, we modelled global annual patterns of pteropod and foraminifer total carbon (TC) biomass and total inorganic carbon (TIC) export fluxes over the top 200m using an ensemble of five species distribution models (SDMs). An exhaustive newly assembled dataset of zooplankton abundance observations was used to estimate the biomass of both plankton groups. With the SDM ensemble we modeled global TC biomass depending on multiple environmental parameters. 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 the biomass of both groups, surface temperature is the strongest environmental correlate, followed by chlorophyll-a. We found mean annual standing stocks of 52 (48-57) Tg TC and 0.9 (0.6-1.1) Tg TC for pteropods and foraminifers, respectively. This translates to mean annual TIC fluxes of 14 (9-22) Tg TIC yr-1 for pteropod shells and 11 (3-27) Tg TIC yr-1 for foraminifer tests. These results are similar to previous estimates for foraminifers standing stocks and fluxes but approximately a factor of ten lower for pteropods. The two zooplankton calcifiers contribute approximately 1.5% each to global surface carbonate fluxes, leaving 40%-60% of the global carbonate fluxes unaccounted for. We make suggestions how to close this gap.

Planktonic Foraminifera have been collected from the water column with different plankton sampling devices equipped with nets of various mesh sizes, which impedes direct comparison of observed quantifications. Here, we use data on the community size structure of planktonic Foraminifera to assess the impact of mesh size on the measured abundance (ind/m 3) of planktonic Foraminifera. We use data from the FORCIS database (Chaabane et al., 2023) on the global ocean at different sampling depths over the past century. We find a global cumulative increase in abundance with size, which is best described using a Michaelis-Menten function. This function yields multiplication factors by which one size fraction can be normalized to any other size fraction equal to or larger than 100 µm. The resulting size normalization model is calibrated over a range of different depth intervals, and validated with an independent dataset from various depth ranges. The comparison to the Berger, 1969 equivalent catch approach shows a significant increase in the predictive skill of the model. The new size normalization scheme enables comparison of Foraminifera abundance data sampled with plankton nets of different mesh sizes, such as compiled in the FORCIS database. The correction methodology may be effectively employed for various other plankton groups such as diatoms and dinoflagellates.