Abstract
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.