The contributions of various calcifying plankton to the South Atlantic calcium carbonate stock

Pelagic calcifying plankton play an important role in the marine carbon cycle. However, field studies quantifying the contributions of multiple calcifying plankton groups to particulate inorganic carbon (PIC) stocks and export into the ocean interior are scarce. Most studies target one specific plankton group and adjust their sampling strategy accordingly, hampering comparisons. Furthermore, the literature is strongly biased towards foraminifera and coccolithophores, so aragonite contributions (e.g., gastropods) remain virtually unconstrained. A holistic view is required for future projections of marine carbon cycle changes. Here, we present the contributions of three main calcifying plankton groups-coccolithophores, foraminifera and planktonic gastropods (comprising heteropods and pteropods)-to PIC stocks and fluxes throughout the water column during a sampling campaign in the South Atlantic Ocean. Coccolithophore calcite dominated the depth-integrated PIC standing stock (∼ 80 %), followed by aragonite from planktonic gastropods (∼ 17 %) and calcite from foraminifera (∼ 3 %). The estimated production and export of the calcifying plankton largely depend on assumed turnover times and sinking speeds, which both have large uncertainties. Coccolithophores contributed 92 %–99 % of the produced PIC, depending on planktonic gastropod turnover time, and from 52 % to 99 % of the exported PIC, depending on their mode of sinking. Both the standing stock and export of planktonic gastropods was significantly larger than that of foraminifera. Similarity between our results and those from different ocean basins suggests that these patterns are global in nature, implying that not only coccolithophores but also gastropods may be a more important contributor to the oceans PIC inventory than foraminifera, challenging a longstanding paradigm.