Global contribution of echinoderms to the marine carbon cycle: CaCO₃ budget and benthic compartments

The contribution of carbonate-producing benthic organisms to the global marine carbon budget has been overlooked, the prevailing view being that calcium carbonate (CaCO₃) is predominantly produced and exported by marine plankton in the “biological pump.” Here, we provide the first estimation of the global contribution of echinoderms to the marine inorganic and organic carbon cycle, based on organism-level measurements from species of the five echinoderm classes. Echinoderms' global CaCO₃ contribution amounts to ~0.861 Pg CaCO₃/yr (0.102 Pg C/yr of inorganic carbon) as a production rate, and ~2.11 Pg CaCO₃ (0.25 Pg C of inorganic carbon) as a standing stock from the shelves, slopes, and abyssal depths. Echinoderm inorganic carbon production (0.102 Pg C/yr) is less than the global pelagic production (0.4–1.8 Pg C/yr) and similar to the estimates for carbonate shelves globally (0.024–0.120 Pg C/yr). Echinoderm CaCO₃ production per unit area is ~27.01 g CaCO₃·m⁻²·yr⁻¹ (3.24 g C·m⁻²·yr⁻¹ as inorganic carbon) on a global scale for all areas, with a standing stock of ~63.34 g CaCO₃/m² (7.60 g C/m² as inorganic carbon), and ~7.97 g C/m² as organic carbon. The shelf production alone is 77.91 g CaCO₃·m⁻²·yr⁻¹ (9.35 g C·m⁻²·yr⁻¹ as inorganic carbon) in contrast to 2.05 g CaCO₃·m⁻²·yr⁻¹ (0.24 g C·m⁻²·yr⁻¹ as inorganic carbon) for the slope on a global scale. The biogeography of the CaCO₃ standing stocks of echinoderms showed strong latitudinal variability. More than 80% of the global CaCO₃ production from echinoderms occurs between 0 and 800 m, with the highest contribution attributed to the shelf and upper slope. We provide a global distribution of echinoderm populations in the context of global calcite saturation horizons, since undersaturated waters with respect to mineral phases are surfacing. This shallowing is a direct consequence of ocean acidification, and in some places it may reach the shelf and upper slope permanently, where the highest CaCO₃ standing stocks from echinoderms originate. These organism-level data contribute substantially to the assessment of global carbonate inventories, which at present are poorly estimated. Additionally, it is desirable to include these benthic compartments in coupled global biogeochemical models representing the “biological pump” and its feedbacks, since at present all efforts have focused on pelagic processes, dominated by coccolithophores. The omission of the benthic processes from modeling will only diminish the understanding of elemental fluxes at large scales and any future prediction of climate change scenarios.