DUSTCO · Effects of atmospheric DUST deposition on COccolithophore production

Effects of atmospheric DUST deposition on COccolithophore production

The deposition of mineral dust originating from continental deserts is thought to provide nutrients that are essential for phytoplankton growth in large areas of the ocean, with implications for primary production and remaining oceanic food chain and subsequent export of Corg. Atmospheric dust deposition is, thus, likely to change the Earth’s climate and atmospheric CO2 either hrough fertilising the ocean (nutrient source) and/or by accelerating the biological carbon pump (ballasting effect). Coccolithophores (Haptophyta) are the most important open-ocean primary producers covering their cells with tiny calcite plates (the coccoliths) through a biogeochemical process that incorporates C in calcite and releases CO2 into the environment. Investigating the response of coccolithophores to ongoing ocean warming and future increased frequency of dust storms is crucial for understanding how this will affect the important biogeochemical cycles that they contribute to. In DUSTCO, we had the unique opportunity to study the impact of Saharan dust deposition on coccolithophores along a transatlantic array lying directly underneath the largest dust plume originating from Africa (12º N). Combining transatlantic observations of coccolithophore export fluxes and cells densities, while interpreting the results on a multi-proxy and multidisciplinary framework, provided an unprecedented spatiotemporal perspective on the ecology of tropical Atlantic calcifying phytoplankton. One of the novel outcomes of DUSTCO is that deep-dwelling species F. profunda and G. flabellatus seem to have a competitive advantage in a future warmer ocean compared to fast-blooming surface-dwelling species, which has the potential to increase the “rain ocean” in the future (Guerreiro et al., 2019). Yet, we provide evidence that dust-triggered pulsed productivity of the latter species may contribute to decrease the “rain ratio” (Korte et al., 2020). It seems that increasing Saharan dust outbreaks are likely to stimulate the biological pump via ballasting, but also via coccolith-ballasting, though enhancing the production of opportunistic species. Combining transatlantic data from in situ and remote sensing observations was crucial to demonstrate the potential of the UPZ/LPZ ratio as proxy for depth variations of the nutricline, both in the present and for paleoclimatic reconstructions. Earlier observations of dust-triggered coccolithophore export productivity in the tropical North Atlantic are confirmed with new insights gained from our experiment simulating Saharan wet dust deposition. E. huxleyi was seen growing in response to dust-born nutrient input in oligotrophic conditions mimicking a natural tropical ocean, without the intervention of nitrogen-fixation by marine diazotrophs. Persistently lower cell densities and a switch from an originally calcified towards a non-calcified E. huxleyi population following the addition of dust in one of the replicates suggests that mineral dust has the potential to limit coccolithogenesis and/or to induce a viral infection in E. huxleyi. Strong links between high coccolith-Sr/Ca ratios and pulsed coccolithophore export productivity triggered by wind-forced water mixing and dry Saharan dust deposition at the western tropical Atlantic supports the potential of this ratio as a biogeochemical productivity proxy. However, decreased coccolith-Sr/Ca ratios during the pulsed coccolithophore bloom triggered by wet Saharan dust and Amazon water inflow in the fall suggests that the ratios were mostly reflecting dust-related sinking processes (ballasting) rather than coccolithophore production (fertilisation).

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз