Periodic Reporting for period 1 - DUSTCO (Effects of atmospheric DUST deposition on COccolithophore production)
Periodo di rendicontazione: 2018-03-01 al 2020-02-29
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).
Part of the results have been reported in the paper “Transatlantic gradients in calcifying phytoplankton (coccolithophore) fluxes” (Guerreiro et al., 2019, Prog. Oceanog.) and contributed to the paper “Multiple drivers of production and particle export in the western tropical North Atlantic” (Korte et al., 2020, Limnol. & Ocean.). Four other publications are still in preparation (Guerreiro et al., A, B, C, D). The results were publicly presented at seminars, workshops and conferences, including the 17th Conference of the International Nannoplankton Association (Sept. 2019, Brazil) and contributed to presentations at the EGU General Assembly, 3rd inDUST Joint Working Group meeting, 20th INQUA (International Union for Quaternary Research), 20th POGO (Partnership for Observation of the Global Ocean) and the Atlantic from Space Workshop (ESA and NOC). Amongst other means, the progress of the project and the main results have been also disseminated through the website/blog www.dustco-online.com created in April 2018 and regularly updated ever since.