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Effects of atmospheric DUST deposition on COccolithophore production

Periodic Reporting for period 1 - DUSTCO (Effects of atmospheric DUST deposition on COccolithophore production)

Reporting period: 2018-03-01 to 2020-02-29

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).
DUSTCO has significantly contributed to advance existing understanding on what drives coccolithophore export production and spatiotemporal ecological dynamics (WP1, WP2), as well as their contribution to carbonate export flux (WP3) in the tropical North Atlantic. Combining transatlantic in situ observations at distinct timescales (photic zone – daily; sediment traps – seasonal) in synergy with a broad range of multi-proxy in situ and remote sensing observations provided an unprecedented perspective calcifying phytoplankton living between NW Africa and the Caribbean (WP1 and WP2). Through linking fieldwork observations with insights from testing wet Saharan dust deposition on a coccolithophore monoculture, DUSTCO also contributed to disclose the impact of nutrient input from chemically processed atmospheric dust on the cells growth (WP5) and biogeochemistry (WP3 and WP4) of coccolithophores. New understanding into the response of this group to ongoing ocean warming (WP1, WP2) and into their role modulating the biological carbon pumps across the tropical North Atlantic is also provided (WP3).
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.
DUSTCO has contributed to fulfil the increasing need for developing fundamental and in situ multidisciplinary research in tropical open-oceans, as these regions provide modern references for future ocean productivity in the face of climate-driven nutrient-depletion. In addition to potentially decelerate the biological pump in lowering atmospheric CO2, climate-driven ecological shifts may also disturb the oceans capacity in providing ecosystem services for socio-economic activities and human welfare. This is crucial for countries deep-rooted in fishing industries and with a strategic vector of development in the economy of the sea. As most models dealing with global ocean productivity are based in surface Chl-a concentrations derived from satellite observations, DUSTCO’s spatiotemporally resolved perspective is an important step forward towards a better understanding on these issues. We also provide evidence that dust-triggered increase of primary productivity in the upper photic zone of the ocean may contribute to compensate for the expected reduced efficiency of the biological carbon pumps. In other words, atmospheric dust is likely to play a key role in alleviating nutrient-limitation of marine productivity in the future, feedbacking onto the Earth’s climate by enhancing CO2 sequestration (and accelerating the biological pumps. The European science base and economy will greatly benefit from DUSTCO’s new insights regarding the link between atmosphere-ocean interactions and marine productivity, and with that, become able to provide far more realistic models for the fate of ocean productivity, fisheries and climate.