Great news! Our new paper based on multidisciplinary data collected during the Atlantic Meridional Transect (AMT28, 2018) and from satellite Earth Observations, has just been published at Frontiers in Marine Science! Our plan was to investigate links between biological, oceanographic, and atmospheric processes influencing the meridional (between 50ºN and 2ºS) and in-depth distribution (along the photic zone of the ocean) of coccolithophore communities (calcifying phytoplankton) across the entire North- and Equatorial Atlantic. The studied transect included, of course, the Atlantic region which is more heavily affected by the deposition of dust originating from the Saharan and Sahel desert regions, in Africa.
Location of the sampling stations across the North- and equatorial Atlantic, collected during AMT28, and a schematic representation of the main surface currents in the region (A); location of aerosol sampling and approximate position of the Intertropical Convergence Zone (ITCZ) during the expedition (B).
Understanding the dynamics of phytoplankton in the modern ocean: why does it matter?
Marine phytoplankton are among the smallest organisms living in out planet. Yet, they are the first link of the ocean food web, accounting for about 50% of the global primary production and producing ca 50% of the oxygen that we breath. Like land plants, these tiny organisms need light and nutrients to survive and reproduce in the photic (illuminated) zone of the ocean. And since they use atmospheric CO2 to photosynthesise their organic (cellular) tissues, marine phytoplankton ends up being the main responsible for the biological carbon pump through which this carbon-enriched material is exported from the upper ocean and sequestered in deep-sea sediments for longer periods of time [Cermeño et al., 2008]. However, ongoing climate warming is likely to reduce nutrient availability for the survival of these tiny yet crucially important planktonic organisms, through making the ocean more thermally stratified as well as acidic. This is an issue because any changes in phytoplankton productivity and/or species composition in response to ocean warming are likely to have cascading biogeochemical effects that feed back to the changing climate.
Why do we focus on coccolithophores?
Perhaps what best distinguishes coccolithophores from other phytoplankton groups is their unique capacity of influencing the cycling of marine carbon in three ways: 1) they act as a natural sink of CO2, while performing photosynthesis to produce particulate organic carbon (POC); 2) they also act as a natural source of CO2 sink, while biomineralizing particulate inorganic carbon (PIC) with which they build their carbonated exoskeleton (the coccosphere); and finally, they also provide a natural source of mineral ballast with which they contribute to facilitate the export and burial of POC (the so-called “coccolith-ballasting effect”) [e.g., Rost and Riebesell, 2004; Guerreiro et al., 2021]. In other words: any changes in coccolithophore productivity are likely to alter the efficiency of the biological carbon pump since the latter is largely determined by how the PIC/POC ratio (also known as “rain ratio”) changes from the surface to the deep ocean.
Picture of a coccolithophore bloom as seen from the Scanning Electron Microscope (SEM). This bloom was captured at the surface of the ocean in the NE tropical North Atlantic, in April 2016 (Photo Credits: Catarina V. Guerreiro).
Why do we care about dust deposition in the ocean?
It’s mostly because there are megatons of these tiny mineral particles being lifted from continental deserts and transported by the wind for thousands of kilometers around the globe, every year, until they are eventually deposited again on the soils of rainforests, glaciers and, also of course, in the ocean. As dust particles are known to carry macronutrients and trace metals that are essential for fertilizing marine phytoplankton, dust deposition may act as an important (alternative) nutrient source in those large, remote, and heavily thermally stratified open-ocean regions where phytoplankton is limited by nutrient-depletion during most of the year. This process is likely to become increasingly important as driver of oxygen production and CO2 sequestration in our current scenario of an ever-warmer ocean due to climate change.
NE tropical Atlantic: a natural laboratory to study the effects of dust!
Being adjacent to NW Africa, a region affected by ongoing desertification (Mirzabaev et al., 2019), the NE tropical Atlantic provides a unique opportunity to investigate the effects of Saharan Desert dust outbreaks on ocean productivity and biogeochemistry. Indeed, of all the currently active sources of dust, the Sahara Desert is the most important, contributing up to 70% of the total dust emissions on a global scale. According to NASA estimates, about 140 megatons of the total Saharan dust particles that are transported yearly across the Atlantic by the trade winds, end up settling in the ocean (e.g., Yu et al., 2019), thereby providing nutrients for phytoplankton living in the tropical North Atlantic (Goudie and Middleton, 2001; Jickells et al., 2005). The biogeochemical effects from such Saharan dust deposition in their ocean are among the main issues that we have been investigating in the context of project CHASE!
Massive plumes of storm-blown Saharan dust as revealed from satellite images (Photo Credits here).
Coccolithophores as biogeochemical tools to unravel atmosphere-ocean interactions: what were our findings?
Despite their lower nutrient requirements (compared to, for instance, diatoms), coccolithophores are susceptible to changes in ocean conditions including, of course, nutrient depletion. Our study indeed confirmed existing understanding of the distribution of ecologically distinct coccolithophorespecies as being related to major latitudinal hydrological gradients across the North Atlantic, which determine whether the ocean is more or less thermally stratified, illuminated and/or nutrient-depleted.
Our observations revealed that dynamic, oxygenated and microphytoplankton-enriched (cell-size > 20 mm) waters at higher-latitudes were characterized by less diverse coccolithophore populations, dominated by fast-blooming coccolithophore species with higher nutrient requirements and typical of the upper photic zone. In contrast, the heavily stratified and picoplankton-enriched (cell-size < 2 mm) waters of the subtropical gyre revealed more diverse populations, dominated by umbelliform coccolithophores and holococcolithophores at the surface, and by floriform taxa in the lower photic zone.
Meridional and vertical distribution of cell concentrations produced by the main coccolithophore taxonomic groups (adapted from Young, 1994) living along the photic zone during across the North Atlantic during AMT28: (A) r-selected placolith-bearing taxa including E. huxleyi, G. oceanica, G. ericsonii, G. muellerae; (B) other placolith-bearing taxa including Umbilicosphaera spp., Oolithotus spp. and C. leptoporus; (C) species within Rhabdosphaera spp. and umbelliform taxa including Umbellosphaera spp. and D. tubifera; (D) deep-dwelling floriform taxa including F. profunda, G. flabellatus and A. robusta; (E) miscellaneous taxa including (ordered from more to less abundant) Syracosphaera spp., Ophiaster spp., S. pulchra, Calciosolenia spp., Helicosphaera spp., Coronosphaera spp., Acanthoica spp., Michaelsarsia spp., R. sessilis, other taxa contributing < 5% to the coccolithophore assemblage, and indetermined taxa; (F) holococcolithophore species. Light-yellow band indicates the approximate location of the North Atlantic Subtropical Gyre at ~15-40° N (Figure from Guerreiro et al., 2023).
On the resilience of subtropical gyre coccolithophore communities
Umbelliform species and holococcolithophores are usually found to be preferably living under the “harsh conditions” of heavily stratified tropical and subtropical open-ocean areas, where phytoplankton production is limited by nutrient-depletion during most of the year (Winter et al., 1994; Poulton et al., 2017). Nevertheless, that these taxa (together) produced mean concentrations of 14.4×1000 cells/L present at 30–12°N, only slightly lower compared to 17.7×1000 cells/L produced by fast-blooming taxa at higher and more nutritious high-latitude ocean conditions (at 50–40°N), suggests that the “tropical and subtropical communities” are well adapted and can be quite productive in the context of an ever-warming ocean due to climate change.
How about the effects of Saharan dust?
While crossing the heavily stratified and oligotrophic tropical Atlantic region offshore NW Africa (at ~12-10°N), our meridional transect showed a clear increase in the abundances of fast-blooming species (Emiliania huxleyi and Gephyrocapsa oceanica) and of cyanobacteria (including both picoplankton and N2-fixing Trichodesmium spp.) at the surface of the region of more persistent Saharan dust deposition. The observed increase s-Fe and s-P found in dust samples collected at these same latitudes clearly suggests an ecological response to atmospheric nutrient input by Saharan dust deposition. As this region of persistently enhanced Saharan-dust inputs off NW Africa was also where we found the highest macronutrient concentrations measured along and below the nutricline, our datafurther suggest that the NE tropical Atlantic may act as a permanent dust-born nutrient depocenter, which is consistent with previous studies.
Meridional distribution of coccolithophore communities in relation to other phytoplankton groups and hydrological and atmospheric conditions: (A) mean cell concentrations of the main coccolithophore taxonomic groups determined from each site (adapted from Young, 1994); (B) holococcolithophore mean percentages (black line) and Chl-a concentration measured at the surface (light green) and DCM (dark green); (C) Shannon-Weaver diversity index (exp), averaged to the photic zone (light purple) and at the surface (dark purple); (D) mean percentages of by r-selected placolith-bearing coccolithophores (green), cell concentrations of N2-fixing Trichodesmium sp. at the surface (black line), and pigment- derived cyanobacteria s.l. (i.e., zea/TChl-a); (E) mixed layer depth (MLD – black), temperature (red line) and salinity (blue) at the surface; (F) photosynthetically available radiation (PAR – black line), and aerosol optical thickness (AOT – orange); (G) concentrations of dust (dark orange) and of atmospheric soluble Fe (red) and soluble P (black); (H) estimated fluxes of dry- and wet dust deposition, calculated according to Baker et al. (2010, 2013) and Powell et al. (2015). NAC=North Atlantic Current, AzC=Azores Current; NASG=North Atlantic Subtropical Gyre, NEC=North Equatorial Current, NECC=North Equatorial Counter-Current. Orange band indicates the region of highest dust deposition during AMT28 (along ~18–10°N) (Figure from Guerreiro et al., 2023).
Based on the deposition estimates that we present in our paper, wet deposition appears to be the dominant delivery mechanism of s-Fe and s-P (see the Figure above), suggesting that the magnitude of coccolithophores’ ecological response is likely to be much higher when nutrients are delivered with rain compared to the studied sampling period. This is in line with previous studies where we provide evidence on the potential of Saharan dust in, at least partly, contribute to counterbalance the effects of climate-driven ocean stratification through stimulating r-selected and ballasting-efficient coccolithophore species (Guerreiro et al., 2021).
For more details about this study, you can download our paper at Frontiers in Marine Science.