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Incubation experiments comprising Saharan dust additions were conducted in the tropical North Atlantic Ocean along an east-west transect at 12°N to study the phytoplankton response to nutrient release in oligotrophic seawater conditions. Experiments were performed at three stations (M1, M3, M4), mimicking wet and dry deposition of low and high amounts of Saharan dust deposition from two different dust sources (paleo-lake and sand dune). Dust particle sizes were adjusted to resemble dust that is naturally deposited over the ocean at the experiment sites. For wet dust deposition, the dust was pre-leached in acidified 'artificial rainwater' (H2SO4) for 16 to 24 hours, mimicking acid cloud processing at different pH values. Experiments were run up to eight days. Daily nutrient measurements of phosphate (PO43-), silicate (SiO44-), nitrate (NO3-) and cell abundances were performed in addition to measurements of concentrations of total dissolved iron (DFe), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) at the start and at the end of the experiments. A significant initial increase and subsequent gradual decrease in PO43-, SiO44- and DFe concentrations were observed after wet dust deposition using high amounts of dust previously leached in low pH rain (H2SO4, pH = 2). Remarkably, the experiments showed no nutrient release (PO43-, SiO44- and DFe) from dry-dust addition and the NO3- concentrations remained unaffected in all (dry and wet) experiments. The prokaryotic cyanobacterium Synechococcus spp. was the most prominent picophytoplankton in all mixed layer experiments. After an initial increase in cell abundance, a subsequent decrease (at M1) or a slight increase (at M3) with similar temporal dynamics was observed for dry and wet dust deposition experiments. The POC concentrations increased in all experiments and showed similar high values after both dry and wet dust deposition treatments, even though wet dust deposition is considered to have a higher potential to introduce bioavailable nutrients (i.e. PO43-, SiO44- and DFe) into the otherwise nutrient-starved oligotrophic ocean. Our observations suggest that such nutrients may be more likely to favor the growth of the phytoplankton community when an additional N-source is also available. In addition to acting as a fertilizer, our results from both dry and wet dust deposition experiments suggest that Saharan dust particles might be incorporated into marine snow aggregates leading to similar high POC concentrations. [ABSTRACT FROM AUTHOR]

Coccolithophores are calcifying phytoplankton and major contributors to both the organic and inorganic oceanic carbon pumps. Their export fluxes, species composition and seasonal patterns were determined in two sediment trap moorings in the open equatorial North Atlantic (M4 at 12°â€‰N 49°â€‰W and M2 at 14°â€‰N 37°â€‰W), which collected settling particles synchronously in successive 16-day intervals from October 2012 to November 2013, at 1200 m water depth. The two trap locations show a similar seasonal pattern in total coccolith export fluxes and a predominantly tropical coccolithophore settling assemblage throughout the monitored year. Species fluxes were yearlong dominated by lower photic zone (LPZ) taxa (Florisphaera profunda, Gladiolithus flabellatus), but also included upper photic zone (UPZ) taxa (Umbellosphaera spp., Rhabdosphaera spp., Umbilicosphaera spp., Helicosphaera spp.). The LPZ flora was most abundant during fall 2012, whereas the UPZ flora was more important during summer. In spite of these similarities, the western part of the study area produced persistently higher fluxes, averaging 241 × 107 coccoliths m-2 d-1 (117 × 107 to 423 × 107 coccoliths m-2 d-1) at station M4, compared to only 66 × 107 coccoliths m-2 d-1 (25 × 107 to 153 × 107 coccoliths m-2 d-1) at station M2. Higher fluxes at M4 were mainly produced by the LPZ species, although most UPZ species also contributed higher fluxes, reflecting enhanced productivity in the western equatorial North Atlantic. In addition, we found two marked flux peaks of the more opportunistic species Gephyrocapsa muellerae and Emiliania huxleyi indicating a fast response to nutrient-enrichment of the UPZ, probably by wind-forced mixing, whereas increased fluxes of G. oceanica and E. huxleyi in October/November 2013 coincided with the occurrence of Amazon River affected surface waters. Since the spring and fall events of 2013 were also accompanied by two dust flux peaks we propose a scenario where atmospheric dust also provided fertilizing nutrients to this area. Enhanced surface buoyancy associated to the river plume indicates that the Amazon acted not only as a nutrient source, but also as a surface density retainer for nutrients supplied from the atmosphere. Still, lower total coccolith fluxes during these events compared to the maxima recorded in November 2012 and July 2013 indicate that transient productivity by opportunistic species was less important than background tropical productivity in the equatorial North Atlantic. This study illustrates how two seemingly similar sites in an open-ocean tropical setting actually differ greatly in ecological and oceanographic terms, and provides valuable insights into the processes governing the ecological dynamics and the downward export of coccolithophores in the tropical North Atlantic. [ABSTRACT FROM AUTHOR]

Massive amounts of Saharan dust are blown from the African coast across the Atlantic Ocean towards the Americas each year. This dust has, depending on its chemistry, direct and indirect effects on global climate including reflection and absorption of solar radiation as well as transport and deposition of nutrients and metals fertilizing both ocean and land. To determine the temporal and spatial variability of Saharan dust transport and deposition and their marine environmental effects across the equatorial North Atlantic Ocean, we have set up a monitoring experiment using deep-ocean sediment traps as well as land-based dust collectors. The sediment traps were deployed at five ocean sites along a transatlantic transect between northwest Africa and the Caribbean along 12â?° N, in a down-wind extension of the land-based dust collectors placed at 19â?° N on the Mauritanian coast in Iwik. In this paper, we lay out the setup of the monitoring experiment and present the particle fluxes from sediment trap sampling over 24 continuous and synchronised intervals from October 2012 through to November 2013. We establish the temporal distribution of the particle fluxes deposited in the Atlantic and compare chemical compositions with the land-based dust collectors propagating to the down-wind sediment trap sites, and with satellite observations of Saharan dust outbreaks. First-year results show that the total mass fluxes in the ocean are highest at the sampling sites in the east and west, closest to the African continent and the Caribbean, respectively. Element ratios reveal that the lithogenic particles deposited nearest to Africa are most similar in composition to the Saharan dust collected in Iwik. Down-wind increasing Al, Fe and K contents suggest a downwind change in the mineralogical composition of Saharan dust and indicate an increasing contribution of clay minerals towards the west. In the westernmost Atlantic, admixture of re-suspended clay-sized sediments advected towards the deep sediment trap cannot be excluded. Seasonality is most prominent near both continents but generally weak, with mass fluxes dominated by calcium carbonate and clear seasonal maxima of biogenic silica towards the west. The monitoring experiment is now extended with autonomous dust sampling buoys for better quantification Saharan dust transport and deposition from source to sink and its impact on fertilization and carbon export to the deep ocean. [ABSTRACT FROM AUTHOR]