Effects of dry and wet Saharan dust deposition in the tropical North Atlantic Ocean

Abstract

<p><strong>Abstract.</strong> 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’ (H<sub>2</sub>SO<sub>4</sub>) 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 (PO<sub>4</sub><sup>3<sup>−</sup></sup>), silicate (SiO<sub>4</sub><sup>4<sup>−</sup></sup>), nitrate (NO<sub>3<sup>−</sup></sub>) 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.</p><p> A significant initial increase and subsequent gradual decrease in PO<sub>4</sub><sup>3<sup>−</sup></sup>, SiO<sub>4</sub><sup>4<sup>−</sup></sup> and DFe concentrations were observed after wet dust deposition using high amounts of dust previously leached in low pH rain (H<sub>2</sub>SO<sub>4</sub>, pH = 2). Remarkably, the experiments showed no nutrient release (PO<sub>4</sub><sup>3<sup>−</sup></sup>, SiO<sub>4</sub><sup>4<sup>−</sup></sup> and DFe) from dry-dust addition and the NO<sub>3<sup>−</sup></sub> concentrations remained unaffected in all (dry and wet) experiments. The prokaryotic cyanobacterium <i>Synechococcus</i> 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. PO<sub>4</sub><sup>3<sup>−</sup></sup>, SiO<sub>4</sub><sup>4<sup>−</sup></sup> 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.</p>

Tropical Rains Controlling Deposition of Saharan Dust Across the North Atlantic Ocean

“…This may stimulate phytoplankton production, which in turn impacts atmospheric CO 2 levels (Guieu et al, ; Jickells et al, ; Mills et al, ; Pabortsava et al, ). Bottle‐incubation experiments with Saharan dust additions along the same transect as in the present study have shown that Saharan dust indeed has the potential to deliver macronutrients and micronutrients (e.g., phosphate, silica, and dissolved iron) if exposed to acidic conditions before deposition (Korte et al, ). Not surprisingly, the concentration of nutrients released is in turn related to the amount of dust deposited, highlighting the importance for accurate dust flux measurements.…”

“…This may lead to higher solubility of iron and other nutrients in wet deposition (Spokes et al, ). In contrast, a study by Korte et al () suggests that dry‐deposited dust has a much lower nutrient bioavailability. Also, since satellite instruments cannot detect wet dust deposition below the clouds, a multidisciplinary approach combining in situ observations and satellite remote sensing with model simulations should provide a better understanding of processes involved in dust transport and deposition type and help to constrain the biogeochemical impact of dust deposition.…”

African biomass burning is a substantial source of phosphorus deposition to the Amazon, Tropical Atlantic Ocean, and Southern Ocean

“…Dust has been assumed to be the dominant source of these nutrients to the TAO year round (10,15); however, the impact of dust on marine productivity is debated. In fact, a recent study (19) showed that dust has to be extensively acidified to supply SP to seawater, suggesting that other aerosol sources may be more important for providing soluble nutrients.…”

“…In terrestrial environments, the residence time of P in soil is longer, facilitating the transformation of insoluble P into more bioavailable forms (9). African dust is thought to be the largest contributor of P to the Amazon and TAO (15), but its solubility is low (16)(17)(18)(19)(20). While P in dust can be transformed into more soluble forms via reactions with acids (20,21), the importance of this process during transport is debated (19,22,23).…”

“…African dust is thought to be the largest contributor of P to the Amazon and TAO (15), but its solubility is low (16)(17)(18)(19)(20). While P in dust can be transformed into more soluble forms via reactions with acids (20,21), the importance of this process during transport is debated (19,22,23). In contrast, P in aerosols from biomass burning (BB) and fossil fuel combustion are more soluble and could be a more important source of SP transported to the TAO (15,18,(23)(24)(25).…”