Nutrient release from dry and wet Saharan dust deposition

Dry and wet Saharan dust deposition incubation experiments were conducted in the tropical North Atlantic Ocean at 12°N to study the nutrient release of Saharan dust under oligotrophic seawater conditions with possible fertilization effects on the phytoplankton. Low and high amounts of Saharan dust deposition from two different dust sources (paleo-lake and sand dune) were used from which the dust's 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) and particulate organic carbon (POC) 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 in the eastern Atlantic or a slight increase in the middle of the Atlantic was observed for allexperiments, independently of dry- and wet-dust deposition. The POC concentrations at the end of the experiments increased in all treatments 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, a clear response of the pico-phytoplankton was absent. However, our observations suggest that the leached nutrients may be more likely to favor the growth of the phytoplankton community when an additional N-source is also available.

本研究于北纬12°的热带北大西洋海域开展撒哈拉沙尘干湿沉降培养实验，旨在探究寡营养海水（oligotrophic seawater）环境下撒哈拉沙尘的营养盐释放过程，及其对浮游植物（phytoplankton）的潜在施肥效应。
实验采用两种不同沙尘源（古湖沉积与沙漠沙丘）的低、高剂量沙尘沉降样本，并对沙尘粒径进行调整，使其与实验海域自然沉降的沙尘特征一致。
针对湿沉降沙尘样本，将沙尘置于酸化‘人工雨水’（硫酸H₂SO₄）中预淋溶16至24小时，以模拟不同pH条件下的酸性云团过程。
实验周期最长为8天。实验期间每日检测磷酸盐（phosphate, PO₄³⁻）、硅酸盐（silicate, SiO₄⁴⁻）、硝酸盐（nitrate, NO₃⁻）浓度及细胞丰度，并在实验起始与结束阶段测定总溶解态铁（total dissolved iron, DFe）与颗粒态有机碳（particulate organic carbon, POC）浓度。
针对经低pH雨水（硫酸体系，pH=2）淋溶的高剂量沙尘湿沉降样本，实验观测到PO₄³⁻、SiO₄⁴⁻与DFe浓度呈现先显著升高、后逐步下降的变化趋势。
值得注意的是，干沉降沙尘添加组未观测到上述营养盐（PO₄³⁻、SiO₄⁴⁻与DFe）的释放，且所有干湿沉降实验中硝酸盐（NO₃⁻）浓度均未发生显著变化。
在所有混合层培养实验中，原核蓝细菌聚球藻属（Synechococcus spp.）为最优势的超微型浮游植物（picophytoplankton）类群。
所有实验的细胞丰度均呈现先升高的初始变化，后续在大西洋东部海域样本中出现丰度下降，而在大西洋中部海域样本中则呈小幅增长，且该变化趋势与沙尘干湿沉降处理无关。
所有处理组的实验结束时POC浓度均有所升高，且干、湿沉降沙尘处理组的POC浓度最终均达到相近的高水平。
尽管湿沉降沙尘被认为更易将生物可利用营养盐（即PO₄³⁻、SiO₄⁴⁻与DFe）引入原本营养盐匮乏的寡营养海域，但超微型浮游植物并未呈现明确的响应特征。
不过，本研究观测结果表明，若同时存在额外氮源，经淋溶释放的营养盐或更有利于浮游植物群落的生长。