Abstract. The surface mixed layer (ML) in the Mediterranean Sea is a well stratified domain characterized by low macro-nutrient and low chlorophyll content, during almost 6 months of the year. Nutrient dynamics in the ML depend on allochthonous inputs, through atmospheric deposition and on biological recycling. Here we characterize the biogeochemical cycling of N in the ML by combining simultaneous in situ measurements of atmospheric deposition, nutrients, hydrological conditions, primary production, heterotrophic prokaryotic production, N₂ fixation and leucine aminopeptidase activity. The measurements were conducted along a 4300 km transect across the central and western open Mediterranean Sea in spring 2017. Dry deposition was measured on a continuous basis while two wet deposition events were sampled, one in the Ionian Sea and one in the Algerian basin. Along the transect, N budgets were computed to compare sources and sinks of N in the mixed layer. On average, phytoplankton N demand was 2.9 fold higher (range 1.5–8.1) than heterotrophic prokaryotic N demand. In situ leucine aminopeptidase activity contributed from 14 to 66 % of heterotrophic prokaryotic N demand, and N₂ fixation rate represented 1 to 4.5 % of the phytoplankton N demand. Dry atmospheric deposition of inorganic nitrogen, estimated from dry deposition of (nitrate + ammonium) in aerosols, was higher than N₂ fixation rates in the ML (on average 4.8 fold). The dry atmospheric input of inorganic N represented a highly variable proportion of biological N demand in the ML, 10–82 % for heterotrophic prokaryotes and 1–30 % for phytoplankton. Stations visited for several days allowed following the evolution of biogeochemical properties in the ML and within the nutrient depleted layers. At the site in the Algerian Basin and on a basis of high frequency sampling of CTD casts before and after a wet dust deposition event, different scenarios were considered to explain a delayed appearance of peaks in dissolved inorganic phosphate in comparison to nitrate within the ML. After the rain, nitrate was higher in the ML than in the nutrient depleted layer below. Estimates of nutrient transfer from the ML to the nutrient depleted layer could explain 1/3 of the nitrate fate out of the ML. Luxury consumption of P by heterotrophic prokaryotes, further transferred in the microbial food web, and remineralized by grazers, is one explanation for the delayed phosphate peak of DIP. The second explanation is a transfer from ML to the nutrient depleted layer below through adsorption/desorption processes on particles. Phytoplankton did not benefit directly from atmospheric inputs in the ML, probably due to a high competition with heterotrophic prokaryotes, also limited by N and P availability at the time of this study. Primary producers, in competition for nutrients with heterotrophic prokaryotes, decreased their production after the rain, recovering their initial state of activity after 2 days lag in the vicinity of the deep chlorophyll maximum layer.

中文翻译：

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贫营养海洋系统中大气沉积对生物地球化学循环的影响

摘要。地中海中的表面混合层（ML）是分层良好的区域，其特征是一年中的大约6个月中大量营养素含量较低且叶绿素含量较低。ML中的营养动力学取决于异源输入，大气沉积和生物循环利用。在这里，我们通过结合大气沉积，营养物，水文条件，初级生产，异养原核生物，N _2的同时原位测量来表征ML中N的生物地球化学循环。固定和亮氨酸氨肽酶活性。这些测量是在2017年春季沿着地中海中部和西部的4300公里横断面进行的。连续测量了干沉降，同时采样了两次湿沉降事件，其中一个在爱奥尼亚海中，一个在阿尔及利亚海盆中。沿着该断面，计算了N个预算以比较混合层中N的源和汇。平均而言，浮游植物对氮的需求量比异养原核生物对氮的需求量高2.9倍（范围为1.5-8.1）。原位亮氨酸氨基肽酶活性占异养原核氮需求的14％至66％，而N ₂固定率占浮游植物氮需求的1％至4.5％。根据气溶胶中（硝酸盐+铵盐）的干燥沉积估算，无机氮的干燥大气沉积高于N _2。ML中的固定率（平均4.8倍）。无机氮的干燥大气输入量表示最大的生物氮比例是可变的，异养原核生物为10–82％，浮游植物为1–30％。参观了几天的台站允许ML中和营养耗尽层中生物地球化学特性的演变。在阿尔及利亚盆地的现场，并基于湿粉尘沉积事件前后的CTD铸件的高频采样，考虑了不同的情况来解释与ML中的硝酸盐相比，溶解的无机磷酸盐的峰出现延迟。雨后，ML中的硝酸盐含量高于下面的营养消耗层。从ML到营养耗尽层的养分转移估算值可以解释ML硝酸盐命运的1/3。异养原核生物对P的奢侈消费，进一步转移到微生物食物网中，并被放牧者再矿化，这是DIP磷酸盐峰延迟的一种解释。第二种解释是通过颗粒上的吸附/解吸过程，从ML转移到下面的营养耗尽层。浮游植物不能直接从ML中的大气输入中受益，这可能是由于与异养原核生物的激烈竞争，本研究时氮和磷的可用性也受到限制。初级生产者为了与异养原核生物竞争营养而在雨后降低了产量，