Influence of atmospheric deposition on biogeochemical cycles in an oligotrophic ocean system

Abstract

Abstract. The surface mixed layer (ML) in the Mediterranean Sea is a well-stratified\ndomain characterized by low macronutrients and low chlorophyll content for almost 6\xa0months of the year. In this study we characterize the\nbiogeochemical cycling of nitrogen (N) in the ML by analyzing simultaneous in\xa0situ measurements of atmospheric deposition, nutrients in seawater, hydrological conditions, primary production, heterotrophic prokaryotic production, N2 fixation and leucine aminopeptidase activity. Dry deposition was continuously measured across the central and western open Mediterranean Sea, and 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 the sources and sinks of N in the mixed layer. In\xa0situ leucine aminopeptidase\nactivity made up 14\u2009% to 66\u2009% of the heterotrophic prokaryotic N demand, and the N2 fixation rate represented 1\u2009% to 4.5\u2009% of the phytoplankton N demand. Dry atmospheric deposition of inorganic nitrogen, estimated from dry deposition of nitrate and ammonium in aerosols, was higher than the 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 among the stations, 10\u2009%–82\u2009% for heterotrophic\nprokaryotes and 1\u2009%–30\u2009% for phytoplankton. As some sites were visited on several days, the evolution of biogeochemical properties in the ML and within the nutrient-depleted layers could be followed. At the Algerian Basin site, the biogeochemical consequences of a wet dust deposition event were monitored through high-frequency sampling. Notably, just after the rain, nitrate was higher in the ML than in the nutrient-depleted layer below. Estimates of nutrient transfer from the ML into the nutrient-depleted layer could explain up to a third of the nitrate loss from the ML. Phytoplankton did not benefit directly from the atmospheric inputs into the ML, probably due to high competition with heterotrophic prokaryotes, also limited by N and phosphorus (P) availability at the time of this study.\nPrimary producers decreased their production after the rain but recovered\ntheir initial state of activity after a 2\u2009d lag in the vicinity of the\ndeep chlorophyll maximum layer.\n