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 domain characterized by low macronutrients and low chlorophyll content for almost 6 months of the year. In this study we characterize the biogeochemical cycling of nitrogen (N) in the ML by analyzing simultaneous in situ 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 situ leucine aminopeptidase activity made up 14 % to 66 % of the heterotrophic prokaryotic N demand, and the 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 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 %–82 % for heterotrophic prokaryotes and 1 %–30 % 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. Primary producers decreased their production after the rain but recovered their initial state of activity after a 2 d lag in the vicinity of the deep chlorophyll maximum layer.

Phytoplankton community response to episodic wet and dry aerosol deposition in the subtropical North Atlantic

“…Few studies have considered the importance of the aerosol deposition pathway, either through dust storms (dry deposition) or rainfall events (wet deposition; Ridame et al 2014; Van Wambeke et al 2021), despite them potentially differing in terms of aerosol acidity (lower pH under wet deposition) and settling efficiency in the air (elevated under wet deposition), which is expected to have a critical impact on the magnitude of nutrient supply (Baker et al 2007, 2010; Jung et al 2011). In addition, we know comparatively little about how aerosol deposition affects specific phytoplankton groups (DiTullio and Laws 1991; Kramer et al 2020), despite this potentially having important implications for carbon export; for instance, phytoplankton blooms dominated by diatoms are more likely to trigger enhanced export of faster‐sinking particulate organic carbon to the ocean interior (Martin et al 2011; Leblanc et al 2018).…”

Impact of dust addition on the microbial food web under present and future conditions of pH and temperature

“…Parallel nutrient enrichment incubations conducted in darkness showed that in situ heterotrophic bacterioplankton communities (initial conditions of the present experiments) were N, P co-limited at TYR, mainly P limited at ION, and N limited at FAST (Van Wambeke et al, 2021). However, after incubation, the HB appeared to be weakly bottom-up controlled (Ducklow, 1992) especially in D and G (Fig.…”

“…Intriguingly, the enhanced viral production did not translate into marked changes in the viral abundance. The abundance of a low DNA virus population, which typically comprises the viruses of bacteria, actually decreased between t0 and t48h pointing to a possible viral decay, potentially related to an adsorption onto dust particles (Weinbauer et al, 2009;Yamada et al, 2020) and the potential export of viral particle to deeper water layers (Van Wambeke et al, 2021). While recurrent patterns emerged from this study, the amplitude of viral responses varied between the experiments.…”

“…All effects might be generally enhanced in future climate scenarios. The impact of dust deposition on metabolic processes and the consequences for the carbon and nitrogen cycles and the biological pump based on these minicosm experiments are further discussed in Gazeau et al (2021b) and Ridame et al (2022), and the in situ effect of a wet dust deposition event is explored in Van Wambeke et al (2021) in this special issue.…”

Nitrogen Cycling Across a Salinity Gradient From the Pearl River Estuary to Offshore: Insight From Nitrate Dual Isotopes

“…This estimate is much higher than in the surface and mid‐layer waters of the offshore area (9%) and river mouth (11%), and slightly higher than in the high‐salinity water of PRE (17%) during the winter (Ye et al., 2016), manifesting the importance of atmospheric nitrate deposition to the offshore ecosystem. Although the atmospheric nitrate may be rapidly assimilated and cycled at the surface, particularly in the oligotrophic areas (Tsunogai et al., 2011; Van Wambeke et al., 2021). However, in our study area in the PRE, the nutrients in the water column are relatively sufficient, particularly ammonium; even the average concentration of NH 4 + in the surface water of the nearshore and offshore area can still reach 1.33 μmol/L (Figure 2).…”