Abstract
Abstract. In low-nutrient low-chlorophyll areas, such as the Mediterranean Sea, atmospheric fluxes represent a considerable external source of nutrients likely supporting primary production, especially during periods of stratification. These areas are expected to expand in the future due to lower nutrient supply from sub-surface waters caused by climate-driven enhanced stratification, likely further increasing the role of atmospheric deposition as a source of new nutrients to surface waters. Whether plankton communities will react differently to dust deposition in a warmer and acidified environment remains; however, an open question. The potential impact of dust deposition both in present and future climate conditions was investigated in three perturbation experiments in the open Mediterranean Sea. Climate reactors (300 L) were filled with surface water collected in the Tyrrhenian Sea, Ionian Sea and in the Algerian basin during a cruise conducted in the frame of the PEACETIME project in May–June 2017. The experiments comprised two unmodified control tanks, two tanks enriched with a Saharan dust analogue and two tanks enriched with the dust analogue and maintained under warmer (+3 ∘C) and acidified (−0.3 pH unit) conditions. Samples for the analysis of an extensive number of biogeochemical parameters and processes were taken over the duration (3–4 d) of the experiments. Dust addition led to a rapid release of nitrate and phosphate, however, nitrate inputs were much higher than phosphate. Our results showed that the impacts of Saharan dust deposition in three different basins of the open northwestern Mediterranean Sea are at least as strong as those observed previously, all performed in coastal waters. The effects of dust deposition on biological stocks were different for the three investigated stations and could not be attributed to differences in their degree of oligotrophy but rather to the initial metabolic state of the community. Ocean acidification and warming did not drastically modify the composition of the autotrophic assemblage, with all groups positively impacted by warming and acidification. Although autotrophic biomass was more positively impacted than heterotrophic biomass under future environmental conditions, a stronger impact of warming and acidification on mineralization processes suggests a decreased capacity of Mediterranean surface plankton communities to sequester atmospheric CO2 following the deposition of atmospheric particles.
Highlights
Atmospheric deposition is well recognized as a significant source of micro- and macro-nutrients for surface waters of the global ocean (Duce et al, 1991; Jickells et al, 2005; Moore et al, 2013)
Our results showed that the impacts of Saharan dust deposition in three different basins of the open northwestern Mediterranean Sea are at least as strong as those observed previously, all performed in coastal waters
Gazeau et al (2021) did not observe an additional impact of future environmental conditions on the export of organic matter after dust addition. These experiments conducted during the PEACETIME cruise represent the first attempt to investigate the impacts of atmospheric deposition on surface plankton communities both under present and future environmental conditions
Summary
Atmospheric deposition is well recognized as a significant source of micro- and macro-nutrients for surface waters of the global ocean (Duce et al, 1991; Jickells et al, 2005; Moore et al, 2013). The potential modulation of the biological carbon pump efficiency and the associated export of carbon by atmospheric deposition events are still poorly understood and quantified (Law et al, 2013) This is especially true for low-nutrient low-chlorophyll (LNLC) areas where atmospheric fluxes can play a considerable role in nutrient cycling and that represent 60 % of the global ocean surface area (Longhurst et al, 1995) as well as 50 % of global carbon export (Emerson et al, 1997). Ternon et al (2010) reported an average annual dust flux over 4 years of 11.4 g m−2 yr−1 (average during the period 2003–2007) at the DYFAMED station in the northwestern Mediterranean Sea. Dust deposition, mostly in the form of pulsed inputs, is mainly associated with wet deposition (Loÿe-Pilot and Martin, 1996). The most important events reported in the 2010 decade amounted to ∼ 22 g m−2 (Bonnet and Guieu, 2006; Guieu et al, 2010b)
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