Abstract
Abstract. Although atmospheric dust fluxes from arid as well as human-impacted areas represent a significant source of nutrients to surface waters of the Mediterranean Sea, studies focusing on the evolution of the metabolic balance of the plankton community following a dust deposition event are scarce, and none were conducted in the context of projected future levels of temperature and pH. Moreover, most of the experiments took place in coastal areas. In the framework of the PEACETIME project, three dust-addition perturbation experiments were conducted in 300 L tanks filled with surface seawater collected in the Tyrrhenian Sea (TYR), Ionian Sea (ION) and Algerian basin (FAST) on board the R/V Pourquoi Pas? in late spring 2017. For each experiment, six tanks were used to follow the evolution of chemical and biological stocks, biological activity and particle export. The impacts of a dust deposition event simulated at their surface were followed under present environmental conditions and under a realistic climate change scenario for 2100 (ca. +3 ∘C and −0.3 pH units). The tested waters were all typical of stratified oligotrophic conditions encountered in the open Mediterranean Sea at this period of the year, with low rates of primary production and a metabolic balance towards net heterotrophy. The release of nutrients after dust seeding had very contrasting impacts on the metabolism of the communities, depending on the station investigated. At TYR, the release of new nutrients was followed by a negative impact on both particulate and dissolved 14C-based production rates, while heterotrophic bacterial production strongly increased, driving the community to an even more heterotrophic state. At ION and FAST, the efficiency of organic matter export due to mineral/organic aggregation processes was lower than at TYR and likely related to a lower quantity/age of dissolved organic matter present at the time of the seeding and a smaller production of DOM following dust addition. This was also reflected by lower initial concentrations in transparent exopolymer particles (TEPs) and a lower increase in TEP concentrations following the dust addition, as compared to TYR. At ION and FAST, both the autotrophic and heterotrophic community benefited from dust addition, with a stronger relative increase in autotrophic processes observed at FAST. Our study showed that the potential positive impact of dust deposition on primary production depends on the initial composition and metabolic state of the investigated community. This impact is constrained by the quantity of nutrients added in order to sustain both the fast response of heterotrophic prokaryotes and the delayed one of primary producers. Finally, under future environmental conditions, heterotrophic metabolism was overall more impacted than primary production, with the consequence that all integrated net community production rates decreased with no detectable impact on carbon export, therefore reducing the capacity of surface waters to sequester anthropogenic CO2.
Highlights
Low-nutrient low-chlorophyll (LNLC) areas represent 60 % of the global ocean surface area (Longhurst et al, 1995; McClain et al, 2004)
Phytoplankton communities at stations Tyrrhenian Sea (TYR) and Ionian Sea (ION) were dominated by prymnesiophytes followed by cyanobacteria, while, at station FAST, the phytoplanktonic community was clearly dominated by photosynthetic prokaryotes
The three experiments were conducted under rather similar conditions in terms of nutrient availability and chlorophyll stock of the tested seawater, contrasting responses were observed following the simulation of a wet dust deposition event
Summary
Low-nutrient low-chlorophyll (LNLC) areas represent 60 % of the global ocean surface area (Longhurst et al, 1995; McClain et al, 2004). Atmospheric dust fluxes represent a significant source of these nutrients to surface waters in LNLC regions and as such could play a significant role in stimulating primary production (e.g., Bishop et al, 2002; Guieu et al, 2014a; Jickells and Moore, 2015), potentially increasing the efficiency of the biological pump in the sequestration of atmospheric CO2. As heterotrophic prokaryotes have been shown to outcompete phytoplankton during nutrient addition experiments (e.g., Guieu et al, 2014b; Mills et al, 2008; Thingstad et al, 2005), dust deposition could induce even stronger enhancements of heterotrophic bacterial production and/or respiration rates, thereby reducing net atmospheric CO2 drawdown and the potential for carbon export outside the euphotic zone (Guieu et al, 2014a). The metabolic balance (or net community production, NCP) is defined as the difference between gross primary production (GPP) of autotrophic organisms and community respiration (CR) of both autotrophic and heterotrophic organisms, revealing the capacity of surface waters to absorb atmospheric CO2
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