Impact of dust addition on the metabolism of Mediterranean plankton communities and carbon export under present and future conditions of pH and temperature

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.

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

“…During an artificial in situ DIP enrichment experiment in the eastern Mediterranean, P rapidly circulated through hprok and heterotrophic ciliates, and the phytoplankton were not directly linked to this "bypass" process . Bioassays conducted in the tropical Atlantic Ocean have also shown that hprok respond more strongly than phytoplankton to nutrients from Saharan aerosols (Marañón et al, 2010), a pattern that has been confirmed in a meta-analysis of dust addition experiments (Guieu et al, 2014a;Guieu and Ridame, 2021;Gazeau et al, 2021). We considered hprokN demand together with phytoN demand and compared it to autochthonous (DON hydrolysis by ectoenzymatic activity) and allochthonous (atmospheric deposition) sources.…”

“…Based on these we used a mean C/P of 130 for phytoplankton. A C/P value of 100 was used for heterotrophic prokaryotes (Godwin and Cotner, 2015).…”

“…The so-called luxury DIP uptake by organisms like hprok is efficient (small cells with high surface / volume ratio and DIP kinetic uptake adapted to low concentrations). It is of course difficult to quantify such in situ variations in comparison to mesocosm/minicosm dust addition experiments, in which heterotrophy was clearly promoted (Marañón et al, 2010;Guieu et al, 2014b;Gazeau et al, 2021). A few field studies have confirmed these trends (Herut et al, 2005;Pulido-Villena et al, 2008), but, as stated in the Introduction, these studies lacked high-frequency sampling.…”

“…Such a C/P ratio suggests that DIP was not limiting these organisms anymore. Indeed a decrease in C/P quotas may highlight a switch from P to C limitation for heterotrophic bacteria (Godwin and Cotner, 2015) and from P to N limitation or increased growth rates for phytoplankton (Moreno and Martiny, 2018). Furthermore, as DIP is also recycled via alkaline phosphatase within the ML, we also consider another source of DIP via alkaline phosphatase activity, which could release 139 µmol DIP m −2 during this period (see Van Wambeke et al, 2021, for in situ estimates).…”

“…Assuming also that DIP resulting from AP hydrolysis was fully assimilated by the plankton, the C/P ratio would be 19. This low ratio seems unrealistic for phytoplankton (Moreno and Martigny, 2018) as well as hprok, even growing in surplus C conditions (Makino et al, 2003;Lovdal et al, 2008;Godwin and Cotner, 2015). Some of the P recycled in or brought into the ML from atmospheric deposition has consequently been exported below the ML.…”