Spatial gradients in trace metal concentrations in the surface microlayer of the Mediterranean Sea

Abstract

The relationship between dust deposition and surface water metal concentrations is poorly understood. Dissolution, solubility, and partitioning reactions of trace metals from dust particles are governed by complex chemical, biological, and physical processes occurring in the surface ocean. Despite that, the role of the sea surface microlayer (SML), a thin, but fundamental component modulating the air-sea exchange of materials has not been properly evaluated. Our study revealed that the SML of the Mediterranean Sea is enriched with bioactive trace metals (i.e., Cd, Co, Cu, and Fe), ranging from 8 (for Cd) to 1000 (for Fe) times higher than the dissolved metal pool in the underlying water column. The highest enrichments were spatially correlated with the atmospheric deposition of mineral particles. Our mass balance results suggest that the SML in the Mediterranean Sea contains about 2 tons of Fe. However, we did not detect any trends between the concentrations of metals in SML with the subsurface water concentrations and biomass distributions. These findings suggest that future studies are needed to quantify the rate of metal exchange between the SML and the bioavailable pool and that the SML should be considered to better understand the effect of atmospheric inputs on the biogeochemistry of trace metals in the ocean.

The response and actions taken to accommodate the reviewers-1’ comments

“…Metal aerosol compositions are shown in Table 1. The range (ng/m 3 ) of concentrations in our study (Cd: 0 -0.2 , Co: 0.05 -0.3 , Cu: 0.4 -4.8, Fe: 23 -266, Ni: 1.6 -7.1, Mo: 0.0 -2.1, V: 3 -18, Zn: 2.0 -14.6 and Pb: 0.2 -7.0) was of the same order of magnitude of previous measurements collected in the same region and season (Becagli et al, 2012;Calzolai et al, 2015, p.10;Guerzoni et al, 1997;Tovar-Sánchez et al, 2014;Heimbürger et al, 2010), and were therefore consistent with Western Mediterranean background concentrations. However, it has been shown that aerosol concentrations of anthropogenic trace metals (i.e.…”

“…Trace metal concentrations of T-SML (Table 1), with the exception of Pb, were lower, although, of the same order of magnitude, than those measured in a previous study carried out in the MS (Tovar-Sánchez et al, 2014). This lower content of metals in the SML is probably related to the lower dust aerosol deposition during our sampling period and to the lack of influence of desert dust aerosols except for the Fast station.…”

“…For example, in regions under the influence of dust events, such as the North Atlantic Ocean or Mediterranean Sea, concentrations of Cu, Fe or Pb in the total pool of the SML are up to 800, 200 and 150 times higher than in the dissolved metal pool of the underlying water (Tovar-Sánchez et al, 2019). It has been estimated that the SML in the MS contains around 2 tons of total Fe, and this amount could be much higher during episodes of dust events (Tovar-Sánchez et al, 2014). However, despite such enrichment in the concentration of trace metals, little is known about their residence times, their influence on the activity of the microbial community within the SML, or their release rates regarding the underlying waters.…”

The biogeochemistry of cobalt in the Mediterranean Sea

“…Despite these indirect evidences on the biological importance of Co, its biogeochemistry is poorly documented in the MS. Previously reported DCo surface concentrations are elevated compared to other oligotrophic domains and increase eastward from the Western Basin to the Aegean Sea [ Tovar‐Sánchez et al , ]. Atmospheric inputs and Gibraltar Strait surface fluxes are thought to be the main sources and scavenging to be the main sink of DCo [ Zhang et al , ; Guieu et al , ; Elbaz‐Poulichet et al , , ].…”

The Impact of Dry Atmospheric Deposition on the Sea-Surface Microlayer in the SE Mediterranean Sea: An Experimental Approach

“…In addition to the aerosol derived N and P input, other micronutrients from the aerosols may also be limiting factors for microbial activity (e.g., Paytan et al, 2009). Tovar-sánchez et al (2014) showed that Fe originating from aerosol particles was found to be unevenly distributed between the SML and SSL following a dust storm event. Thus, when considering the results described here and the work presented by Tovar-sánchez and collegues, we assume that aerosol depositions may trigger different impacts in these distinct layers.…”