Abstract
Abstract. The Sea Surface Microlayer (SML) is known to be enriched by trace metals relative to the underlying water and harbor diverse microbial communities (i.e., neuston). However, the processes linking metals and biota in the SML are not yet fully understood. The metal (Cd, Co, Cu, Fe, Ni, Mo, V, Zn and Pb) concentrations in aerosol samples in the SML (dissolved and total fractions) and in subsurface waters (SSWs; dissolved fraction at ∼1 m depth) from the western Mediterranean Sea were analyzed in this study during a cruise in May–June 2017. The composition and abundance of the bacterial community in the SML and SSW, the primary production, and Chl a in the SSW were measured simultaneously at all stations during the cruise. Residence times in the SML of metals derived from aerosol depositions were highly variable and ranged from minutes for Fe (3.6±6.0 min) to a few hours for Cu (5.8±6.2 h). Concentrations of most of the dissolved metals in both the SML and SSW were positively correlated with the salinity gradient and showed the characteristic eastward increase in the surface waters of the Mediterranean Sea (MS). In contrast, the total fraction of some reactive metals in the SML (i.e., Cu, Fe, Pb and Zn) showed a negative correlation with salinity and a positive correlation with microbial abundance, which might be associated with microbial uptake. Our results show a strong negative correlation between the dissolved and total Ni concentration and heterotrophic bacterial abundance in the SML and SSW, but we cannot ascertain whether this correlation reflects a toxicity effect or is the result of some other process.
Highlights
The Mediterranean Sea (MS) is enriched in many trace metals relative to similar nutrient-depleted waters in the open ocean (e.g., concentrations in our study (Cd), Cr, Co, Cu, Ni, Fe, Zn) (Bonnet et al, 2013; Boyle et al, 1985; Sarthou and Jeandel, 2001; Sherrell and Boyle, 1988)
The composition of metal aerosols was mainly influenced by air masses from northern Europe and the Atlantic Ocean (Fig. S1 in the Supplement), except between 1 and 5 June when African air masses were loaded with dust
Some rain events occurred during the cruise, one measured when the vessel was at station, 5 June from 02:36 to 03:04 UTC
Summary
The Mediterranean Sea (MS) is enriched in many trace metals relative to similar nutrient-depleted waters in the open ocean (e.g., Cd, Cr, Co, Cu, Ni, Fe, Zn) (Bonnet et al, 2013; Boyle et al, 1985; Sarthou and Jeandel, 2001; Sherrell and Boyle, 1988). A. Tovar-Sánchez et al.: Characterizing the surface microlayer in the Mediterranean Sea atmospheric deposition, both dry and wet, is the dominant pathway for the large-scale transport of trace metals to the water column and sediments in the MS (Guieu et al, 2002, 2010; Jordi et al, 2012; Ternon et al, 2010; Tovar-Sánchez et al, 2010, 2014). Tovar-Sánchez et al.: Characterizing the surface microlayer in the Mediterranean Sea atmospheric deposition, both dry and wet, is the dominant pathway for the large-scale transport of trace metals to the water column and sediments in the MS (Guieu et al, 2002, 2010; Jordi et al, 2012; Ternon et al, 2010; Tovar-Sánchez et al, 2010, 2014) Many of these metals play an important role in biogeochemical processes of this sea. Copper from aerosol deposition has been demonstrated to have toxic effects on marine phytoplankton (Jordi et al, 2012; Paytan et al, 2009), while Ni and Zn concentrations in Posidonia oceanica are considered good proxy for aerosol deposition (Tovar-Sánchez et al, 2010)
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