Abstract
Aeolian inputs of organic and inorganic nutrients to the ocean are important as they can enhance biological production in surface waters, especially in oligotrophic areas like the Mediterranean. The Mediterranean littoral is particularly exposed to both anthropogenic and Saharan aerosol depositions on a more or less regular basis. During the last few decades experimental studies have been devoted to examining the effect of inorganic nutrient inputs from dust on microbial activity. In this study, we performed experiments at two different locations of the NW Mediterranean, where we evaluated the changes in the quality and quantity of dissolved organic matter due to atmospheric inputs of different origin (Saharan and anthropogenic) and its subsequent transformations mediated by microbial activities. In both experiments the humic-like and protein-like substances, and the fluorescence quantum yield increased after addition. In general, these changes in the quality of dissolved organic matter did not significantly affect the prokaryotes. The recalcitrant character of the fluorescent dissolved organic matter (FDOM) associated with aerosols was confirmed, as we found negligible utilization of chromophoric compounds over the experimental period. We framed these experiments within a two-year time series data set of atmospheric deposition and coastal surface water analyses. These observations showed that both Saharan and anthropogenic inputs induced changes in the quality of organic matter, increasing the proportion of FDOM substances. This increase was larger during Saharan dust events than in the absence of Saharan influence.
Highlights
The Mediterranean Sea, due to its low nutrient and chlorophyll concentration, is considered one of the most oligotrophic marine systems (McGill 1965, Krom et al 1991, Lucea et al 2003)
In order to evaluate the potential role of atmospheric deposition on the dynamics of coastal fluorescent dissolved organic matter (FDOM), we calculated its proportion with respect to the in situ seawater concentration for 23 months (September 2012 to July 2014)
The results revealed that the deposition of humic-like compounds and protein-like compounds contributed to an increase in FDOM in surface waters that represented between 0.2% and 3% per m3 per day (Fig. 1)
Summary
The Mediterranean Sea, due to its low nutrient and chlorophyll concentration, is considered one of the most oligotrophic marine systems (McGill 1965, Krom et al 1991, Lucea et al 2003). Much less attention has been paid to the potential impact of aerosols on the quality of dissolved organic matter (DOM), and in particular on the quality of the optically active fraction (CDOM). This fraction is a key parameter regulating the penetration of ultraviolet radiation in the water column, so changes in its concentration can alter both primary and secondary production (Smith and Cullen 1995). A sub-fraction of CDOM that emits light when excited by UV radiation is called fluorescent dissolved organic matter (FDOM)
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