Occurrence of α, ω-dicarboxylic acids and ω-oxoacids in surface waters of the Rhone River and fluxes into the Mediterranean Sea

Abstract

The Mediterranean Sea is a semi-enclosed marine environment surrounded by densely populated areas. This ecosystem is under strong anthropogenic pressure at present. Riverine waters are important input pathways of water-soluble organic compounds that potentially contribute to the dissolved organic carbon (DOC) pool. Here, we report the first ever measurements of bi-functional carboxylic acids such as α, ω -dicarboxylic acids (or diacids) and related polar compounds ω -oxoacids (or ketoacids) (BCAs=α, ω-dicarboxylic acids+ω -oxoacids), along with the DOC levels in Rhone River waters. Surface water samples were collected from February 2006 to June 2009 approximately 50km above the Rhone River mouth, which is the main supplier of freshwater to the Mediterranean Sea. The BCA concentrations averaged 32.4±15.3µgl−1, and exhibited a wide range of values from 13.2µgl−1 (Spring 2008) to 71.2µgl−1 (winter 2007). The contribution of carbon from BCAs to the DOC pool (BCA-C) accounted for 0.28 to 1.42% of the DOC. Although no seasonal trend was evident in the studied period, our results showed that the highest BCA concentrations did not always follow high water discharges. α,ω -dicarboxylic acids are the most abundant compound class (20.7±10.6µgl−1), which is followed by ω -oxoacids (11.7±6.2µgl−1). The Rhone River was estimated to deliver between 1.37 and 16.4 t d−1 of BCA to the Gulf of Lions. Assuming a mean water discharge of ∼1790m3s−1, a broad estimate suggests a loading of 750–4000 t yr−1 BCAs to the northwestern Mediterranean Sea. Our results indicated that glyoxylic acid (ωC2) was the most abundant BCA followed by oxalic acid (C2di) and fumaric acid (trans configuration of unsaturated C4 diacid). This result indicates that there is a different molecular distribution between the aquatic and atmospheric compartments, including rainwaters and aerosols, for which previous studies highlighted a predominance of oxalic acid, which was followed by malonic and/or succinic acid. Runoff and riverine biological process (rather than photochemical oxidation reactions) are hypothesized to control the BCA occurrence and molecular distribution in the Rhone River and thus their inputs to the coastal NW Mediterranean Sea, although the environmental implications of this stock of BCA are still unknown.