Biogeochemical characterization of the riverine particulate organic matter transferred to the NW Mediterranean Sea

[摘要] A large amount of terrestrial organic matter is annually delivered by riversto the continental shelf, where this material is either degraded, buried ortransferred to the deep sea by hydrodynamic processes such as storms. Therelative amount of terrestrial organic matter in the marine sediments isoften determined by analysing the stable isotopes (δ¹³C andδ¹⁵N) and the C / N ratio of organic matter because the variousparticulate organic matter (POM) sources have distinct isotopiccompositions. With the objective to refine and better interpret POM sourcesin the marine environment, we have characterized monthly terrestrial POMdelivered by eight rivers discharging to the NW Mediterranean Sea:the Rhône, Hérault, Orb, Aude, Têt, Fluvià, Ter and Tordera rivers. These rivers were simultaneously sampled from November 2008 toDecember 2009 and the concentrations of total suspended matter (TSM),particulate organic carbon (POC) and nitrogen (PN), as well as their stableisotopic ratios (δ¹³C and δ¹⁵N) were determined.During the survey, three rainstorm events with winds coming from the E–NE andthe S–SE impacted the NW Mediterranean. Depending on the direction ofincoming winds, the fluvial response (amount of water discharge and TSM) wasdifferent. Rivers draining the Alps (Rhône River) and Central Massif(Hérault, Orb, and Aude rivers) were mostly impacted by rainstormsassociated with winds coming from the S–SE, while rivers draining thePyrenees (Têt, Fluvià, and Ter rivers) and the Montseny Massif(Tordera River) were impacted by rainstorms associated with winds coming fromthe E–NE. In addition, the spatial evolution of water discharges showsa different hydrological regime of the Rhône River, with relativelyconstant and high water stages and TSM concentrations when compared tocoastal rivers, characterized by long periods of low water stages. TSMconcentrations are positively correlated to water discharges (high waterflows resuspended riverbed sediments) but show an inverse relationship withPOC and PN relative contents (mostly due to dilution and by low availabilityof light in river waters during flood events). TSM in most of the coastalrivers have on average 2.5–3 times higher POC and PN mean contents than theRhône River (8.5 and 1.5%, respectively, for coastal rivers comparedto3.6 and 0.5%, respectively, for the Rhône River). This discrepancymay be caused by the long drought periods in small coastal Mediterraneanwatersheds that enhance the eutrophication in studied coastal rivers. Theδ¹³C ratios of organic matter also reflect this discrepancy betweenhigh and low water stages with values ranging from −33.2 to−24.5‰. The enriched ¹³C values(−26.3 ± 0.4‰ for the Rhône River and−26.9 ± 1.2‰ for coastal rivers), measured during highwater stages, express mostly a mixture of terrestrial source (plant remainsand soils) whereas depleted ¹³C values (∼ −30‰)associated with low water stages exhibit a source with predominant freshwateralgae. The high δ¹⁵N mean values (>8‰) found inTêt, Ter and Tordera rivers may underline the importance ofdenitrification processes as a consequence of the eutrophication andanthropogenic impact.

[学科分类] 地球化学与岩石