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

Abstract

Abstract.A large amount of terrestrial organic matter is annually delivered by rivers to the continental shelf, where this material is either degraded, buried or transferred to the deep sea by hydrodynamic processes such as storms. The relative amount of terrestrial organic matter in the marine sediments is often determined by analysing the stable isotopes (δ 13 C and δ 15 N) and the C / N ratio of organic matter because the various particulate organic matter (POM) sources have distinct isotopic compositions. With the objective to refine and better interpret POM sources in the marine environment, we have characterized monthly terrestrial POM delivered 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 to December 2009 and the concentrations of total suspended matter (TSM), particulate organic carbon (POC) and nitrogen (PN), as well as their stable isotopic ratios (δ 13 C and δ 15 N) were determined.During the survey, three rainstorm events with winds coming from the E-NE and the S-SE impacted the NW Mediterranean. Depending on the direction of incoming winds, the fluvial response (amount of water discharge and TSM) was different. Rivers draining the Alps (Rhône River) and Central Massif (Hérault, Orb, and Aude rivers) were mostly impacted by rainstorms associated with winds coming from the S-SE, while rivers draining the Pyrenees (Têt, Fluvià, and Ter rivers) and the Montseny Massif (Tordera River) were impacted by rainstorms associated with winds coming from the E-NE. In addition, the spatial evolution of water discharges shows a different hydrological regime of the Rhône River, with relatively constant and high water stages and TSM concentrations when compared to coastal rivers, characterized by long periods of low water stages. TSM concentrations are positively correlated to water discharges (high water flows resuspended riverbed sediments) but show an inverse relationship with POC and PN relative contents (mostly due to dilution and by low availability of light in river waters during flood events). TSM in most of the coastal rivers have on average 2.5-3 times higher POC and PN mean contents than the Rhône River (8.5 and 1.5 %, respectively, for coastal rivers compared to 3.6 and 0.5 %, respectively, for the Rhône River). This discrepancy may be caused by the long drought periods in small coastal Mediterranean watersheds that enhance the eutrophication in studied coastal rivers. The δ 13 C ratios of organic matter also reflect this discrepancy between high and low water stages with values ranging from −33.2 to −24.5 ‰. The enriched 13 C values (−26.3 ± 0.4 ‰ for the Rhône River and −26.9 ± 1.2 ‰ for coastal rivers), measured during high water stages, express mostly a mixture of terrestrial source (plant remains and soils) whereas

Flow Regime and Nutrient-Loading Trends from the Largest South European Watersheds: Implications for the Productivity of Mediterranean and Black Sea’s Coastal Areas

“…Fifty kilometers upstream the mouth, the Rhône splits in two branches of "Grand Rhône" and "Petit Rhône", which carry about 90% and 10% of the total flow, respectively. After the damming of the Nile in 1968, the Rhône has become the major source of freshwater in the Mediterranean [42]. This river is characterized by large loads of nitrogen and phosphorus, which has increased in the period 1970-1990 [43,44].…”

Origin of remineralized organic matter in sediments from the Rhone River prodelta (NW Mediterranean) traced by Δ 14 C and δ 13 C signatures of pore water DIC

“…Therefore, the organic matter in the suspended particles in the Rhone River has a variable D 14 C ranging from À90 to +150‰ (Cathalot et al, 2013;Toussaint, 2013). It also displays a mean d 13 C isotopic signature of À27.1 ± 0.6‰ (Cathalot et al, 2013;Higueras et al, 2014;Kerhervé et al, 2001;Toussaint, 2013).…”

Aged riverine particulate organic carbon in four UK catchments

“…The organic matter components of SPM have been studied by a variety of analytical techniques and with stable isotopes and element ratios (da Cunha et al, 2000;Onstad et al, 2000;Kendall et al, 2001;Higueras et al, 2014) to obtain information on molecular structure and insight into the materials from which the POM is derived. Determination of the radiocarbon content of POM provides further information, not only on sources but also apparent age.…”