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

Abstract

A large amount of terrestrial organic matter is annually delivered by rivers to the continental shelf, where this material is either 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 analyzing the stable isotopes (δ¹³C and &delta;¹⁵N) 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 monthly characterized terrestrial POM delivered by eight rivers discharging to the NW Mediterranean Sea: 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 (δ¹³C and &delta;¹⁵N) were determined. <br><br> 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 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 in average 2.5–3 times higher POC and PN mean contents than the Rhône River (8.5% and 1.5%, respectively for coastal rivers against 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 δ¹³C ratios of organic matter reflect also this discrepancy between high and low water stages with values ranging from −33.2&permil; to −24.5&permil;. The enriched ¹³C values (&minus;26.3 ± 0.4&permil; for the Rhône River and &minus;26.9 ± 1.2&permil; for coastal rivers), measured during high water stages, express mostly a mixture of terrestrial source (plant remains and soils) whereas depleted <sup>13</s...

Environmental controls on particle fluxes in the Gulf of Valencia, NW Mediterranean.

“…This preferential sediment transport path along the slope would prevent this flux of particles to reach intermediate water column depths at the central part of the GoV (station Ct) and even deeper parts of the GoV basin (station Cb). Similar events of storm-related sediment resuspension and its subsequent along-current transport have also been observed in the Gulf of Lions (Bourrin et al, 2008;Higueras et al, 2014), even with wave heights of 2.5 m (Palanques et al, 2012). The time decoupling between meteorological events and the magnitude of the TMF in each of the traps suggest that TMF peaks in the GoV can develop by a sequential combination of Ebro River discharges and sporadic storm events.…”

River organic matter shapes microbial communities in the sediment of the Rhône prodelta

“…The d 13 C value, explaining significant amounts of variability in the microbial communities, is also an indicator of the source of the organic carbon. The d 13 C values in our samples ranged from À 27.67 to À 25.83, indicating a predominant contribution of the Rhô ne River particulate OM (POM) (Darnaude et al, 2004;Higueras et al, 2014) at the expense of the marine contribution (Harmelin-Vivien et al, 2008). Higueras et al (2014) demonstrated that depleted d 13 C values (o À 27%) characterized freshwater production in several Mediterranean rivers whereas enriched d 13 C values (4 À 27%) traced predominantly the soil OM.…”

“…*Some samples are missing because they contained o450 sequences. a m À 3 s À 1 ; b mg g À 1 dry weight (DW) sediment; c mg g À 1 DW sediment; d terrestrial fraction was calculated using the d 13 C value and a two end-members model, where terrestrial end-member was d 13 C ¼ À 27.4 ± 0.7% and marine end member is d 13 C ¼ À 20.1 ± 0.8% (Harmelin-Vivien et al, 2008;Higueras et al, 2014); e median particle diameter (mm); f the coloring is different only if the values were significantly different (Po0.05); g samples where the variance was too large to determine whether differences/similarities were significant.…”