Alexandra Coynel

Bio: Alexandra Coynel is an academic researcher from University of Bordeaux. The author has contributed to research in topics: Estuary & Sediment. The author has an hindex of 25, co-authored 66 publications receiving 1999 citations. Previous affiliations of Alexandra Coynel include Centre national de la recherche scientifique.

Spatial and seasonal dynamics of total suspended sediment and organic carbon species in the Congo River

Abstract: [1] The Congo (Zaire) River, the world's second largest river in terms both of water discharges and of drainage area after the Amazon River, has remained to date in a near-pristine state. For a period between 2 and 6 years, the mainstream near the river mouth (Brazzaville/Kinshasa station) and some of the major and minor tributaries (the Oubangui, Mpoko, and Ngoko-Sangha) were monitored every month for total suspended sediment (TSS), particulate organic carbon (POC), and dissolved organic carbon (DOC). In this large but relatively flat equatorial basin, TSS levels are very low and organic carbon is essentially exported as DOC: from 74% of TOC for the tributaries flowing in savannah regions and 86% for those flowing in the rain forest. The seasonal patterns of TSS, POC, and DOC show clockwise hysteresis in relation to river discharges, with maximum levels recorded 2 to 4 months before peak flows. At the Kinshasa/Brazzaville station, the DOC distribution is largely influenced by the input from the tributaries draining the large marshy forest area located in the center of the basin. There is a marked difference between specific fluxes, threefold higher in the forest basins than in the savannah basins. The computation of inputs to the Atlantic Ocean demonstrates that the Congo is responsible for 14.4 × 106 t/yr of TOC of which 12.4 × 106 t/yr is DOC and 2 × 106 t/yr is POC. The three biggest tropical rivers (the Amazon, the Congo, and the Orinoco), with only 10% of the exoreic world area drained to world oceans, contribute ∼4% of its TSS inputs but 15–18% of its organic carbon inputs. These proportions may double when considering only world rivers discharging into the open ocean.

Mapping the world’s free-flowing rivers

Abstract: Free-flowing rivers (FFRs) support diverse, complex and dynamic ecosystems globally, providing important societal and economic services. Infrastructure development threatens the ecosystem processes, biodiversity and services that these rivers support. Here we assess the connectivity status of 12 million kilometres of rivers globally and identify those that remain free-flowing in their entire length. Only 37 per cent of rivers longer than 1,000 kilometres remain free-flowing over their entire length and 23 per cent flow uninterrupted to the ocean. Very long FFRs are largely restricted to remote regions of the Arctic and of the Amazon and Congo basins. In densely populated areas only few very long rivers remain free-flowing, such as the Irrawaddy and Salween. Dams and reservoirs and their up- and downstream propagation of fragmentation and flow regulation are the leading contributors to the loss of river connectivity. By applying a new method to quantify riverine connectivity and map FFRs, we provide a foundation for concerted global and national strategies to maintain or restore them. A comprehensive assessment of the world’s rivers and their connectivity shows that only 37 per cent of rivers longer than 1,000 kilometres remain free-flowing over their entire length.

The role of terrestrially derived organic carbon in the coastal ocean: A changing paradigm and the priming effect

Abstract: One of the major conundrums in oceanography for the past 20 y has been that, although the total flux of dissolved organic carbon (OC; DOC) discharged annually to the global ocean can account for the turnover time of all oceanic DOC (ca. 4,000–6,000 y), chemical biomarker and stable isotopic data indicate that there is very little terrestrially derived OC (TerrOC) in the global ocean. Similarly, it has been estimated that only 30% of the TerrOC buried in marine sediments is of terrestrial origin in muddy deltaic regions with high sedimentation rates. If vascular plant material—assumed to be highly resistant to decay—makes up much of the DOC and particulate OC of riverine OC (along with soil OC), why do we not see more TerrOC in coastal and oceanic waters and sediments? An explanation for this “missing” TerrOC in the ocean is critical in our understanding of the global carbon cycle. Here, I consider the origin of vascular plants, the major component of TerrOC, and how their appearance affected the overall cycling of OC on land. I also examine the role vascular plant material plays in soil OC, inland aquatic ecosystems, and the ocean, and how our understanding of TerrOC and “priming” processes in these natural systems has gained considerable interests in the terrestrial literature, but has largely been ignored in the aquatic sciences. Finally, I close by postulating that priming is in fact an important process that needs to be incorporated into global carbon models in the context of climate change.

Illuminated darkness: molecular signatures of Congo River dissolved organic matter and its photochemical alteration as revealed by ultrahigh precision mass spectrometry.

Abstract: Congo River water was filtered and then irradiated for 57 d in a solar simulator, resulting in extensive photodegradation of dissolved organic matter (DOM). Whole-water (i.e., unfractionated) DOM was analyzed pre- and post-irradiation using ultrahigh resolution Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), revealing the following three pools of DOM classified based upon their photoreactivity: (1) photo-resistant, (2) photo-labile, and (3) photo-produced. Photo-resistant DOM was heterogeneous, with most molecular classes represented, although only a small number of aromatics and no condensed aromatics were identified. The photoproduced pool was dominated by aliphatic compounds, although it included a small number of aromatics, including condensed aromatics. Aromatic compounds were the most photoreactive, with . 90% being lost upon irradiation. Photochemistry also resulted in a significant drop in the number of molecules identified and a decrease in their structural diversity. The FT-ICR MS signatures of two classes of refractory organic matter, black carbon and carboxylic-rich alicyclic molecules (CRAM), were present in the sample prior to irradiation, indicating that the Congo River could be a significant exporter of recalcitrant DOM to the ocean. All black carbon–like molecules identified in the initial sample were lost during irradiation. Molecular signatures consistent with CRAM were also highly photo-labile, demonstrating that environmental solar irradiation levels are capable of removing these refractory compounds from aquatic systems. Irradiation also shifted the molecular signature of terrestrial DOM toward that of marine DOM, thereby complicating the task of tracking terrestrial DOM in the ocean. The transfer of terrestrial dissolved organic matter (TDOM) via rivers to the oceans is a significant component in the global and oceanic carbon budgets (Schlesinger and Melack 1981; Hedges 1992). The , 0.25 Pg of dissolved organic carbon (DOC) discharged from rivers annually can account for the mean radiocarbon-based turnover times of oceanic DOC (, 4000–6000 yr; Williams and Gordon 1970). However, only very small amounts of TDOM have been identified in seawater using both organic biomarker and stable carbon isotopic approaches (Meyers-Schulte and Hedges 1986; Moran et al. 1991; Opsahl and Benner 1997). Estimates of TDOM contributions to the oceans have been based on comparisons of the biochemical and isotopic compositions of open-ocean dissolved organic matter (DOM) to freshwater riverine end-member counterparts (Meyers-Schulte and Hedges 1986; Hernes and Benner 2006) and have not typically accounted for changes during transit within rivers, estuaries, or the ocean (Cole and Caraco 2001; Raymond and Bauer 2001; Benner 2002). Without a better understanding of the types and magnitudes of modifications that components of TDOM undergo in lower rivers, estuaries, and the ocean, we may be misinterpreting the effective molecular signatures of TDOM in the ocean. Three massive tropical rivers, the Congo, Amazon, and Orinoco, are responsible for over a quarter of global DOC input to the oceans (Coynel et al. 2005). The Congo is second only to the Amazon as a DOC conduit between the terrestrial and marine biogeochemical cycles, exporting , 12.4 Tg DOC yr21, equivalent to , 5% of the global