Oxic/anoxic oscillations and organic carbon mineralization in an estuarine maximum turbidity zone (The Gironde, France)

Abstract

The study of vertical particle dynamics in the highly turbid Gironde Estuary has shown intense cycles of sedimentation and resuspension at both diurnal and neap-spring time scales. Fluid mud, with suspended particulate matter (SPM) concentrations between 50 and 500 g liter ) preferentially reworked at the neap-spring time scale. Due to the alternation of sedimentation and resuspension periods, most of the sediment experienced oxic/anoxic oscillations throughout the neap-spring cycle. Fluid mud resuspension occurred without any observable incidence on the surface-water oxygenation. An increase in total alkalinity was found in the fluid mud, due to both anaerobic respiration and a carbonate dissolution coupled to aerobic respiratory CO 2 generation. This phenomenon significantly affected the inorganic carbon budget of the estuary, increasing the HCO input to the coastal ocean and reducing the CO 2 flux to the atmosphere. An Ϫ 3 accumulation of labile-dissolved organic carbon observed in the fluid mud suggests that these oscillations result in an acceleration of particulate organic matter (POM) decomposition. In the Gironde MTZ, a net loss of refractory land-derived POM occurs. This system acts as an efficient oxic/suboxic ''fluidized bed reactor,'' similar to mobile deltaic muds.Understanding processes affecting carbon distribution through estuaries is of major importance for a better assessment of the contribution of world rivers to the carbon budget of the coastal ocean. The influence of tides in macrotidal estuaries increases the residence times of both water and 1 Present address: Université de Liège, Mécanique des Fluides Géophysiques, Unité d'Océanographie Chimique, Institut de Physique (B5), B-4000 Sart Tilman, Belgium. AcknowledgmentsThis work has been funded by the European Commission in the framework of the Biogest project (ENV4-CT96-0213); this is a contribution of the Eloise Projects network. It was also supported by the URM 13 project, linking IFREMER and the University of Bordeaux I.-DGO-UMR 5805. Fieldwork was carried out onboard the RV Gwen Drez (IFREMER, France), the RVs Côte d'Aquitaine and Côte de la Manche (INSU, France), and the RV Belgica (Belgium); we thank captains and crews. A. De Resseguier (DGO) built the fluid mud sampler and was present on the field for sampling assistance. We are grateful to P. Castaing and A. Sottolichio (DGO) for help in using the OBS and to A. V. Borges (University of Liège)

Excess atmospheric carbon dioxide transported by rivers into the Scheldt estuary

“…The flows of carbon through the Scheldt estuary are now well documented, both by field and model approaches [8,12,13,23,24,27,31]. In contrast to highly turbid French estuaries (Gironde and Loire [1,2]), there is no evidence for carbonate dissolution or precipitation in the Scheldt estuary [23]. Heterotrophic activity, that mineralizes a large fraction of the organic carbon carried by the rivers, and acidification, due to nitrification, are the major processes that produce CO 2 .…”

Atmospheric CO2 evasion, dissolved inorganic carbon production, and net heterotrophy in the York River estuary

“…Spatial variation in sulfate reduction rates in estuaries is highly variable with maximum rates in salt marsh sediments and shallow subtidal areas (Howarth 1984), but it is evident that rates of net sulfate reduction in these areas could balance the alkalinity source necessary for DIC accumulation. It is worth noting that Abril et al (1999) recently measured alkalinity generation at the maximum turbidity zone in the Gironde estuary. These workers proposed that in the Gironde, 14% of the total HCO 3 Ϫ exported to the coastal ocean was produced in the maximum turbidity zone, and a significant percentage of the generated HCO 3 Ϫ was balanced by denitrification and manganese reduction.…”

Sulfetos volatilizáveis por acidificação e metais extraídos simultaneamente na avaliação de sedimentos de água doce

“…O CO 2 produzido nesta decomposição é o precursor do ânion bicarbonato, principal responsável pela alcalinidade. 20,21,23,27 É na etapa final de degradação da matéria orgânica que substân-cias oxidantes, normalmente presentes em sedimentos eutróficos, passam a ser relevantes. O consumo destes oxidantes se dá em função da profundidade, seguindo uma ordem crescente de produção de energia livre de Gibbs por mol de carbono orgânico oxidado, obedecendo à sequência: O 2 > óxidos de Mn ~ nitratos > óxidos de Fe > sulfatos.…”