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International audience; We report on diurnal, tidal, and seasonal variations of dissolved inorganic carbon (DIC), water partial pressure of CO2 (pCO2), and associated water–air CO2 fluxes in a tidal creek of a temperate coastal lagoon with 70% of intertidal flats, during eight tidal/diurnal cycles and two consecutive years covering all seasons. Surface waters of the lagoon were always slightly oversaturated in CO2 with respect to the atmosphere with an average pCO2 value of 496 ± 36 ppmv. Seasonally, subsurface water pCO2 values were controlled by both temperature and biological/tidal advection effects that compensated each other and resulted in weak annual variations. High-resolution temporal pCO2 records reveal that the highest fluctuations (192 ppmv) occurred at the tidal/diurnal scale as a result of biological activity, advection from the tidal flat, and porewater pumping that all contributed to water pCO2 and carbonate chemistry variations. Total alkalinity (TA) versus salinity plots suggest a net production of alkalinity in the lagoon attributed to benthic carbonate dissolution and/or anaerobic degradation of organic matter. We specifically highlighted that for the same salinity range, during flooding, daytime pCO2 were generally lower than nighttime pCO2 values because of photosynthesis, whereas during ebbing, daytime pCO2 were higher than nighttime pCO2 values because of heating. Waters in the lagoon were a relatively weak CO2 source to the atmosphere over the year compared to other estuarine and lagoon waters elsewhere, and to sediment-air fluxes measured simultaneously by atmospheric Eddy Covariance (EC) in the Arcachon lagoon. Because of low values and small variations of the air-sea pCO2 gradient, the variability of fluxes calculated using the piston velocity parameterization was greatly controlled by the wind speed at the diurnal and, to a lesser extent, seasonal time scales. During the emersion, the comparison of these pCO2 data in the tidal creek with EC fluxes measured 1.8 km away on the tidal flat suggests high heterogeneity in air-sea CO2 fluxes, both spatially and at short time scales according to the inundation cycle and the wind speed. In addition to tidal pumping when the flat becomes emerged, our data suggest that lateral water movement during the emersion of the flat generates strong spatial heterogeneity in water–air CO2 flux.

International audience; The carbonate chemistry was investigated in the semiarid eutrophic Araruama Lagoon (Brazil), one of the largest hypersaline coastal lagoons in the world. Spatial surveys during winter and summer periods were performed, in addition to a diurnal sampling in summer. The hypersaline waters have higher concentrations of total alkalinity (TA) and dissolved inorganic carbon (DIC) than the seawater that feed the lagoon, due to evaporation. However, TA and DIC concentrations were lower than those expected from evaporation. Calcium carbonate (CaCO3) precipitation partially explained these deficits. The negative correlation between the partial pressure of CO2 (pCO2) and chlorophyll a (Chl a) indicated that DIC was also consumed by primary producers. The uptake by photosynthesis contributes to 57–63% of DIC deviation from evaporation, the remaining credited to CaCO3 precipitation. Marked pCO2 undersaturation was prevalent at the innermost region with shallow, confined, and phytoplankton-dominated waters, with a strong enrichment of heavier carbon isotope (δ13C-DIC up to 5.55‰), and highest pH (locally counter-acting the process of ocean acidification). Oversaturation was restricted to an urbanized region, and during night-time. The lagoon behaved as a marked CO2 sink during winter (− 15.32 to − 10.15 mmolC m−2 day−1), a moderate sink during summer (− 5.50 to − 4.67 mmolC m−2 day−1), with a net community production (NCP) of 93.7 mmolC m−2 day−1 and prevalence of net autotrophic metabolism. A decoupling between CO2 and O2 exchange rate at the air–water interface was attributed to differences in gas solubility, and high buffering capacity. The carbonate chemistry reveals simultaneous and antagonistic actions of CaCO3 precipitation and autotrophic metabolism on CO2 fluxes, and could reflect future conditions in populated and semiarid coastal ecosystems worldwide.

Projections for the next 50 years predict a widespread distribution of hypoxic zones in the open and coastal ocean due to environmental and global changes. The Tidal Garonne River (SW France) has already experienced few episodic hypoxic events. However, predicted future climate and demographic changes suggest that summer hypoxia could become more severe and even permanent near the city of Bordeaux in the next few decades. A 3D model, which couples hydrodynamic, sediment transport, and biogeochemical processes, is applied to assess the impact of factors submitted to global and regional climate changes on oxygenation in the turbidity maximum zone (TMZ) of the Tidal Garonne River during low-discharge periods. The model simulates an intensification of summer hypoxia with an increase in temperature, a decrease in river flow or an increase in the local population, but not with sea level rise, which has a negligible impact on dissolved oxygen. Different scenarios were tested by combining these different factors according to the regional projections for 2050 and 2100. All the simulations showed a trend toward a spatial and temporal extension of summer hypoxia that needs to be considered by local water authorities to impose management strategies to protect the ecosystem.

We measured spatial and temporal variations in carbon concentrations, isotopic compositions and exports during a complete hydrological cycle in nine watercourses draining a lowland forested podzolized catchment, flowing into the Arcachon lagoon (France). In addition, integrated fluxes of CO2 across the water-atmosphere interface were estimated to assess the relative importance of CO2 evasion versus lateral carbon transport at the catchment scale. Watercourse similarities and specificities linked to the local catchment characteristics are discussed and compared with other riverine systems. Low concentrations of suspended particulate matter and particulate organic carbon (POC) were generally measured in all the watercourses (8. 4 ± 3. 4 and 1. 6 ± 0. 6 mg L-1, respectively), reflecting limited mechanical soil erosion. The generally high POC content in the suspended matter (20 %), low Chl a concentrations (1. 3 ± 1. 4 μg L-1) and the relatively constant δ13C-POC value (near -28 ‰) throughout the year reveal this POC originates from terrestrial C3 plant and soil detritus. The presence of podzols leads to high levels of dissolved organic carbon (DOC; 6. 6 ± 2. 2 mg L-1). Similarly, high dissolved inorganic carbon (DIC) concentrations were measured in the Arcachon lagoon catchment (5. 9 ± 2. 2 mg L-1). The δ13C-DIC value around -20 ‰ throughout the year in many small watercourses reveals the predominance of terrestrial carbon mineralisation and silicate rock weathering in soils as the major DIC source. With pCO2 between 1,000 and 10,000 ppmv, all watercourses were a source of CO2 to the atmosphere, particularly during the low river stage. Organic carbon parameters remained relatively stable throughout the year, whereas DIC parameters showed strong seasonal contrasts closely linked to the hydrological regime and hyporheic flows. In total, the carbon export from the Arcachon watershed was estimated at 15,870 t C year-1 or 6 t C km-2 year-1, mostly exported to the lagoon as DOC (35 %), DIC (24 %) and lost as CO2 degassing to the atmosphere (34 %). © 2012 Springer Basel AG. ispartof: Aquatic Sciences vol:75 issue:2 pages:299-319 status: published

We report a data-set of dissolved methane (CH4) in three rivers (Comoe, Bia and Tanoe) and five lagoons (Grand-Lahou, Ebrie, Potou, Aby and Tendo) of Ivory Coast (West Africa), during the four main climatic seasons (high dry season, high rainy season, low dry season and low rainy season). The surface waters of the three rivers were over-saturated in CH4 with respect to atmospheric equilibrium (2221–38719%), and the seasonal variability of CH4 seemed to be largely controlled by dilution during the flooding period. The strong correlation of CH4 concentrations with the partial pressure of CO2 (pCO2) and dissolved silicate (DSi) confirm the dominance of a continental sources (from soils) for both CO2 and CH4 in these rivers. Diffusive air–water CH4 fluxes ranged between 25 and 1187 μmol m−2 day−1, and annual integrated values were 288 ± 107, 155 ± 38, and 241 ± 91 μmol m−2 day−1 in the Comoe, Bia and Tanoe rivers, respectively. In the five lagoons, surface waters were also over-saturated in CH4 (ranging from 1496 to 51843%). Diffusive air–water CH4 fluxes ranged between 20 and 2403 μmol m−2 day−1, and annual integrated values were 78 ± 34, 338 ± 217, 227 ± 79, 330 ± 153 and 326 ± 181 μmol m−2 day−1 in the Grand-Lahou, Ebrie, Potou, Aby and Tendo lagoons, respectively. The largest CH4 over-saturations were observed in the Tendo and Aby lagoons that are permanently stratified systems (unlike the other three lagoons), leading to anoxic bottom waters favorable for a large CH4 production. In addition, these two stratified lagoons showed low pCO2 values due to high primary production, which suggests an efficient transfer of organic matter across the pycnocline. As a result, the stratified Tendo and Aby lagoons were respectively, a low source of CO2 to the atmosphere and a sink of atmospheric CO2 while the other three well-mixed lagoons were strong sources of CO2 to the atmosphere but less over-saturated in CH4.

We report partial pressure of CO2 (pCO2) and ancillary data in three rivers (Bia, Tanoe, and Comoe) and five lagoons (Tendo, Aby, Ebrie, Potou, and Grand-Lahou) in Ivory Coast (West Africa), during four cruises covering the main climatic seasons. The three rivers were oversaturated in CO2 with respect to atmospheric equilibrium, and the seasonal variability of pCO2 was due to dilution during the flooding period. Surface waters of the Potou, Ebrie, and Grand-Lahou lagoons were oversaturated in CO2 during all seasons. These lagoons behaved similarly to the oligohaline regions of macrotidal estuaries that are CO2 sources to the atmosphere due to net ecosystem heterotrophy and inputs of riverine CO2 rich waters. The Aby and Tendo lagoons were undersaturated in CO2 with respect to the atmosphere because of their permanent haline stratification (unlike the other lagoons) that seemed to lead to higher phytoplankton production and export of organic carbon below the pycnocline.

A study of the particulate organic carbon (POC) in the estuarine turbidity maxima (ETMs) of the three major French macrotidal estuaries shows that the average contents are 1.5, 3.3 and 3.1% (expressed in % of dry suspended sediment) in the Gironde, Loire and Seine Estuaries, respectively. There is no seasonal variation of POC contents in the Gironde Estuary, whereas, they often increase in the Loire and the Seine Estuaries in spring and summer. The lability of the estuarine particulate organic matter was estimated by two analyses: 1/labile organic matter was measured as the organic carbon loss during incubation tests over one month; 2/ the hydrolysable organic fraction was determined after 6N HCl digestion. The organic fractions of the ETMs are mainly refractory. Any increase in the amount of POC as compared to the background levels (cited above) is always correlated to an increase of organic matter lability. The yearly average fluvial contributions by various particulate organic pools (soil and litter organic matter; organic matter of phytoplanktonic and human origin) that enter the three estuaries were quantified. In the Garonne River, soil and litter are the major (90%) POC sources. In the Loire system, due to the eutrophication of the river water, phytoplankton contributes up to 50% of the total POC load. In the Seine river, soil and litter contribute 70% of the total POC input; POC of human origin is also significant (10%), due to the impact of the City of Paris (10 million inhabitants). The lability of the different types of organic matter ranks as follows: phytoplankton ~litter > human-origin organic matter >> soil. By combining the POC budgets and the lability of each type of organic fraction, it was possible to explain why the POC of the three ETMs is different and characterizes its refractory vs. labile nature.

International audience; The Nivelle River, a typical Pyrenean mountainous watershed reaching the Bay of Biscay (Atlantic Ocean), was sampled with high resolution during 1996. The particulate organic carbon (POC) contents during successive floods shows that there is a graduated impoverishment of the organic fraction of suspended particulate matter (SPM) from the first flood to the next ones, reaching a threshold value (3%) attributed to allochtonous fraction (soil). On the basis of the high frequency data of water discharge and POC concentration, an annual POC flux was established: 845 tons, corresponding to a specific POC flux of 5.3 tC km−2 yr−1. This value was obtained during a dry period and must be considered as a minimum value for longer time scale. The POC originated mostly from soil (55%) and riparian/litter (~40%) with a very minor (−2 yr−1, if extended to the whole mountainous area of the southern coast of the Bay of Biscay (19,000 km2), leads to an estimated POC flux around 100,000 t yr−1. Although small Cantabrian mountainous rivers contributed to only 28% of the freshwater discharge in the Bay of Biscay, their POC load was estimated to account for 70% of the total POC inputs in the Bay.

The occurrence of alkylated volatile iodide andselenide species was evidenced and investigatedin water, sediments and overlying atmosphere ofthree major European estuaries, such as theGironde (F), the Rhine (NL) and the Scheldt(B/NL), along with the salinity gradient. Foriodine, up to eight volatile species wereobserved as alkyl-iodides in estuarine waters.The major one (ca. 40%) was methyl-iodide(MeI) with average seasonal concentrationsranging from 1 to 100 pmol l−1. Otherspecies observed were found to correspond toseveral halomethane derivatives and lightalkyl-iodide species. For selenium, dimethylselenide (Me2Se) was the main compound(ca. 90%) over three methyl-selenidesencountered in estuarine waters. Me2Seaverage seasonal concentrations were found torange between 0.2 and 100 pmol l−1 in thewater column. The occurrence of methylatediodides and selenides seems to be mainlyrelated to the algae's biomass turnover asindicated by photosynthetic pigment tracers(i.e. chlorophyll a and phaeopigments) andseasonal variation of surface waterconcentrations. The production and release ofgaseous iodide and selenide compounds may thenresult from natural biological pathways leadingto the methylation of their inorganic form.Finally, significant volatilisation rates wereevaluated leading to average seasonal fluxrates for total volatile iodide and selenidecompounds ranging from 4 to 100 and from 1 to75 nmol m−2 d−1, respectively.Estuarine mass balance estimated from MeI andMe2Se distributions indicates thatvolatilisation to the atmosphere represents aprimary sink for MeI and Me2Se fromestuarine surface waters.

Phytoplankton pigments were studied by Liquid Chromatography (HPLC) in nine West European estuaries. Three estuaries, i.e. the Rhine, Scheldt and the Gironde were sampled four times to cover the different seasons, whereas the other six estuaries were sampled once. Pigment distributions in estuaries reflect both riverine inputs as well as autochthonous blooms. Fucoxanthin was the most common accessory photosynthetic pigment showing that Diatoms were the most common group in the studied estuaries and were particularly dominant during autumn and winter. In the very turbid Gironde estuary, degradation processes were predominant between salinities 1 and 20, while Diatoms, Dinoflagellates and Crypto- phytes bloomed above 20 salinity during spring and summer. This contrasted with the highly eutrophic but less turbid Scheldt, where phytoplanktonic blooms occurred at low salinities close to the city of Antwerp. In the Scheldt, we observed both a tenfold fluctuation of phytoplankton biomass and a fluctuating pigment diversity index. In contrast, chlorophyll a was always low in the Gironde, but we observed large variations of pigment diversity among samplings during different seasons. Distribution of pheopigments showed that the maximum turbidity zone (MTZ) was a highly reactive region for heterotrophic phytoplankton degradation. The Scheldt and the Thames were the most anthropogenic influenced estuaries contrasting with the Gironde estuary that has a less urbanised watershed. An estuarine typo- logy is proposed based on three clusters emerging from a correspondence analysis of pigment variables and variables characterising the anthropogenic impact and physical forcing.