Henri Etcheber

Inorganic and organic carbon biogeochemistry in the Gautami Godavari estuary (Andhra Pradesh, India) during pre‐monsoon: The local impact of extensive mangrove forests

tidal mangrove creeks in the delta. Here, variations in the concentration and d 13 Co f the DIC pool were shown to result largely from the mineralization of organic matter. The present study clearly identifies the mangrove creeks as an active site of mineralization and CO2 efflux to the atmosphere, but shows that these changes in the aquatic biogeochemistry are a localized feature, rapidly fading in the adjacent Kakinada Bay. Our data indicate that mineralization of dissolved organic carbon (DOC) of mangrove origin, and its subsequent efflux as CO2 to the atmosphere may represent an important fate for mangrove carbon. Although further quantification of this process in a variety of systems is required, we suggest that some of the current ideas on the role of mangroves in the carbon budget of the coastal zone may need to be reconsidered. INDEX TERMS: 4235 Oceanography: General: Estuarine processes; 4504 Oceanography: Physical: Air/sea interactions (0312); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4815 Oceanography: Biological and Chemical: Ecosystems, structure and dynamics; KEYWORDS: mangroves, mineralization, stable isotopes

Particulate organic carbon in the estuarine turbidity maxima of the Gironde, Loire and Seine estuaries: origin and lability.

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.

Analysis of total and organic carbon and total nitrogen in settling oceanic particles and a marine sediment : an interlaboratory comparison

Abstract 10 laboratories, using their routine methods, collaborated in a comparison of analyses of total carbon, nitrogen and organic carbon in a single sample of marine sediment and a pooled sample of settling particulate material collected by sediment trap. The standard error of the means from all individual laboratories was ±3% of the mean for all total carbon results, and for total nitrogen in the marine sediment, but was ±7% of the mean for total nitrogen in the sediment trap material. Larger errors (±8% of the mean) were found for organic carbon results from both the marine sediment and the sediment trap material, with a range of results (expressed as a percentage of the untreated dry weight) for the sediment trap sample analyzed here of 5.60–8.03%. When this range is compared with that of particulate organic carbon concentration in sediment trap material obtained by different workers at various deep ocean sites (2.4–10.2%) [Wefer, G., 1989. Particle flux in the ocean: Effects of episodic production, in: W.H. Berger, V.S. Smetacek, G. Wefer (Eds.), Productivity of the Ocean: Present and Past, John Wiley & Sons, pp. 139–154.] it is apparent that a large proportion of the observed oceanic variability could be accounted for by difference in analytical technique. We suggest that the problem stems from the difficulty of accurate separation of the organic and inorganic phases and discuss the inaccuracies involved in the separation. We present evidence that the range of results is partly due to either volatilisation of organic carbon during acid treatment, or incomplete removal of organic matter during thermal treatment. If results from different laboratories are to be compared, as is presently required for basin scale and global studies of ocean carbon cycling and budgets, there is clearly a need for suitable reference materials, rigorous intercalibration and technique development.

Long-term monitoring (1960–2008) of the river-sediment transport in the Red River Watershed (Vietnam): Temporal variability and dam-reservoir impact

The Red River (China/Vietnam, A=155,000 km(2)) is a typical humid tropics river originating from the mountainous area of Yunnan Province in China. Based on information on daily discharge (Q) and suspended particulate matter (SPM) concentration between 1960-2008 for the SonTay gauging station (outlet of the River and entry to the Delta) provided by the National Institute IMHE-MONRE, the mean annual SPM flux was estimated at 90 Mt/yr, corresponding to a sediment yield of 600 t/km(2)/yr. The temporal variability of annual SPM fluxes (ranging from 24 to 200 Mt/yr) is strongly related to the interannual hydrological conditions. However, some years of high water flow were not associated with high sediment fluxes, especially after 1989 when the HoaBinh dam came into operation. Therefore, the median discharge pre- (3389 m(3)/s) and post 1989 (3495 m(3)/s) are similar indicating there was little or no change between both periods. Sediment rating curves (power law-type; SPM=aQ(b)) were fitted for both periods (1960-1989; 1990-2008). The analysis of the pre- and post-1989 sediment rating parameters (a, b) suggests a downshift of b-parameter values after 1989, attributed to a decrease of the sediment supply due to the commissioning of the HoaBinh dam. A single sediment rating curve derived from 1960-1989 data was used to simulate the annual variability of former sediment delivery, generating excellent cumulative flux estimates (error ~1%). In contrast, applying the same rating curve to the 1990-2008 data resulted in systematic and substantial (up to 109%) overestimation. This suggests that the HoaBinh dam reduces annual SPM delivery to the delta by half, implying important metal/metalloid storage behind the HoaBinh dam.

Algal and microbial processes involved in particulate organic matter dynamics in the Loire estuary

Algal and bacterial biomass and activities have been studied in the Loire estuary (France) by means of particulate organic matter proteins and chlorophyll pigments determination, electron transport system activity measurement, 14C-bicarbonate incorporation and tritiated thymidine incorporation determination. These data collected throughout the estuary in various hydrological conditions allow to characterize two opposite typical situations: (i) During winter and early spring, when discharge increases beyond 1000 m3s-1, heterotrophic activity always dominates over primary production but remains moderate because of low temperature and non-biodegradable quality of organic carbon; (ii) at drought situations, phytoplankton production develops in the river causing increases of pH and oxygen concentration. In these situations, accumulation of phytoplanktonic material in the highly turbid inner estuary results in very high heterotrophic activities causing pH decrease and complete depletion of dissolved oxygen. Budgets of POC show that the anthropogenic inputs contribute only for less than 5% to the organic load of the inner estuary.

Distribution of phytoplankton pigments in nine European estuaries and implications for an estuarine typology

Phytoplankton pigments were studied by LiquidChromatography (HPLC) in nine West Europeanestuaries. Three estuaries, i.e. the Rhine,Scheldt and the Gironde were sampled four timesto cover the different seasons, whereas theother six estuaries were sampled once. Pigmentdistributions in estuaries reflect bothriverine inputs as well as autochthonousblooms. Fucoxanthin was the most commonaccessory photosynthetic pigment showing thatDiatoms were the most common group in thestudied estuaries and were particularlydominant during autumn and winter. In the veryturbid Gironde estuary, degradation processeswere predominant between salinities 1 and 20,while Diatoms, Dinoflagellates and Cryptophytesbloomed above 20 salinity during spring andsummer. This contrasted with the highlyeutrophic but less turbid Scheldt, wherephytoplanktonic blooms occurred at lowsalinities close to the city of Antwerp. In theScheldt, we observed both a tenfold fluctuationof phytoplankton biomass and a fluctuatingpigment diversity index. In contrast,chlorophyll a was always low in theGironde, but we observed large variations ofpigment diversity among samplings duringdifferent seasons. Distribution of pheopigmentsshowed that the maximum turbidity zone (MTZ)was a highly reactive region for heterotrophicphytoplankton degradation. The Scheldt and theThames were the most anthropogenic influencedestuaries contrasting with the Gironde estuarythat has a less urbanised watershed. Anestuarine typology is proposed based on threeclusters emerging from a correspondenceanalysis of pigment variables and variablescharacterising the anthropogenic impact andphysical forcing.

Seasonal variation and origin of Particulate Organic Carbon in the lower Garonne River at La Reole (southwestern France)

Using a databank, grouping results on dailymeasurements of Total Suspended Matter (TSM) and Particulate Organic Carbon (POC) on the lower Garonne river at La Reole (upstream limit of the dynamic tide) over the last 20 years, we studied seasonal variations in particulate organic carbon concentrations and fluxes, using TSM concentration classes. The results show seasonal variations in POC concentrations and fluxes, and especially a strong impact of flood events. A qualitative and quantitative hypothesis on the differentiation of POC origin is suggested. On every time scale (daily, seasonally or yearly), this hypothesis allows the estimation of POC inputs from the three different pools: soil, litter and autochthonous production. The Garonne River exports 78 000 tons of POC annually, 54% from soil (0.8 t km-2 yr-1), 38% from litter (about 0.55 t km-1 yr-1) (km of permanent stream) and 8% from autochthonous production. Most of this material is probably mineralized in the Gironde estuary.

Assessment of dam trapping efficiency from water residence time: Application to fluvial sediment transport in the Adour, Dordogne, and Garonne River Basins (France)

Dam-trapping efficiency can be estimated by using the hydraulic residence time. On the regional scale, the global impact of several dams can be assessed by taking into account the spatial organization of dams along the river network. Therefore, in this study, a method is proposed to estimate the global-trapping efficiency, TEw, for any watershed containing more than one dam. This method is applied to the Adour, Dordogne, and Garonne River watersheds (southwestern France). The spatial organization of dams and its impact on TEw and on sediment yields measured at 37 TSM sampling stations over 2 years are discussed. Positive correlation between drainage areas and river sediment loads corrected from dam regulation using TEw, as well as comparisons between TEw-corrected sediment yields and sediment yields measured upstream from dams, point out the interest of the method in order to reconstitute the natural sediment yields.

Fluorescence and DOC contents of pore waters from coastal and deep-sea sediments in the Gulf of Biscay

Abstract Fluorescence of waters from the Gulf of Biscay was investigated. Pore waters fluoresced more intensely and exhibited red-shifted spectra relative to overlying seawaters. Also, a blue-shift was observed going from coastal to open sea sites. Results indicate that continental inputs of fluorescent material reach the sea bed at all sites studied. Organic matter (OM) modifications within sediments were also observed. In the uppermost layer (6 cm), fluorescence intensity and dissolved organic carbon (DOC) concentrations decrease, followed by a red-shift in emission spectra with increasing depth. This may reflect the increase in OM molar mass due to humification. The reverse of these trends in the deepest sub-oxic sediments might be related to the degradation of OM released from the solid phase, resulting in dissolved fluorescent material with a relative paucity of oxygen-containing functional groups. A very good correlation of DOC with fluorescence was observed in all cores.

Factors contributing to hypoxia in a highly turbid, macrotidal estuary (the Gironde, France)

Dissolved oxygen (DO) is a fundamental parameter of coastal water quality, as it is necessary to aquatic biota, and it provides an indication of organic matter decomposition in waters and their degree of eutrophication. We present here a 7 year time series of DO concentration and ancillary parameters (river discharge, water level, turbidity, temperature, and salinity) from the MAGEST high-frequency monitoring network, at four automated stations in the central and fluvial regions of the macrotidal Gironde Estuary, one of the largest European estuaries. The spatio-temporal variability of DO at different time scales was first related to the migration and position of the maximum turbidity zone in this extremely turbid estuary. Since 2005, the Gironde Estuary has recorded several borderline hypoxic situations (DO close to 2 mg L(-1)) and a 7 day-long hypoxic event (DO < 2 mg L(-1)) in July 2006. Summer hypoxia occurred exclusively in the fluvial, low salinity, and high turbidity sections of the estuary and was significantly more pronounced in front of the large urban area of Bordeaux (715 000 inhabitants). Detailed analysis of the data at the seasonal, neap-spring and semi-diurnal tidal time scales, reveals that hypoxia in this area occurred: (i) in the maximum turbidity zone; (ii) during the spring to neap tide transition; (iii) at highest water temperature; and (iv) at lowest river discharge; there was also evidence of an additional negative impact of sewage treatment plants of Bordeaux city. Enhancement of respiration by turbidity, temperature and inputs of domestic biodegradable organic matter and ammonia, versus renewal of waters and dispersion of reduced pollutants with the river discharge, appeared as the dominant antagonist processes that controlled the occurrence of summer hypoxia. In the context of long-term environmental changes (increase in temperature and population, decrease in summer river discharge), the occurrence of severe hypoxia could not be excluded in the next decades in the upstream reach of the Gironde Estuary.