We report a dataset of the partial pressure of CO2 (pCO2) and methane concentrations (CH4) in the surface waters of Lake Kivu obtained during four cruises covering the two main seasons (rainy and dry). Spatial gradients of surface pCO2 and CH4 concentrations were modest in the main basin. In Kabuno Bay, pCO2 and CH4 concentrations in surface waters were higher, owing to the stronger influence of subaquatic springs from depth. Seasonal variations of pCO2 and CH4 in the main basin of Lake Kivu were strongly driven by deepening of the epilimnion and the resulting entrainment of water characterized by higher pCO2 and CH4 concentrations. Physical and chemical vertical patterns in Kabuno Bay were seasonally stable, owing to a stronger stratification and smaller surface area inducing fetch limitation of wind driven turbulence. A global and regional cross-system comparison of pCO2 and CH4 concentrations in surface waters of lakes highlights the peculiarity of Kabuno Bay in terms of pCO2 values in surface waters. In terms of surface CH4 concentrations, both Kabuno Bay and the main basin of Lake Kivu are at the lower end of values in lakes globally, despite the huge amounts of CH4 and CO2 in the deeper layers of the lake.

CIESM Congress, 10-14 May 2010 • In the frame of the European Integrated Project “Southern European Seas: Assessing and Modelling Ecosystem changes” (SESAME), two sets of cruises were carried out in spring and fall 2008 to cover all of the Southern European Sea (SES) regions. Here, we report on the dynamics in surface waters of the partial pressure of CO2 (pCO2) either measured directly by equilibration or computed from pH and total alkalinity. • Sea surface temperature (SST) ranged between 12°C in the Black Sea in spring to 29°C in the Cilician basin in fall. In all the SES regions SST increased from spring to fall, on average by 8.0°C, ranging between 6°C (Alboran) to 11°C (Cilician). • pCO2 at in-situ temperature (pCO2@SST) ranged from 260 ppm in the Black Sea in fall to 450 ppm in the Cilician bassin in fall. In most the SES regions pCO2@SST increased from spring to fall, except in the Adriatic (pCO2@SST ± stable, despite large spatial variability (not shown)) and except in the Black Sea (decrease of pCO2@SST) • Since an increase of 8°C in SST leads to an increase of pCO2 of 145 ppm we normalized the pCO2 data to a temperature of 20°C (pCO2@20°C). pCO2@20°C ranged from 540 ppm in spring to 270 ppm in fall in the Black Sea. In the Adriatic, Aeagean, Black Sea, Cilician, Levantine, pCO2@20°C decreased from spring to fall, indicating a cumulated drawdown of CO2 by biological activity. In the Alboran pCO2@20°C increased from spring to fall possibly indicative of upwelling (in agreement with a lesser increase of SST from spring to fall than the other SES regions). In the Maramara Sea, pCO2@20°C increased from spring to fall due to a strong decline in primary production (not shown). • Air-sea CO2 fluxes ranged from -5.7 mmolC m-2 d-1 in the Black Sea in fall to +2.5 mmolC m-2 d-1 in the Cilician bassin in fall. In Spring, all the SES regions were sinks for atmospheric CO2. In fall, the more « open » SES areas were sources of atmospheric CO2 (Alboran, Levantine, Cilician & Aegean). In fall, the more « coastal » SES areas remained sinks for atmospheric CO2 (Adriatic, Marmara, Black Sea), but only in the Black Sea the CO2 sink increased in fall. Alboran Sea SST (°C)

Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40μmolL−1h−1, dissolved inorganic carbon (DIC) production of 9–12μmolL−1h−1, very low NH4+ and Mn2+ concentrations, and constant NO3− production rates (0.4 - 0.7μmolL−1h−1) due to coupled ammonification and nitrification. The beginning of anoxic periods (24h following oxic to anoxic switches) showed DIC production rates of 2.5–8.6μmolL−1h−1 and very fast NO3− consumption (5.6–6.3μmolL−1h−1) and NH4+ production (1.4–1.5μmolL−1h−1). The latter rates were positively correlated to NO3− concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO3− concentrations and DIC and NH4+ productions of less than 1.3 and 0.1μmolL−1h−1, respectively. Mn2+ and CH4 were produced at constant rates (respectively 0.3 and 0.015μmolL−1h−1) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24–36h following anoxic to oxic switches) showed rapid NH4+ and Mn2+ decreases to zero (1.6 and 0.8–2μmolL−1h−1, respectively) and very fast NO3− production (3μmolL−1h−1). This NO3− production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface.

In estuaries, the gas transfer velocity (k) is driven by a combination of two major physical drivers, wind and water current. The k values for CO2 in the macrotidal Gironde Estuary were obtained from 159 simultaneous pCO2 and floating chamber flux measurements. Values of k increased with wind speed and were significantly greater when water currents and wind were in opposing directions. At low wind speeds (<1m s−1), k increased with water current velocities (0–1.5m s−1) following an exponential trend. The latter was a good proxy for the Y-intercept in a generic equation for k versus wind speed in estuaries. We also found that, in this turbid estuary, k was significantly lower at high turbidity. The presence of suspended material in great concentrations (TSS>0.2g L−1) had a significant role in attenuating turbulence and therefore gas exchange. This result has important consequences for modeling water oxygenation in estuarine turbidity maxima. For seven low turbidity estuaries previously described in the literature, the slope of the linear regression between k and wind speed correlates very well with the estuary surface area due to a fetch effect. In the Gironde Estuary, this slope follows the same trend at low turbidity (TSS<0.2g L−1), but is on average significantly lower than in other large estuaries and decreases linearly with the TSS concentration. A new generic equation for estuaries is proposed that gives k as a function of water current velocity, wind speed, estuarine surface area and TSS concentration.

The hypothesis of nutrient-rich pore-waters seeping at low tide through sediments to channel waters, which drain tidal flats during ebb, was evaluated in the Arcachon lagoon. The back of the bay is affected by freshwater inputs and underground freshwater discharges. The upper part of tidal flat consists of permeable sandy sediments, which are covered by a muddy sediment layer on the lower part. Permeable sediments outcrop in the bed of channel web. Surface water chemistry and early diagenesis processes in sediment were estimated by collecting channel web waters and cores on a tidal flat and in channels at different seasons and time scales. Waters from tidal creeks are under-oxygenated, and enriched in reduced solutes. Muddy sediments showed evidences of strong organic matter mineralization and bioturbation. Underlying permeable sandy sediments revealed as well evidences of an enrichment of inorganic nutrients, and dilution with fresh continental groundwater. During ebb, tidal creek waters stem from mudflats by seeping of anoxic pore-waters, and from permeable sediments by advection of reduced waters. A rough estimation shows that the yearly contribution of this tidal pump of pore-waters for dissolved inorganic phosphorus (DIP) and ammonia inputs is of the same order of magnitude than river inputs for the studied part of the bay. Extrapolated to the whole Arcachon lagoon, pore-water discharge at low tide supplies to water column at least 556 kmol yr(-1) and 18300 kmol yr(-1) of DIP and NH(4)(+), respectively. Tidal drainage at low tide represents therefore a minimal contribution of recycled nutrient of 55% for DIP and 15% for dissolved inorganic nitrogen to the lagoon. (c) 2008 Elsevier B.V. All rights reserved.

Suspended particles and surface waters were collected in the Gironde estuary (southwestern France) along the salinity gradient. Dissolved inorganic phosphorus (DIP) was analyzed in the filtered surface waters. The suspended particles were sequentially leached to determine five fractions of phosphorus: exchangeable or loosely sorbed P, reactive Fe‐bound P, refractory Fe oxide‐bound P, P associated with apatite and carbonates, and organic P. Experiments were conducted to determine the effects of temperature, salinity, and DIP concentration on phosphorus sorption/desorption processes. The concentration and distribution of particulate phosphorus was homogeneous along the salinity gradient of the estuary, whereas it was variable in time and space in the freshwater part. DIP showed a rapid desorption of loosely sorbed P at low salinities. Desorption depended on suspended particle concentration, but not on DIP concentration. At high suspended particle concentrations, some exchangeable P remained linked to particles, until suspended particle concentration decreased downstream of the estuary. This delayed desorption of bioavailable P occurs in waters in which the penetration of light is sufficient to support photosynthesis and probably plays a major role in primary production at high salinities. Mineralization of organic phosphorus also releases available phosphorus, but this process is slow relative to the water residence time in the estuary. The budget of DIP, the loss of particulate P from the organic fraction, and the water residence time suggest that the dynamics of P in the Gironde estuary is well explained by the two processes of P release from particles to waters without P uptake.

Methane and suspended particulate matter (SPM) concentrations, monitored bimonthly during one hydrological year (2003–2004) along 70 km transects in the tidal regions of the Garonne and Dordogne rivers (SW France), showed a significant negative correlation, both spatially and temporally. During spring in clear waters (SPM < 50 mg L−1), methane production was first evidenced by a net increase in methane concentrations, in parallel with temperature and a decrease in river flow. In summer, as soon as the estuarine turbidity maximum (ETM) appeared and SPM concentrations exceeded 100 mg L−1, methane concentrations decreased from ~600 to ~30 nmol L−1 in one month. More downstream in the turbid Gironde estuary, methane concentrations were occasionally below atmospheric equilibrium. In dark microcosms, high methane consumption was observed in samples from the ETM with SPM concentrations >2,000 mg L−1, but not after removing the SPM by settling (SPM = 16 mg L−1), nor in a sample collected few kilometers upstream, with SPM = 3 mg L−1. Methane oxidation was also able to draw down methane concentrations below half the atmospheric equilibrium value in an ETM sample. Suspended clays in the ETM enhance methane oxidation and strongly reduce methane fluxes to the atmosphere.

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.

In the Bay of Revellata (Corsica, Mediterranean Sea), we investigate since late-2006 metabolic rates (gross pri-mary production (GPP) and community respiration (CR)) related to various communities (Posidonia oceanica seagrass meadow, Posidonia oceanica litter, epilithic macro-algae) using optodes on benthic chambers over Posidonia oceanica seagrass meadow, Posidonia oceanica litter, epilithic macro-algae. Over the Posidonia oceanica seagrass meadow, these incubations allow to analyse changing rates of nighttime CR, and to evaluate the difference be-tween daytime and nighttime CR. Over the Posidonia oceanica litter, these incubations reveal surprisingly highly variables GPP and CR values. Finally, these incubations also allow deriving GPP and CR values from epilithic macro-algae, the second most important benthic compartment of in the Bay of Calvi