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Natural lakes are thought to be globally important sources of greenhouse gases (CO 2 , CH 4 , and N 2 O) to the atmosphere although nearly no data have been previously reported from Africa. We collected CO 2 , CH 4 , and N 2 O data in 24 African lakes that accounted for 49% of total lacustrine surface area of the African continent and covered a wide range of morphology and productivity. The surface water concentrations of dissolved CO 2 were much lower than values attributed in current literature to tropical lakes and lower than in boreal systems because of a higher productivity. In contrast, surface water–dissolved CH 4 concentrations were generally higher than in boreal systems. The lowest CO 2 and the highest CH 4 concentrations were observed in the more shallow and productive lakes. Emissions of CO 2 may likely have been substantially overestimated by a factor between 9 and 18 in African lakes and between 6 and 26 in pan-tropical lakes.

Lake Edward is one of the African Rift Valley lakes draining into the Nile River basin. We conducted three sampling series in Lake Edward in October-November 2016, March-April 2017 and January 2018, in distinct seasonal conditions and in several sites varying by depth and proximity to river outlets, including the Kazinga Channel, which connects the hypertrophic Lake George to Lake Edward. The phytoplankton was examined using microscopy and marker pigment analysis by high performance liquid chromatography (HPLC) and subsequent CHEMTAX processing for estimating abundance of phytoplankton groups. Chlorophyll a concentration in the pelagic and littoral open lake sites barely exceeded 10 µg L−1 whereas, in contrast, in the semi-enclosed Bay of Katwe influenced by the Kazinga Channel chlorophyll a was up to 100 µg L−1. Despite substantial seasonal variations of limnological conditions such as photic and mixed layer depths, cyanoprokaryotes/cyanobacteria represented on average 60% of the phytoplankton biomass, followed by diatoms, which contributed ~25% of chlorophyll a, and by green algae, chrysophytes and cryptophytes. 248 taxa were identified with clear prevalence of cyanobacteria (104 taxa), from the morphological groups of coccal and filamentous species (non-heterocytous and heterocytous). The high proportion of heterocytous cyanobacteria, along with a relatively high particulate organic carbon to nitrogen (C:N) ratio, suggest N limitation as well as light limitation, most pronounced in the pelagic sites. During the rainy season, the most abundant diatoms in the plankton were needle-like Nitzschia. Comparison with previous studies found differences in water transparency, total phosphorus, and phytoplankton composition.

We report a data set of dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition (stable carbon isotope signatures, absorption and fluorescence properties) obtained from samples collected in Lake Victoria, a large lake in East Africa. Samples were collected in 2018–2019 along a bathymetric gradient (bays to open waters), during three contrasting seasons: long rainy, short rainy and dry, which corresponded to distinctly water column mixing regimes, respectively, stratified, semi-stratified and mixed regimes. Eight DOM components from parallel factor analysis (PARAFAC) were identified based on three-dimensional excitation–emission matrices (EEMs), which were aggregated into three main groups of components (microbial humic-like, terrestrial humic-like, protein-like). Spatially, the more productive bays were characterized by higher DOM concentration than deeper more offshore waters (fluorescence intensity and DOC were ~ 80% and ~ 30% higher in bays, respectively). Seasonally, the DOM pool shifted from protein-like components during the mixed regime to microbial humic-like components during the semi-stratified regime and to terrestrial humic-like components during the stratified regime. This indicates that pulses of autochthonous DOM derived from phytoplankton occurred when the lake was mixing, which increased the availability of dissolved inorganic nutrients. Subsequently, this freshly produced autochthonous DOM was microbially processed during the following semi-stratified regime. In the open waters, during the stratified regime, only terrestrial refractory DOM components remained because the labile and fresh stock of DOM created during the preceding mixed season was consumed. In the bays, the high terrestrial refractory DOM during the stratified regime may be additionally due to the allochthonous DOM input from the runoff. At the scale of the whole lake, the background refractory DOM probably comes mainly from precipitation and followed by river inputs.

We characterized the spatio-temporal dynamics of carbon (C) in rivers of the tropical Nyong catchment (South Cameroon). In 2016, we measured fortnightly at 6 locations along an upstream-downstream gradient from groundwater to the main stream of order 6, total alkalinity, dissolved inorganic C (DIC) used together with pH to compute pCO2, dissolved and particulate organic C (DOC and POC) and total suspended matter. Forest, groundwater had low DOC content (< 1 mg L−1) as its leaching was probably prevented in the overlaying lateritic soils. Forest groundwater was supersaturated in CO2 (~50 times the atmospheric value) because of the solubilisation of the CO2 originating from soil respiration. Wetlands water exhibited higher DOC (> 14 mg L−1) and similar DIC concentrations than the forest groundwater. Surface runoff was considered negligible in the basin due to low slopes and high infiltration capacity of the soils, making wetlands and forest groundwater the two main sources of C for surface waters. The influence of wetlands on C dynamics in rivers was significant during periods of high waters when the hydrological connectivity between surface waters and wetlands was enhanced. On annual scale, wetlands exported 60 % (15.4 ± 7.2 t C km−2 yr−1) of the total amount of C transferred laterally to surface waters, the remaining 40 % (12.1 ± 5.8 t C km−2 yr−1) being transferred from forest groundwater. Heterotrophic respiration in rivers averaged 89 mmol m−2 d−1 whereas CO2 degassing was 1260 mmol m−2 d−1, which shows that it is unlikely that the river heterotrophic respiration was the main process sustaining CO2 emission. The comparison of the hydrological export of terrestrial C via forest groundwater with the net terrestrial C sink in the Nyong watershed shows that only ~4 % of the net terrestrial C sink reach the aquatic ecosystem. The carbon mass balance of the Nyong watershed highlights that attributing to a unique terrestrial source the whole amount of riverine carbon emitted to the atmosphere and exported to the ocean and ignoring the river–wetland connectivity can lead to the misrepresentation of C dynamics in tropical watersheds.

International audience; High-energy tidal beaches are exposed to strong physical forcings. The submarine groundwater discharge (SGD) that occurs in intertidal sandy sediments includes both terrestrial, fresh groundwater flow and seawater recirculation, and plays a significant role in regulating biogeochemical cycles in some coastal zones. In this transition zone between land and sea, complex biogeochemical reactions alter the chemical composition of pore waters that discharge to the coastal ocean. Recent studies highlight that SGD can be a significant source of carbon to the coastal ocean but very few have investigated SGD in high-energy environments. We have characterized the dissolved carbon dynamics in such a high-energy environment (Truc Vert Beach, SW France) through pore water sampling in key compartments of the SGD system. Dissolved organic carbon (DOC), pH, total alkalinity (TA), and the isotopic composition of dissolved inorganic carbon (d 13 C-DIC) were measured in pore waters sampled at regular intervals between 2011 and 2014 in the intertidal zone of the beach, the mixing zone of the subterranean estuary (STE), and the freshwater aquifer upstream from the beach. Results reveal that SGD exports dissolved carbon mostly as DIC to the Aquitaine coast some of which originates from the aerobic respiration of marine organic matter within the beach aquifer. This is highlighted by the opposite spatial trend of DOC, which is consumed, and DIC, which is produced. Saline pore waters expelled from the beach through tidally-driven recirculation of seawater provide about 4400 tons of carbon per year to the coastal zone of the 240 km-long Aquitaine sandy coast. Terrestrial groundwater, characterized by high pCO2 values, is also a significant contributor to the DIC flux to the coastal ocean (16200 tons per year). This flux is abated by CO2 evasion in the upper beach, at the onset of the salinity gradient in the STE, and within the surficial freshwater aquifer along the forest-beach transect below the coastal foredune. Accordingly, the DIC:TA ratio evolves to below 1, suggesting that this SGD increases the buffer capacity of coastal seawater against acidification. This study demonstrates that high-energy beaches are active vectors of DIC from the land to the coastal ocean as well as significant sources of CO2 to the atmosphere, and must therefore be taken into consideration in SGD carbon budgets.

Sandy beaches are places of active organic matter mineralization due to water renewal providing organic matter and electron acceptors in the porous and permeable sands. Recycled biogenic compounds are efficiently transferred to the coastal marine environment via wave and tidal-driven advective flows. The biogeochemical processes in beach aquifers were mainly studied in semi enclosed systems with low tidal amplitude, and with a connection to continental aquifers contributing to solute fluxes to the coast from terrestrial groundwater. We present here the study of a pocket beach isolated from terrestrial aquifers with a high tidal amplitude and a medium energy wave regime. In situ measurements, cross-shore profiles and vertical sampling were conducted during several tidal cycles in spring and autumn. Cross-shore transects, obtained at low tide from holes that represent a mixture of the upper 20 cm of the water saturated zone, showed concentration gradients of redox and recycled compounds. Increase in pCO2, dissolved phosphate and ammonium concentrations downslope revealed that more products from organic matter mineralization accumulated in the lower beach. The related increase in total alkalinity downslope indicated that the part of anaerobic processes in organic matter oxidation was higher in the lower beach. Concentration and δ13C of dissolved inorganic carbon in pore waters suggested that the carbon mineralized in pore waters came from marine plant debris that were mixed with the sand. Continuous probe records of dissolved oxygen saturation and vertical profiles revealed a tidally-driven dynamics of pore water in the first centimetres of the lower beach aquifer. Ventilation of pore waters corresponded to wave pumping and swash-induced infiltration of seawater in the upper 10–20 cm of sediment. Nutrients and reduced compounds produced through organic matter mineralization remained stored in pore water below the layer disturbed by wave. The flux of these components to seawater is possible when this interface is eroded, for example when wave energy increases after a less energetic period. The low extension of the studied aquifer, typical of pocket beaches, limits the connection with continental groundwater. Both tidally-driven and wave-driven recirculation of seawater allows pocket beaches to be efficient bioreactors for marine organic matter mineralization. As such, they provide the coastal environment with recycled nutrients, and not new nutrients.

Dissolved inorganic carbon (DIC), its stable isotope composition (δ13C-DIC) and ancillary parameters of the water column were investigated in a eutrophic tropical marine-dominated estuary surrounded by a large urban area (Guanabara Bay, Rio de Janeiro, Brazil). Most negative δ13C-DIC signatures (down to −6.1‰) were found in polluted regions affected by direct sewage discharges where net heterotrophy induces high partial pressure of CO2 (pCO2) and DIC concentrations. Keeling plot was applied to this polluted region and determined the δ13C-DIC sewage signature source of −12.2‰, which is very consistent with isotopic signature found in wastewater treatment plans. These negative δ13C-DIC signatures (i.e., DIC depleted in 13C) were restricted to the vicinity of urban outlets, whereas in the largest area of the bay δ13C-DIC signatures were more positive (i.e., DIC enriched in 13C). The most positive δ13C-DIC signatures (up to 4.6‰) were found in surface waters dominated by large phytoplankton blooms, with positive correlation with chlorophyll a (Chl a). In the largest area of the bay, the preferential uptake of the lighter stable carbon isotope (12C) during photosynthesis followed the Rayleigh distillation, and appeared as the most important driver of δ13C-DIC variations. This reveals an important isotopic fractionation (e) by phytoplankton due to successive algal blooms that has turned the remaining DIC pool enriched with the heavier stable carbon isotope (13C). The calculated diel apparent e showed higher values in the morning (18.7‰–21.6‰) and decreasing in the afternoon (6.8‰–11.1‰). e was positively correlated to the pCO2 (R2 = 0.88, p = 0.005) and DIC concentrations (R2 = 0.73, p = 0.02), suggesting a decline in carbon assimilation efficiency and decreasing uptake of the lighter carbon under CO2 limiting conditions. The eutrophic coastal waters of Guanabara Bay have δ13C-DIC signatures well above that found in estuaries, shelf and ocean waters worldwide.

During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O-2). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO3-; on average 1140 +/- 485 mu mol L-1) and low methane (CH4; 40 +/- 25 nmol L-1) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO3- (25 +/- 40 mu mol L-1) and higher CH4 (1770 +/- 1830 nmol L-1) concentrations. The partial pressure of carbon dioxide (pCO(2); 30,650 +/- 11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 +/- 26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO2 to the water table. In contrast, pCO(2) was not significantly different in crop (4480 +/- 2680 ppmv) and forest (4900 +/- 4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO3- concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH4+) and CH4 in stream waters and concomitant decreased in NO3- concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH4+ and CH4. In forest streams, dissolved iron (Fe2+), NH4+ and CH4 were negatively correlated with O-2 reflecting the gradual oxygenation of stream water and associated oxidations of Fe2+, NH4+ and CH4. The results overall showed that forest groundwater behaved as source of CO2 and CH4 to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH4 production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO2 to streams (but less so than forest groundwater). Conversely, in streams, pCO(2) was not significantly affected by land use while CH4 production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics.

International audience; We collected samples from sea water, runnel water, beach pore waters, water from the unconfined surficial aquifer discharging at the beach face, groundwater, and rainwater from the Aquitanian coast in order to determine the flux of dissolved inorganic nitrogen (DIN), phosphorus and silica from terrestrial submarine groundwater discharge (SGD). The flux of fresh groundwater was obtained from a water balance calculation based on precipitation and evapotranspiration and assessment of the coastal watershed from hydrograph separation. Waters with intermediate salinities between sea water and freshwaters are found all along the 240-km-long coast, indicating that SGD is ubiquitous. The estimated fresh water flux is 2.25m3d−1m−1 longshore. Terrestrial SGD provides a DIN flux of 9·106mol each year to the adjacent coastal zone. This flux is about four times lower than the release of DIN due to tidally driven saline SGD. The freshwater DIN flux is low because the upland land use consists almost exclusively of pine forest. Dissolved organic nitrogen represents more than 60% of the total dissolved nitrogen flux. Dissolved iron, phosphorus and silica have much higher concentrations in the anoxic forest aquifer than in the fresh-water end-member of the subterranean estuary sampled in the upper beach aquifer. This suggests that the salinity gradient of the estuary does not correspond to a redox gradient. The redox front between anoxic groundwater and fresh oxic waters occurs below the soil-depleted foredune/yellow dune. Anoxic P- and Si-rich waters seep directly on the beach face only in the north Gironde, where the foredunes are eroded. This study reveals the role of the sandy foredune aquifer in biogeochemical fluxes from SGD, which is to dilute and oxidize waters from the unconfined surficial upland aquifer.

Streams and rivers emit significant amounts of CO2 and constitute a preferential pathway of carbon transport from terrestrial ecosystems to the atmosphere. However, the estimation of CO2 degassing based on the water-air CO2 gradient, gas transfer velocity and stream surface area is subject to large uncertainties. Furthermore, the stable isotope signature of dissolved inorganic carbon (delta C-13-DIC) in streams is strongly impacted by gas exchange, which makes it a useful tracer of CO2 degassing under specific conditions. For this study, we characterized the annual transfers of dissolved inorganic carbon (DIC) along the groundwater-stream-river continuum based on DIC concentrations, stable isotope composition and measurements of stream discharges. We selected a homogeneous, forested and sandy lowland watershed as a study site, where the hydrology occurs almost exclusively through drainage of shallow groundwater (no surface runoff). We observed the first general spatial pattern of decreases in pCO(2) and DIC and an increase in delta C-13-DIC from groundwater to stream orders 1 and 2, which was due to the experimentally verified faster degassing of groundwater C-12-DIC compared to C-13-DIC. This downstream enrichment in C-13-DIC could be modelled by simply considering the isotopic equilibration of groundwater-derived DIC with the atmosphere during CO2 degassing. A second spatial pattern occurred between stream orders 2 and 4, consisting of an increase in the proportion of carbonate alkalinity to the DIC accompanied by the enrichment of C-13 in the stream DIC, which was due to the occurrence of carbonate rock weathering downstream. We could separate the contribution of these two processes (gas exchange and carbonate weathering) in the stable isotope budget of the river network. Thereafter, we built a hydrological mass balance based on drainages and the relative contribution of groundwater in streams of increasing order. After combining with the dissolved CO2 concentrations, we quantified CO2 degassing for each stream order for the whole watershed. Approximately 75% of the total CO2 degassing from the watershed occurred in first- and second-order streams. Furthermore, from stream order 2-4, our CO2 degassing fluxes compared well with those based on stream hydraulic geometry, water pCO(2), gas transfer velocity, and stream surface area. In first-order streams, however, our approach showed CO2 fluxes that were twice as large, suggesting that a fraction of degassing occurred as hotspots in the vicinity of groundwater resurgence and was missed by conventional stream sampling.

Inland waters have been recognized as a significant source of carbon dioxide (CO2) to the atmosphere at the global scale. Fluxes of CO2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water–air gradient of the partial pressure of CO2 (pCO2). Currently, direct measurements of water pCO2 remain scarce in freshwaters, and most published pCO2 data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibrator and headspace) water pCO2 in a large array of temperate and tropical freshwaters. The 761 data points cover a wide range of values for TA (0 to 14 200 μmol L−1), pH (3.94 to 9.17), measured pCO2 (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 μmol L−1). Calculated pCO2 were >10% higher than measured pCO2 in 60% of the samples (with a median overestimation of calculated pCO2 compared to measured pCO2 of 2560 ppmv) and were >100% higher in the 25% most organic-rich and acidic samples (with a median overestimation of 9080 ppmv). We suggest these large overestimations of calculated pCO2 with respect to measured pCO2 are due to the combination of two cumulative effects: (1) a more significant contribution of organic acids anions to TA in waters with low carbonate alkalinity and high DOC concentrations; (2) a lower buffering capacity of the carbonate system at low pH, which increases the sensitivity of calculated pCO2 to TA in acidic and organic-rich waters. No empirical relationship could be derived from our data set in order to correct calculated pCO2 for this bias. Owing to the widespread distribution of acidic, organic-rich freshwaters, we conclude that regional and global estimates of CO2 outgassing from freshwaters based on pH and TA data only are most likely overestimated, although the magnitude of the overestimation needs further quantitative analysis. Direct measurements of pCO2 are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters.

Export of soil carbon to superficial water through the drainage of groundwater is a significant but poorly documented component of the continental carbon budget. We monitored the concentrations of dissolved organic and inorganic carbon (DOC and DIC) in groundwaters and first order streams of a small temperate, forested and sandy watershed where hydrology occurs exclusively through drainage (no surface runoff). The studied watershed was also implemented for continuous measurements of groundwater table, precipitation, evapotranspiration, river discharge, and net ecosystem exchanges of sensible and latent heat fluxes as well as CO2. On a monthly basis, we found a good consistency between precipitation and the sum of evapotranspiration, drainage and groundwater storage. DOC and DIC temporary storage in groundwater and export to streams varied drastically during the hydrological cycle, the residence times of these two carbon forms varying from one month to several years. DOC concentrations in groundwater and streams were maximal at high water table and high stream discharge, when the water table reached the superficial organic rich layer of the soil. A large fraction of this winter DOC maximum was temporarily stored and further mineralized to DIC in the groundwater and only about 15 % was exported to streams during winter periods. In contrast, DIC, which was present in majority in the form of dissolved CO2 in groundwater and streams, was apparently diluted at high water table: DIC concentrations were maximum at low water table and low discharge in late summer and maximum pCO2 in groundwater corresponded to the late summer period of heterotrophic conditions (i.e., Reco > GPP). Groundwater DIC peaked in late summer and was followed by a rapid loss of excess CO2 from stream surface to the atmosphere. Overall, mean carbon export was 7.5 g C m−2 yr−1 (50 % as DOC and 50 % as DIC) and represented only 1.5 % of the NEE. About 65 % of the DIC exported from groundwaters returned to the atmosphere in the form of CO2 in first order streams.

International audience; The importance of P-flux on aquatic ecosystems bears out the study of P dynamics in river catchments. Aqueous P geochemistry depends on biogeochemical processes and Fe geochemistry. Our study focused on P and Fe behaviour at the groundwater-river water boundary. We monitored the distribution of dissolved and particulate P and Fe in the Leyre river from order 0 streams to order 4 main-river. We also analysed porewaters from river bed sediments and from wells located in the two main land-use types (pine forest and corn field). We determined the particulate speciation of P using chemical extractions. The particulate-dissolved P exchange capacity was measured using in-vitro experiments and 32 P radioactivity. Results show that the studied catchment groundwaters supplied dissolved Fe and P with a high Fe/P ratio. Dissolved Fe was subjected to oxidative precipitation at the groundwater/river water interface. This oxidation was enhanced by aquatic plants. P was trapped on Fe(III) particles so much so that dissolved P remained at concentrations >0.2 µM. This means that the coastal river outlet receives particulate P, which can be solubilized after anoxic diagenetic processes.