Depositional fluxes and sources of particulate carbon and nitrogen in natural lakes and a young boreal reservoir in Northern Que´bec

We investigated the depositional trends of total particles, carbon and nitrogen in a newly created, 600-km2 hydroelectric reservoir in Northern Québec, and compared the results with those observed in lakes of the surrounding region. We show that particulate fluxes exhibit a large degree of spatial heterogeneity in both the reservoir (68–548 mg POC m−2 d−1 and 5–33 mg PN m−2 d−1) and the natural lakes (30–150 mg POC m−2 d−1 and 3–12 mg PN m−2 d−1) and that on average, settling fluxes of the reservoir (211 ± 46 mg POC m−2 d−1 and 14 ± 3 mg PN m−2 d−1) exceeded lake deposition (79 ± 13 mg POC m−2 d−1 and 7 ± 1 mg PN m−2 d−1) by approximately two-fold. Our results also show that the nature of the organic matter reaching the sediments was significantly different between lakes and the reservoir, which can have consequences for benthic metabolism and the long-term storage. We found that sinking fluxes in the reservoir were mostly regulated by local morphological and hydrological conditions, with higher fluxes along or in the vicinity of the old riverbed (average 400 ± 73 mg POC m−2 d−1 and 24 ± 5 mg PN m−2 d−1) and lower fluxes in calmer zones such as side bays (average 106 ± 10 mg POC m−2 d−1 and 8 ± 1 mg PN m−2 d−1). In lakes, where settling fluxes were not linked to the trophy, or dissolved organic carbon, the actual nature of the sedimenting organic material was influenced by lake morphometry and the relative contribution of algal versus terrestrial sources. We conclude that re-suspension and erosion play a major role in shaping the reservoir sinking fluxes which explain both, the higher reservoir deposition and also some of the qualitative differences between the two systems. Despite all these differences, sinking particulate organic carbon fluxes were small and surprisingly similar relative to the surface carbon dioxide emissions in both the reservoir and lakes, representing approximately 16–17 % of the carbon efflux estimated for these same systems in 2008.status: publishe

Dynamics of greenhouse gases (CO2, CH4, N2O) along the Zambezi River and major tributaries, and their importance in the riverine carbon budget

© Author(s) 2015. Spanning over 3000 km in length and with a catchment of approximately 1.4 million km2, the Zambezi River is the fourth largest river in Africa and the largest flowing into the Indian Ocean from the African continent. We present data on greenhouse gas (GHG: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) concentrations and fluxes, as well as data that allow for characterization of sources and dynamics of carbon pools collected along the Zambezi River, reservoirs and several of its tributaries during 2012 and 2013 and over two climatic seasons (dry and wet) to constrain the interannual variability, seasonality and spatial heterogeneity along the aquatic continuum. All GHG concentrations showed high spatial variability (coefficient of variation: 1.01 for CO2, 2.65 for CH4 and 0.21 for N2O). Overall, there was no unidirectional pattern along the river stretch (i.e., decrease or increase towards the ocean), as the spatial heterogeneity of GHGs appeared to be determined mainly by the connectivity with floodplains and wetlands as well as the presence of man-made structures (reservoirs) and natural barriers (waterfalls, rapids). Highest CO2 and CH4 concentrations in the main channel were found downstream of extensive floodplains/wetlands. Undersaturated CO2 conditions, in contrast, were characteristic of the surface waters of the two large reservoirs along the Zambezi mainstem. N2O concentrations showed the opposite pattern, being lowest downstream of the floodplains and highest in reservoirs. Among tributaries, highest concentrations of both CO2 and CH4 were measured in the Shire River, whereas low values were characteristic of more turbid systems such as the Luangwa and Mazoe rivers. The interannual variability in the Zambezi River was relatively large for both CO2 and CH4, and significantly higher concentrations (up to 2-fold) were measured during wet seasons compared to the dry season. Interannual variability of N2O was less pronounced, but higher values were generally found during the dry season. Overall, both concentrations and fluxes of CO2 and CH4 were well below the median/average values for tropical rivers, streams and reservoirs reported previously in the literature and used for global extrapolations. A first-order mass balance suggests that carbon (C) transport to the ocean represents the major component (59%) of the budget (largely in the form of dissolved inorganic carbon, DIC), while 38% of the total C yield is annually emitted into the atmosphere, mostly as CO2 (98%), and 3% is removed by sedimentation in reservoirs.status: publishe

The age of river-transported carbon: a global perspective

©2015. American Geophysical Union. All Rights Reserved. The role played by river networks in regional and global carbon (C) budgets is receiving increasing attention. Despite the potential of radiocarbon measurements (Δ 14 C) to elucidate sources and cycling of different riverine C pools, there remain large regions for which no data are available and no comprehensive attempts to synthesize the available information and examine global patterns in the 14 C content of different riverine C pools. Here we present new 14 C data on particulate and dissolved organic C (POC and DOC) from six river basins in tropical and subtropical Africa and compiled >1400 literature Δ 14 C data and ancillary parameters from rivers globally. Our analysis reveals a consistent pattern whereby POC is progressively older in systems carrying higher sediment loads, coinciding with a lower organic carbon content. At the global scale, this pattern leads to a proposed global median Δ 14 C signature of -203‰, corresponding to an age of ∼1800 years B.P. For DOC exported to the coastal zone, we predict a modern (decadal) age (Δ 14 C=+22 to +46‰), and paired data sets confirm that riverine DOC is generally more recent in origin than POC - in contrast to the situation in ocean environments. Weathering regimes complicate the interpretation of 14 C ages of dissolved inorganic carbon, but the available data favor the hypothesis that in most cases, more recent organic C is preferentially mineralized.status: publishe

Contrasting biogeochemical characteristics of the Oubangui River and tributaries (Congo River basin)

The Oubangui is a major tributary of the Congo River. We describe the biogeochemistry of contrasting tributaries within its central catchment, with watershed vegetation ranging from wooded savannahs to humid rainforest. Compared to a 2-year monitoring record on the mainstem Oubangui, these tributaries show a wide range of biogeochemical signatures, from highly diluted blackwaters (low turbidity, pH, conductivity, and total alkalinity) in rainforests to those more typical for savannah systems. Spectral analyses of chromophoric dissolved organic matter showed wide temporal variations in the Oubangui compared to spatio-temporal variations in the tributaries, and confirm that different pools of dissolved organic carbon are mobilized during different hydrological stages. δ(13)C of dissolved inorganic carbon ranged between -28.1‰ and -5.8‰, and was strongly correlated to both partial pressure of CO2 and to the estimated contribution of carbonate weathering to total alkalinity, suggesting an important control of the weathering regime on CO2 fluxes. All tributaries were oversaturated in dissolved greenhouse gases (CH4, N2O, CO2), with highest levels in rivers draining rainforest. The high diversity observed underscores the importance of sampling that covers the variability in subcatchment characteristics, to improve our understanding of biogeochemical cycling in the Congo Basin.status: publishe

Technical Note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters

© 2015 Author(s). Inland waters have been recognized as a significant source of carbon dioxide (CO 2 ) to the atmosphere at the global scale. Fluxes of CO 2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water-air gradient of the partial pressure of CO 2 (pCO 2 ). Currently, direct measurements of water pCO 2 remain scarce in freshwaters, and most published pCO 2 data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibrator and headspace) water pCO 2 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 pCO 2 (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 μmol L -1 ). Calculated pCO 2 were >10% higher than measured pCO 2 in 60% of the samples (with a median overestimation of calculated pCO 2 compared to measured pCO 2 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 pCO 2 with respect to measured pCO 2 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 pCO 2 to TA in acidic and organic-rich waters. No empirical relationship could be derived from our data set in order to correct calculated pCO 2 for this bias. Owing to the widespread distribution of acidic, organic-rich freshwaters, we conclude that regional and global estimates of CO 2 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 pCO 2 are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters.status: publishe

Globally significant greenhouse gas emissions from African inland waters

© 2015 Macmillan Publishers Limited. Carbon dioxide emissions to the atmosphere from inland waters - streams, rivers, lakes and reservoirs - are nearly equivalent to ocean and land sinks globally. Inland waters can be an important source of methane and nitrous oxide emissions as well, but emissions are poorly quantified, especially in Africa. Here we report dissolved carbon dioxide, methane and nitrous oxide concentrations from 12 rivers in sub-Saharan Africa, including seasonally resolved sampling at 39 sites, acquired between 2006 and 2014. Fluxes were calculated from published gas transfer velocities, and upscaled to the area of all sub-Saharan African rivers using available spatial data sets. Carbon dioxide-equivalent emissions from river channels alone were about 0.4 Pg carbon per year, equivalent to two-thirds of the overall net carbon land sink previously reported for Africa. Including emissions from wetlands of the Congo river increases the total carbon dioxide-equivalent greenhouse-gas emissions to about 0.9 Pg carbon per year, equivalent to about one quarter of the global ocean and terrestrial combined carbon sink. Riverine carbon dioxide and methane emissions increase with wetland extent and upland biomass. We therefore suggest that future changes in wetland and upland cover could strongly affect greenhouse-gas emissions from African inland waters.status: publishe