Climate Change and...
- Climate Variability
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Effects of Climate Change
A. Baum, T. Rixen, J. Samiaji (2007). Relevance of peat draining rivers in central Sumatra for the riverine input of dissolved organic carbon into the ocean. Estuarine, Coastal and Shelf Science 73 (3-4): 563-570
ABSTRACT: Sources and discharges of dissolved organic carbon (DOC) from the central Sumatran river Siak were studied. DOC concentrations in the Siak ranged between 560 and 2594μmol l−1 and peak out after its confluence with the river Mandau. The Mandau drains part of the central Sumatran peatlands and can be characterized as a typical blackwater river due to its high DOC concentration, its dark brown-coloured, acidic water (pH 4.4–4.7) and its low concentration of total suspended matter (12–41 mg l−1 ). The Mandau supplies about half of the DOC that enters the Siak Estuary where it mixes conservatively with ocean water. The DOC input from the Siak into the ocean was estimated to be 0.3 Tg C yr−1 . Extrapolated to entire Indonesia the data suggest a total Indonesian DOC export of 21 Tg yr−1 representing 10% of the global riverine DOC input into the ocean.
T. S. Bianchi, J. J. Galler, M. A. Allison (2007). Hydrodynamic sorting and transport of terrestrially derived organic carbon in sediments of the Mississippi and Atchafalaya Rivers. Estuarine, Coastal and Shelf Science 73 (1-2): 211-222
ABSTRACT: Over the course of two years, four cruises were conducted at varying levels of discharge in the lower Mississippi and Atchafalaya Rivers (MR and AR) where grab samples were collected from sand- and mud-dominated sediments. The tetramethylammonium hydroxide (TMAH) thermochemolysis method was used to determine sources of terrestrially derived organic carbon (OC) in these two sediment types, to examine the effects of hydrodynamic sorting on lignin sources in river sediments.
Average lignin concentrations in the lower MR were 1.4 ± 1.1 mg gOC−1 at English Turn (ET) and 10.4 ± 27.4 mg gOC−1 at Venice. Using these concentrations, annual lignin fluxes to the Gulf of Mexico, from tidal and estuarine mud remobilization at ET and Venice, were 3.1 ± 2.5 × 105 kg and 11.4 ± 30.0 × 105 kg, respectively. Much of the lignin-derived materials in muddy sediments appeared to be derived from non-woody grass-like sources – which should decay more quickly than the woody materials typically found in the sandy deposits. The average total OC% (1.93 ± 0.47) of English Turn sands yields an annual flux of 0.34 ± 0.09 × 109 kg. Lignin flux in the English Turn sands (3.6 ± 2.6 mg gC−1 ) using the numbers above would be 12.2 ± 9.4 × 105 kg. The extensive amounts of sand-sized woody materials (coffee-grinds) found in the sandy sediments in both the AR and MR are likely derived from woody plant materials. This is the first time it has been demonstrated that sandy sediments in the MR provide an equally important pathway (compared to muds) for the transport of terrestrially derived organic matter to the northern Gulf of Mexico. Using the AR average %OC in sand (1.16 ± 0.72), we estimated an annual flux of OC to the shelf of 0.13 ± 0.07 × 109 kg. Lignin flux for AR sands was estimated to be 12.4 ± 12.1 × 105 kg. Despite the high error associated with these numbers, we observe for the first time that the flux of lignin in sandy sediments in the AR to the northern Gulf of Mexico is comparable to that found in the MR. These results further support the likelihood of grain-size related hydrodynamic sorting of terrestrially derived organic carbon in the lower Mississippi and Atchafalaya Rivers, suggesting that there is a distinct sandy sediment organic fraction contributed by major rivers to the global carbon cycle.
ABSTRACT: Data were collected on POC, DOC and phytoplankton in the Zaire river, estuary and plume. Mean river value for POC was 1.1 mg·l−1 , 4.7% of the suspended matter. Average DOC content of the river water was 8.5 mg·l−1 . These values are in accordance with the calculations of TOC input from rivers to the world's ocean. Within the estuary POC and chlorophyll decreased regularly up to a salinity of 20. Between salinities of 20 and 32 small phytoplankton bloom occurred resulting also in higher POC values.
DOC mixed conservatively up to a salinity of 25; at salinities above 25, values indicate DOC production. This DOC production occurred partly in the bottom water of the canyon where low oxygen values indicated mineralization and conversion of the accumulated POC into DOC. Another area of DOC production observed inside and outside the surface waters of the plume, was partly related to autolysis and degradation of the phytoplankton bloom.
This study shows that the influence of rivers on the organic carbon in the ocean will not be confined to the amount introduced directly, but that we have to add the amounts of POC and DOC resulting from enhanced phytoplankton primary production by nutrient input from rivers and by river induced upwelling.
ABSTRACT: Stable carbon isotope ratios have been used to study the sources of particulate organic carbon (POC) in the Amazon River and its tributaries as well as to examine the transport of the riverine POC into the oceanic environment. POC in the upper reaches of the Amazon River has more positiveδ13C values (−24·5 to −28·0‰) than that in the middle and lower reaches (−27·9 to −30·1‰). Thed13 C content of POC from the tributaries is generally more negative than that observed in the Amazon main channel. Thisd13 C evidence shows that the POC in the Amazon main channel is predominantly of terrestrial origin rather than a result of in situ production. A large range ofd13 C values (−17·5 to −28·4‰) is observed in the Amazon estuary and plume and is attributed to the mixing of riverine and marine POC. POCd13 C measurements detect riverine organic detritus as far as 290 km offshore in a direction parallel to the river channel; the isotopic signal of riverine POC can be seen as far as 1100 km to the north-west of the river mouth in the Amazon River plume.
W. Cai, M. Dai, Y. Wang, W. Zhai, T. Huang, S. Chen, F. Zhang, Z. Chen, Z. Wang (2004). The biogeochemistry of inorganic carbon and nutrients in the Pearl River estuary and the adjacent Northern South China Sea. Continental Shelf Research 24 (12): 1301-1319
ABSTRACT: The Zhu-jiang (Pearl River) estuary and its adjacent continental shelf in the Northern South China Sea (SCS) is unique in that its drainage basin is located entirely in a subtropical zone with heavy population development, and therefore represents an important regime for biogeochemical studies on how large rivers influence continental shelves. The near-zero salinity end member has high nutrient concentrations (silicate 130–140μM, nitrate 75–100μM and phosphate 0.2–1.2μM) and relatively high total dissolved inorganic carbon (DIC) (1500 μM) and alkalinity (1650 μM) values. Water column DIC, alkalinity, and nutrient in the estuary are largely controlled by mixing of waters from different tributaries with different drainage basin chemistry, anthropogenic influence, and degree of estuarine recycling. Biological uptake of nutrients and inorganic carbon occur in the outer estuary and inner shelf areas supported by riverine nutrients. The N/P and Si/P ratios are generally very high within the estuary. The summertime area-integrated biological production rate of 0.8 gC m−2 d−1 is estimated based on the depletion of DIC and alkalinity relative to the conservative mixing line and a plume travel time. This estimate agrees reasonably well with 14C based primary production rates (PP) and with that from effective river phosphate flux. Biological production decreases about 10-fold in the open continental shelf and slope and is largely supported by mixing with subsurface water. A comparison of DIC, phosphate, and nitrate concentrations in the surface mixing layer and at the bottom of the euphotic zone with the 14C-based PP (0.13 gC m−2 d−1) suggests that the surface water residence time in the Northern SCS is 1.3 years. The N/P, Si/P, and Si/C ratios are 15, 25, and 0.15, respectively. The subtropical Pearl River study is also compared to other large rivers with regard to differences in both natural processes (i.e., weathering rates) and anthropogenic influences (i.e., nutrient input) between these different river–estuary systems.
ABSTRACT: Total evaporation couples water and energy balance equations, while transpiration, which is the major component of total evaporation over most of the global land surface, is strongly determined by the rate of carbon assimilation. A model combining the rate of carbon assimilation with water and energy balance equations has been developed, and run using satellite and ancillary data for 48-month period (January 1987 to December 1990) over global land surface. Relations between net carbon accumulation by terrestrial plant communities, evaporation, and net radiation are presented for 15 largest river basins of the world, and evaluated against those derived from field measurements.
J. J. Cole, Y. T. Prairie, N. F. Caraco, W. H. McDowell, L. J. Tranvik, R. G. Striegl, C. M. Duarte, P. Kortelainen, J. A. Downing, J. J. Middelburg, J. Melack (2007). Plumbing the global carbon cycle: integrating inland waters into the terrestrial carbon budget. Ecosystems 10 (1): 172-185
ABSTRACT: Because freshwater covers such a small fraction of the Earth’s surface area, inland freshwater ecosystems (particularly lakes, rivers, and reservoirs) have rarely been considered as potentially important quantitative components of the carbon cycle at either global or regional scales. By taking published estimates of gas exchange, sediment accumulation, and carbon transport for a variety of aquatic systems, we have constructed a budget for the role of inland water ecosystems in the global carbon cycle. Our analysis conservatively estimates that inland waters annually receive, from a combination of background and anthropogenically altered sources, on the order of 1.9 Pg C y−1 from the terrestrial landscape, of which about 0.2 is buried in aquatic sediments, at least 0.8 (possibly much more) is returned to the atmosphere as gas exchange while the remaining 0.9 Pg y−1 is delivered to the oceans, roughly equally as inorganic and organic carbon. Thus, roughly twice as much C enters inland aquatic systems from land as is exported from land to the sea. Over prolonged time net carbon fluxes in aquatic systems tend to be greater per unit area than in much of the surrounding land. Although their area is small, these freshwater aquatic systems can affect regional C balances. Further, the inclusion of inland, freshwater ecosystems provides useful insight about the storage, oxidation and transport of terrestrial C, and may warrant a revision of how the modern net C sink on land is described.
ABSTRACT: Total organic carbon (TOC) levels in the Columbia River measured monthly from May 1973 to December 1974 ranged from a maximum of 270μmol l−1 during late spring and early summer to a minimum of 150μmol l−1 during late autumn. Sampling locations were directly behind the spillway at the Bonneville Dam, 230 km upstream, and at Kalama, Washington, 128km upstream from the river mouth. The average annual TOC contribution from the Columbia River drainage to the north-eastern Pacific is 4·9×1010 mol with an average concentration of approximately 195μmol l−1 . Of this TOC annual export, 89% is dissolved organic carbon (DOC) and 11% is particulate organic carbon (POC). The TOC and DOC levels were most highly correlated with increased oxygen saturation and dischange, while POC correlated more closely to high instream primary productivy as indicated by higher pH and oxygen supersaturation.
Variability of DOC in the main channel of the Columbia River from Portland, Oregon, to the estuary during a June 1974 cruise was minimal. The DOC concentrations ranged from 221–260μmol l−1 with no significant upstream or downstream gradients. Diel variation also was slight, varying randomly during 24h between 235–257μmol l−1 . The relative annual constancy of the DOC is indicative of the refractory nature of a significant proportion of the dissolved organic load of the Columbia River.
Dornblaser, M. M., Striegl, R. G. (2009). Suspended sediment and carbonate transport in the Yukon River Basin, Alaska : fluxes and potential future responses to climate change. Water Resources Research 45 (6): W06411
ABSTRACT: Loads and yields of suspended sediment and carbonate were measured and modeled at three locations on the Yukon, Tanana, and Porcupine Rivers in Alaska during water years 2001–2005 (1 October 2000 to 30 September 2005). Annual export of suspended sediment and carbonate upstream from the Yukon Delta averaged 68 Mt a−1 and 387 Gg a−1 , respectively, with 50% of the suspended sediment load originating in the Tanana River Basin and 88% of the carbonate load originating in the White River Basin. About half the annual suspended sediment export occurred during spring, and half occurred during summer-autumn, with very little export in winter. On average, a minimum of 11 Mt a−1 of suspended sediment is deposited in floodplains between Eagle, Alaska, and Pilot Station, Alaska, on an annual basis, mostly in the Yukon Flats. There is about a 27% loss in the carbonate load between Eagle and Yukon River near Stevens Village, with an additional loss of about 29% between Stevens Village and Pilot Station, owing to a combination of deposition and dissolution. Comparison of current and historical suspended sediment loads for Tanana River suggests a possible link between suspended sediment yield and the Pacific Decadal Oscillation.
Evans, C. D., Freeman, C., Cork, L. G., Thomas, D. N., Reynolds, B., Billett, M. F., Garnett, M. H., Norris, D. (2007). Evidence against recent climate-induced destabilisation of soil carbon from14 C analysis of riverine dissolved organic matter. Geophysical Research Letters 34 (L07407): doi:10.1029/2007GL029431
ABSTRACT; The stability of global soil carbon (C) represents a major uncertainty in forecasting future climate change. In the UK, substantial soil C losses have been reported, while at the same time dissolved organic carbon (DOC) concentrations in upland waters have increased, suggesting that soil C stocks may be destabilising in response to climate change. To investigate the link between soil carbon and DOC at a range of sites, soil organic matter, soilwater and streamwater DOC were analysed for radiocarbon (14 C). DOC exported from C-rich landscapes appears younger than the soil C itself, much of it comprising C assimilated post-1950s. DOC from more intensively managed, C-poor soils is older, in some cases >100 years. Results appear consistent with soil C destabilisation in farmed landscapes, but not in peatlands. Reported C losses may to a significant extent be explained by mechanisms other than climate change, e.g. recovery from acidification in peatlands, and agricultural intensification in managed systems.
ABSTRACT: Carbon sequestration is a temporal process in which carbon is continuously being stored/released over a period of time. Different methods of carbon accounting can be used to account for this temporal nature including annual average carbon, annualized carbon, and ton-year carbon. In this paper, starting by exposing the underlying connections among these methods, we examine how the comparisons of sequestration projects are affected by these methods and the major factors affecting them. We explore the empirical implications on carbon sequestration policies by applying these accounting methods to the Upper Mississippi River Basin, a large and important agriculture area in the US. We found that the differences are significant in terms of the location of land that might be chosen and the distribution of carbon sequestration over the area, although the total amount of carbon sequestered does not differ considerably across programs that use different accounting methods or different values of the major factors.
H. Freitag, P. R. Ferguson, K. Dubois, E. Kofi Hayford, V. von Vordzogbe, J. Veizer (2008). Water and carbon fluxes from savanna ecosystems of the Volta River watershed, West Africa. Global and Planetary Change 61 (1-2): 3-14
ABSTRACT: The fluxes of water and carbon from terrestrial ecosystems are coupled via the process of photosynthesis. Constraining the annual water cycle therefore allows first order estimates of annual photosynthetic carbon flux, providing that the components of evapotranspiration can be separated. In this study, an isotope mass-balance equation is utilized to constrain annual evaporation flux, which in turn, is used to determine the amount of water transferred to the atmosphere by plant transpiration. The Volta River watershed in West Africa is dominated by woodland and savanna ecosystems with a significant proportion of C4 vegetation. Annually, the Volta watershed receives 380 km3 of rainfall, 50% of which is returned to the atmosphere via transpiration. An annual photosynthetic carbon flux of 170 × 1012 g C yr−1 or 428 g C m−2 was estimated to be associated with this water vapor flux. Independent estimates of heterotrophic soil respiration slightly exceeded the NPP estimate from this study, implying that the exchange of carbon between the Volta River watershed and the atmosphere was close to being in balance or that terrestrial ecosystems were a small annual source of CO2 to the atmosphere. In addition to terrestrial carbon flux, the balance of photosynthesis and respiration in Volta Lake was also examined. The lake was found to evade carbon dioxide to the atmosphere although the magnitude of the flux was much smaller than that of the terrestrial ecosystems.
A. C. Gebhardt, B. Gaye-Haake, D. Unger, N. Lahajnar, V. Ittekkot (2004). Recent particulate organic carbon and total suspended matter fluxes from the Ob and Yenisei Rivers into the Kara Sea (Siberia). Marine Geology 207 (1-4): 225-245
ABSTRACT: The Ob and Yenisei Rivers account for more than one-third of the total fresh water supply to the Arctic Ocean. In the past, their sediment load and particulate organic carbon (POC) discharge into the Kara Sea has been measured at stations in the hinterland far south of the estuaries. Suspended matter has been sampled in the estuaries and southern Kara Sea within the framework of the joint Russian–German “SIRRO” program (Siberian River Run-Off), allowing a reliable new estimate of fluxes from the rivers into the Kara Sea. Our estimates of annual supplies of sediment (3.76×106 t), particulate organic carbon (0.27×106 t) and particulate nitrogen (PN) (0.027×106 t) from the Ob River to the Kara Sea are lower than earlier estimates from the northernmost gauging station in the hinterland due to deposition of particulate matter in the Ob Bay. On the other hand, our estimates of the Yenisei's annual sediment (5.03×106 t), particulate organic carbon (0.57×106 t) and particulate nitrogen (0.084×106 t) supplies to the Kara Sea are probably too high, as they suggest a pure bypass system in the investigated area. We differentiate between an area of recent deposition in the south of the Kara Sea and an area of recent organic matter degradation further north.
J. Hejzlar, M. Dubrovský, J. Buchtele, M. Růžička (2003). The apparent and potential effects of climate change on the inferred concentration of dissolved organic matter in a temperate stream (the Malše River, South Bohemia). Science of The Total Environment 310 (1-3): 143-152
ABSTRACT: Long-term and seasonal changes in concentration of dissolved organic matter (DOM) and their possible drivers were evaluated for an upland stream in central Europe during 1969–2000. Two periods have been detected within this data set—years with decreased DOM until the middle of 1980s and then years with increased DOM until 2000. Temperature, hydrological regime of runoff from the catchment (namely the amount of interflow), and changes in atmospheric deposition of acidity coincided with the variations in DOM concentrations. The analysis of single runoff events confirmed the relation between the export of increased DOM concentrations from the catchment and interflow. A multiple linear regression model based on monthly averages of temperature and interflow explained 67% of DOM variability. This model suggested a 7% increase in DOM concentration under the scenarios of possible future climate change related to doubled CO2 concentration in the atmosphere. The scenarios were based on results of several global circulation models.
ABSTRACT: This study provides the first detailed estimate of riverine organic carbon fluxes in British rivers, as well as highlighting major gaps in organic carbon data in national archives. Existing data on organic carbon and suspended solids concentrations collected between 1989 and 1993, during routine monitoring by the River Purification Boards (RPBs) in Scotland and the National River Authorities (NRAs) in England and Wales, were used with annual mean flows to estimate fluxes of dissolved and particulate organic carbon (DOC and POC) in British rivers. Riverine DOC exports during 1993 varied from 7·7-103·5 kg ha-1 year-1 , with a median flux of 31·9 kg ha-1 year-1 in the 85 rivers for which data were available. There was a trend for DOC fluxes to increase from the south and east to the north and west. A predictive model based on mean soil carbon storage in 17 catchments, together with regional precipitation totals, explained 94% of the variation in the riverine DOC exports in 1993. This model was used to predict riverine DOC fluxes in regions where no organic carbon data were available. Calculated and predicted fluxes were combined to produce an estimate for exports of DOC to tidal waters in British rivers during 1993 of 0·68±0·07 Mt. Of this total, rivers in Scotland accounted for 53%, England 38% and Wales 9%. Scottish blanket peats would appear to be the largest single source of DOC exports in British rivers. An additional 0·20 Mt of organic carbon were estimated to have been exported in particulate form in 1993, approximately two-thirds of which was contributed by English rivers. It is suggested that riverine losses of organic carbon have the potential to affect the long-term dynamics of terrestrial organic carbon pools in Britain and that rivers may regulate increases in soil carbon pools brought about by climate change.
ABSTRACT: General circulation models predict that freshwater discharge from the Mississippi River (USA) to the coastal ocean would increase 20% if atmospheric CO2 concentration doubles. Here we use a coupled physical-biological 2-box model to investigate the potential impacts of increased freshwater and nutrient inputs on the production and decay of organic matter in the coastal waters of the northern Gulf of Mexico. Model results for a doubled CO2 climate indicate that the annual net productivity of the upper water column (NP, 0 to 10 m) is likely to increase by 65 g C m-2 yr-1 , relative to a 1985-1992 average (122 g C m-2 yr-1 ). Interestingly, this projected increase is of the same magnitude as the one that has occurred slnce the 1940s due to the introduction of anthropogenlc nutrients. An increase in annual NP of 32 g C m-2 yr-1 was observed during the Great Mississippi River Flood of 1993, thus indicating the general validity of a doubled CO2 scenario. The total oxygen uptake in the lower water column (10 to 20 m), in contrast, is likely to remain at its present value of about 200 g O2 m-2 yr-1 . Thus, carbon export and burial, rather than in situ respiration, are likely to be the dominant processes balancing coastal carbon budgets, leading perhaps to an expanded extent of the hypoxic zone.
McClelland, J.W., Stieglitz, M., Pan, F., Holmes, R.M., Peterson, B.J. (2007). Recent changes in nitrate and dissolved organic carbon export from the upper Kuparuk River, North Slope, Alaska. Journal of Geophysical Research G: Biogeosciences 112 (G04S60): doi:10.1029/2006JG000371
ABSTRACT: Export of nitrate and dissolved organic carbon (DOC) from the upper Kuparuk River between the late 1970s and early 2000s was evaluated using long-term ecological research (LTER) data in combination with solute flux and catchment hydrology models. The USGS Load Estimator (LOADEST) was used to calculate June–August export from 1978 forward. LOADEST was then coupled with a catchment-based land surface model (CLSM) to estimate total annual export from 1991 to 2001. Simulations using the LOADEST/CLSM combination indicate that annual nitrate export from the upper Kuparuk River increased by ~5 fold and annual DOC export decreased by about one half from 1991 to 2001. The decrease in DOC export was focused in May and was primarily attributed to a decrease in river discharge. In contrast, increased nitrate export was evident from May to September and was primarily attributed to increased nitrate concentrations. Increased nitrate concentrations are evident across a wide range of discharge conditions, indicating that higher values do not simply reflect lower discharge in recent years but a significant shift to higher concentration per unit discharge. Nitrate concentrations remained elevated after 2001. However, extraordinarily low discharge during June 2004 and June–August 2005 outweighed the influence of higher concentrations in determining export during these years. The mechanism responsible for the recent increase in nitrate concentrations is uncertain but may relate to changes in soils and vegetation associated with regional warming. While changes in nitrate and DOC export from arctic rivers reflect changes in terrestrial ecosystems, they also have significant implications for Arctic Ocean ecosystems.
ABSTRACT: Data on carbon river fluxes recently obtained by the authors for the Congo basin within the framework of the PIRAT Program (INSU-CNRS/ORSTOM) are compared with results previously obtained for the Amazon basin. A special interest is devoted to the bicarbonate river fluxes and to their relationships with river discharges. The flux of atmospheric and soil CO2 consumed by rock weathering is estimated to be 3.1 × 105 and 0.5 × 105 moles/a/km2 respectively for the Amazon and the Congo basin. These CO2 fluxes represent, respectively, 67.4% and 74.7% of the total bicarbonate river fluxes. A comparison to other large river basins shows that this contribution is directly related to the proportion of carbonate rock areas. A transfer function between the weathering CO2 flux and the river discharge is calculated for each basin and allows the reconstitution of the variations of this flux using the river discharge fluctuations during the last century. These interannual CO2 fluctuations present average increasing trends of 10% for Amazon basin and only 0.7% for the Congo basin during the last century.
P.A. Raymond, J. E. Bauer, N. F. Caraco, J. J. Cole, B. Longworth, S. T. Petsch (2004). Controls on the variability of organic matter and dissolved inorganic carbon ages in northeast US rivers. Marine Chemistry 92 (1-4): 353-366
ABSTRACT: The use of natural14 C in aquatic and marine studies provides unique source, turnover and processing information with respect to local, regional and global carbon budgets. The number of14 C analyses in many aquatic ecosystems is small, however, thus limiting the full potential of14 C as a tool in carbon studies in these systems. This is particularly true for rivers, which form key linkages between terrestrial and oceanic systems. As part of our on-going studies, radiocarbon-based measurements of dissolved and particulate organic carbon (DOC and POC, respectively), as well as dissolved inorganic carbon (DIC), are being conducted in a group of northeast US rivers. The values for bulkΔ14 C-DOC, and Δ14 C-POC, Δ14 C-DIC and their associatedd13 C ratios are reported for 64 measurements on seven different river systems. While the examined rivers occupy a relatively small geographic area, they individually export both ancient and fully modern post-bomb C to the coastal ocean. The large ranges in Δ14 C for DOC, POC and DIC point to large spatial and temporal variability in the relative inputs of fossil vs. contemporary C, and the processing of each, in this geographically contiguous set of rivers. Although this represents a fairly well-studied group of rivers with respect to carbon ages, the sample number still precludes an in-depth understanding of specific sources of different aged components to these rivers, or to river systems in general. The collective data permit some preliminary assessments concerning the relative importance of wetlands, ancient uplifted sedimentary marine OM, and within-system modification as important potential controls on riverine carbon ages.
Raymond, P. A., Oh, N.-H., Turner, R. E., Broussard, W. (2008). Anthropogenically enhanced fluxes of water and carbon from the Mississippi River (includes supplementary information online). Nature 451 (7177): 449-452
ABSTRACT: The water and dissolved inorganic carbon exported by rivers are important net fluxes that connect terrestrial and oceanic water and carbon reservoirs1 . For most rivers, the majority of dissolved inorganic carbon is in the form of bicarbonate. The riverine bicarbonate flux originates mainly from the dissolution of rock minerals by soil water carbon dioxide, a process called chemical weathering, which controls the buffering capacity and mineral content of receiving streams and rivers2 . Here we introduce an unprecedented high-temporal-resolution, 100-year data set from the Mississippi River and couple it with sub-watershed and precipitation data to reveal that the large increase in bicarbonate flux that has occurred over the past 50 years (ref. 3) is clearly anthropogenically driven. We show that the increase in bicarbonate and water fluxes is caused mainly by an increase in discharge from agricultural watersheds that has not been balanced by a rise in precipitation, which is also relevant to nutrient and pesticide fluxes to the Gulf of Mexico. These findings demonstrate that alterations in chemical weathering are relevant to improving contemporary biogeochemical budgets. Furthermore, land use change and management were arguably more important than changes in climate and plant CO2 fertilization to increases in riverine water and carbon export from this large region over the past 50 years.
W.R. Rouse, M.S.V. Douglas, R.E. Hecky, A.E. Hershey, G.W. Kling, L.E> Lesack, P. Marsh, M. McDonald, B.J. Nicholson, N.T. Roulet, J.P. Smol (1997). Effects of climate change on the freshwaters of Arctic and subarctic North America. Hydrological Processes 11 (8): 873-902
ABSTRACT: Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally influenced by arctic air masses while Pacific air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring flood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which affects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 4°C in summer and 9°C in winter for a 2 × CO2 scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of > 1°C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can affect every aspect of the environment. The normal spring flooding of ice-jammed north-flowing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would affect the chemical, mineral and nutrient status of lakes and most likely have deleterious effects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warmer and drier conditions would most likely change them from a sink to a source for atmospheric carbon. There is some evidence that this may be occurring already. Region 2 is very vulnerable to global warming. Its freshwater systems are probably the least studied and most poorly understood in North America. There are clear needs to improve our current knowledge of temperature and precipitation patterns; to model the thermal behaviour of wetlands, lakes and rivers; to understand better the interrelationships of cold region rivers with their basins; to begin studies on the very large lakes in the region; to obtain a firm grasp of the role of northern peatlands in the global carbon cycle; and to link the terrestrial water balance to the thermal and hydrological regime of the polar sea. Overall, there is a strong need for basic research and long-term monitoring.
B. Schlünz, R. R. Schneider, P. J. Müller, G. Wefer (2000). Late Quaternary organic carbon accumulation south of Barbados: influence of the Orinoco and Amazon rivers?. Deep Sea Research Part 1: Oceanographic Research Papers 47 (6): 1101-1124
ABSTRACT: Isotopic and geochemical proxies measured in bulk sediment samples of two gravity cores south of Barbados were used to develop a model for the organic carbon accumulation during the last 250 kyr with respect to the influence of terrestrial sources (e.g. the Orinoco and Amazon rivers) as well as the marine contributions, sea-level, surface currents, and morphological features. Total organic carbon (TOC) content and the stable organic carbon isotopes of the organic matter (d13 Corg ) show no glacial to interglacial variability. TOC content is generally very low in both cores but increases between 40 and 120 kyr. A comparable pattern is detected in accumulation rates of the organic matter but is only hinted in thed13 Corg ratios. The results suggest that during the last 250 kyr the organic carbon accumulation south of Barbados has been controlled by glacioeustatic sea-level changes and the general morphologic settings. A sea-level stand of 15–80 m below present day seems generally to favour the accumulation of organic matter south of Barbados. Althoughd13 Corg ratios reveal no clear trend in the organic matter composition, terrestrial organic carbon discharged by rivers (Orinoco or Amazon) seems not to be a major component in the sediments of that area during the last 250 kyr.
Scott, D. T., Baisden, W. T., Davies-Colley, R., Gomez, B., Hicks, D. M., Page, M. J., Preston, N. J., Trustrum, N. A., Tate, K. R., Woods, R. A. (2006). Localized erosion affects national carbon budget. Geophysical Research Letters 33 (L01402): doi:10.1029/2005GL024644
ABSTRACT: Small mountainous rivers discharge disproportionate amounts of sediment and carbon to the Earth's oceans. Our New Zealand data demonstrates that localized erosion plays a greater role in C budgets than has been recognized in national and global studies. We estimate that New Zealand's rivers export 4 ± 1 Mg C km−2 yr−1 of dissolved organic carbon (DOC) and 10 ± 3 Mg C km−2 yr−1 of particulate organic carbon (POC) (2 and 6 times the global average), which is equivalent to 40% of New Zealand's fossil fuel emissions. Under intact native vegetation in mountain-belt hot spots, POC export greatly exceeds CO2 consumption from mineral weathering. Moreover, deforestation of fertile steepland greatly accelerates POC loss, evidenced by 1.7% of New Zealand's land area which generates 20% of exported POC. Thus, localized erosion deserves increased attention in C budgets and accounting.
ABSTRACT: We have developed a mass balance analysis of organic carbon (OC) across the five major river subsystems of the Mississippi (MS) Basin (an area of 3.2 × 106 km2 ). This largely agricultural landscape undergoes a bulk soil erosion rate of 480 t·km−2 ·yr−1 (1500 × 106 t/yr, across the MS Basin), and a soil organic carbon (SOC) erosion rate of 7 t·km−2 ·yr−1 (22 × 106 t/yr). Erosion translocates upland SOC to alluvial deposits, water impoundments, and the ocean. Soil erosion is generally considered to be a net source of CO2 release to the atmosphere in global budgets. However, our results indicate that SOC erosion and relocation of soil apparently can reduce the net SOC oxidation rate of the original upland SOC while promoting net replacement of eroded SOC in upland soils that were eroded. Soil erosion at the MS Basin scale is, therefore, a net CO2 sink rather than a source.
ABSTRACT: Isotopic composition of dissolved inorganic carbon (d13 CDIC) in the Ottawa River basin is about−8 and −16‰ for lowland carbonate and upland silicate tributaries, respectively. This suggests that (1) the source of DIC to the Ottawa River is soil respiration and carbonate weathering, (2) exchange with the atmosphere is unidirectional or volumetrically unimportant, and (3) in-river respiration and photosynthesis are not significant influences on the river carbon budget. Accepting these constraints, chemical and isotopic data are used to reconstitute soilp CO2 for tributary catchments. Averages for upland silicate, mixed, and lowland carbonate basins are calculated to be roughly 2000, 5000, and 30,000 ppm, respectively. These values are used as input to model the pathway of carbon through the watershed—rain water to soil water to river water. The flux of carbon from the Ottawa River as DIC is calculated to be 4.3×1010 mol C/a. Utilizing carbon isotopes, 75% and 25% of the Ca2 ++ Mg2 + flux is calculated to originate from carbonate and silicate weathering, respectively, and 61% of the DIC is calculated to originate from organic respiration. The latter represents some 6% of respired carbon in the basin, assuming an average respiration rate of 0.5 mmol C m−2 h−1 . Based on a diffusion model, CO2 evasion to the atmosphere from the Ottawa River and its tributaries is estimated to be 1.3×1010 mol C/a or 30% of the DIC flux.
Vallières, C., Retamal, L., Ramlal, P., Osburn, C.L., Vincent, W.F. (2008). Bacterial production and microbial food web structure in a large arctic river and the coastal Arctic Ocean. Journal of Marine Systems 74 (3-4): 756-773
ABSTRACT: Globally significant quantities of organic carbon are stored in northern permafrost soils, but little is known about how this carbon is processed by microbial communities once it enters rivers and is transported to the coastal Arctic Ocean. As part of the Arctic River-Delta Experiment (ARDEX), we measured environmental and microbiological variables along a 300 km transect in the Mackenzie River and coastal Beaufort Sea, in July-August 2004. Surface bacterial concentrations averaged 6.7 × 105 cells mL-1 with no significant differences between sampling zones. Picocyanobacteria were abundant in the river, and mostly observed as cell colonies. Their concentrations in the surface waters decreased across the salinity gradient, dropping from 51,000 (river) to 30 (sea) cells mL-1 . There were accompanying shifts in protist community structure, from diatoms, cryptophytes, heterotrophic protists and chrysophytes in the river, to dinoflagellates, prymnesiophytes, chrysophytes, prasinophytes, diatoms and heterotrophic protists in the Beaufort Sea. Size-fractionated bacterial production, as measured by 3H-leucine uptake, varied from 76 to 416 ng C L-1 h-1 . The contribution of particle-attached bacteria (> 3μm fraction) to total bacterial production decreased from > 90% at the Mackenzie River stations to < 20% at an offshore marine site, and the relative importance of this particle-based fraction was inversely correlated with salinity and positively correlated with particulate organic carbon concentrations. Glucose enrichment experiments indicated that bacterial metabolism was carbon limited in the Mackenzie River but not in the coastal ocean. Prior exposure of water samples to full sunlight increased the biolability of dissolved organic carbon (DOC) in the Mackenzie River but decreased it in the Beaufort Sea. Estimated depth-integrated bacterial respiration rates in the Mackenzie River were higher than depth-integrated primary production rates, while at the marine stations bacterial respiration rates were near or below the integrated primary production rates. Consistent with these results, P CO2 measurements showed surface water supersaturation in the river (mean of 146% of air equilibrium values) and subsaturation or near-saturation in the coastal sea. These results show a well-developed microbial food web in the Mackenzie River system that will likely convert tundra carbon to atmospheric CO2 at increasing rates as the arctic climate continues to warm.
ABSTRACT: Concentrations of NO3 −, NO2 − , NH4 + , PO4 3− , Si(OH)4 − , DOC, DON, DOP, POC, PON, POP, chlorophyl a, phaeopigments, carbohydrates and proteins were measured for one year in the Morlaix river waters (Brittany coast, France). By comparison with the averages known for the unpolluted rivers of the world, concentrations of NO3 −, NH4 + , PO4 3− , NO2 − and DON are found to be much higher, and those of nitrate and ammonium exceed even those known for highly polluted rivers. The selective pollution by N compounds is caused by an excessive use of fertilizers in the hinterland although with NH4+, however, urban pollution can also be an important factor. Allochthonous sources supply a major fraction of particulate organic compounds to the river waters. Dissolved fractions of N and P in river waters are very large compared to the particulate fractions, whereas carbon is more or less equally distributed between DOC and POC. Transport of these nutrients to the estuary show three different patterns in relation to river discharge changes. Areal loading of the estuary with NO3 and NH4 + is greater than that known for other temperate estuaries. The deleterious effects of N pollution are, however, offset by the low freshwater flow, high tidal prism volume, estuarine basin geomorphology and uptake by benthic microalgae.
Y. Wu, J. Zhang, S. M. Liu, Z. F. Zhang, Q. Z. Yao, G. H. Hong, L. Cooper (2007). Sources and distribution of carbon within the Yangtze River system. Estuarine, Coastal and Shelf Science 71 (1-2): 13-25
ABSTRACT: Dissolved, particulate, soil and plant samples were collected from the Yangtze River (Changjiang) system in May 1997 and May 2003 to determine the sources and distribution of organic and inorganic matter within the river system. Average dissolved organic carbon (DOC) concentrations within the main stream were 105μM C in 1997 and 108 μM C in 2003. Particulate organic carbon (POC) ranged from 0.5% to 2.5% of total suspended matter (TSM). Both dissolved inorganic carbon (DIC) and particulate inorganic carbon (PIC) concentrations decreased from upper to lower reaches of the river, within the ranges 1.2–2.7 mM and 0.08–4.3% of TSM, respectively.d13 C andd15 N values for tributaries and the main stream varied from−26.8‰ to −25.1‰ and 2.8‰ to 6.0‰, respectively. A large spatial variation in particulate organic matter (POM) is recorded along the main stream, probably due to the contributions of TSM from major tributaries and POM input from local vegetation sources. The dominance of C-3 plants throughout the entire basin is indicated byd13 C andd15 N values, which range from−28.8‰ to −24.3‰ and from −0.9‰ to 5.5‰, respectively. Thed13 C andd15 N values of organic matter within surface soil from alongside tributaries and the main stream vary from−28.9‰ to −24.3‰ and 2.7‰ to 4.5‰, respectively. Although these differences are subtle, there is a slight enrichment of15 N in soils along the main stream. Various approaches, such as C/N and stable isotopes, were used to trace the sources of organic matter within the river. Riverine POM is mostly derived from soil; the contribution from phytoplankton is minor and difficult to trace via the composition of particles. POC flux has decreased from >5 × 106 t yr−1 during the period 1960–1980 to about 2 × 106 t yr−1 in 1997. This trend can be explained by decreasing sediment load within the Yangtze River. The export of TOC from the Yangtze River at the end of the 20th Century is approximately equivalent to that of the Zaire River, less than that of the Amazon River, and higher than that of other large rivers such as the Mississippi. Large amounts of DOC and POC were transported to coastal areas of the East China Sea over a short period during 1998 flood events, containing large amounts of nutrients and pollutants. Such an event could be an important trigger for coastal environmental problems and changes to the health of ecosystems.
L. Zou, M.-Y. Sun, L. Guo (2006). Temporal variations of organic carbon inputs into the upper Yukon River: Evidence from fatty acids and their stable carbon isotopic compositions in dissolved, colloidal and particulate phases. Organic Geochemistry 37 (8): 944-956
ABSTRACT: To understand the variations of organic carbon inputs in high latitude rivers, we tracked the changes in fatty acid concentrations and their stable carbon isotopic compositions in dissolved (DOM, <1 kDa), colloidal (COM, 1 kDa to 0.45μm) and particulate (POM, >0.45 μm) organic matter collected from the upper Yukon River (Alaska, USA) during ice-open season from May to September 2002. In spite of the high variability in discharge, total organic carbon in the river continuously declined from the beginning of snowmelt, while fatty acid contents (normalized to organic carbon in each pool) varied independently in the three organic pools with the largest fraction of total fatty acids in the POM. Fatty acid compositions in each pool were similarly dominated by short-chain saturated (14:0, 16:0, and 18:0) and monounsaturated [16:1 and 18:1(n−9)] fatty acids (70–80%), while bacterial fatty acids [normal and branched 15:0 and 17:0 plus 18:1(n − 7)] comprised an important fraction (16–30% of the total) in all samples. The concentrations of individual fatty acids in the three pools varied greatly during the ice-open season, but the compound-specific stable carbon isotopic ratios of fatty acids in POM were less variable compared to those in DOM and COM. Mass balance calculation showed that land-derived fatty acids dominated in May (snowmelt) while aquatic produced fatty acids peaked in July, consistent with the maximum Chl-a concentration. In addition, bacteria-specific fatty acids (e.g., iso-15:0) had similard13 C ratios in all three pools, with little difference from those of bulk carbon. Our results suggest that relative inputs of organic matter from various sources into the upper Yukon River are significantly affected by snowmelt, rainfall, soil erosion, discharge, aquatic production, and biochemical degradation.