Total Organic Carbon Export Research Articles

Dynamics of dissolved and particulate organic carbon in a saline and semiarid stream of southeast Spain (Chicamo stream)

Annual variations in the concentration of dissolved (DOC) and particulate organic carbon (CPOC = Coarse; FPOC = Fine; UPOC = Ultrafine) were studied in a 100 m-reach of the Chicamo stream, an intermittent saline stream in southeast Spain. DOC represented the most important fraction of organic carbon flowing in the Chicamo stream (>98%), with concentrations of about 1.7 mgC l−1 during most of the year, reaching 2.5 mgC l−1 in summer. One high flow episode during a rain storm in winter was characterized by a considerably increased concentration of DOC (9.4 mgC l−1). CPOC was the dominant POC fraction. Positive and significant correlations were found for DOC and discharge, which support the idea of allochthonous inputs due to floods. There was no significant correlation between POC and discharge. No significant correlations were found for DOC or POC with the physico-chemical parameters measured, while a negative significant correlation was found between DOC and temperature. The export of total organic carbon from the drainage basin of the Chicamo stream was low (6.2 × 10−4 gC m−2 yr−1) and typical of streams in arid and semi-arid regions. The results of a Principal Component Analysis defined three different phases. The first consisted of short periods, during which floods provide pulses of allochthonous organic carbon and nutrients, the second a dry phase (summer), defined by biotic interactions, during which the stream could acts as a `sink' of organic matter, and the third and final phase which is characterised by hydrological stability.

Was a carbon balance measured in the equatorial Pacific during JGOFS?

The likelihood that the carbon fluxes measured as part of the US-JGOFS field program in the equatorial Pacific ocean (EgPac) during 1992 yielded a balanced carbon budget for the surface ocean was determined. The major carbon fluxes incorporated into a surface carbon budget were: new production, particulate organic carbon (POC) and dissolved organic carbon (DOC) export, CaC0 3 export, C0 2 gas evasion, dissolved inorganic carbon (DIC) supply, and the time rate of charge. The ratio of the measured concentration gradients of DOC and DIC provided a constraint on the ratio of POC/DOC export. Uncertainties of ±30–50% for individual carbon flux measurements reduce the likelihood that a carbon balance can be measured during a JGOFS process-type study. As a benchmark, carbon fluxes were prescribed to yield a hypothetical surface carbon budget that was, on average, balanced. Given the typical errors in the individual carbon fluxes, however, there was only about a 30% chance that this hypothetical budget could be measured to be balanced to ±50%. Using this benchmark, it was determined that there was a ∼ 95 % chance that the carbon flux measurements yielded a surface DIC budget balanced (to ±50%) during El Nino conditions in boreal spring 1992, when the total organic carbon export rate was - 5 mmol C m-2 day- 1 and the POC export was ∼ 3 mmol C m −2 day −1. In boreal fall 1992, during cold period conditions, there was a ∼70% chance that the surface carbon DIC budget was balanced when the total organic carbon export rate was ∼ 20 mmol C m −2 day −1 and export was -13 mmol C m-2 day-'. The DOC to DIC concentration gradient ratio of - -0.15, measured in depth profiles down to 100m and in surface waters, was used as an important constraint that most (> 70%) of the organic carbon exported from the euphotic zone was POC rather than DOC. If a balanced surface DIC budget was used to test the compatibility of individual carbon fluxes measured during EgPac, then a three- to four-fold increase in total and particulate organic carbon export between spring and fall is indicated. This increase was not reflected in the POC loss rates measured by drifting sediment trap collections or estimated by 234Th deficiencies coupled with the C/Th measured on suspended particles.

The total organic carbon export rate based on 13C and 12C of DIC budgets in the equatorial Pacific region

Carbon stable isotopic composition (δ 13C) of total dissolved inorganic carbon (DIC) is independent of DIC as a tracer for net organic carbon production rate in the surface ocean due to the Significantly different equilibration times for CO 2 and 13CO 2 during air-sea exchange. The rates of the organic carbon export and net physical supply of DIC, can therefore, be determined from the mixed layer mass balances for DIC and DIC 13 (DIC· 13C/ 12C), given an air-sea C0 2 flux estimated from wind speed and measured pCO 2. This method does not rely on specifying the absolute advetive and diffusive transports of DIC. The mass balance approach was applied to measurements of concentration and δ 13C of DIC made in the equatorial Pacific ocean in the Spring and Fall of 1991–1992 during WOCE P16C and US JGOFS EqPac cruises. The calculated organic carbon export rate out of mixed layer during Spring 1992 El Nin˜o was 3.2±1.4 mmol C m −2 d −1 (between 2°N to 2°S and 170°W to 110°W). No spatial trend for the total organic carbon export rate was determined despite the large variation in C02 degassing rate from ∼0.1 mmol C m −2 d −1 at 170°W to ∼4.0 mmol Cm −2 d −1 at 110°W over this region. Under the cold tongue conditions in the end of August 1992, a higher organic carbon export rate, about 11.5±6.3 mmol C m -2 d −1 was obtained on the equator at 140°W while the rate for the previous September (in 1991) was 4.9 ± 2.4 mmol C m −2 d −1. Most of the uncertainty in our estimated results from the uncertainty in gas exchange coefficient. The comparison between the estimated organic carbon export rates and the particulate organic carbon (POC) export rates derived from thorium budget and sediment drifting traps at the same stations (Buesseler et al., 1995; Murray et al., 1996) suggests that particulate organic carbon probably accounts for about 70% of the organci export. Since physical, rather than biological, processes primarily control the distribution of DIC, NO 3 O 2 and other nutrient-related properties in this region, it is very difficult to accurately obtain the net organic carbon export rate from advection rates and nutrient gradients, as demonstrated by a 3-D calculation.

Spatial and temporal variability of total organic carbon along 140°W in the equatorial Pacific Ocean in 1992

Total organic carbon (TOC) was analyzed on four transects along 140°W in 1992 using a high temperature combustion/discrete injection (HTC/DI) analyzer. For two of the transects, the analyses were conducted on-board ship. Mixed-layer concentrations of organic carbon varied from about 80 μM C at either end of the transect (12°N and 12°S) to about 60 μM C at the equator. Total organic carbon concentrations decreased rapidly below the mixed-layer to about 38–40 μM C at 1000 m across the transect. Little variation was observed below this depth; deep water concentrations below 2000m were virtually monotonic at about 36 μM C. Repeat measurements made on subsequent cruises consistently found the same concentrations at 1000 m or deeper, but substantial variations were observed in the mixed-layer and the upper water column above 400 m depth. Linear mixing models of total organic carbon versus σ θ exhibited zones of organic carbon formation and consumption. TOC was found to be inversely correlated with apparent oxygen utilization (AOU) in the region between the mixed-layer and the oxygen minimum. In the mixed-layer, TOC concentrations varied seasonally. Part of the variations in TOC at the equator was driven by changes in the upwelling rate in response to variations in physical forcing related to an El Niño and to the passage of tropical instability waves. TOC export fluxes, calculated from simple box models, averaged 8±4 mmol C m −2day −1 at the equator and also varied seasonally. These export fluxes account for 50–75% of the total carbon deficit and are consistent with other estimates and model predictions.

Biogeochemistry of carbon in the Amazon River

Depth‐integrated, discharge‐weighted water samples were collected over 1,800 km of the Amazon River on eight cruises at different stages of the hydrograph, 1982–1984. Fine (FPOC, <63 µm) and coarse (CPOC, >63 µm) particulate organic carbon as weight percentage of suspended sediment varied between 0.9–1.5% for FPOC and 0.5–3.4% for CPOC. Concentrations of FPOC ranged from 5 mg liter−1 upriver to 2 mg liter−1 downriver in the mainstem and from 6 mg liter−1 in the Rio Madeira to <1 in the Rio Negro. CPOC had similar distribution patterns, but with concentrations 15–30% those of FPOC. Dissolved organic carbon (DOC) averaged 4–6 mg liter−1 in the mainstem and up to 12 mg liter−1 in the Rio Negro. Upriver dissolved inorganic carbon (DIC) concentrations of about 1,200 µM were diluted by tributaries and floodplain drainage to 600 µM at the most downriver site. Evasion of CO2, invasion of O2, and in situ oxidation were of comparable magnitude, 3–8 µmol m−2 s−1.The average export of total organic carbon (TOC) was 36.1 Tg yr−1 (8.5 g m−2 yr−1), of which 62% was DOC, 34% was FPOC, and 4% was CPOC. TOC inputs were insufficient to support in situ oxidation by a factor of at least two. A relatively small, rapidly cycling pool of labile organic matter may coexist with a much larger pool of more refractory material.

The exchange of organic carbon in basin mangrove forests in a southwest Florida estuary

The magnitude and seasonality of organic carbon exchange was estimated for two basin mangrove forests in Rookery Bay, Florida. Runoff and tidal inundation in the forests were seasonal with half the annual total of each occurring from August to October. In each forest there were 152 tides yr −1 with a cumulative depth of about 12 m. Total organic carbon increased in bay waters exporting from the mangroves following a flood tide and peak concentrations were associated with export due to rainfall. The amount of net export from each basin forest was similar, although the concentration of organic carbon in each were different. Monthly net organic carbon export was proportional to the cumulative tidal amplitude within the forest. Total organic carbon export was 64 gC m −2 yr −1 and DOC was 75% of the total. A comparison of organic carbon export among riverine, fringe and basin mangroves suggests that tidal hydrology influences the proportion of litter fall that is exported from mangroves; and the magnitude of this organic carbon export from mangroves is related to the cumulative tidal amplitude within the forests.

Characteristics of Sediment and Organic Carbon Export from Pristine Boreal Forest Watersheds

Estimates of the amount of material moving annually from terrestrial ecosystems to the ocean are largely based on an incomplete understanding of events occurring throughout the hydrologic year, and only a vague comprehension of in-stream processes controlling that export. Discharge, suspended sediment, particulate organic matter (POM; > 0.5 μm), dissolved organic carbon (DOC; <0.5 μm diameter), and the percentage of organic matter were measured from 1979 to 1981 in five pristine Quebec streams: First Choice Creek (1st order; watershed area: 0.25 km2), Beaver Creek (2nd order; 1.83 km2), Muskrat River (5th order; 204 km2), Matamek River (6th order; 673 km2), and the Moisie River (9th order; 19 871 km2). All streams, with the exception of First Choice Creek, have a strong spring freshet when 43–55% of the annual discharge occurs. By describing sediment and organic carbon export throughout the annual hydrologic cycle, 1 showed that during the 2-mo spring freshet 71–92% of the annual sediment load is exported but only 59–65% of the annual POM load, and only 47–51% of the annual DOC load. Sediment yield is relatively constant between watersheds (1.5–7.6 g∙m−2∙yr−1), as is POM export (1.0–6.7 g ash-free dry-weight [AFDW]∙m−2∙yr−1); however, export DOC varies from 3.1 g C∙m−2∙yr−1 in First Choice Creek to 48.4 g C∙m−2∙yr−1 in Beaver Creek. There appears to be rapid loading of carbon between 1st- and 2nd-order streams in boreal forests, followed by biological and physical processing as watershed area increases. Thus, for the Moisie River watershed, export of total organic carbon (TOC) is reduced to only 4.7 g C∙m−2∙yr−1. Export of coarse particulate organic matter (> 1 mm) is negligible (normally < 0.1 mg∙L−1), as is oxidation of the suspended load (< 0.5%∙d−1). Effects of summer storms, natural diel variations, and depth of sample from the water column are shown to have a minimal influence on concentrations. Rating curves (kg∙d−1 vs. discharge) are developed to estimate the annual yield of sediment, POM, and DOC, and to evaluate long-term variations. From the results I suggest that in-stream processing and retention devices exert considerable control over the quantity and nature of suspended organic material. Physical processes such as the discharge regime and stream power are relatively less important in determining organic concentrations, but more important in determining sediment concentrations.Key words: seston, carbon, sediment, boreal forest, watershed, river, stream, export

Transport of organic carbon to the oceans by rivers of North America: a synthesis of existing data

Transport of organic carbon in rivers of North America to the oceans was assessed by compiling and analyzing data from 82 North American rivers draining 60% of Canada and the United States. These data were collected by the U.S. Geological Survey and the Water Resources Branch of Canada's Inland Waters Directorate. Mean annual concentrations of total organic carbon showed considerably less variation than did annual specific export (gC m -2 yr -1 ). Regional variation in annual specific export of total organic carbon was attributed primarily to differences in annual runoff. Transport of total organic carbon to coastal regions by North American rivers totalled about 35 × 10 12 ± 5 × 10 12 gC yr -1 in 1977 and 1978. The total organic carbon flux to the oceans associated with the long-term average annual flow in North American rivers was computed to be about 40 × 10 12 gC yr -1 , slightly larger than that in 1977 and 1978 because of the lower-than-average runoff in some regions those years. DOI: 10.1111/j.2153-3490.1982.tb01805.x

Transport of organic carbon to the oceans by rivers of North America: a synthesis of existing data

Transport of organic carbon in rivers of North America to the oceans was assessed by compiling and analyzing data from 82 North American rivers draining 60% of Canada and the United States. These data were collected by the U.S. Geological Survey and the Water Resources Branch of Canada's Inland Waters Directorate. Mean annual concentrations of total organic carbon showed considerably less variation than did annual specific export (gC m^-2 yr-1). Regional variation in annual specific export of total organic carbon was attributed primarily to differences in annual runoff. Transport of total organic carbon to coastal regions by North American rivers totalled about 35 × 10¹² ± 5 × 10¹² gC yr^-1 in 1977 and 1978. The total organic carbon flux to the oceans associated with the long-term average annual flow in North American rivers was computed to be about 40 × 1012 gC yr^-1, slightly larger than that in 1977 and 1978 because of the lower-than-average runoff in some regions those years.