The effect of microorganisms and seasonal factors on the isotopic composition of particulate organic carbon from the black sea

The isotopic composition of particulate organic carbon (POC) from the Black Sea deep-water zone was studied during a Russian-Swiss expedition in May 1998. POC from the upper part of the hydrogen sulfide zone (the C-layer) was found to be considerably enriched with the¹²C isotope, as compared to the POC of the oxycline and anaerobic zone. In the C-layer waters, the concurrent presence of dissolved oxygen and hydrogen sulfide and an increased rate of dark CO₂ fixation were recorded, suggesting that the change in the POC isotopic composition occurs at the expense of newly formed isotopically light organic matter of the biomass of autotrophic bacteria involved in the sulfur cycle. In the anaerobic waters below the C-layer, the organic matter of the biomass of autotrophs is consumed by the community of heterotrophic microorganisms; this results in weighting of the POC isotopic composition. Analysis of the data obtained and data available in the literature allows an inference to be made about the considerable seasonable variability of the POC δ¹³C value, which depends on the ratio of terrigenic and planktonogenic components in the particulate organic matter.     

Geochemical characteristics of the carbonate constructions formed during microbial oxidation of methane under anaerobic conditions

The aragonite constructions of the Black Sea are formed in a stable anaerobic zone and are a perfect object to study the natural mechanism of anaerobic methane oxidation. The most probable pathway of methane oxidation is its methanogen-mediated reaction with bicarbonates, dissolved in seawater, with the formation of water and acetate, which is then consumed by other components of the anaerobic community. Comparison of the delta 13C values of carbonate minerals and organic matter once more demonstrated that the formation of the organic matter of biomass is accompanied by intense fractionation of carbon isotopes, as a result of which the total organic matter of biomass acquires an extremely light isotopic composition, characterized by delta 13C values as low as -83.8@1000.     

Seasonal and site-specific variability in terrigenous particulate organic carbon concentration in near-shore waters of Lake Biwa, Japan

Identifying sources of particulate organic matter (POM) is important for clarifying fundamental mechanisms by which lake food webs are sustained. We determined carbon and nitrogen stable isotope ratios of POM in near-shore waters of Lake Biwa, a large, meso-eutrophic lake in Japan, to estimate relative contributions of terrigenous particulate organic carbon (T-POC), plankton-derived POC (P-POC) and epilithon-derived POC (E-POC) to POC in near-shore waters. Samples were collected during different months (November, February, May and July) at 29 sites located near the mouth of tributary rivers with different discharge and catchment land use. The data revealed that POC mainly consisted of P-POC and T-POC, with relative contributions varying widely over season and among locations. E-POC generally contributed little to the near-shore POC. Path analyses revealed that the concentration of riverine POC whose isotopic signatures were similar to those of rice straws increased with a larger %paddy field area in the catchment of tributary rivers, which subsequently enhanced T-POC inputs to near-shore waters through riverine transportation. Furthermore, our results suggested that T-POC contribution was influenced, with a time lag, by wave-driven turbulence and shore topography, which appear to affect sedimentation and resuspension of T-POC.     

Biogeochemical properties of a tropical swamp forest ecosystem in southern Thailand

The distributions of organic matter in the tropical swamps in southern Thailand are reported. The concentrations of particulate and dissolved organic carbon (POC and DOC) in the Bang Nara River, which drains swamp forests and nearby paddy fields, were 2.9 ± 2.0 and 6.2 ± 1.3 mg C l⁻¹, respectively. Although the variation was large, DOC concentration in the Bang Nara River seemed to be higher than POC in November 1992 (DOC/POC ratio, 2.8 ± 2.2). River waters from the upland areas were characterized by low POC and DOC concentrations as compared with Bang Nara River water. The δ¹³C values of POC and river sediments were useful to distinguish between organic matter originating in upland and swamp areas. It is suggested that the distributions of organic matter and its isotopic composition reflect the difference in drainage characteristics between lowland swamp and upland areas. Isotopic analyses of plant leaves and soils revealed that the swamp forest ecosystems were characterized by low δ¹³C and low δ¹⁵N values, which suggested low efficiency of water use by plants and large contributions of atmospheric deposition of nitrogen, respectively. Although CO₂ recycling in the forest might be an important factor determining the δ¹³C values of understory plants, the main process in carbon metabolism of tropical swamp forests would be CO₂ exchange between the atmosphere and forest canopy. Received: May 1, 2001 / Accepted: September 28, 2001     

Enzymatic Activities and Prokaryotic Abundance in Relation to Organic Matter along a West–East Mediterranean Transect (TRANSMED Cruise)

The distribution of extracellular enzymatic activities (EEA) [leucine aminopeptidase (LAP), ?-glucosidase (GLU), alkaline phosphatase (AP)], as well as that of prokaryotic abundance (PA) and biomass (PB), dissolved organic carbon (DOC), particulate organic carbon and particulate total nitrogen (POC, PTN), was determined in the epi-, meso-, and bathypelagic waters of the Mediterranean Sea along a West-East transect and at one Atlantic station located outside the Strait of Gibraltar. This study represents a synoptical evaluation of the microbial metabolism during early summer. Decreasing trends with depth were observed for most of the parameters (PA, PB, AP, DOC, POC, PTN). Significant differences between the western and eastern basins of the Mediterranean Sea were found, displaying higher rates of LAP and GLU and lower C/N ratios more in the eastern than in the western areas. Conversely, in the epipelagic layer, PA and PB were found to be higher in the western than in the eastern basins. PB was significantly related to DOC concentration (all data, n = 145, r = 0.53, P < 0.01), while significant correlations of EEA with POC and PTN were found in the epipelagic layer, indicating an active response of microbial metabolism to organic substrates. Specific enzyme activities normalized to cell abundance pointed out high values of LAP and GLU in the bathypelagic layer, especially in the eastern basin, while cell-specific AP was high in the epi- and bathypelagic zone of the eastern basin indicating a rapid regeneration of inorganic P for both prokaryotes and phytoplankton needs. Low activity and abundance characterized the Atlantic station, while opposite trends of these parameters were observed along the Mediterranean transect, showing the uncoupling between abundance and activity data. In the east Mediterranean Sea, decomposition processes increased probably in response to mesoscale structures which lead to organic matter downwelling.     

Distribution and activity of microorganisms in the deep repository for liquid radioactive waste at the Siberian Chemical Combine

The physicochemical conditions, composition of microbial communities, and the rates of anaerobic processes in the deep sand horizons used as a repository for liquid radioactive wastes (LRW) at the Siberian Chemical Combine (Seversk, Tomsk oblast), were studied. Formation waters from the observation wells drilled into the horizons used for the radioactive waste disposal were found to be inhabited by microorganisms of different physiological groups, including aerobic organotrophs, anaerobic fermentative, denitrifying, sulfate-reducing, and methanogenic bacteria. The density of microbial population, as determined by cultural methods, was low and usually did not exceed 10⁴ cells/ml. Enrichment cultures of microorganisms producing gases (hydrogen, methane, carbon dioxide, and hydrogen sulfide) and capable of participation in the precipitation of metal sulfides were obtained from the waters of the disposal site. The contemporary processes of sulfate reduction and methanogenesis were assayed; the rates of these terminal processes of organic matter destruction were found to be low. The denitrifying bacteria from the deep repository were capable of reducing the nitrates contained in the wastes, provided sources of energy and biogenic elements were available. Biosorption of radionuclides by the biomass of aerobic bacteria isolated from groundwater was demonstrated. The results obtained give us insight into the functional structure of the microbial community inhabiting the waters of repository horizons. This study indicates that the numbers and activity of microbial cells are low both inside and outside the zone of radioactive waste dispersion, in spite of the long period of waste discharge.     

Distribution and activity of microorganisms in the deep repository for liquid radioactive waste at the Siberian Chemical Combine

The physicochemical conditions, composition of microbial communities, and the rates of anaerobic processes in the deep sandy horizons used as a repository for liquid radioactive wastes (LRW) at the Siberian Chemical Combine (Seversk, Tomsk oblast), were studied. Formation waters from the observation wells drilled into the production horizons of the radioactive waste disposal site were found to be inhabited by microorganisms of different physiological groups, including aerobic organotrophs, anaerobic fermentative, denitrifying, sulfate-reducing, and methanogenic bacteria. The density of microbial population, as determined by cultural methods, was low and usually did not exceed 10(4) cells/ml. Enrichment cultures of microorganisms producing gases (hydrogen, methane, carbon dioxide, and hydrogen sulfide) and capable of participation in the precipitation of metal sulfides were obtained from the waters of production horizons. The contemporary processes of sulfate reduction and methanogenesis were assayed; the rates of these terminal processes of organic matter destruction were found to be low. The denitrifying bacteria from the underground repository were capable of reducing the nitrates contained in the wastes, provided sources of energy and biogenic elements were available. Biosorption of radionuclides by the biomass of aerobic bacteria isolated from groundwater was demonstrated. The results obtained give us insight into the functional structure of the microbial community inhabiting the waters of repository production horizons. This study indicates that the numbers and activity of microbial cells are low both inside and outside the zone of radioactive waste dispersion, in spite of the long period of waste discharge.     

Is the copepod egg production in a highly turbid estuary (the Gironde, France) a function of the biochemical composition of seston?

The influence of the biochemical composition of particles originating from surface waters of the Gironde estuary on egg production rates of Eurytemora affinis zooplanktonic population was studied. In the high turbidity zone, suspended particulate matter had a low nutritional quality because the easily available organic fraction represented less than 15% of the overall particulate organic matter. In waters located seaward of the high turbidity zone, a slight increase in nutritional quality was observed. As a result, the sum of easily extractable organic macromolecules represented 15 to 33% of the overall particulate organic matter. The present study suggests that the low egg-production rate of Eurytemora affinis, occurring in the high turbidity zone, results from combined effects of temperature and bad feeding conditions in the area. Low copepod production can be explained by little phytoplankton growth due to light limitation and, therefore, restricted food availability, as well as difficulties in food selection, non-living particle may being dominant.     

Geochemical Peculiarities of the Carbonate Constructions Formed during Microbial Oxidation of Methane under Anaerobic Conditions

The aragonite constructions of the Black Sea are formed in a stable anaerobic zone and are a perfect object to study the natural mechanism of anaerobic methane oxidation. The most probable pathway of methane oxidation is its methanogen-mediated reaction with bicarbonates, dissolved in seawater, with the formation of water and acetate, which is then consumed by other components of the anaerobic community. Comparison of the ¹³C values of carbonate minerals and organic matter once more demonstrated that the formation of the organic matter of biomass is accompanied by intense fractionation of carbon isotopes, as a result of which the total organic matter of biomass acquires an extremely light isotopic composition, characterized by ¹³C values as low as –83.8.     

Temporal variability of particulate organic carbon,nitrogen and phosphorus in the Northern Adriatic Sea

The particulate organic carbon (POC), nitrogen (PN) and phosphorus (PP) vertical distribution along the water column and temporal variability in coastal and offshore waters of the Northern Adriatic Sea were related to the hydrodynamic conditions and biological processes. Fresh water inputs from the Po and Adige rivers enhance primary production, resulting in high POC, PN and PP concentrations at the surface. In offshore waters, POC and PN concentrations were about 3–4 times less than in the coastal waters, while PP were up to 10 times lower, highlighting a marked phosphorus depletion. In the bottom layer, the POC content decreases due to the strong density gradients which separate bottom waters with prevailing degradation processes. Short term 48 h-variability of POC, PN and PP in the coastal waters was determined to a great extent by variations in the spreading of river plumes at the surface and by nepheloid layers and resuspension processes in the bottom waters. The particulate matter in the Adriatic offshore waters is extremely depleted as regards particulate phosphorus and is characterised by C_org:P and N:P ratios higher than the Redfield ratio.     

Organic matter transport across a macrotidal estuary grandient: the Seine estuary,Fance

The macrotidal Seine estuary receives mineral and organic fluxes from a strongly industrialized basin. Upstream-downstream analysis of POC, polysaccharides and phenolic compounds in the particulate fraction indicated clearly the continental inflow. Within the maximum turbidity zone, the organic matter/suspended particulate matter ratio is lower than elsewhere which results from the dilution effect. Particulate tracers were quantified using grain size analysis and S.E.M. techniques. These sedimentary data defined the ratio of marine to continental particulate flux and mechanisms controlling the suspended particulate matter load within the estuarine mixing zone (resuspension, frontal enrichment and turbulent mixing processes). Organic parameters confirm the data on the mineral matter and were used to distinguish between the marine and continental inputs, from the upstream and downstream samples. Between the two endmembers, variations in organic parameters were controlled mainly by the hydrodynamics in the estuarine zone, rather than by salinity changes.     

Biogeochemistry of platelet ice: its influence on particle flux under fast ice in the Weddell Sea, Antarctica

An array of four sediment traps and one current meter was deployed under a well-developed platelet layer for 15 days in the Drescher Inlet in the Riiser Larsen ice shelf, in February 1998. Traps were deployed at 10 m (just under the platelet layer), 112 m (above the thermocline), 230 m (below thermocline) and 360 m (close to sea floor). There was a substantial flux of particulate organic material out of the platelet layer, although higher amounts were collected in the traps either side of the thermocline. Material collected was predominantly composed of faecal pellets containing diatom species growing within the platelet layer. The size classes of these pellets suggest they derive from protists grazing rather than from larger metazoans. Sediment trap material was analysed for particulate organic carbon/nitrogen/phosphorus (POC/PON/POP) and '¹³C_POC (carbon isotopic composition of POC). These were compared with organic matter in the overlying platelet layer and the water column. In turn, the biogeochemistry of the platelet layer and water column was investigated and the organic matter characteristics related to inorganic nutrients (nitrate, nitrite, ammonium, silicate, phosphate), dissolved organic carbon/nitrogen (DOC/DON), pH, dissolved inorganic carbon (DIC), oxygen and '¹³C_DIC (carbon isotopic composition dissolved inorganic carbon).     

Terrestrial subsidies to lake food webs: an experimental approach

Cross-ecosystem movements of material and energy are ubiquitous. Aquatic ecosystems typically receive material that also includes organic matter from the surrounding catchment. Terrestrial-derived (allochthonous) organic matter can enter aquatic ecosystems in dissolved or particulate form. Several studies have highlighted the importance of dissolved organic carbon to aquatic consumers, but less is known about allochthonous particulate organic carbon (POC). Similarly, most studies showing the effects of allochthonous organic carbon (OC) on aquatic consumers have investigated pelagic habitats; the effects of allochthonous OC on benthic communities are less well studied. Allochthonous inputs might further decrease primary production through light reduction, thereby potentially affecting autotrophic resource availability to consumers. Here, an enclosure experiment was carried out to test the importance of POC input and light availability on the resource use in a benthic food web of a clear-water lake. Corn starch (a C(4) plant) was used as a POC source due to its insoluble nature and its distinct carbon stable isotope value (δ(13)C). The starch carbon was closely dispersed over the bottom of the enclosures to study the fate of a POC source exclusively available to sediment biota. The addition of starch carbon resulted in a clear shift in the isotopic signature of surface-dwelling herbivorous and predatory invertebrates. Although the starch carbon was added solely to the sediment surface, the carbon originating from the starch reached zooplankton. We suggest that allochthonous POC can subsidize benthic food webs directly and can be further transferred to pelagic systems, thereby highlighting the importance of benthic pathways for pelagic habitats.     

Vertical distributions of stable isotopic compositions and bacteriochlorophyll homologues in suspended particulate matter in saline meromictic Lake Abashiri

We determined chloropigment composition as well as stable carbon and nitrogen isotopic compositions of dissolved and particulate species in saline meromictic Lake Abashiri. We observed a sharp peak of bacteriochlorophyll e in a narrow redox boundary zone and the upper monimolimnion, indicating a dense population of brown-colored strains of green sulfur bacteria around the chemocline. Nitrogen isotopic records of particulate nitrogen and dissolved ammonium suggested that the green sulfur bacteria in the redox boundary zone assimilated either ammonium or dinitrogen through the nitrogen fixation pathway. In the anoxic monimolimnion, several lines of evidence suggest that a major portion of particulate organic matter originated from the overlying mixolimnion and redox boundary zone.     

Origin, biochemical composition and vertical flux of particulate organic matter under the pack ice in Terra Nova Bay (Ross Sea, Antarctica) during late summer 1995

Water samples and particulate materials settling under the pack ice were collected in an ice-covered area near the Terra Nova Bay Italian Station during late summer 1995, in order to study short-term changes in the biochemical composition of particulate organic matter. At the end of the study period the phytoplankton biomass increase (up to >3.0 μg chlorophyll-a l⁻¹) was probably related to the intrusion under the pack ice of chlorophylls-enriched surface waters coming from the near ice-free area. Such increase was associated also with a notable increase in particulate organic matter concentrations, as well as in particulate organic matter vertical fluxes (up to >100 mg C m⁻² day⁻¹). Proteins were the most abundant biochemical class of particulate organic matter (on average about 49%), followed by lipids (29%) and carbohydrates (22%). By contrast, organic matter collected in the sediment trap was characterized by the dominance of lipids (about 55% of the total biopolymeric carbon flux) over carbohydrates (28%) and proteins (17%). The hydrolizable particulate biopolymeric carbon accounted for about 23% of total biopolymeric carbon. This value was about one-half of that found in ice-free waters, suggesting that the suspended particulate organic material under the pack ice was less digestible than in ice-free waters or was already partially digested. Despite this, and the decay of labile organic compounds in the sediment trap during the deployment, material settling towards the sea bottom under the pack ice in Terra Nova Bay, owing to its high lipid content, might represent an important high-quality food source for benthic consumers. Finally, assuming as possible the intrusion under sea ice of primary organic matter-enriched waters, we hypothesize the occurrence of a “fertilization” effect deriving from ice-melting areas towards under-ice waters, supplying the latter with an additional rate of primary organic matter. Accepted: 18 February 1999     

Carbon isotopic composition in suspended organic matter and bottom sediments of the East Arctic seas

The samples of water and bottom sediments of the East Siberian and Chukchi Seas collected during the second Russian-American RUSALCA expedition were used to analyze patterns of the isotopic composition of carbon in the organic matter (OM) of suspended material (SOM) and bottom sediments (BOM). Similar to other marine environments, the SOM isotopic composition depended on the ratio between the terrigenous and planktonic OM, both in the water body as a whole and in its parts. Thus, in the East Siberian Sea the carbon of SOM was poorer in ¹³C (??¹³C = ?24.51??) than the open part of the more productive Chukchi Sea (??¹³C = ?22.16??). In the less productive coastal waters of the Chukchi Sea, the ratio of terrigenous OM increased, resulting in a ??¹³C shift to lower values (?23.40??). Due to the influx of reduced products of anaerobic diagenesis of the sediments, elevated total number of microorganisms and dark CO₂ fixation were found in the near-bottom water at the water-sediment biogeochemical barrier. The newly formed biomass of autotrophic microorganisms shifted the carbon isotopic composition of the near-bottom suspended material to more positive ??¹³C values, with the average values of ?23.39 and ?20.37?? for the East Siberian and Chukchi Sea, respectively. Changes in the carbon isotopic composition of OM resulting from microbial activity continued in the upper sediment layers. When the rate of biomass synthesis increased that of biomass consumption, the ¹³C content increased further. At higher rates of OM mineralization, ¹²C accumulated in its remaining part.     

Spatial and seasonal changes of dissolved and particulate organic C in the North Sea

Sampling of the central region of the North Sea was carried out to study the spatial and seasonal changes of dissolved and particulate organic C (DOC and POC, respectively). The surface waters were collected during four cruises over a year (Autumn 2004–Summer 2005). DOC and POC concentrations were measured using high temperature catalytic oxidation methods. The surface water concentrations of DOC and POC were spatially and temporally variable. There were significantly different concentrations of DOC and POC between the inshore and offshore waters in winter and summer only, with no clear trend in autumn and spring. Highest mean concentrations of DOC were measured in spring with lower and similar mean concentrations for other seasons. POC showed a clear seasonal cycle throughout the year with highest surface mean concentrations found in autumn and spring, but lowest in winter and summer. The DOC distributions during autumn and spring were strongly correlated with chlorophyll suggesting extracellular release from phytoplankton was an important DOC source during these two seasons. The lower concentrations of DOC in summer were probably due to the heterotrophic uptake of labile DOC. The seasonal changes in the C:N molar ratios of surface DOM (dissolved organic matter) resulted in higher mean C:N molar ratios in spring and lower ratios in winter. These high ratios may indicate nutrient limitation of heterotrophic uptake immediately after the spring bloom. There is limited data available for DOC cycling in these productive shelf seas and these results show that DOC is a major component of the C cycle with partial decoupling of the DOC and DON cycling in the central North Sea during the spring bloom. Handling editor: Luigi Naselli-Flores     

长白山原始阔叶红松林土壤有机质组分小尺度空间异质性

土壤有机质(SOM)对于维持生态系统生产力具有非常重要的意义,有机质的组成、空间分布和空间关联性是影响和控制诸多生态系统过程的重要因素。应用地统计学方法,对长白山原始阔叶红松林局部尺度内0—20 cm土壤有机质与活性有机质的空间异质性进行了研究,并通过交叉半方差分析探讨了二者之间的相关性。研究结果表明:(1)总体上来说,土壤有机碳(SOC)、全氮(TN)、颗粒态有机碳(POC)和颗粒态有机氮(PON)空间异质性较小;而土壤微生物量碳(MBC)、微生物量氮(MBN)和表层(0—10 cm)溶解性有机碳(DOC)的空间异质性较大;(2)SOC、TN、MBC、DOC、POC和PON随着深度的增加空间自相关性增加;而溶解性有机氮(DON)的空间自相关性随深度的增加变化不大;(3)SOC与TN在表层和下层(10—20 cm)均存在空间上的正相关关系;(4)SOC、TN在表层和下层分别与MBC、MBN、DOC、DON和POC呈空间上的正相关性,但是与PON之间的空间相关关系较差;(5)不同土层深度的土壤活性有机质之间的相关关系存在差异。在表层,除POC,PON外,其余土壤活性有机质组分在空间上两两相关;但是随着土壤深度的增加,活性有机质变量之间在空间上两两相关。研究结果表明土壤有机质组分在长白山原始阔叶红松林小尺度内存在不同空间异质性和空间关联性,这为人们更好的理解森林生态系统功能(如土壤养分循环)提供了重要的理论依据。     

The composition and distribution of the particulate matter in the Strait of Magellan (Chile) during the 1991 and 1995 Italian campaigns

During 1991 and 1995, the Italian National Program for Antarctic Research carried out two oceanographic campaigns in the framework of the International (Strait of) Magellan Project. In this paper, we describe the distribution, biochemical composition, and mineralogical characteristics of particulate matter and the characteristics of the water masses defining microbasins in the Strait of Magellan. The data analyses highlighted differences in quality and quantity of the suspended matter and its organic component in the basins that make up the Strait and the Pacific Ocean. The westernmost basin is subject to an eastward flow from the Pacific Ocean, and continental runoff in the surface layers, which are consequently rich in organic matter with a high C:N ratio. The central basin, Isla Carlos III–Segunda Angostura, is characterized by the mixing of Sub-Antarctic Pacific waters, continental runoff, and glacio-fluvial waters: The basin has the lowest particulate matter concentration, but at Paso Ancho showed high concentrations of organic matter with a lower C:N ratio. The easternmost basin, Segunda Angostura-Atlantic entrance, is characterized by the mixing of the water column due to strong Atlantic tides and showed high concentration of particulate matter with a high detritic component. Generally, the organic matter concentrations showed significant differences during the cruises, being higher in 1991. Using the multivariate discriminant factor analysis to determine whether statistically significant differences existed between defined sampling areas, we determined that temperature, particulate organic carbon, and total particulate matter were the variables most important for the difference between areas.     

Particulate organic carbon in the estuarine turbidity maxima of the Gironde,Loire and Seine estuaries: origin and lability

A study of the particulate organic carbon (POC) in the estuarine turbidity maxima (ETMs) of the three major French macrotidal estuaries shows that the average contents are 1.5, 3.3 and 3.1% (expressed in % of dry suspended sediment) in the Gironde, Loire and Seine Estuaries, respectively. There is no seasonal variation of POC contents in the Gironde Estuary, whereas, they often increase in the Loire and the Seine Estuaries in spring and summer. The lability of the estuarine particulate organic matter was estimated by two analyses: 1/labile organic matter was measured as the organic carbon loss during incubation tests over one month; 2/ the hydrolysable organic fraction was determined after 6N HCl digestion. The organic fractions of the ETMs are mainly refractory. Any increase in the amount of POC as compared to the background levels (cited above) is always correlated to an increase of organic matter lability. The yearly average fluvial contributions by various particulate organic pools (soil and litter organic matter; organic matter of phytoplanktonic and human origin) that enter the three estuaries were quantified. In the Garonne River, soil and litter are the major (90%) POC sources. In the Loire system, due to the eutrophication of the river water, phytoplankton contributes up to 50% of the total POC load. In the Seine river, soil and litter contribute 70% of the total POC input; POC of human origin is also significant (10%), due to the impact of the City of Paris (10 million inhabitants). The lability of the different types of organic matter ranks as follows: phytoplankton ∼litter > human-origin organic matter >> soil. By combining the POC budgets and the lability of each type of organic fraction, it was possible to explain why the POC of the three ETMs is different and characterizes its refractory vs. labile nature.