Transport and settlement of organic matter in small streams

1. After it enters streams, terrestrially derived organic matter (OM) rapidly absorbs water. Using field and laboratory experiments, we examined how this process affected the buoyancy, settling velocity, transport distance and retention locations of four types of organic matter typically found in Pacific coastal streams (‘flexible’ red alder leaves and three ‘stiff’ particle types – Douglas‐fir needles, red cedar fronds and Douglas‐fir branch pieces). 2. Immersion in water rapidly changed the physical characteristics of alder leaves, Douglas‐fir needles and red cedar fronds, which all reached constant still‐water settling velocities after only a few days of soaking. In contrast, the settling velocity of branch pieces continued to increase for 13 days, eventually reaching much higher values than any other OM type. Dried alder leaves became negatively buoyant after only two days of immersion, while other types took substantially longer (up to 24 days) before the specific gravity of all particles was >1. 3. We released saturated OM particles in an experimental channel and found that all particle types travelled further in a fast, shallow ‘riffle’ than a slow, deep ‘pool’. Comparisons with a passive settlement null model indicated that leaves were retained more rapidly than expected in the riffle (by large protruding stones), while the three stiff particle types travelled further than expected (probably due to turbulent suspension) and were retained when they settled in deeper water between larger stones. In pools, passive settlement appeared to dominate the retention of all OM types, with leaves travelling furthest. 4. These retention patterns corresponded well with those observed when saturated OM particles collected in the field were released in two pools and two riffles in a second‐order coastal stream. 5. When the experimental channel and in‐stream data were combined, the retention rates of the three stiff OM types were closely related to calculated Rouse numbers (Rouse number = particle settling velocity/shear velocity), whereas the retention rate of alder leaves was not. This suggests that different physical mechanisms are responsible for the retention of leaves and stiff OM types in shallow streams.     

Total PCB-organic matter correlation in sediments from three estuarine areas of Portugal

Levels of tPCB and organic matter content were determined in surface sediments from Sado and Tagus estuaries and Ria Formosa lagoon, and in particles collected in sediment traps in Ria Formosa. Significant relationships (p<0.001) between concentrations of tPCB and organic matter content were obtained when the most contaminated samples and the less contaminated organic-rich sediments were excluded. The excluded samples originated from nearby PCB and organic matter sources. Organic matter appears, thus, the most important indicator of a sediment's sorptive capacity in these estuarine systems, except near the sources where the organic seeking effect is masked. On the basis of the relationship slopes one may conclude that Tagus and Sado estuaries are more contaminated than Alcacer channel (upper Sado estuary) and Ria Formosa.     

Redistribution of sediments in three Swedish lakes

Sedimentation and redistribution of fine sediments in three Swedish lakes of different character have been investigated using settling sediment traps. The bottom shear stress from wind generated waves are calculated and the extension of erodable bottom area is related to wind conditions. Wave induced erosion and deposition during and after cessation of storms in different parts of a lake are discussed theoretically. It is shown that a single one year storm may redistribute more bottom material than the accumulated resuspension caused by frequent but smaller wind events. The settling sediment trap deposition and the concentration of suspended solids are related to the extension of erodable bottom area of particular storms. It is found that in lakes where there are relatively large areas of erosion bottoms, resuspended material from the part of the lake most susceptible to strong winds of large fetch constitutes a major part of the settled material on deep bottoms.     

Organic matter transport in an Appalachian Mountain river in Virginia,U.S.A.

Organic material transport in the New River, Virginia, was investigated over a 12 month period. Collections were made using drift nets and grab water samples from bridges at two sites about 210 km apart. About midway between the two sampling sites is a 1920 ha impoundment used for flood control and power generation. Dissolved organic matter (DOM) ranged 1–50 mg l^–1 at Site 1, upstream from the impoundment, and 11–19 mg l^–1 at Site 2 and was the most abundant form of organic matter at both sites during most periods of the year. Fine particulate organic matter (FPOM) ranged 1–45 mg l^–1 at Site 1 and 1–9 mg l^–1 at Site 2. Concentration of coarse particulate organic matter (CPOM) ranged 0.1–0.7 mg l^–1 at Site 1 and 0.1–0.2 mg l^–1 at Site 2. On an annual basis, the organic matter loads at Site 1 and Site 2 were computed to be 67 000 and 76 800 T y^–1, respectively, suggesting that the impoundment trapped and processed POM, and that municipal and industrial treatment facilities between the study sites supplemented DOM in the river.     

Organic matter in the Elbe estuary

In the low salinity region of the Elbe estuary in March–April 1992 the turbidity zone was characterized by high loads of suspended matter, 7% of which was organic material (750 μM C) at the surface. Particulate nitrogen, phosphorus and carbohydrates concentrations reached 55 μM N, 10 μM P and more than 15 μM glc. eq., corresponding to 13% of total C, at the surface and increasing threefold near the bottom. In spite of the peaking of particulate organic material levels in the maximum turbidity zone, there were only consistent qualitative changes in total particulate C, N, P, and carbohydrates along the Elbe estuary. Downstream, both the percentage of particulate organic material and the turbidity: organic material ratio decreased, indicating decomposition in the upper estuary and dilution with inorganic suspended matter from the lower estuary. Diatoms, the dominant phytoplankton group, decreased from the upper reaches towards the turbidity zone by 0.3 (surface) and 1.5 mg C l⁻¹ (bottom). This corresponded to 12 and 60% of the decrease in total particulate carbon. Estimated local input of organic carbon by primary production (21 μg Cl⁻¹d⁻¹) was almost compensated by calculated minimum grazing (14 μg C l⁻¹d⁻¹). Considering net primary production and grazing, the dissimilation by zooplankton (5 μg C l⁻¹d⁻¹) and heterotrophic bacterial decomposition (48 μg C l⁻¹d⁻¹), when summed over the estimated flushing time (12 days) represented a loss of suspended organic matter of 0.6 mg Cl⁻¹. Since this was only 20% of the observed decrease in particulate carbon, significant dilution processes must be assumed. Dissolved organic nitrogen decreased from 35 to 10 μM N and dissolvd organic phosphorus from 0.6 to 0.1 μM P towards the sea, mainly due to dilution. The distribution of phosphate, with highest loads in the turbidity maximum of 2.4 μM, suggested an interaction with the accumulated load of particulate material.     

Dissolved Organic Matter concentration and composition in the forests and streams of Olympic National Park, WA

Dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) character were investigated in soil water (15 and 40 cm) and streams at eleven sites in Olympic National Park. In addition, the effect of added nitrogen on soil water DOM concentration and composition was tested. Forested plots covering a gradient of precipitation, climate, slope, and aspect in Olympic National Park were fertilized with the addition of 20, 10 and zero (control) kg urea-N ha^–1 y^–1. Seven sites had the two different fertilizer treatments and control plots, while the additional four sites had no fertilizer treatments. Soil water DOC concentrations ranged from 0.5 mg C/L to 54.1 mg C/L, with an average value of 14.1 mg C/L. Streams had low DOC concentrations ranging from 0.2 mg C/L to 4.4 mg C/L, with an average value of 1.2 mg C/L. DOM composition was examined with regard to molar ratios, H:C, O:C and N:C, index of unsaturation, average carbon oxidation state, and specific absorbance. Fertilizer had no consistent effect on either DOM concentration or composition across the study sites. Soil depth influenced both DOM concentration and composition. Shallow soil water DOM had greater concentrations, higher specific absorbance, a higher degree of unsaturation, and had lower molar ratios compared to deep soil water samples. Overall, changes in DOM stoichiometry and specific absorbance as a function of soil depth were consistent despite the diversity of the forested study sites sampled.     

Impact of Anthropogenic Organic Matter on the Distribution Patterns of Sediment Microbial Community from the Yangtze River,China

Abstract

How microbes respond to substantial and increasing anthropogenic disturbance remains an open question in river systems. We tested the hypothesis that the source and distribution of anthropogenic organic matter (OM) were significant factors affecting the spatial variation of the microbial community composition of the Yangtze River sediments. Bulk geochemical proxies and lignin phenols suggested a general decrease of terrestrial C3 plants or soil OM input from the middle to the lower reaches. Fecal sterols inferred higher sewage contamination levels in the middle reaches. Polycyclic aromatic hydrocarbons (PAHs) distribution indicated a dominant biomass and coal combustion signal in the middle reaches, whereas a mixed source including petroleum combustion in the lower reaches. Phylogenetic analysis revealed a large portion of Methanobacteria and Verrucomicrobia enriched in the middle reaches, whereas OM-degrading bacteria, including Flavobacteria, Acidobacteria, and Alphaproteobacteria were dominant in the lower reaches. Quantitative PCR analyses and multivariate analysis further demonstrated that sources and distribution of OM had combined effects in shaping alpha and beta-diversity of sediment microbial communities. Sewage discharge and incomplete OM combustion, respectively, were associated with Methylococcaceae, Chloroflexi, and Bacteroidetes groups. This study provides a foundation for further understanding of the river sediment microbial composition, considering the continued increase of anthropogenic influences.     

Storage and dynamics of organic matter in different springs of small floodplain streams

The flow of groundwater through the sediment layer (underflow or hyporheic zone) of streams and at the origin of streams can influence organic matter uptake dynamics of floodplain. The River Rhône floodplain has limestone foothills. Here we studied 2 karstic and 2 interstitial springs differing by aquifer geology. Organic matter, physico-chemical conditions were compared between these springs during two seasons (from March to September 1989) and at different depths (0, –20 cm, –40 cm).Temperatures indicated large differences in underflow between springs, in their relation to the surrounding environment, and between seasons. Springs are well oxygenated, with differences between layers. Cultivated fields supply interstitial springs with nitrates, and pools are nutrient traps. DOC was heterogeneous in space and time and correlates with VFPOC. Particulate nutrients were correlated with available surface area of sediment grains. Physical conditions of each spring were prominent in determining storage and turnover of organic matter. Each spring, by its own characteristics and dynamics regulating stability and turnover, had an effect or control on storage, transport and retention of organic matter (quality, quantity). These springs offer an example of the heterogeneity, and give a view of the diversity of patches within a floodplain. The data suggest that groundwater flow of springs may be a major factor in the functioning of floodplain tributaries.     

Organic matter composition and degree of humification in lignite-rich mine soils under a chronosequence of pine

In the Lusatian mining district, in the eastern part of the Federal Republic of Germany, organic matter of reclaimed mine soils consists of a mixture of lignite and recently formed soil organic matter (recent carbon). The aim of the study was to investigate the recent carbon accumulation and the degree of humification of a chronosequence of young mine soils under forest. The lignite content of the forest floor, Ai (0–5 cm) and Cv horizons (1 m depth) was determined by ¹⁴CU activity measurements and the structural composition of the organic matter was characterised by ¹³C CPMAS NMR spectroscopy. To obtain a characterisation of the degree of humification, the soil samples were analysed for the content of polysaccharides, proteins, lignin and lipids by wet chemical methods. ¹⁴C activity measurements indicate that at the oldest site, comparable amounts of carbon accumulated in the first few centimetres of the soil profile than in natural forest soils. ¹³C CPMAS NMR spectra of the organic matter in the Ai horizons of the three soil profiles were dominated by aromatic and alkyl carbon species characteristic for lignite, but indicated as well an increasing contribution of carbon species from decomposing plant litter with soil age. When the results from wet chemical analyses were normalised to the total carbon content no changes with age could be noticed. After normalisation of the amount of litter compounds to the recent carbon content, the carbon identified by plant litter compound analysis decreased with increasing depth and increasing age of the soils. After 32 years the values are comparable to those of natural forest soils. These observations were confirmed by increasing degree of lignin alteration with stand age and soil depth. The data of wet chemical analyses complement data obtained by ¹⁴C activity measurements and ¹³C CPMAS NMR spectroscopy and lead to the conclusion that 32 years after reforestation the degree of humification of the soil organic matter is in the same range as those of natural sites. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Role of transported particulate organic matter in the macroinvertebrate colonization of litter bags in streams

1. The exposure of mesh litter bags has been widely used to investigate the role of benthic macroinvertebrates in leaf processing in freshwaters. In this sense, several studies have related litter bag breakdown rates to the presence of colonizing invertebrates. A possible confounding factor in such experiments is that the litter bags trap suspended particulate organic matter that can itself attract invertebrate colonists, irrespective of the intended experimental treatment.
2. We attempted to quantify the accumulation of particulate organic matter (POM) within litter bags and to investigate its possible impact on macroinvertebrate density and richness. In seven headwater forested streams we exposed mesh bags filled either with beech leaves ( Fagus sylvatica ) or with plastic strips of an equal surface area.
3. Principal component analysis (PCA) showed that bag type and stream were the main explanatory variables for invertebrate colonization and POM accumulation within the bags. In contrast, there was little variation among sampling dates (6.4% of the total inertia).
4. The accumulated POM within the bags was substantial (up to 8.83 g ash-free dry mass (AFDM)) but highly variable among sites (mean from 0.32 to 4.58 g AFDM). At each of the seven sites, both richness and abundance of invertebrates were positively correlated with the mass of accumulated POM in bags. Macroinvertebrate colonization (notably taxon richness) was directly linked with the quantity of POM accumulated.
5. Our findings provide evidence of a potential pitfall in linking invertebrates to litter processing in mesh bags, particularly when large amounts of POM, entrained in stream flow, accumulate within the bags. An evaluation of the POM mass trapped in litter bags could account for the erratic patterns sometimes observed in their colonization by invertebrates.     

Tropical tree species composition affects the oxidation of dissolved organic matter from litter

Plant species effects on soil nutrient availability are relatively well documented, but the effects of species differences in litter chemistry on soil carbon cycling are less well understood, especially in the species-rich tropics. In many wet tropical forest ecosystems, leaching of dissolved organic matter (DOM) from the litter layer accounts for a significant proportion of litter mass loss during decomposition. Here we investigated how tree species differences in soluble dissolved organic C (DOC) and nutrients affected soil CO₂ fluxes in laboratory incubations. We leached DOM from freshly fallen litter of six canopy tree species collected from a tropical rain forest in Costa Rica and measured C-mineralization. We found significant differences in litter solubility and nutrient availability. Following DOM additions to soil, rates of heterotrophic respiration varied by as much as an order of magnitude between species, and overall differences in total soil CO₂ efflux varied by more than four-fold. Variation in the carbon: phosphorus ratio accounted for 51% of the variation in total CO₂ flux between species. These results suggest that tropical tree species composition may influence soil C storage and mineralization via inter-specific variation in plant litter chemistry.     

Microbial destruction of organic matter in the bottom sediments of Lithuanian lakes

The rates of the processes of bacterial sulfate reduction (SR) and total destruction of organic matter (D_total) were studied in the bottom sediments (BS) of 14 lakes in Lithuanian national and regional parks in the summers of 1998–2002. Anaerobic processes accounted for an average of 92% of D_total in the depressions of deep-water lakes; for the sediments of shallow lakes, high rates of oxygen uptake were noted. The SR rate in different lakes varied from 0.09 to 2.60 mg S^2?/(dm³ day). At low sulfate concentrations (13.3–70.6 mg S-SO ₄ ^2? /dm³), characteristic of the BS of freshwater ecosystems, the main factor that affected the SR rate in the BS of the lakes studied was the content of readily available organic matter; only in special cases, was it affected by a change in the sulfate ion concentration. In shallow lakes, temperature-dependent activation of sulfate-reducing bacteria and their inhibition by acidification of the environment were recorded. The contribution of SR to D_total was 0.2 to 11.0%.     

Bacterial abundance and production in river sediments as related to the biochemical composition of particulate organic matter (POM)

The major proportion of heterotrophic activity in running waters islocalized on the solid surfaces of sediments in the benthic and hyporheic zoneand is dominated by microorganisms. However, this assertion is based on thestudies of small streams, and little is known about the microbial metabolism oforganic matter in river ecosystems. We therefore explored the relationshipsbetween bacterial abundance and production and the gradients of organic matterquality and quantity in sediments of a sixth-order lowland river (Spree,Germany). We found vertical gradients of detrital variables (particulateorganicmatter (POM), particulate organic carbon (POC), nitrogen (PN), and protein) andof bacterial variables (abundance, production, turnover time, and proportion ofbacterial carbon in total POC) in two different sediment types. These gradientswere steeper in stratified sediments than in the shifting sediments. Detritalvariables correlated strongly with bacterial abundance and production. The bestcorrelation was found for detrital variables indicating substrate quantity andquality (r_S = 0.90 for PN with abundance). Although bacterialbiomasscomprised only 0.7% of the POC (1.9% of PN, 3.4% of the protein) in sediments,the turnover of sedimentary organic carbon was fast (median = 62d), especially in the shifting sediments. Our findings demonstratethat sediment dynamics significantly foster organic carbon metabolism in riversystems. Thus, these sediments, which are typical for lowland rivers, stronglyinfluence the metabolism of the whole ecosystem.     

Loss-on-ignition estimates of organic matter and relationships to organic carbon in fluvial bed sediments

Fluvial bed sediments represent an important sink and source for a variety of organic and inorganic compounds. Their most important constituent is organic matter (OM) and its primary component organic carbon (OC). Few studies have been conducted in fluvial environments examining bed-associated OM or OC. This is surprising given the recent interest in global carbon cycling and the importance of bed-associated organics as ecosystem energy sources. The objective of this study was to examine the relationship between OM, determined by loss-on-ignition (LOI), and OC in fluvial bed sediments determined by a dry combustion analyzer. The wide adoption of the LOI method in soil science reflects its ease of use, it is inexpensive, it is rapid, requires no specialized training, and strong statistical relationships commonly exist between OM and OC estimated by standard dry combustion procedures. Regression models were developed between OC and OM for six bed sediment size fractions (≤2.0 mm) for 113 sample sites in a tropical stream on Oahu, Hawaii. All models were highly significant (p < 0.0001), with coefficients of determination ranging from 35 to 79%. Measurement of LOI explained 64% of the variation in OC for all grouped data. The black-box LOI approach may be useful for rapid reconnaissance surveys of drainage systems. Examination of OM to OC conversion factors for Manoa bed sediments indicates that values typically observed in the soils literature (1.7–2.2) are far too low. Values of OM/OC were found to increase with increasing grain size, and decrease with increasing LOI percentage. Conversion factors obtained for grouped data had a mean of 14.9, a coefficient of variation of 21%, and a range of values between 6.2 and 27.4. It is suggested that these high conversion factors reflect significant water loss by dehydration of Fe, Al, and Mn oxides at a muffle furnace temperature of 450 °C. Therefore, the blind application of conversion factors developed from soils should be avoided when converting from OM to OC for fluvial bed sediments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Organic matter, heterotrophic activity, and NO· consumption in soils

When the effect of water content was minimized, soil CO₂ evolution and soil organic matter content were good predictors of aerobic NO. uptake rate constants across a wide range of soil types. Field manure application to a Gleysol stimulated NO. uptake rate constants and lowered NO. compensation points compared to unfertilized or NH₄NO₃-fertilized soil. This effect lasted for months after manure application. In a laboratory experiment, addition of manure reduced the NO. efflux associated with nitrification of NH₄ Cl fertilizer, and manured soils had a greater capacity to remove NO. from polluted air. Evidence is presented that these observations result from NO. oxidation during heterotrophic microbial activity in soil.     

Storage and decomposition of particulate organic matter along the longitudinal gradient of an agriculturally-impacted stream

An agriculturally-impacted stream in northern Idaho was examined over a two-year period to determine seasonal and longitudinal patterns of the storage and decomposition of particulate organic matter. Biomass of benthic organic matter (BOM) was considerably less than values reported in the literature for comparable, undisturbed streams. Coarse, fine, and total benthic particulate organic matter were not correlated with parameters pertaining to stream size (e.g., stream order), but were correlated with sample site and amount of litterfall. The association of BOM with site and litterfall suggests that storage of particulate organic matter is a function of local characteristics rather than stream size. Low biomass of stored organic matter is a response to the low input of terrestrially-derived organic matter resulting from removal of climax vegetation.Leaf packs of alder, Alnus sp., were placed in the stream seasonally for 30 and 60 d. While there were significant differences for months, there was no significant difference among sites for leaf packs exposed for 30 d. Significant differences were observed among both sites and months for leaf packs exposed for 60 d; however, differences among sites accounted for only 5% of the variance. The absence of differences in decomposition of organic matter along the gradient of Lapwai Creek, despite heterogeneity of the drainage basin and availability of organic matter, may be in response to the overall low biomass of stored benthic organic matter. This study demonstrates that agricultural activity can substantially influence instream heterotrophic processes through reduced availability of organic matter and can shape community structure and ecosystem dynamics of streams flowing through agricultural drainage basins.     

Sulphide release from anoxic sediments in relation to iron availability and organic matter recalcitrance and its effects on inorganic phosphorus recycling

This research aims to analyse the sediment capacity to buffer free sulphide release in three coastal lagoons which differ in terms of eutrophication level, tide influence and primary producer communities. A preliminary estimate of soluble reactive phosphorus (SRP) regeneration coupled with sulphide fluxes is also made. Sediment profiles of ferrous and ferric iron and reduced sulphur pools were determined in three stations in the Bassin d'Arcachon (South West France), in one site in the Etang du Prévost lagoon (Southern France), and in three stations in the Sacca di Goro lagoon (Northern Italy). Laboratory experiments were also conducted by incubating sediment slurries. Slurries from the French lagoons were also enriched with about 2% d.w. of organic detritus obtained from the dominant macrophytes of each site, namely Zostera noltii and Ruppia cirrhosa (Bassin d'Arcachon), and Ulva rigida (Etang du Prévost). In the Sacca di Goro, slurry experiments were conducted at two sites with different salinity range, sediment composition and hydrodynamics.Field data showed that concentrations of available iron (Fe(II)+Fe(III)) ranged from a minimum of 28.5 µmol cm^–3 (Etang du Prévost) to a maximum of 275.7 µmol cm^–3 (Sacca di Goro). Moreover, in the French lagoons, acid volatile sulphide (AVS) accumulation in the superficial sediment was related to ferrous iron concentrations. Laboratory experiments showed that in spite of strong reducing conditions, sulphide and SRP release was weaker in iron-rich sediments and in those enriched with the most refractory organic matter. The highest fluxes were detected in sediment slurries from the Etang du Prévost, which had the lowest iron content, supplied by 2% of the labile detritus from Ulva rigida. In this case, SRP release was directly related to sulphide production.Two factors seem significant to evaluate the buffer capacity against free sulphide and SRP release from anoxic sediment: organic matter biodegradability, which forces sediment toward reducing conditions, and iron availability, which can affect sulphide mobility as well as the iron hydroxide-phosphate-sulphide system.     

Early stages of leaf decomposition are mediated by aquatic fungi in the hyporheic zone of woodland streams

1. Leaf litter constitutes the major source of organic matter and energy in woodland stream ecosystems. A substantial part of leaf litter entering running waters may be buried in the streambed as a consequence of flooding and sediment movement. While decomposition of leaf litter in surface waters is relatively well understood, its fate when incorporated into river sediments, as well as the involvement of invertebrate and fungal decomposers in such conditions, remain poorly documented. 2. We tested experimentally the hypotheses that the small interstices of the sediment restrict the access of the largest shredders to buried organic matter without compromising that of aquatic hyphomycetes and that fungal decomposers in the hyporheic zone, at least partly, compensate for the role of invertebrate detritivores in the benthic zone. 3. Alder leaves were introduced in a stream either buried in the sediment (hyporheic), buried after 2 weeks of exposure at the sediment surface (benthic‐hyporheic), or exposed at the sediment surface for the entire experiment (benthic). Leaf decomposition was markedly faster on the streambed surface than in the two other treatments (2.1‐ and 2.8‐fold faster than in the benthic‐hyporheic and hyporheic treatments, respectively). 4. Fungal assemblages were generally less diverse in the hyporheic habitat with a few species tending to be relatively favoured by such conditions. Both fungal biomass and sporulation rates were reduced in the hyporheic treatment, with the leaves subject to the benthic‐hyporheic treatment exhibiting an intermediate pattern. The initial 2‐week stage in the benthic habitat shaped the fungal assemblages, even for leaves later subjected to the hyporheic conditions. 5. The abundance and biomass of shredders drastically decreased with burial, except for Leuctra spp., which increased and was by far the most common leaf‐associated taxon in the hyporheic zone. Leuctra spp. was one of the rare shredder taxa displaying morphological characteristics that increased performance within the limited space of sediment interstices. 6. The carbon budgets indicated that the relative contributions of the two main decomposers, shredders and fungi, varied considerably depending on the location within the streambed. While the shredder biomass represented almost 50% of the initial carbon transformed after 80 days in the benthic treatment, its contribution was <0.3% in the hyporheic one and 2.0% in the combined benthic‐hyporheic treatment. In contrast, mycelial and conidial production in the permanently hyporheic environment accounted for 12% of leaf mass loss, i.e. 2–3 times more than in the two other conditions. These results suggest that the role of fungi is particularly important in the hyporheic zone. 7. Our findings indicate that burial within the substratum reduces the litter breakdown rate by limiting the access of both invertebrate and fungal decomposers to leaves. As a consequence, the hyporheic zone may be an important region of organic matter storage in woodland streams and serve as a fungal inoculum reservoir contributing to further dispersal. Through the temporary retention of litter by burial, the hyporheic zone must play a significant role in the carbon metabolism and overall functioning of headwater stream ecosystems.     

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.