Optical signatures of dissolved organic matter in the watershed of a globally large river (Yangtze River,China)

Parallel factor analysis of fluorescence excitation emission matrices of surface water samples of a globally large river (Yangtze River, China) watershed identified three classes of fluorescent dissolved organic matter (FDOM) that had ex/em = 280/330 nm, 305/385 nm and 350/450 nm respectively, resembling “peak T”, “peak M” and “peak C” commonly identified in natural water, respectively. Peak T (a tyrosine/tryptophan-like FDOM) did not show correlations to peak M or C which were humic-like substances, while a positive correlation (r = 0.935, p < 0.001) was present between the natural log-transformed maximum fluorescence intensity (F_max) of peaks T and M indicating a tight link during their production and processing. F_max values (in Raman unit nm^?1) normalized to dissolved organic carbon (DOC) concentration were low, varying in ranges 15.93–85.95, 29.83–83.54 and 19.73–51.05 × 10^?5 nm^?1 (μmol/L)^?1 for peaks T, M and C, respectively, in line with the history of strong photobleaching of the water samples as indicated by fairly high absorption spectral slope ratios (0.75–1.53 with a mean 1.03). Intermediate fluorescence index (FI) (1.46–1.83 with a mean 1.61) and small specific absorption at 254 nm (0.64–1.93 with a mean 1.15 m^?1 mg^?1 L) of the water samples, indicated the presence of both aquatic microbial DOM (e.g. peak T) and soil DOM (e.g. peak C). Peak C could be substantially removed by UV-A (320–400 nm) irradiation, while peak M was slightly increased when a microbe-containing water was exposed to the same UV-A irradiation. Taken together, peak C was attributed to diffuse soil source while peak M was likely attributed to joint effects of microbial activities and solar irradiation on the chromophores in the sample.     

Photoproduction of hydrogen peroxide in aqueous solution from model compounds for chromophoric dissolved organic matter (CDOM)

To explore whether quinone moieties are important in chromophoric dissolved organic matter (CDOM) photochemistry in natural waters, hydrogen peroxide (H₂O₂) production and associated optical property changes were measured in aqueous solutions irradiated with a Xenon lamp for CDOM model compounds (dihydroquinone, benzoquinone, anthraquinone, napthoquinone, ubiquinone, humic acid HA, fulvic acid FA). All compounds produced H₂O₂ with concentrations ranging from 15 to 500 μM. Production rates were higher for HA vs. FA (1.32 vs. 0.176 mM h⁻¹); values ranged from 6.99 to 0.137 mM h⁻¹ for quinones. Apparent quantum yields (Θ_app; measure of photochemical production efficiency) were higher for HA vs. FA (0.113 vs. 0.016) and ranged from 0.0018 to 0.083 for quinones. Dihydroquinone, the reduced form of benzoquinone, had a higher production rate and efficiency than its oxidized form. Post-irradiation, quinone compounds had absorption spectra similar to HA and FA and 3D-excitation–emission matrix fluorescence spectra (EEMs) with fluorescent peaks in regions associated with CDOM.     

Total,methyl and organic mercury in sediments of the Southern Baltic Sea

Distribution of sedimentary mercury in the Southern Baltic was investigated. Sediment samples were collected from the Southern Baltic in the period from 2009 to 2011, and concentrations of sedimentary total mercury (average 102 ng/g, range 5.8–225 ng/g) and methyl mercury (average 261 pg/g, range 61–940 pg/g) were measured in the manner that the influence of both patchiness and seasonal changes were assessed. Moreover, sedimentary mercury extracted with organic solvent- the so-called organic mercury was also analyzed (average 425 pg/g, range 100–1440 pg/g). There is a statistically significant dependence between organic mercury and both methyl mercury and total mercury concentrations in the sediments. Methyl mercury contribution to total mercury varied from 0.12% to 1.05%, while organic mercury contributed to 2% of total concentration on average. The area studied, although mercury concentrations exceed threefold the geochemical background, can be regarded as moderately contaminated with mercury, and methylmercury.     

Chemical evolution of Macondo crude oil during laboratory degradation as characterized by fluorescence EEMs and hydrocarbon composition

The fluorescence EEM technique, PARAFAC modeling, and hydrocarbon composition were used to characterize oil components and to examine the chemical evolution and degradation pathways of Macondo crude oil under controlled laboratory conditions. Three major fluorescent oil components were identified, with Ex/Em maxima at 226/328, 262/315, and 244/366 nm, respectively. An average degradation half-life of ～20 d was determined for the oil components based on fluorescence EEM and hydrocarbon composition measurements, showing a dynamic chemical evolution and transformation of the oil during degradation. Dispersants appeared to change the chemical characteristics of oil, to shift the fluorescence EEM spectra, and to enhance the degradation of low-molecular-weight hydrocarbons. Photochemical degradation played a dominant role in the transformation of oil components, likely an effective degradation pathway of oil in the water column. Results from laboratory experiments should facilitate the interpretation of field-data and provide insights for understanding the fate and transport of oil components in the Gulf of Mexico.     

Ecotoxicity of Musa paradisiaca leaf extract-coated ZnO nanoparticles to the freshwater microcrustacean Ceriodaphnia cornuta

Currently, nanotechnology has gained much interest due to the unique properties of nanomaterials in science and technology. Different types of metallic nanoparticles are routinely synthesized. However, their release into the aquatic environments is a major ecotoxicological concern. In this scenario, it is important to study the potential impact of engineered nanoparticle in aquatic organisms especially freshwater microcrustaceans, such as Ceriodaphnia cornuta. In this study, ZnO NPs were synthesized using the aqueous leaf extracts of Musa paradisiaca and physico-chemically characterized by UV–Vis spectroscopy, X-ray diffraction (XRD), Fourier transform infra red (FTIR), scanning electron microscopy (SEM) and atomic force microscopy (AFM). UV–Vis spectroscopy recorded the absorbance peak of ZnO NPs at 338 nm. XRD analysis showed the various Bragg’s reflection peaks at 100, 002, 101, 102, 110, 103, 200, 112, 201, 004 and 202 lattice planes. FTIR spectroscopy outlined sharp intense peaks at 3416 cm⁻¹, 1388 and 1416 cm⁻¹. SEM showed the spherical shape of ZnO NPs with mean particle size of 23.3 nm. AFM confirmed the spherical shape, nanosize and 3D topography of NPs. The ecotoxicity of ZnO NPs was tested on the freshwater crustacean C. cornuta. ZnO NPs were comparatively less toxic than zinc acetate. ZnO NPs caused 42% mortality of C. cornuta at 50 μg mL⁻¹. However, 80% mortality was observed at 50 μg mL⁻¹ of zinc acetate after 24 h. Light and confocal laser scanning microscopic images evidenced the uptake and accumulation of ZnO NPs in the gut of C. cornuta at 50 μg mL⁻¹ after 24 h. Structural deformities were observed on C. cornuta after treatment with 50 μg mL⁻¹ of ZnO NPs. Overall, this study describes the potential impact of the biologically synthesized ZnO NPs in comparison with zinc acetate in the freshwater crustacean C. cornuta.     

Arsenic and heavy metals in native plants at tailings impoundments in Queretaro,Mexico

Ten native plants species that grow in three tailings dams from Ag, Pb, Cu and Zn mine in Queretaro, Mexico were studied. Total concentrations in tailings were 183–14,660 mg/kg As, 45–308 mg/kg Cd, 327–1754 mg/kg Pb, 149–459 mg/kg Cu and 448–505 mg/kg Zn. In the three tailings dams, the solubility of these elements is low. Tailings in dam 1 are acid generating while tailings in dams 2 and 3 are not acid-generating potential. Plants species that accumulate arsenic and heavy metals was identified; Nicotina glauca generally presented the highest concentrations (92 mg/kg As, 106 mg/kg Cd, 189 mg/kg Pb, 95 mg/kg Cu and 1985 mg/kg Zn). Other species that accumulate these elements are Flaveria pubescens, Tecoma stans, Prosopis Sp, Casuarina Sp and Maurandia antirrhiniflora. Two species were found that accumulates a large amount of metals in the root, Cenchrus ciliaris and Opuntia lasiacantha. Concentrations in soils in which plants grow were 488–5990 mg/kg As, 5–129 mg/kg Cd, 169–3638 mg/kg Pb, 159–1254 mg/kg Cu and 1431–13,488 mg/kg Zn. The Accumulation Factor (AF) determined for plants was less than 1, with exception of N. glauca for Cd. The correlation between arsenic and heavy metals found in soils and plants was low. Knowledge of plant characteristics allows it use in planning the reforestation of tailings dams in controlled manner. This will reduce the risk of potentially toxic elements are integrated into the food chain of animal species.     

Content and distribution of trace metals in surface sediments from the northern Bering Sea,Chukchi Sea and adjacent Arctic areas

Concentrations of trace metals (Zn, Cr, Cu, V, Cd and Pb), total organic carbon (TOC), black carbon (BC) and their granulometry were examined in 25 surface sediment samples from the northern Bering Sea, Chukchi Sea and adjacent areas. Trace metal concentrations in the sediments varied from 21.06–168.21 mg kg⁻¹ for Zn, 8.91–46.94 mg kg⁻¹ for Cr, 2.69–49.39 mg kg⁻¹ for Cu, 32.46–185.54 mg kg⁻¹ for V, 0.09–0.92 mg kg⁻¹ for Cd, and 0.95–15.25 mg kg⁻¹ for Pb. The geoaccumulation index (I_geo) indicated that trace metal contamination (Zn and Cd) existed in some stations of the study area. The distribution of grain size plays an important role in influencing the distribution of trace metals (Zn, Cr, Cu, V, and Pb) in sediments from the Chukchi Sea and adjacent areas.     

The effect of Hurricane Lili on the distribution of organic matter along the inner Louisiana shelf (Gulf of Mexico,USA)

On October 3, 2002 Hurricane Lili made landfall on a previously studied region of the inner Louisiana shelf as a Category 2 storm with winds over 160 km/h. A week after the hurricane, major impacts of the storm were not evident in the water column except for the lower than expected inshore salinities (∼12 psu) for this time of year, which was characterized by low river discharge. Turbidity profiles were typical of those measured during previous investigations with suspended sediment concentrations >75 mg/L at inshore stations and <50 mg/L in surface waters and offshore. The implication is that the sediments resuspended during the hurricane settled soon after the storm passage. Water column particulate organic carbon (POC) concentrations ranged from 0.1 to over 2.0 mg/L, with the highest concentrations measured near the seabed and in the inshore portions of the study area. Suspended particles were characterized by low organic matter content (%POC of 0.5–2 wt%), low chlorophyll:POC ratios (Chl:POC<4 mg/g) and moderately elevated POC:particulate nitrogen ratios (POC:PN of 10–14 mol/mol), all suggesting their source was locally resuspended seabed sediment rather than from algal biomass or land-derived vascular plant detritus.Post hurricane sediment deposition throughout the study area resulted in a storm layer that ranged from <0.5 to 20 cm in thickness. In most locations sediment accumulation ranged from 3 to 10 cm. The storm deposits were generally composed of silty clays with a coarser, somewhat sandy 1–2 cm basal layer. Surface sediments from the storm layer were characterized by relatively high mineral surface areas (SA of 30–50 m²/g) and elevated OC contents (%OC of 1.0–2.0%). The dispersal of fine sediments following the hurricane resulted in marked changes in the SA and %OC values of surface sediments from offshore locations, which prior to the storm contained coarser, organic-poor particles (SA of 5–15 m²/g and %OC of 0.2–0.6%). The OC:SA and OC:N ratios of storm layer sediments ranged from 0.4 to 0.6 mg OC/m² and from 10 to 12 mol/mol, respectively, and were comparable to those measured in surface sediments prior to the hurricane. Such similarities in the composition of the organic matter reinforce the idea that the source of the storm deposits was the finer fraction of resuspended seabed sediments, with little evidence for inputs from local land-derived sources or autochthonous algal production. Overall, the magnitude of sediment and organic matter deposition on the seabed after the storm greatly exceeded the annual inputs from the Atchafalaya River and coastal primary production. The combined effects of hurricane-driven erosion and post-storm deposition represent a major perturbation to the benthic community of the region, which is already subject to these types of disturbances due to the combined effects of peaks in river discharge and the passage of storm fronts.     

Determination of selected persistent organic pollutants in wastewater from landfill leachates,using an amperometric biosensor

Landfill leachates that contain persistent organic pollutants (POPs) are a big threat to groundwater systems and are projected to have hazardous effects in the long term if proper management strategies of the landfills are not put in place by those responsible. Monitoring the levels of POPs in landfill leachates is very crucial. This work presents an amperometric biosensor for determination of selected POPs in landfill leachates. The biosensor is based on kinetic inhibition of horseradish peroxidase (HRP). The enzyme was immobilised by electrostatic attachment on a polyaniline-modified Pt electrode surface. Selected POPs inhibited HRP enzyme activity and the decrease in the enzyme activity was used to determine these environmental pollutants. Selected polybrominated diphenyl ethers (PBDEs), polybrominated biphenyls (PBBs) and polychlorinated biphenyls (PCBs) were the analytes of choice because they are commonly found in South Africa water systems. Limits of detection for the amperometric biosensor were established as 0.014, 0.018, 0.022, 0.016 and 0.019 μg l⁻¹ for BDE-100, PBB-1, PCB-1, PCB-28 and PCB-101, respectively. The HRP biosensor system gave different linear ranges for; BDE-100 (0.424–25.8 μg l⁻¹), PBB-1 (0.862–13.4 μg l⁻¹), PCB-1 (0.930–18.1 μg l⁻¹), PCB-28 (0.730–15.7 μg l⁻¹) and PCB-101 (0.930–27.1 μg l⁻¹). Inhibition studies on HRP biosensor response toward the reduction of H₂O₂ in the absence and presence of the selected POPs were carried out to investigate the inhibition kinetics and its mechanism. The results obtained indicated that the inhibition mechanism was competitive for PBDEs and non-competitive for biphenyls (PCBs and PBBs). The application of the biosensor was tested on wastewater samples obtained from landfill leachate for determination of selected POPs. The leachate samples were found to contain PCB-28 (0.28 ± 0.03 μg l⁻¹) and PCB-101 (0.31 ± 0.02 μg l⁻¹). The samples were also analysed by GC–MS as a cross-check method and the two sets of results were in close agreement.     

Nutrient inputs from submarine groundwater discharge on the Santiago reef flat,Bolinao, Northwestern Philippines

Submarine groundwater discharge (SGD) on the reef flat of Bolinao, Pangasinan (Philippines) was mapped using electrical resistivity, ²²²Rn, and nutrient concentration measurements. Nitrate levels as high as 126 μM, or 1–2 orders of magnitude higher than ambient concentrations, were measured in some areas of the reef flat. Nutrient fluxes were higher during the wet season (May–October) than the dry season (November–April). Dissolved inorganic nitrogen (DIN = NO₃ + NO₂ + NH₄) and soluble reactive phosphorus (SRP) fluxes during the wet season were 4.4 and 0.2 mmoles m⁻² d⁻¹, respectively. With the increase population size and anthropogenic activities in Bolinao, an enhancement of SGD-derived nitrogen levels is likely. This could lead to eutrophic conditions in the otherwise oligotrophic waters surrounding the Santiago reef flat.     

Late Pleistocene genesis of the middle Yarlung Zhangbo Valley,southern Tibet (China), as deduced by sedimentological and luminescence data

Knowledge on valley formation and palaeohydrology of the Yarlung Zhangbo (named Brahmaputra south of the Himalayas) is still in an early stage. Research was conducted in the middle valley reach around Gonggar in order to investigate the sediment properties, age and palaeoenvironmental implication of a widespread aggradational valley terrace. This heavily dissected terrace, lying c. 25 m above the present floodplain, consists of topping aeolian and colluvial deposits, intermediate lacustrine fines and basal fluvial sands. Sedimentological properties of the lacustrine layers suggest a cold-climate depositional environment. Geochemical data refer to a common provenance of all sediments investigated from a regional catchment area, implying a local provenance of topping loesses and sands by aeolian sorting of nearby fluvial deposits. OSL dates, representing the first luminescence dating effort partly applied to non-aeolian sediments in this valley, cluster closely in the interval 11.7 ± 0.8–17.4 ± 1.4 ka. During the Late Pleistocene (until c. 16 ka) the investigated middle valley reach around Gonggar was occupied by a lake attaining an extension of at least 100 km.     

Balancing the global oceanic neodymium budget: Evaluating the role of groundwater

The distinctly different, ε_Nd(0) values of the Atlantic, Indian, and Pacific Oceans requires that the residence time of Nd in the ocean (i.e., τ_Nd) be on the order of, or less than, the ocean mixing time of ∼ 500–1500 yr. However, estimates of τ_Nd, based on river influxes, range from 4000 to 15,000 yr, thus exceeding the ocean mixing time. In order to reconcile the oceanic Nd budget and lower the residence time by roughly a factor of 10, an additional, as yet unidentified, and hence “missing Nd flux” to the ocean is necessary. Dissolution of materials deposited on continental margins has previously been proposed as a source of the missing flux. In this contribution, submarine groundwater discharge (SGD) is examined as a possible source of the missing Nd flux. Neodymium concentrations (n = 730) and ε_Nd(0) values (n = 58) for groundwaters were obtained from the literature in order to establish representative groundwater values. Mean groundwater Nd concentrations and ε_Nd(0) values were used along with recent estimates of the terrestrial (freshwater) component of SGD (6% of river discharge on a global basis) to test whether groundwater discharge to the coastal oceans could account for the missing flux. Employing mean Nd concentrations of the compiled data base (i.e., 31.8 nmol/kg for all 730 analyses and 11.3 nmol/kg for 141 groundwater samples from a coastal aquifer), the global, terrestrial-derived SGD flux of Nd is estimated to range between 2.9 × 10⁷ and 8.1 × 10⁷ mol/yr. These estimates are of the same order of magnitude, and within a factor of 2, of the missing Nd flux (i.e., 5.4 × 10⁷ mol/yr). Applying the SGD Nd flux estimates, the global average ε_Nd(0) of SGD is predicted to be − 9.1, which is similar to our estimate for the missing Nd flux (− 9.2), and in agreement with the mean (± S.D.) ε_Nd(0) measured in groundwaters (i.e., ε_Nd(0) = −8.9 ± 4.2). The similarities in the estimated SGD Nd flux and corresponding ε_Nd(0) values to the magnitude and isotope composition of the missing Nd flux are compelling, and suggest that discharge of groundwater to the oceans could account for the missing Nd flux. Future investigations should focus on quantifying the Nd concentrations and isotope compositions of groundwater from coastal aquifers from a variety of coastal settings, as well as the important geochemical reactions that effect Nd concentrations in subterranean estuaries in order to better constrain contributions of SGD to the oceanic Nd budget.     

Tolerance of Brazilian brain coral Mussismilia braziliensis to sediment and organic matter inputs

In Brazil, where reefs occur in markedly turbid environments, the relationship between sedimentation/organic matter and corals is poorly known. Thus, the ex situ effects of sediment with and without organic matter over the ΔF/Fm and physical state of Mussismilia braziliensis were analyzed. The ΔF/Fm and coral physical state, evaluated through the susceptibility index to sedimentation (SI), were measured in seven colonies exposed to sedimentation (0–450 mg cm⁻² day⁻¹) free of organic matter after 45 days of exposure, and in 12 colonies exposed to sedimentation (0–500 mg cm⁻² day⁻¹) with organic matter content (10%), in which case ΔF/Fm was measured after 72 h and SI after 120 h. In both cases there were effects of increasing sedimentation on the SI with no effect on ΔF/Fm. Despite the tolerance to high sedimentation rates shown by this coral, we noted that the presence of organic matter might reduce its tolerance to sedimentation stress.     

Impact of environmental factors on bacterial communities in floodplain lakes differed by hydrological connectivity

We analysed total bacterial number and mean volume of cells at three sites in each of ten floodplain lakes in the Middle Basin of the Biebrza River, North-Eastern Poland to test bacterioplankton communities change according to the distance to the river. The composition of the bacterial communities was determined by fluorescent in situ hybridization method. Total number of bacteria in the lakes ranged from 4.0 to 7.48 cells × 10⁶ mL⁻¹ with dominance by Actinobacteria, the contribution of which was positively correlated with water level. Old river channels (side-arms) featured Alpha- and Gammaproteobacteria. The community of Betaproteobacteria was limited by concentration of dissolved organic carbon. Archaea, in spite of a minor role (<3.65% of DAPI-4′,6-diamidino-2-phenylindole) in the communities, showed a positive relation to floodplain lake isolation. Multivariate analysis demonstrated that bacterioplankton in riverine lakes was similar to that in rivers, while lakes with limited water exchange showed a similarity to fertile lakes. Water level and nutrients were among the factors determining bacterial community structure.     

Occurrence of endocrine-disrupting chemicals in riverine sediments from the Pearl River Delta,China

The endocrine-disrupting chemicals (EDCs) was investigated in 28 riverine sediments from the Pearl River system, China and analyzed by an ultrasonication extraction and GC–MS method. The concentrations of 4-tert-octylphenol (OP), 4-nonylphenol (NP), and bisphenol A (BPA) in the sediments were in the ranges of <2.0–210, 107–16198 and <1.7–430 ng/g dw, respectively. The steroid estrogens estrone (E1) and 17β-estradiol (E2) in the sediments ranged from <1.3 to 10.9 ng/g dw and from <0.9 to 2.6 ng/g dw, respectively. The spatial distribution of these chemicals was related to the discharge of domestic and industrial wastewater along the rivers. The positive correlation between EDCs and total organic carbon indicates that sedimentary organic carbon is an important factor in controlling the distributions of EDCs. Compared with other previous studies, the ZR and DR rivers from the PRD were heavily contaminated by APs and BPA.     

Selected trace metals (As,Cd and Hg) distribution and contamination in the coastal wetland sediment of the northern Beibu Gulf,South China Sea

Contamination with As, Cd and Hg, their spatial and temporal distribution are reported from the coastal wetland sediments of the northern Beibu Gulf, South China Sea. The content of As, Cd, Hg and TOC in surface sediments is 8.1 ± 5.8 μg g^?1, 0.08 ± 0.14 μg g^?1, 0.034 ± 0.028 μg g^?1 and 0.45 ± 0.39%, respectively. The mean sedimentation rates are 0.93–1.37 cm year^?1 during 1920s to 2008 determined by ²¹⁰Pb and ¹³⁷Cs dating in three cores. The vertical profiles of As, Cd and Hg content in the cores retrieved from Qin and Nanliu River estuaries show increasing trends during 1985–2008 due to anthropogenic impact caused by local economic development. Locally the surface sediments have potential ecological risk of As to benthos according to the NOAA sediment quality guidelines.     

Long-term annual groundwater storage trends in Australian catchments

The period of direct groundwater storage measurements is often too short to allow reliable inferences of groundwater storage trends at catchment scales. However, as groundwater storage sustains low flows in catchments during dry periods, groundwater storage can also be estimated indirectly from daily streamflow based on hydraulic groundwater theory; this idea was applied herein to 17 selected Australian catchments to examine their long-term (half a century or longer) groundwater storage trends. On average, over past 45 years, groundwater storage exhibited negative trends in all the selected catchments, except in the Katherine River catchment located in the Northern Territory. These negative trends persisted over longer periods, close to 100 years in some catchments and the strongest decreasing trend of 0.241 mm per year was observed in the Barron River catchment in New South Wales. However, groundwater storage exhibited different trends over the different shorter periods. Thus, while during the period of 1997–2007, 15 out of the 17 catchments showed negative trends in groundwater storage, during the period of 1980–2000, 12 out of the 17 catchments exhibited positive trends in groundwater storage; this underscores the fact that record lengths of one or even two decades are inadequate to derive meaningful trends. Strong consistencies in the trends exist across most catchments, indicating that groundwater storage is affected by large-scale climate factors.     

Distribution,inventory and turnover of benthic organic biomass in the Strait of Georgia,Canada, in relation to natural and anthropogenic inputs

Recently compiled databases facilitated estimation of basin-wide benthic organic biomass and turnover in the Strait of Georgia, an inland sea off western Canada. Basin-wide organic biomass was estimated at 43.1 × 10⁶ kg C and production was 54.6 × 10⁶ kg C yr⁻¹, resulting in organic biomass turnover (P/B) of 1.27 × yr⁻¹. Organic biomass and production for sub-regions were predictable from modified organic flux (r² > 0.9). P/B declined significantly with increasing modified organic flux, suggesting greater biomass storage in high flux sediments. Biomass and production were highest, and P/B lowest near the Fraser River. Annual basin-wide benthic production was 60% of previously estimated oxidized organic flux to substrates, which agrees with proportional measurements from a recent, localized study.Deviations from expected patterns related to organic enrichment and other stressors are discussed, as are potential impacts to benthic biomass and production, of declining bottom oxygen, increasing bottom temperature and potential changes in riverine input.     

Carbon remineralization in the Amazon–Guianas tropical mobile mudbelt: A sedimentary incinerator

The Amazon River spawns a vast mobile mudbelt extending ∼1600 km from the equator to the Orinoco delta. Deposits along the Amazon–Guianas coastline are characterized by some of the highest C_org remineralization rates reported for estuarine, deltaic, or shelf deposits, however, paradoxically, except where stabilized by mangroves or intertidal algal mats, they are usually suboxic and nonsulfidic. A combination of tides, wind-driven waves, and coastal currents forms massive fluid muds and mobile surface sediment layers ∼0.5–2 m thick which are dynamically refluxed and frequently reoxidized. Overall, the seabed functions as a periodically mixed batch reactor, efficiently remineralizing organic matter in a gigantic sedimentary incinerator of global importance. Amazon River material entering the head of this dynamic dispersal system carries an initial terrestrial sedimentary C_org loading of ∼ 0.7 mg C m⁻² particle surface area. Total C_org loading is lowered to ∼ 0.2 mg C m⁻² in the proximal delta topset, ∼60–70% of which remains of terrestrial origin. Loading decreases further to 0.12–0.14 mg C m⁻² (∼60% terrestrial) in mudbanks ∼600 km downdrift along French Guiana, values comparable to those found in the oligotrophic deepsea. DOC/ΣCO₂ ratios in pore waters of French Guiana mudbanks indicate that >90% of metabolized organic substrates are completely oxidized. Within the Amazon delta topset at the head of the dispersal system, both terrestrial and marine organic matter contribute substantially to early diagenetic remineralization, although reactive marine substrate dominates (∼60–70%). The conditional rate constant for terrestrial C_org in the delta topset is ∼0.2 a⁻¹. As sedimentary C_org is depleted during transit, marine sources become virtually the exclusive substrate for remineralization except very near the mangrove shoreline. The δ¹³C and Δ¹⁴C values of pore water ΣCO₂ in mudbanks demonstrate that the primary source of remineralized organic matter within ∼1 km of shore is a small quantity of bomb signature marine plankton (+80‰). Thus, fresh marine organic material is constantly entrained into mobile deposits and increasingly drives early diagenetic reactions along the transit path. Relatively refractory terrestrial C_org is lost more slowly but steadily during sedimentary refluxing and suboxic diagenesis. Amazon Fan deposits formed during low sea level stand largely bypassed this suboxic sedimentary incinerator and stored material with up to ∼3X the modern high stand inner shelf C_org load (Keil et al., 1997b. Proceedings of the Ocean Drilling Program, Scientific Results. Vol. 155. pp. 531–537). Sedimentary dynamics, including frequency and magnitude of remobilization, and the nature of dispersal systems are clearly key controls on diagenetic processes, biogeochemical cycling, and global C storage along the continental margins.     

Short time-scale impacts of hydropeaking on benthic invertebrates in an Alpine stream (Trentino,Italy)

The impact of a single hydropeaking event was studied in the Alpine stream Noce Bianco. Four stations were selected, one upstream and three, respectively, at 0.25, 6, and 8 km downstream from a hydropower plant. We collected drifting invertebrates during a planned water release that increased the discharge 7-fold. At the onset of the hydropeaking wave the number of invertebrates lost from the riverbed per minute to the drift increased 9-fold at the first downstream station and the same effects propagated 8 km downstream. The drift was composed mainly of aquatic insect larvae (Chironomidae, Plecoptera, Ephemeroptera Baetidae, and Psychodidae, with Chironomidae as the most abundant taxon at all stations) and partly by larval and adult riparian insects, and by Oligochaeta, which were particularly abundant at the station 6 km downstream. We monitored drift for 30 min from the start of the water release: peaks in drifting invertebrates occurred within 5–10 min of the beginning of the hydropeaking wave, and most of the invertebrates were washed out within the first 15 min of the water release. The different timeframes were possibly due to habitat preferences (most of the taxa that increased in the drift at the arrival of the wave were associated with algae and organic debris, which were washed off quickly by the increase in discharge) and/or behavioral adaptations (other taxa initially resisted the shear stress and began to drift with a delay of 5–10 min). The temporal pattern and drift composition corresponded well with those reported in literature, and indicate that repeated high-flow events of similar magnitude cause considerable losses from benthic populations to drift.