Spectroscopic characterization and molecular weight distribution of dissolved organic matter in sediment porewaters from Lake Erhai, Southwest China

Dissolved organic matter (DOM) in sediment porewaters from Lake Erhai, Southwest China was investigated using dissolved organic carbon (DOC) concentration, UV absorbance, fluorescence and molecular weight distribution. DOC exhibited a high concentration at the sediment–water interface with a rapid decrease to the oxic–anoxic interface at approximately 7 cm, and then increased with depth. Similar trends were also found for the UV absorption coefficients at 254 and 280 nm in the porewaters. DNA in the sediment was also measured, which confirmed the high abundance of aerobic bacteria in the upper layer of the sediment. Both humic-like (peaks A and C) and protein-like (peaks B and D) fluorescence were observed in the porewater DOM, and their fluorescence intensities exhibited a similar porewater profile as DOC concentration. A strong correlation was found between the peak fluorescence intensity ratio r(A, C) and r(D, B). Both the fluorescence index and UV absorption coefficient at 254 nm suggested a dramatic increase in aromaticity of porewater DOM across the oxic–anoxic interface. Porewater DOM exhibited a multimodal distribution of molecular weight with a relatively low polydispersity. The results of this study offer significant insight into the nature and properties of DOM in freshwater ecosystems.     

Geochemistry and Microbial Populations in Sediments of the Northern Baffin Bay,Arctic

The Northern Baffin Bay between Greenland and Canada is a remote Arctic area restricted in primary production by seasonal ice cover, with presumably low sedimentation rates, carbon content and microbial activities in its sediments. Our aim was to study the so far unknown subseafloor geochemistry and microbial populations driving seafloor ecosystems. Shelf sediments had the highest organic carbon content, numbers of Bacteria and Archaea, and microcosms inoculated from Shelf sediments showed highest sulfate reduction and methane production rates. Sediments in the central deep area and on the southern slope contained less organic carbon and overall lower microbial numbers. Similar 16S rRNA gene copy numbers of Archaea and Bacteria were found for the majority of the sites investigated. Sulfate in pore water correlated with dsrA copy numbers of sulfate-reducing prokaryotes and differed between sites. No methane was found as free gas in the sediments, and mcrA copy numbers of methanogenic Archaea were low. Methanogenic and sulfate-reducing cultures were enriched on a variety of substrates including hydrocarbons. In summary, the Greenlandic shelf sediments contain vital microbial communities adapted to their specific environmental conditions.     

Genotoxicity assessment in aquatic environments under the influence of heavy metals and organic contaminants

The genotoxicity of river water and sediment including interstitial water was evaluated by microscreen phage-induction and Salmonella/microsome assays. Different processes used to fractionate the sediment sample were compared using solvents with different polarities. The results obtained for mutagenic activity using the Salmonella/microsome test were negative in the water and interstitial water samples analysed using the direct concentration method. The responses in the microscreen phage-induction assay showed the presence of genotoxic or indicative genotoxic activity for at least one water sample of each site analysed using the same concentration method. Similar results were obtained for interstitial water samples, i.e. absence of mutagenic activity in the Salmonella/microsome test and presence of genotoxic activity in the microscreen phage-induction assay. Metal contamination, as evidenced by the concentrations in stream sediments, may also help explain some of these genotoxic results. Stream sediment organic extracts showed frameshift mutagenic activity in the ether extract detected by Salmonella/microsome assay. The concentrates evaluated by microscreen phage-induction assay identified the action of organic compounds in the non-polar, medium polar and polar fractions. Thus, the microscreen phage-induction assay has proven to be a more appropriate methodology than the Salmonella/microsome test to analyse multiple pollutants in this ecosystem where both organic compounds and heavy metals are present.     

Quantifying the potential for flow to remove the emergent aquatic macrophyte Sparganium erectum from the margins of low-energy rivers

The emergent macrophyte species Sparganium erectum occurs commonly at the margins of low- to medium- energy river systems across the northern temperate zone. It is considered as an invasive species along low-energy water courses in many parts of the US and Australia. The life-cycle and biomechanical properties of this species make it very well adapted to such environments, allowing rapid growth and sediment trapping, such that encroachment into the channel occurs as the growing season progresses. The widespread growth of species such as S. erectum is therefore of particular concern, when considering the flood risk potential of many rivers. As such, the conditions required for survival or uprooting and scouring of this plant are of interest, as are the times of the year and processes by which these plants spread to increase the size of current stands, and to form new stands. It is known that S. erectum reproduces by several vegetative methods including rhizome growth, dispersal of detached rhizomes, and relocation of entire plants. However, the mechanisms and flow conditions necessary for uprooting or scouring of entire plants, and the separation of fragments of this species, at different times of the year, are largely unknown. The aim of this paper is to model the uprooting resistance of S. erectum plants as reported by Liffen et al. (in press), and to investigate the manner by which this species is adapted to proliferate in low-energy, low-gradient streams. The results presented here show that Monte Carlo simulations using the RipRoot root-reinforcement model can be used to accurately model plant pullout forces, rhizome interconnectivity and length changes for S. erectum plants throughout the growing season. Analysis presented here also suggests that plant uprooting forces are several orders of magnitude larger than potential drag forces that could act on the S. erectum plants at the River Blackwater site modeled, and even at sites with much higher channel slopes. This result suggests that the ability of these plants to thrive in low-energy rivers, but not in higher-energy river environments, is less related to driving forces causing drag on the plants, and more related to the energy conditions controlling erosion and deposition of the fine substrate materials these plants thrive in. The critical shear stress of the fine within-vegetation material was shown here to only be exceeded by the average boundary shear stress within the vegetation, during winter months when above-ground biomass and thus Manning's n values were at their lowest. For example, during March and April average boundary shear stress was predicted to exceed critical boundary shear stress for 6% of the time. Erodibility measurements from jet-tests conducted at the River Blackwater fieldsite suggested that this excess in boundary shear stress could result in potential vertical scour of up to 0.09 m in both March and April. During the majority of the growing season sediment trapping rather than erosion dominated, with enough deposition occurring over the summer to protect all but the shallowest, weakest and least interconnected rhizomes and plants from being scoured in the winter months. The balance between erosion and deposition within stands of S. erectum in these low-energy environments therefore allows for the maintenance of established stands of vegetation, whilst still allowing for scouring of weaker S. erectum plants that can establish previously un-colonized channel margins further downstream.     

微生态制剂改善对虾养殖池塘底质的效果

研究了在117 d的养殖周期中微生态制剂对南美白对虾池塘底质的改良效果.结果表明,与对照组相比,施用微生态制剂可使底质中总氮、总磷和硫化物的含量显著下降;总菌数量无显著变化,而芽孢杆菌、氨化细菌以及硫氧化细菌、硫还原细菌、弧菌数量差异显著,其中弧菌数量在施用微生态制剂处理和对照条件下分别为3.65×10³ cfu·g^-1和1.16×10⁵ cfu·g^-1.表明施用微生态制剂可以减少氮、磷、硫等营养物质的积累,改善池塘底质的菌相,为南美白对虾的健康养殖提供良好的池塘底质环境.     

Predictions of stream nutrient and sediment yield changes following restoration of forested riparian buffers

Riparian tree planting is widely recognised as a means to improve water quality and stream habitat. However, shading of riparian pasture grasses can lead to channel widening, and riparian shade may limit the growth of macrophytes and algae that assimilate dissolved nutrients from the water column. We investigated concerns that riparian management could lead to increased yields of nutrients and sediments through a conceptual modelling exercise. A simple model of the trade-off between interception of nutrients in runoff by forest buffers versus reduction of in-stream uptake due to shade, predicted that a buffer strip alongside a small headwater stream would reduce nutrient export, while a buffer strip instigated as an isolated patch alongside a larger stream (c. >2.5 km² upstream catchment size) would increase nutrient export, as the relative amount of nutrients trapped by the buffer decreases as the nutrient load present in the stream water increases. However, in these larger streams with width exceeding approximately 6 m, sufficient light may reach the streambed for plant and algal growth, which in turn would promote instream nutrient processing. At the peak of streambank erosion after planting, predicted total sediment yield (hillslope plus bank sources) was appreciably higher than the hillslope pasture yield, but sediment yield stabilised c. 35–40 years after planting. When planting was extended over 40 years in the model, the sediment yield never exceeded that in pasture before planting. This conceptual modelling exercise shows that riparian tree planting programmes should commence in the headwaters and progress downstream to avoid nutrient yield increases. Significant sediment yield from bank stored sediment of small streams can be expected until the channel reaches the more stable, original forested width, but progressive planting may decrease the peak loads of sediment.     

The influence of water quality and sediment geochemistry on the horizontal and vertical distribution of phosphorus and nitrogen in sediments of a large,shallow lake

Distinct horizontal water column concentration gradients of nutrients and chlorophyll a (Chl a) occur within large, shallow, eutrophic Lake Taihu, China. Concentrations are high in the north, where some of the major polluted tributaries enter the lake, and relatively low in the south, where macrophytes generally are abundant. It is not clear, however, whether these water column concentration gradients are similarly reflected in spatial heterogeneity of nutrient concentrations within the bottom sediments. The main objective of this study was therefore to test if horizontal and vertical variations in the phosphorus and nitrogen content in bottom sediments of Lake Taihu are significantly related to (1) horizontal variations in overlying water column nutrient concentrations and (2) other sediment geochemical constituents. We measured the concentration of total phosphorus (TP) and total nitrogen (TN) in surficial sediments (0–2 cm) and TP, TN and Chl a concentrations in water column samples, collected from 32 sites in 2005. In 2006 sediment, TP, TN, carbon, iron and manganese concentrations were measured vertically at 2 cm intervals, extending to a depth of approximately 20 cm, at an additional eight sites. Linear correlation analysis revealed that surficial sediment TP concentrations across the 32 stations were related significantly, though weakly, to annual mean water column concentrations of TP, TN as well as Chl a. Correlations of surficial sediment TN with water column variables were, however, not significant (P > 0.05). Amongst the geochemical variables tested, the vertical variability of sediment TP concentrations was most strongly related to sediment manganese and carbon concentrations. A multiple stepwise linear regression revealed that the combination of sediment manganese and carbon concentrations explained 91% of the horizontal variability in sediment TP concentrations and 65% of the vertical variability. Handling editor: Luigi Naselli-Flores     

Sediment quality criteria in use around the world

 There have been numerous sediment quality guidelines (SQGs) developed during the past 20 years to assist regulators in dealing with contaminated sediments. Unfortunately, most of these have been developed in North America. Traditionally, sediment contamination was determined by assessing the bulk chemical concentrations of individual compounds and often comparing them with background or reference values. Since the 1980s, SQGs have attempted to incorporate biological effects in their derivation approach. These approaches can be categorized as empirical, frequency-based approaches to establish the relationship between sediment contamination and toxic response, and theoretically based approaches that attempt to account for differences in bioavailability through equilibrium partitioning (EqP) (i.e., using organic carbon or acid volatile sulfides). Some of these guidelines have been adopted by various regulatory agencies in several countries and are being used as cleanup goals in remediation activities and to identify priority polluted sites. The original SQGs, which compared bulk chemical concentrations to a reference or to background, provided little insight into the ecosystem impact of sediment contaminants. Therefore, SQGs for individual chemicals were developed that relied on field sediment chemistry paired with field or laboratory-based biological effects data. Although some SQGs have been found to be relatively good predictors of significant site contamination, they also have several limitations. False positive and false negative predictions are frequently in the 20% to 30% range for many chemicals and higher for others. The guidelines are chemical specific and do not establish causality where chemical mixtures occur. Equilibrium-based guidelines do not consider sediment ingestion as an exposure route. The guidelines do not consider spatial and temporal variability, and they may not apply in dynamic or larger-grained sediments. Finally, sediment chemistry and bioavailability are easily altered by sampling and subsequent manipulation processes, and therefore, measured SQGs may not reflect in situ conditions. All the assessment tools provide useful information, but some (such as SQGs, laboratory toxicity and bioaccumulation, and benthic indices) are prone to misinterpretation without the availability of specific in situ exposure and effects data. SQGs should be used only in a “screening” manner or in a “weight-of-evidence” approach. Aquatic ecosystems (including sediments) must be assessed in a “holistic” manner in which multiple components are assessed (e.g., habitat, hydrodynamics, resident biota, toxicity, and physicochemistry, including SQGs) by using integrated approaches. Received: December 26, 2000 / Accepted: December 28, 2001