Increasing Sediment Depth Research Articles

Speciation of organic phosphorus in a sediment profile of Lake Taihu I: Chemical forms and their transformation

Organic phosphorus (nonreactive P, NRP) is a major component of P in sediments, but information about its chemical forms and dynamic transformation is limited. The chemical forms and dynamic behaviors of NRP in a sediment profile from Lake Taihu, a freshwater and eutrophic lake in China, were investigated. Five forms of NRP in the sediments were extracted based on a chemical fractionation technique, including easily labile NRP (NaHCO3-NRP), reactive metal oxide-bound NRP (HC1-NRP), humic acid-associated NRP (NaOH-NRPHA), fulvic acid-associated NRP (NaOH-NRPFA) and residual NRP (Res-TP). There were notable transformations with increasing sediment depth from the labile NaHCO3-NRP and NaOH-NRP pools to the recalcitrant HC1-NRP and Res-TP pools, which caused the NRP to become increasingly recalcitrant as the early diagenetic processes proceeded. Further analyses showed that the relative changes in contents of organic matter and reactive Fe oxides in the sediment profile triggered a competition for binding NRP fractions and led to the transformation of NRP. The results highlighted the importance of abiotic processes in regulating the diagenesis of organic P and its stability in sediments.

Vertical Distribution and Flux of Nutrients in the Sediments of the Mangrove Reclamation Region of Muara Angke Kapuk, Jakarta

The reclaimed mangrove estuary in Muara Angke Kapuk is a reclaimed area that has not evaded the impacted of pollution and waste in the areas surrounding Cengkareng, Jakarta. This is apparent from the fact that almost all sediments under the mangrove trees are buried under heaps of plastic trash. However, the reclaimed region still has variety of organism, which indicating that the region still has an internal carrying capacity, especially nutrients from sediment. The purpose of this research was to examine the condition of sediment nutrients in this mangrove reclamation region. The research was conducted by taking water samples using a modification of the stratified cup at a sediment depth of 0-15 cm with depth intervals of 2.5 cm, and taking sediment samples using the sediment ring. Pore water samples were measured for dissolved oxygen (DO) and concentrations of ammonia, nitrite, nitrate, and phosphate. Sediment samples were used to obtain porosity values. The data obtained is used to make vertical concentration profiles and analysis of vertical nutrient flux. Vertical nutrient flux analysis was performed with the aid of QUAL2K software version 2.11. The results showed different vertical distributions and flux of nutrients, where influx for ammonia and phosphate and an increase in line with increasing sediment depth, while nitrate efflux and a decreased concentration. The flux calculation of nitrite as transitory nutrient was not done, but the concentration decreased after a depth of 2.5 cm. This indicates that the high contamination on the surface does not prevent the natural chemical processes so the reclaimed region can still provide nutritional support for its organism.

Thickness of the Earth's crust in the deep Arctic Ocean: Results of a 3D gravity modeling

The employed method of 3D gravity modeling is based on calculation of the gravity effects of the main density boundaries of the lithosphere, subtraction of these effects from the observed gravity field, and the subsequent conversion of the residual gravity anomalies first to the Moho depth and then to the total thickness of the Earth's crust and the thickness of its consolidated part. On the modeling, we also took into account the gravity effects due to an increase in the sediment density with increasing sediment depth and a rise of the top of the asthenosphere beneath the mid-ocean Gakkel Ridge. The resulting 3D models of the Moho topography and crustal thickness are well consistent with the data of deep seismic investigations. They confirm the significant differences in crustal structure between the Eurasian and Amerasian Basins and give an idea of the regional variations in crustal thickness beneath the major ridges and basins of the Arctic Ocean.

Bacterial biodiversity in deep-sea sediments from two regions of contrasting surface water productivity near the Crozet Islands, Southern Ocean

The relationship between surface-derived particulate organic matter (POM) and deep-sea sediment bacterial abundance, community structure and composition was investigated in two different sediment layers from two zones of contrasting surface water productivity in the southern Indian Ocean. Bacterial sediment communities from high chlorophyll (HC) and low chlorophyll (LC) sites were characterized and compared using direct counts, clone library construction, denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH). Of the 1566 bacterial clones generated from the sediment communities, 1010 matched published 16S rDNA sequences at ≥97% identity. A comparison of surface sediment clone libraries showed that at least one third of all identified operational taxonomic units (OTUs) were common to both HC and LC sites. DGGE community profiles were consistent (82% similar) and evenness of the major phylogenetic groups was 96% similar between surface sediment communities, where gamma- and alpha-Proteobacteria were dominant. Sediment communities shared similarly high biodiversity, while species richness was marginally higher at the LC site. Intra-site shifts in bacterial abundance and composition were observed with increasing sediment depth. Despite the differences in organic matter input between sites, the consistency observed between HC and LC sediment communities pointed to (1) the extent of remineralisation by mega and meio-fauna as a potential factor affecting the quantity and quality of POM available to sediment bacteria, (2) sampling during the early ‘nutrient assimilation phase’ of the bacterial response to freshly deposited POM or (3) that the action of bacteria in the water column could affect the quantity and quality of POM available to sediment bacteria. Although factors other than these may explain the observed similarities, this first comparison of such deep-sea sediment communities in relation to surface-derived productivity may be useful in further elucidating the role of sediment bacteria in carbon remineralisation in the deep-sea environment.

Metabolically active microbial communities in marine sediment under high-CO2 and low-pH extremes

Sediment-hosting hydrothermal systems in the Okinawa Trough maintain a large amount of liquid, supercritical and hydrate phases of CO(2) in the seabed. The emission of CO(2) may critically impact the geochemical, geophysical and ecological characteristics of the deep-sea sedimentary environment. So far it remains unclear whether microbial communities that have been detected in such high-CO(2) and low-pH habitats are metabolically active, and if so, what the biogeochemical and ecological consequences for the environment are. In this study, RNA-based molecular approaches and radioactive tracer-based respiration rate assays were combined to study the density, diversity and metabolic activity of microbial communities in CO(2)-seep sediment at the Yonaguni Knoll IV hydrothermal field of the southern Okinawa Trough. In general, the number of microbes decreased sharply with increasing sediment depth and CO(2) concentration. Phylogenetic analyses of community structure using reverse-transcribed 16S ribosomal RNA showed that the active microbial community became less diverse with increasing sediment depth and CO(2) concentration, indicating that microbial activity and community structure are sensitive to CO(2) venting. Analyses of RNA-based pyrosequences and catalyzed reporter deposition-fluorescence in situ hybridization data revealed that members of the SEEP-SRB2 group within the Deltaproteobacteria and anaerobic methanotrophic archaea (ANME-2a and -2c) were confined to the top seafloor, and active archaea were not detected in deeper sediments (13-30 cm in depth) characterized by high CO(2). Measurement of the potential sulfate reduction rate at pH conditions of 3-9 with and without methane in the headspace indicated that acidophilic sulfate reduction possibly occurs in the presence of methane, even at very low pH of 3. These results suggest that some members of the anaerobic methanotrophs and sulfate reducers can adapt to the CO(2)-seep sedimentary environment; however, CO(2) and pH in the deep-sea sediment were found to severely impact the activity and structure of the microbial community.

Phosphorus fractions and bioavailability in relation to particle size characteristics in sediments from Lake Hongfeng, Southwest China

Information on the chemical composition of phosphorus (P) fractions in sediments is fundamental to understanding P bioavailability and eutrophication in lake ecosystems. Phosphorus fractions and its bioavailability in sediments cores of Lake Hongfeng, southwest China, were investigated using a chemical sequential extraction scheme. Relationships between P fractions, P bioavailability and particle sizes were discussed. P fractions concentrations were ranked in the order: Residual-P > NaOH–rP > NaOH–NRP > HCl–P > BD–P > NH4Cl–P, and all of them decreased with increasing sediment depth. Statistical analysis showed that concentrations of bioavailable P (BAP) which includes the NH4Cl–P, BD–P, NaOH–rP and NaOH–NRP fractions ranged from 404.68 to 1,591.99 mg/kg and accounted for 26.8–71.8 % of the concentrations of total phosphorus (TP) in the top 5 cm sediments, whereas in the whole sediment cores, their concentrations ranged from 239.70 to 1,591.99 mg/kg and accounted for 26.8–76.0 % of TP. The results suggested that the sediments were a large potential source of P for algae blooms in Lake Hongfeng. Phosphorus fractions and their potential bioavailability were influenced by the sediment particle sizes, especially the bioavailability of the NH4Cl–P fraction, which was strongly affected by the presence of fine particle sizes in the sediments.

Chlorinated Paraffins in Sediments from the Pearl River Delta, South China: Spatial and Temporal Distributions and Implication for Processes

Short-chain and medium-chain chlorinated paraffins (SCCPs and MCCPs) were measured in sediments from ponds, rivers and tributaries, and an estuary, as well as a sediment core in the Pearl River Delta (PRD), South China, to comprehensively investigate the spatial and temporal distributions of CPs. The concentrations of SCCPs and MCCPs in sediment were varied from 320 to 6600 ng/g and from 880 to 38,000 ng/g, respectively. Elevated CP concentrations were found in pond sediments (means of 2800 and 21,000 ng/g for SCCPs and MCCPs) in the e-waste recycling area and in river sediments (means of 1200 and 3900 ng/g for SCCPs and MCCPs) in the highly industrialized areas. The significant positive correlations between SCCP concentration and MCCPs/SCCPs in the highly industrialized areas reflected the emission of local industry activities, while the significant negative correlations in the low industrial activity areas could be linked to long-range transportation of CPs. An increased abundance of short chain and low chlorinated congeners was observed in the low industrial activity areas compared to the industrialized areas. The preferred transportation of short chain and low chlorinated congener CPs and the dechlorination of higher chlorinated congeners CPs were the most likely reasons. The vertical profile of CPs in the sediment core indicated a rapid increase in the usage of CPs and a shift to more MCCPs in recent years. The decreased chlorine content of CPs with increasing sediment depth indicated the possibility of dechlorination of higher chlorinated congeners (Cl(9) and Cl(10)) after deposition in sediments with greater dechlorination potential for short chain CPs than long chain CPs.

Cesium, manganese and cobalt water–sediment transfer kinetics and diffusion into mangrove sediments inferred by radiotracer experiments

A study on the trace elements transfer from tidal water to mangrove sediments from Guanabara Bay (southeastern Brazil) was performed in laboratory microcosms. Sediment cores were covered with tidal water spiked with 137Cs, 54Mn and 57Co during 5-h experiments, and water samples were taken at regular intervals to measure uptake kinetics at the sediment–water interface. At the end of the experiments, the uptake and diffusional penetration into the sediments was evaluated. Half-removal times from water to sediments were slightly higher for 137Cs (3.4 ± 1.7 h) than observed for 54Mn (2.3 ± 0.2 h) and 57Co (2.6 ± 1.1 h). After these experiments, all radioisotopes presented decreasing activities with increasing sediment depth, being the distribution of 137Cs indicative of higher diffusion within the upper 2 cm. This study on the removal of 137Cs, 54Mn and 57Co from tidal water by mangrove sediments suggests that while 57Co and 54Mn presented closer behaviors, there was a slightly higher mobility of 137Cs.

Analysis of temperature variability and determination of apparent thermal diffusivity in sandy intertidal sediments at the German North Sea coast

Temperature was measured at depths of 1, 10, 30, 75, and 170 cm in fine sandy intertidal sediments by means of specially-designed “temperature lances”. The measurements cover a period from February to October 2007 and have a temporal resolution of 5 min. Stochastic as well as recurrent processes due to the solar cycle and due to tide induces flooding and drying of the sediment surface lead to a complex composition of the time series curves. Spectral analyses based on Fast Fourier Transformation (FFT) reveal that temperature variability at the sediment water/air interface is widely controlled by recurrent processes with period lengths of 4.93, 6.1, 8.19, ∼12, ∼24 h, and 354 h (14.7 days). The importance of the higher frequencies decreases with increasing sediment depth. At a depth of 30 cm the 24 h and the 14.7 days cycles mainly determine the temperature development over time, while at 75 cm sediment depth contour temperature varies only along the 14.7 days cycle, as well as within the seasonal cycle. Using cross-correlation-analysis the time necessary for a temperature signal at the surface to trigger a response at a sediment depth of 10, 30, and 75 cm was calculated as 1.4, 7.0, and 73.1 h respectively.Utilizing an alternate approach, FFT derived temperature peak-to-peak amplitude values and phase angles of up to 9 different cycles were used to calculate apparent thermal diffusivity in different sediment depths. The thermal diffusivity decreases from approximately 6–9 × 10−7 m2 s−1 from the surface down to a sediment depth of 75 cm.The specially-designed instrumentation has proven to be robust and precise enough to record high resolution time series of sediment temperature in different depths. The time series analysis of the data clearly shows that the temperature variability in the intertidal sediments to a high degree can be explained by recurrent solar and/or tidal effects. So the methods and results presented in this paper can help to answer questions, which are related to sediment temperature in tidal flat environments.

Source-specific variability in post-depositional DNA preservation with potential implications for DNA based paleoecological records

Recent studies have shown that genotyping preserved plankton DNA in marine and lacustrine sediment records using ancient DNA methods is a promising approach for refining paleoenvironmental information. However, the extent to which the preservation of fossil plankton DNA differs between species is poorly understood. Using a continuous 2700 year sediment record from Watts Basin in Ellis Fjord (Antarctica), we compared the level of preservation of fossil DNA derived from important plankton members with varying cellular architecture. The amount of preserved small subunit ribosomal DNA (SSU rDNA; ca. 500 base pair fragments) of dinoflagellates (as extracellular DNA rather than as preserved cysts) that could be amplified by way of PCR declined up to five orders of magnitude with increasing sediment depth and age. In contrast, the amount of similar-sized, PCR-amplifiable, diatom SSU rDNA (predominantly from a cyst-forming Chaetoceros sp.) declined only up to tenfold over 2700 years of deposition. No obvious decline in copy numbers with increasing sediment age was observed for similar-sized SSU rDNA of past chemocline-associated photosynthetic green sulfur bacteria (GSB), which do not have a protective resting stage. In good agreement with the quantitative data, the extent of post-depositional natural degradation to fragments too small to serve as a template for the quantitative PCR assays was greatest for dinoflagellates and lowest for GSB. An increase in the ratio between GSB-derived DNA and intact carotenoids with sediment depth implies that short GSB DNA fragments were better preserved than intact carotenoids and provide a more accurate view into paleoproductivity and the sediment flux of GSB in Watts Basin. We discuss the possible causes behind the variation in the level of DNA preservation among the plankton groups investigated, as well as consequences for the use of using fossil DNA records in paleoecology studies.

Petroleum degradation and associated microbial signatures at the Chapopote asphalt volcano, Southern Gulf of Mexico

At the Chapopote Knoll in the Southern Gulf of Mexico, deposits of asphalt provide the substrate for a prolific cold seep ecosystem extensively colonized by chemosynthetic communities. This study investigates microbial life and associated biological processes within the asphalts and surrounding oil-impregnated sediments by analysis of intact polar membrane lipids (IPLs), petroleum hydrocarbons and stable carbon isotopic compositions (δ 13C) of hydrocarbon gases. Asphalt samples are lightly to heavily biodegraded suggesting that petroleum-derived hydrocarbons serve as substrates for the chemosynthetic communities. Accordingly, detection of bacterial diester and diether phospholipids in asphalt samples containing finely dispersed gas hydrate suggests the presence of hydrocarbon-degrading bacteria. Biological methanogenesis contributes a substantial fraction to the methane captured as hydrate in the shallow asphalt deposits evidenced by significant depletion in 13C relative to background thermogenic methane. In sediments, petroleum migrating from the subsurface stimulates both methanogenesis and methanotrophy at a sulfate–methane transition zone 6–7 m below the seafloor. In this zone, microbial IPLs are dominated by archaeal phosphohydroxyarchaeols and archaeal diglycosidic diethers and tetraethers. Bacterial IPLs dominate surface sediments that are impregnated by severely biodegraded oil. In the sulfate-reduction zone, diagnostic IPLs indicate that sulfate-reducing bacteria (SRB) play an important role in petroleum degradation. A diverse mixture of phosphohydroxyarchaeols and mixed phospho- and diglycosidic archaeal tetraethers in shallow oil-impregnated sediments point to the presence of anaerobic methane-oxidizing ANME-2 and ANME-1 archaea, respectively, or methanogens. Archaeal IPLs increase in relative abundance with increasing sediment depth and decreasing sulfate concentrations, accompanied by a shift of archaeol-based to tetraether-based archaeal IPLs. The latter shift is suggested to be indicative of a community shift from ANME-2 and/or methanogenic archaea in shallower sediments to ANME-1/methanogenic archaea and possibly benthic archaea in deeper sediments.

What is the relative importance of sediment granulometry and vertical gradients to nematode morphometrics?

Nematode morphometrics were compared at 11 fine-sand and three muddy-sediment sites on the Louisiana, USA continental shelf to explore nematode–sediment relationships. Over 1800 nematodes were measured, and morphometric profiles through 4 cm were examined. The mean length of nematodes did not differ between muds and sands, although nematode diameter was greater in muds below 2 cm in sediment depth. Sand-dwelling nematodes, which slide through interstitial pore spaces, were longer relative to their diameter compared to nematodes that burrow through muddy sediments lacking interstices. Nematode diameter increased with both increasing particle size and particle heterogeneity among sandy stations. In muds, nematode length, diameter and biovolume increased with sediment depth, although length:width ratio did not vary with depth in the sediment. In sands, nematode length and length:width ratio increased with increasing sediment depth, but neither diameter nor biovolume varied with depth in the sediment. These results suggest that nematode body size and shape are related both to variation in sediment pore spaces and to vertical gradients within sediments. Within sands, body size variation attributable to granulometry was greater than variation associated with vertical gradients, while within muds variation in body size with sediment depth was greater than variation associated with particle size. Variation in body shape was similar along the vertical sediment profile and with variation in particle size in both sands and muds. We conclude that nematodes are excellent test organisms with which to test hypotheses regarding the confounded effects of sediment granulometry and chemical gradients in benthic ecosystems.

Scale-dependent relations between bacteria, organic matter and invertebrates in a headwater stream

Microbial activity, bacterial and invertebrate densities and particulate organic matter (POM) content were assessed using a modified freeze-coring technique in a Welsh mountain stream. Bacterial cell densities ranged between 0.22 x 10 8 and 4.47 x 10 8 cells cm ―3 and increased with sediment depth reaching maxima in depths between 30 and 40-cm. In contrast, the highest microbial activity (ETS) was found at depths between 5 and 15 cm, sharply declining with depths > 20 cm. POM content was substantially higher in the upper sediment layer (0―5 cm) and declined with increasing sediment depth. Higher nitrogen and organic carbon contents were recorded for particles < 63 μm than for larger size fractions. Meio- and macroinvertebrate densities were significantly and negatively correlated with bacterial number. The results of a multiple regression analysis suggest that bacterial densities, porewater volume and particulate organic carbon (POC) accounted for more than 50 % of the density variations of harpacticoids and larval chironomids. The results underline a strong link between physical habitat properties in a scale-dependent fashion.

Fine particulate organic matter (FPOM) transport and processing in littoral interstices – use of fluorescent markers

A fluorescent labelling method is presented as a new tool for the investigation of organic particle transport and biogenic carbon cycling processes in sandy littoral interstices at Lake Tegel, Berlin, Germany. Passive particle transport through the pore system was studied by in situ exposition of 2.4 μm monodisperse polymeric resin microparticles stained with 7-amino-4-methylcoumarin (AMC). Uptake of fluorescein-5-isothiocyanate (FITC)-labelled Chlorella vulgaris and fine particulate organic matter (FPOM) by the interstitial fauna was investigated in laboratory and field experiments. The major portion (>85%) of the FITC-labelled particles added to sediment cores was recovered from the topmost centimetre of sediment during the study period of 14 days. Uptake of FITC-labelled FPOM was observed in several benthic groups, e.g. chironomids, microcrustaceans, oligochaetes and tardigrads, whereas C. vulgaris was ingested by oligochaetes only. There is evidence to suggest that both are suitable materials for investigating the fragmentation and ingestion of organic material by herbivorous and detritivorous fauna. Field experiments with inert microparticles and FITC-labelled FPOM (<1 mm) prepared from dried alder leaves were carried out in plexiglass tubes as in situ whole core technique. Within the investigation period of two weeks, the transport of FPOM was restricted to the topmost 2–3 cm of sediment in conjunction with a distinct fragmentation to finer size classes with respect to increasing sediment depth. Vertical FPOM transport was hindered by a high interstitial concentration of natural POM and an intensive settlement of the interstices by algae (mainly epispammic algae, 65–96% of algae cell number) and extra-cellular polymeric substances (EPS), which formed a dense three-dimensional structure.

Distribution of migration (state) forms of microelements in the sediments of the ports of Klaipėda and Gdańsk

Abstract During investigations of the Baltic Sea basin significant differences have become manifest in the distribution of certain chemical elements in different chains of the geo (-eco) system (soils, river, lagoon and marine sediments). This paper presents data on the quantitative relationships of elements resulting from interactions of natural sedimentation processes and the possible human impact on the sediments from two port areas. The total (T), lithogenic (LG; stable) and hydrogenic (HG; mobile, reactive) migration forms of Zn, Cr, Cd, Cu, Pb and Ni were analysed. The HG component comprises that pool of the elements contained in readily degrading minerals; increases in this form potentially being an indicator of adverse human impacts. The distribution of total forms depends on the quantity of the sediment fraction with grain size &lt; 0.063 mm (fr. &lt; 0.063 mm) and on the concentration of organic content (Corg). With increasing sediment depth, the concentration of elements decreases sharply, although HG concentrations were seen to increase on several occasions. One explanation for such findings is the mineralisation and decomposition of metals from the remnants of biological activities of organisms (pellets, globules) into the sediments. Element concentrations in the natural sedimentation zone (NSZ) of the marine-influenced North Port at Gdańsk are less than in the NSZ and technogenic sedimentation zone (TSZ) of the riverine part of the port of Gdańsk. The ratios between HG, LG and T in the two sedimentation zones are different for different elements, but are always higher in the TSZ than in the NSZ. Multi-site analyses of the port basins allowed NSZ and TSZ to be distinguished. The TSZ is typified by anomalous concentrations of elements, the specific distribution of their migration forms, the semi-stagnant conditions prevailing there and the proximity of human impact sources. The data presented in this paper indicate that the concentrations of only some of the elements in the TSZ sediments of the ports of Klaipėda and Gdańsk exceed the permitted norms for spoil dredged from port basins. By no means can the total load of elements contained in the spoil dumped at sea be treated as an indicator of marine pollution. For an objective assessment of the real pollution threat to port environments, detailed investigations of the various migration forms of chemical elements need to be carried out, and previously established levels revised.

Vertical distribution of nitrite reductase genes (nir S) in continental margin sediments of the Gulf of Mexico

Marine sediments account for up to 66% of the loss of nitrogen load to coastal areas. Sedimentary denitrification is the main sink for fixed nitrogen in the global nitrogen budget, and thus it is important to understand the structure and composition of denitrifying communities. To understand the structure and composition of denitrifying communities, the diversity of nitrite reductase (nirS) genes from sediments along the Gulf of Mexico was examined using a PCR-based cloning approach. Sediments were collected at three different depths (0-0.5, 4-5 and 19-21 cm). Geochemical analysis revealed decreasing nitrate and oxygen concentrations with increasing sediment depth. This trend coincided with the decrease in diversity of denitrifying bacteria. LIBSHUFF analysis indicated that the clone library in the shallowest sediment (depth, 0-0.5 cm) was significantly different from that in the deepest sediment (depth, 19-21 cm), and that the deeper sediments (depths of 4-5 and 19-21 cm) were significantly similar. Community structural shifts were evident between the shallowest (oxic zone) and deepest (anoxic zone) sediments. Community changes within the deepest sediments were more subtle, with the presence of different nirS clone sequences gradually becoming dominant or, alternatively, decreasing with depth. The changes in community structure at this depth are possibly driven by nutrient availability, with lower quality sources of carbon and energy leading to the disappearance of nirS sequences common in the top layer. The majority of recovered nirS sequences were phylogenetically divergent relative to known denitrifying bacteria in the database.

Amino acid biogeo- and stereochemistry in coastal Chilean sediments

The spatial distribution of total hydrolysable amino acids (THAA) and amino acid enantiomers ( d- and l-forms) was investigated in sediments underlying two contrasting Chilean upwelling regions: at ∼23 °S off Antofagasta and at ∼36 °S off Concepción. The contribution of amino acids to total organic carbon (%T AAC: 7–14%) and total nitrogen (%T AAN: 23–38%) in surface sediments decreased with increasing water depth (from 126 to 1350 m) indicating that organic matter becomes increasingly decomposed in surface sediments at greater water depth. Changes in the ratio between the protein amino acid aspartate and its non-protein degradation product β-alanine confirmed this observation. Furthermore, estimates of THAA mineralization showed that sedimentary amino acid reactivity decreased with both increasing water depth as well as progressive degradation status of the organic matter that was incorporated into the sediment. Reactivity of organic matter in the sediment was also assessed using the Degradation Index (DI) developed by [Dauwe, B., Middelburg, J.J., 1998. Amino acids and hexosamines as indicators of organic matter degradation state in North Sea sediments. Limnol. Oceanogr. 43, pp. 782–798.]. Off Concepción, DI was successfully applied to examine the degradation status of sedimentary organic matter at different water depths. However, unexpected results were obtained at the Antofagasta stations as DI increased with sediment depth, suggesting more degraded organic matter at the surface than deeper in the cores. The contribution of peptidoglycan amino acids to THAA was estimated from the concentrations of d-aspartate, d-glutamic acid, d-serine, and d-alanine. Peptidoglycan amino acids accounted for >18% of THAA in all investigated samples. In surface sediments peptidoglycan amino acids accounted for a progressively larger fraction of THAA at increasing water depths (up to >26%). Further, the contribution of peptidoglycan amino acids to THAA increased with increased sediment depth and age (up to 288-year-old) reaching up to 59%. Independent estimates based on d-amino acid concentrations in selected laboratory strains, bacterial counts and the sedimentary concentrations of d-amino acids indicate that a large fraction of the measured d-amino acids (>47 to >97%) originated from cell wall residues rather than from enumerated cells.

Vertical distribution of benthic community responses to fish predators, and effects on algae and suspended material

The vertical positioning of benthic invertebrates should be a trade-off between the risky, but productive, sediment surface and the safer, but physiologically harsher, conditions deeper down in the sediment. This is because the foraging efficiency of benthic fish decreases with sediment depth, whereas the sediment surface is generally better oxygenated and has a higher resource quality than lower layers. We studied how two benthic fish predators, bream (Abramis brama) and ruffe (Gymnocephalus cernuus), affected the community composition and vertical distribution of benthos, and their indirect effects on algae and suspended material, in field enclosures. Whereas bream had significant effects on the density, composition and distribution of the benthos, ruffe had no such effects. The total benthos biomass in bream treatments was an-order of magnitude lower in the upper sediment layer (0–1 cm) and three times lower in the middle layer (1–3 cm) than in the controls, whereas there were no significant effects in the deepest layer (3–10 cm). Bivalves persisted in the deepest layer although their density was reduced in shallow sediment, whereas gastropods faced the risk of local extinction in the presence of bream. As indirect effects, small-bodied cladocerans, phytoplankton, periphyton and both organic and inorganic suspended material were higher in the bream treatments. We␣conclude that the impact of bream diminished substantially with increasing sediment depth, enabling invertebrates to survive in the sediment and to persist in the presence of bream. However, there were␣no␣indications of any group adjusting their vertical position behaviourally as a response to predation threat.

Lac Dufault sediment core trace metal distribution, bioavailability and toxicity to Hyalella azteca

To determine changes in metal distribution, bioavailability and toxicity with sediment depth, two 20-cm-long replicate cores were collected from a lake historically subjected to the influence of metal mining and smelting activity. The vertical distribution of Pb, Cd and Cu in sediment was similar for all three metals, with the surface layers showing enrichment and the deeper (pre-industrial) layers showing lower concentrations. Toxicity of each sediment core section was determined in laboratory tests with the freshwater amphipod Hyalella azteca. Bioavailable metal in each sediment slice was estimated from metal concentrations in overlying water in these toxicity tests and, for Cd, also from metal bioaccumulation. The profile for Cd in tissue was comparable to Cd in sediment and overlying water, but relative Cd bioavailability from sediment increased with sediment depth. Survival increased with increasing sediment depth, suggesting that surface sediments were probably less or non-toxic before industrialization.

Sediment Depth Attenuation of Biogenic Phosphorus Compounds Measured by 31P NMR

Being a major cause of eutrophication and subsequent loss of water quality, the turnover of phosphorus (P) in lake sediments is in need of deeper understanding. A major part of the flux of P to eutrophic lake sediments is organically bound or of biogenic origin. This P is incorporated in a poorly described mixture of autochthonous and allochthonous sediment and forms the primary storage of P available for recycling to the water column, thus regulating lake trophic status. To identify and quantify biogenic sediment P and assess its lability, we analyzed sediment cores from Lake Erken, Sweden, using traditional P fractionation, and in parallel, NaOH extracts were analyzed using 31P NMR. The surface sediments contain orthophosphates (ortho-P) and pyrophosphates (pyro-P), as well as phosphate mono- and diesters. The first group of compounds to disappear with increased sediment depth is pyrophosphate, followed by a steady decline of the different ester compounds. Estimated half-life times of these compound groups are about 10 yr for pyrophosphate and 2 decades for mono- and diesters. Probably, these compounds will be mineralized to ortho-P and is thus potentially available for recycling to the water column, supporting further growth of phytoplankton. In conclusion, 31P NMR is a useful tool to asses the bioavailability of certain P compound groups, and the combination with traditional fractionation techniques makes quantification possible.