Measurement of Acid Volatile Sulphide and Simultaneously Extracted Metals in Sediment from Lake Albufera (Valencia,Spain)

Lake Albufera (Valencia, Spain) is part of a legally protected wetland of international importance. However, it has deteriorated as a result of urban, industrial, and farming pollution. It is highly eutrophic, and its sediment contains persistent pollutants, such as heavy metals. In anoxic sediments, sulphides represent an important binding phase for heavy metals. In this study, acid volatile sulphide (AVS) and simultaneously extracted metals (SEM) were analyzed in surface sediment extracted from Lake Albufera; organic matter and total metals were also analyzed. Twelve sites were sampled in each of three sampling campaigns conducted in March and September 2007 and September 2008. The results revealed elevated organic matter contents varying between 6.9 and 16.7%. The concentrations of AVS in the lake were high, ranging from 8.5 to 48.5 μmol/g; the lowest concentrations were found in the central sites. The AVS results displayed significant differences between the samples from the winter and summer of 2007 (p < 0.05) but not between the two summer samples. The results obtained for SEM varied from 1.4 to 4.8 μmol/g. The difference SEM-AVS was less than zero for all sampling locations and campaigns, indicating the existence of a sulphide pool able to bind metals.     

水体沉积物中酸可挥发性硫化物(AVS)研究进展

水体沉积物中酸可挥发性硫化物 (AVS)是总硫含量中活性最高的部分 ,是沉积物中有毒重金属的重要结合形态 ,它的含量在很大程度上影响着沉积物重金属的生物有效性 ,从而作为沉积物中有毒重金属环境污染评价的一个重要指标 ;就十多年来水体沉积物中酸可挥发性硫化物 (AVS)的研究进行了综述。概述了 AVS的测定方法及其影响因素 ;探讨了水体沉积物中 AVS含量时空变化的规律 ;同时就目前“同时可提取重金属”(SEM)与 AVS摩尔浓度比值和水体沉积物重金属生物毒性关系的研究进行了概括和分析。     

Concentration,Distribution, Source,and Risk Assessment of PAHs and Heavy Metals in Surface Water from the Three Gorges Reservoir,China

Fifteen polycyclic aromatic hydrocarbons (PAHs) and heavy metals (Cr, Ni, As, Cd, Pb, and Hg) were quantified in 19 surface water sites of the Three Gorges Reservoir, China. The total concentrations of 15 PAHs and six heavy metals in the 19 sample sites ranged from 130.8 ng L^?1 to 227.5 ng L^?1 and 3.2 μg L^?1 to 6.0 μg L^?1, respectively. The mean concentration of As was the highest among the six heavy metals (2.1 ± 0.3 μg L^?1), followed by Cr (0.5 ± 0.3 μg L^?1), Ni (1.3 ± 0.1 μg L^?1), Cd (0.2 ± 0.01 μg L^?1), Pb (0.07 ± 0.08 μ g L^?1) and Hg (0.05 ± 0.08 μg L^?1). The isomer ratio results suggest that PAHs at most sites were mainly from petroleum combustion, while coal and biomass combustion was the main source at sites 1, 2, 6, 7, 9, 14, and 17. Based on principal component analysis, the main source of heavy metals was anthropogenic activities and weathering of bedrocks. Depending on characteristic of RQ_(NCs) ≥ 1 and RQ_(MPCs) < 1, BaA showed higher potential ecological risk than other PAHs, therefore, all sampling site needed to be paid much more attention, included some remedial actions. Meanwhile, after assessing human health risk of heavy metal, it was unlikely to experience adverse health effects, even exposing through more pathways and six kinds of heavy metals simultaneously.     

Bioavailability of zinc in the sediment to the estuarine amphipod <Emphasis Type="Italic">Grandidierella</Emphasis><Emphasis Type="Italic">japonica</Emphasis>

The utility of interstitial water concentrations of metals and simultaneously extracted metals/acid-volatile sulfide differences (SEM–AVS) in two seasons were investigated to explain the biological availability of zinc in sediments to benthic organisms exposed in the laboratory. The amphipod Grandidierella japonica was exposed, in 10-day acute toxicity tests, to clean sediment spiked with zinc to obtain nominal treatments ranging from 0.25 to 74.4 mol g^–1 dry weight with respect to the molar difference between SEM_Zn and AVS. When the molar difference between SEM_Zn and AVS (i.e., SEM–AVS) was <0 mol g^–1, the concentration of zinc in the sediment interstitial water was low and few adverse effects were observed for any of the biological endpoints measured. Conversely, when SEM_Zn–AVS exceeded 0 mol g^–1, the concentration of zinc in the interstitial water and amphipod mortality increased. These data compare favorably with observations made in short-term exposures and thus support the use of AVS as a normalization phase for predicting toxicity in metal-contaminated sediments in different season.     

Geochemical distribution of heavy metals in sediments from sewage fed fish ponds from Kolkata Wetlands,India

Abstract

Kolkata wetlands are the largest sewage fed wetlands in the world. They have been used for aquaculture since 1960. Geochemical distribution of heavy metals (Cr, Cu, Mn, Fe, Zn, Pb, Ni and Al) has been studied in surface sediments using single and sequential extractions techniques. The metal concentrations in sediments were in the following order: Fe>Al> Mn>Zn>Cu> Pb>Cr> Ni, and the average concentrations were 29 μg g^?1, 54 μg g^?1, 328 μg g^?1, 32747 μg g^?1, 169 μg g^?1, 38 μg g^?1, 25 μg g^?1 and 23371 μg g^?1 dry weights for Cr, Cu, Mn, Fe, Zn, Pb, Ni and Al, respectively. Water-soluble percentages of the trace elements are quite low (<0.01–3.75%) but in the presence of chelating agents in the sediments increase the bioavailability of trace elements (2–58%). About 40% of trace elements are in the stable form as a residual fraction of the sediment and more than 50% (nonresidual fraction) metal contamination of the Kolkata wetland sediments were from anthropogenic inputs. The contamination risks of Cr, Mn, Zn, Pb, and Ni are high as their potential availabilities are 70.96%, 58.01%, 63.13%, 55.62%, and 52.15% respectively. The mean concentration of most of the heavy metals in sediments does not exceed the recommended reference values. Zinc and lead concentrations were greater than background level and Interim Sediment Quality Guidelines but lower than Probable Effect Level. Therefore a regular program for monitoring the distribution of heavy metals in water, sediments and biota should be imposed on sewage fed fish ponds of the Kolkata wetland ecosystem.     

Soil quality around a solid waste dumpsite in Port Harcourt, Nigeria

The levels of soil parameters and selected heavy metals around a solid waste dumpsite receiving untreated wastes from all sources and a control site within Port Harcourt, Nigeria have been examined. Top soil (0–15 cm) and sediment samples were collected and analysed for pH value, particle size, total nitrogen, potassium, available phosphorus, organic matter, effective cation exchange capacity, cadmium, nickel and lead using standard methods. The results showed that the waste dump contributed to the high levels of nutrients and heavy metals. The dry season mean concentrations were: organic matter (5.28 ± 1.34% or 132,422.4 kg ha^?1), K (1.60 ± 0.52 meq per 100 g), N (0.09 ± 0.06% or 2257.2 kg ha^?1), Av.P (15.11 ± 7.57 μg g^?1), Cd (1.34 ± 0.72 μg g^?1), Ni (4.10 ± 1.63 μg g^?1) and Pb (38.85 ± 22.18 μg g^?1) while the wet season mean concentrations were organic matter (5.46 ± 1.39% or 136,936.8 kg ha^?1), K (2.79 ± 0.81 meq per 100 g), N (0.10 ± 0.05% or 2508 kg ha^?1), Av.P (9.22 ± 2.69 μg g^?1), Cd (1.72 ± 1.22 μg g^?1), Ni (14.95 ± 14.94 μg g^?1) and Pb (53.50 ± 40.09 μg g^?1). There was efficient mineralization process in the area. The texture of soil on the main dumpsite was loamy sand, which suggests that the ground water in the area is susceptible to contamination by surface pollutants. The texture of soil at the control site is sandy loam while sediment has the textural class of sand. Decomposed organic materials and agricultural activities influenced the texture of soils. The soils from the main dump and sediment were slightly alkaline while the control soil was moderately acidic. In both seasons, a significant variation exists (P < 0.05) between the metal concentrations in soil at the main dump and those in the sediments with a positive correlation (r = 0.572149) in the wet season and (r = 0.956647) in the dry season. The presence of liming materials and activities of microorganisms on the waste dump increased the pH of the soils. The accumulation of nutrients results in the luxuriant growth of plants/crops on the waste dump.     

MMHg production and export from intertidal sediments to the water column of a tidal lagoon (Arcachon Bay,France)

Hg cycling in biologically productive coastal areas is of special importance given the potential for bioaccumulation of monomethylmercury (MMHg) into aquatic organisms. Field experiments were performed during three different seasons in Arcachon Bay, a mesotidal lagoon (SW France), to assess the variability of the water column concentrations, sediment–water exchanges and potential formation and degradation of MMHg. The objectives were to evaluate the contribution of intertidal mudflats to MMHg production and the various pathways of Hg species export. Dissolved and bulk concentrations of Hg species in the water column downstream of tidal flats were measured throughout several tidal cycles. The Hg benthic fluxes at the sediment–water interface were determined by means of benthic chambers for three different stations. Hg methylation and demethylation potentials were determined in surficial sediments and the water column using isotopic tracers. The tidal surveys demonstrated that benthic remobilization of Hg occurs primarily in association with sediment erosion and advection during ebb tide. However, elevated dissolved Hg concentrations observed at low tide were found to be caused by a combination of pore-waters seeping, benthic fluxes and methylation in the water column. Benthic fluxes were more intense during late winter conditions (median MMHg and inorganic Hg (IHg) fluxes: 64 and 179 pmol m^?2 h^?1, respectively) and subsequently decreased in spring (median 0.7 and ?5 pmol m^?2 h^?1, respectively) and fall (median ?0.4 and ?1.3 pmol m^?2 h^?1, respectively). The trends in methylation and demethylation potentials were at the opposite of the fluxes, two times lower during winter than for spring or fall conditions. In this tidal environment, MMHg production in surface sediments and its subsequent release is estimated to be the major source of MMHg to the water column during winter and spring time. However, during the more productive summer period, the Hg methylation extent in the water column may be very significant and equivalent to the sediment contribution.     

Nitrogen removal in coastal sediments of the German Wadden Sea

Although sediments of the German Wadden Sea are suspected to eliminate a considerable share of nitrate delivered to the SE North Sea, their denitrification rates have not been systematically assessed. We determined N₂ production rates over seasonal cycles (February 2009–April 2010) at two locations with two sediments types each, the first site (Meldorf Bight) receiving nitrate during all seasons from the Elbe river plume, and a second site on the island of Sylt, where nitrate is depleted during summer months. In sediments from the Sylt site, N₂ production ranged from 15 to 32 μmol N₂ m^?2 h^?1 in the fine sand station and from 7 to 13 μmol N₂ m^?2 h^?1 in the coarse sand station; N₂ production was not detected when nitrate was depleted in May and July of 2009. N₂ production in the Meldorf Bight sediments were consistently detected at higher rates (58–130 μmol N₂ m^?2 h^?1 in the very fine sand station and between 14 and 30 μmol N₂ m^?2 h^?1 in the medium sand station). Analysis of ancillary parameters suggests that major factors controlling N₂ production in coastal sediments of the German Wadden Sea are the nitrate concentrations in the overlying water, the ambient temperature, and the organic matter content of the sediment. Extrapolating our spot measurements to the zone of nitrate availability and sediment types, we estimate an annual nitrogen removal rate around 16 kt N year^?1 for the entire northern sector of the German Wadden Sea area. This corresponds to 14% of the annual Elbe river nitrogen load.     

Microbial sulfate-reducing activities in anoxic sediment from Marine Lake Grevelingen: screening of electron donors and acceptors

Sulfate-reducing bacteria in marine sediments mainly utilize sulfate as a terminal electron acceptor with different organic compounds as electron donors. This study investigated microbial sulfate-reducing activity of coastal sediment from Marine Lake Grevelingen (MLG), the Netherlands using different electron donors and electron acceptors. All four electron donors (ethanol, lactate, acetate and methane) showed sulfate-reducing activity with sulfate as electron acceptor, suggesting the presence of an active sulfate-reducing bacterial population in the sediment, even at dissolved sulfide concentrations exceeding 12 mM. Ethanol showed the highest sulfate reduction rate of 55 µmol g _VSS ^?1 day^?1 compared to lactate (32 µmol g _VSS ^?1 day^?1), acetate (26 µmol g _VSS ^?1 day^?1) and methane (4.7 µmol g _VSS ^?1 day^?1). Sulfide production using thiosulfate and elemental sulfur as electron acceptors and methane as the electron donor was observed, however, mainly by disproportionation rather than by anaerobic oxidation of methane coupled to sulfate reduction. This study showed that the MLG sediment is capable of performing sulfate reduction by using diverse electron donors, including the gaseous and cheap electron donor methane.     

Effects of sunlight on the production of dissolved organic and inorganic nutrients from resuspended sediments

A series of laboratory-based and field experiments was conducted to address the effects of sunlight-exposed resuspended sediments on dissolved nutrient fluxes in two different water bodies. In suspensions of tidal creek sediments in 0.2 μm-filtered creek water, measurable increases in dissolved nutrients occurred after only 2 h of exposure to simulated sunlight. During a 6-h irradiation, nutrient release rates for total dissolved nitrogen (TDN) and phosphate were 2.2 ± 0.5 (standard error; S.E.) μmol g^?1 h^?1 and 0.09 ± 0.005 μmol g^?1 h^?1 (S.E.), compared to no significant changes in dark controls. The majority of nitrogen was released as dissolved organic nitrogen (87% on average) with lesser amounts of ammonium (13%). Continental shelf sediments resuspended in unfiltered seawater also released phosphate and TDN when exposed to sunlight, suggesting that this process can occur in a variety of marine and estuarine environments. The source material for inorganic nutrients appears to be associated with sediments rather than dissolved organic matter, as no significant changes in nutrient concentrations occurred in experiments with 0.2 μm-filtered creek water or seawater alone. Results suggest that photoproduction of dissolved nutrients from resuspended sediments could be an episodically significant and previously unrecognized source of dissolved organic and inorganic nutrients to coastal ecosystems. This process may be especially important for continental margins where episodic resuspension events occur, as well as in regions experiencing high riverine sediment fluxes resulting from erosion associated with deforestation and desertification.     

Myriophyllum aquaticum as a biomonitor of water heavy metal input related to agricultural activities in the Xanaes River (Córdoba,Argentina)

The aim of the present study was to assess the temporal variation of the heavy metal content (Co, Cu, Fe, Mn, Ni, Pb, and Zn) in surface water and sediments in relation to agricultural practices in the Xanaes River (Córdoba, Argentina). A second objective was to analyze possible relationships between the input of heavy metals on surface water and sediment, heavy metal accumulation and physiological changes in the aquatic plant Myriophyllum aquaticum. Samples were taken from the river at two contrasting sites (between April 2010 and August 2010): (1) a pristine area (mountain site), and (2) an area with intensive agricultural activity located at 60 km down river (agricultural site). The total concentration of heavy metals in surface water was higher in samples collected at the agricultural site but in sediments only the Mn concentration was higher than at the mountain site. The Fe and Mn concentrations in surface water at the agricultural site exceeded the recommended values for Argentinean Legislation of 300 μg L⁻¹ for Fe and 100 μg L⁻¹ for Mn. The accumulations of Zn and Mn in M. aquaticum were higher at the agricultural site and more elevated than the Zn and Mn concentrations in sediments at the same sites and sampling times. At the agricultural site, temporal variations of Cu, Fe and Zn were relatively similar for plants and water column, but the levels of the metals in plants were displaced over time. These results suggest that the levels of pollutants in the river came in pulses from the riverbank. These results show the potential use of M. aquaticum as a suitable accumulation biomonitor at the early stages of heavy metal pollution in rivers.     

Effects of sediment-bound Cd, Pb, and Ni on the growth, feeding, and survival of Capitella sp. I

Anthropogenic metal pollutants bioaccumulated in benthic animals by means of feeding and osmotic diffusion. These metals may affect the physiology of the benthos. In this study, we exposed Capitella sp. I to three metals (Cd, Pb, and Ni), each in eight different concentrations, to determine the effects of metals on the animals. Growth rate, ingestion rate, and percent survival were estimated in three separated experiments. The growth and feeding of the worms were sensitive to even the lowest concentrations of each metal added to the sediments. The lowest observable adverse effect levels for Cd, Ni, and Pb were 0.03, 1.59, and 0.41 μmol g^− 1 sediment, respectively. Growth rates in the elevated metal contaminant treatments decreased drastically at slightly contaminated levels, lessened detrimental effects at moderately contaminated levels, and showed incompensable intoxication at heavily contaminated levels. The trends in ingestion rates were similar to those of growth rates. No significant difference in survivorship was found among the different contaminant levels for any of the three heavy metals. Capitella sp. I was most sensitive to Cd, followed by Ni and Pb, which had similar effects. The rapid physiological responses of Capitella sp. I allowed the animals to survive metal exposure. Sediment productivity remained unchanged at different contamination levels of Ni and Pb, but was drastically reduced at 4.75 μmol g^− 1 Cd in the sediment. This further demonstrated Capitella sp. I can adjust their ingestion rates to maintain constant sediment productivities in moderate pollution conditions; however, when threshold concentration was exceeded, homeostasis collapsed.     

Distribution of heterotrophic aerobic marine bacteria in sediment in Eastern Harbour of Alexandria

In Eastern Harbour (EH) of Alexandria (Egypt), where an under-water museum is planned to be built, the distribution of some heterotrophic bacteria and their relationships to physical parameters and biochemical composition of the sedimentary organ ic matter were investigated. The Eastern Harbour is a relatively shallow semi-closed basin and is sheltered from the sea by an art ificial breakwater leaving two openings, El-Boughaz and El-Silsila. Seven stations covering the area of the EH were selected and surface sediments were collected seasonally from spring to winter 2002. The near bottom temperature varied seasonally with a minimum value in winter and a maximum in summer. In contrast to the temperature values, dissolved oxygen exhibited maximum values in cold seasons. The seasonal average of the total organic carbon ranged from 0.48 ± 0.16 to 4.42 ± 2.46%, while the total organic nitroge n ranged from 0.07±0.08 to 0.42±0.38%. The total carbohydrate had minimum and maximum values of 273 μg g^?1 and 6539 μg g^?1. The combined amino acids represented the dominant biochemical class of organic matter in the EH sediments with an average of 365 ± 1911 μg g^?1. The total bacterial count ranged from 1.4 × 10⁴ to 1.4 × 10⁷ colony forming unit (CFU) g^?1 sediment dry weight. Amylolytic bacterial group was recorded in almost all sites and seasons, while proteolytic bacteria were dominant in spring and au tumn. The variation in the abundance of amylolytic and proteolytic bacterial groups was found to be parallel to the variation in soluble carbohydrates and free amino acids. High percentage of H₂S-producing bacteria was reported during summer at some stations confirming the low oxygen content of the sediment at these sites. Agar-degrading bacteria were found only in warm seasons. The count of co liform bacteria in the EH sediments was very low (<10 CFU g^?1) during all monitored seasons indicating that the EH marine environment was almost free of domestic waste discharge during this period.     

Polycyclic aromatic hydrocarbons in traffic soil and Pinus needles of Beijing,China

Abstract

This study was conducted to examine the impact of vehicular traffic on the contamination status of urban traffic sites in Beijing with respect to polycyclic aromatic hydrocarbons (PAHs). The soil and Pinus pinea needle samples were collected at 12 sites on Three Ring Road in Beijing, China, from July 2009 to March 2010. Total PAH concentrations at traffic sites ranged from 113.86 to 2226.24 ng g^?1 with an average value of 658.71 ± 742.41 ng g^?1 in soil samples and 112.03 to 2016.80ng g^?1 with an average value of 680.29 ± 485.61 ng g^?1 in needle samples. The PAH pattern was dominated by two- and three-ring PAHs (contributing >80% to the total PAHs) at all the 12 traffic sites. Seasonal variations of PAHs compounds indicated the PAHs concentrations in autumn and winter were higher than those in spring and summer due to temperature effects. This work indicates that the low ring PAHs were the major profile at the roadside of a busy street with heavy traffic in Beijing.     

Freeze–thaw processes and intense rainfall: the one-two punch for high sediment and nutrient loads from mid-Atlantic watersheds

Large runoff, sediment, and nutrient exports from watersheds could occur due to individual extreme climate events or a combination of multiple hydrologic and meteorological conditions. Using high-frequency hydrologic, sediment, and turbidity data we show that freeze–thaw episodes followed by intense winter (February) rainstorms can export very high concentrations and loads of suspended sediment and particulate organic carbon (POC) and nitrogen (PN) from mid-Atlantic watersheds in the US. Peak suspended sediment (> 5000 mg L^?1), POC (> 250 mg L^?1) and PN (> 15 mg L^?1) concentrations at our 12 and 79 ha forested watersheds for the February rainfall-runoff events were highest on record and the fluxes were comparable to those measured for tropical storms. Similar responses were observed for turbidity values (> 400 FNU) at larger USGS-monitored watersheds. Much of the sediments and particulate nutrients likely originated from erosion of stream bank sediments and/or channel storage. Currently, there is considerable uncertainty about the contribution of these sources to nonpoint source pollution, particularly, in watersheds with large legacy sediment deposits. Future climate projections indicate increased intensification of storm events and increased variability of winter temperatures. Freeze–thaw cycles coupled with winter rain events could increase erosion and transport of streambank sediments with detrimental consequences for water quality and health of downstream aquatic ecosystems.     

Influence of an intermittent food supply on energy storage by the subpolar deposit feeder Yoldia hyperborea (Bivalvia: Nuculanidae)

Food supply for deposit feeders varies from highly seasonal phytodetritus to a steady source of older organic matter, resulting in contrasting patterns of nutrient uptake and storage. To identify patterns in energy storage and feeding behaviour driven by different food conditions for the circumpolar deposit-feeding protobranch bivalve Yoldia hyperborea, we measured variations in cytological (digestive cell height) and biochemical (lipid class, fatty acid, glycogen, and protein content) components during controlled experiments. Three treatments with organisms in sediment with high refractory organic matter (12 % OM) were exposed to different feeding regimes resembling (a) the annual spring bloom settlement, (b) low food availability during winter, and (c) sporadic resuspension events. Yoldia exposed to a diatom-supplemented diet showed significantly higher mean values for digestive cell height (28.44 μm), glycogen (30.4 mg g^?1 dry mass, DM), diatom-specific fatty acids, and total lipid (TL) levels (14.4 mg g^?1 DM), but lower protein concentrations, than in non-supplemented treatments (digestive cell height 20.34 μm; glycogen 9.23 mg g^?1 DM; TL 6.7 mg g^?1 DM). All analyses showed no effect of resuspension events; thus, it was unlikely that resuspension improved sediment nutritional value. In the absence of recently deposited diatoms, Y. hyperborea did not increase nutrient storage, suggesting that significant amounts of older refractory OM are not used for growth or reproduction. The rapid storage of nutrients derived from diatoms demonstrates the role of seasonal episodic events of settling algae in the nutrition of subpolar Y. hyperborea and in the transfer of energy from the water column to the benthos.     

Melatonin affects the growth and cadmium accumulation of Malachium aquaticum and Galinsoga parviflora

Phytoremediation technology has become one of the main techniques for remediating soils polluted by heavy metals because it does not damage the environment, but heavy metal-tolerant plants have the disadvantages of low biomass and slow growth. A pot experiment was conducted to study the effects of melatonin (Mel) on growth and cadmium (Cd) accumulation in the Cd accumulator Malachium aquaticum and hyperaccumulator Galinsoga parviflora by spraying different concentrations of Mel on them. The results showed that shoot biomass, photosynthetic pigment content and antioxidant enzyme activity were increased in both species after Mel was sprayed on their leaves. Mel reduced the Cd content in shoots of M. aquaticum and increased it in those of G. parviflora. In general, Cd accumulation was greatest in M. aquaticum when Mel was 200 μmol L^?1 (120.71 μg plant^?1, increased by 15.97% than control) and in G. parviflora when Mel was 100 μmol L^?1 (132.40 μg plant^?1, increased by 68.30% than control). Our results suggest it is feasible to improve the remediation efficiency of lightly Cd-contaminated soil by spraying G. parviflora with100 μmol L^?1 Mel.     

Implications of temperature and sediment characteristics on methane formation and oxidation in lake sediments

Methane emissions from aquatic environments depend on methane formation (MF) and methane oxidation (MO) rates. One important question is to what extent increased temperatures will affect the balance between MF and MO. We measured potential MF and MO rates simultaneously at 4, 10, 20 and 30°C in sediment from eight different lakes representing typical boreal and northern temperate lake types. Potential MF rates ranged between 0.002 and 3.99 μmol CH₄ g_d.w. ^?1 day^?1, potential MO rates ranged from 0.01 to 0.39 CH₄ g_d.w. ^?1 day^?1. The potential MF rates were sensitive to temperature and increased 10 to 100 fold over the temperature interval studied. MF also differed between lakes and was correlated to sediment water content, percent of organic material and C:N ratio. Potential MO did not depend on temperature or sediment characteristics but was instead well explained by MF rates at the in situ temperature. It implies that elevated temperatures will enhance MF rates which may cause increased methane release from sediments until MO increases as well, as a response to higher methane levels.     

Influence of hydrological connectivity of riverine wetlands on nitrogen removal via denitrification

Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from <5 to 1,133 μM). Benthic metabolism and denitrification rates were investigated by two overarching factors: season and hydrological connectivity. SOD and DIC fluxes resulted in respiratory quotients greater than one at most sampling sites. Sediment respiration was coupled to both ammonium efflux, which increased from winter to summer, and nitrate consumption, with higher rates in river-connected wetlands. Denitrification rates measured in river-connected wetlands (35–1,888 μmol N m^?2 h^?1) were up to two orders of magnitude higher than rates measured in isolated wetlands (2–231 μmol N m^?2 h^?1), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9–1,888 μmol N m^?2 h^?1) than in winter (2–365 μmol N m^?2 h^?1). Denitrification supported by water column nitrate (D_W) accounted for 60–100% of total denitrification (Dtot); denitrification coupled to nitrification (D_N) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification.     

A silicon budget for an Alaskan arctic lake

The cycling of dissolved reactive silicate (DRS) and sedimentary biogenic SiO₂ has been examined in ultraoligotrophic Toolik Lake, Alaska. Watershed output of DRS (∼ 7 mmol m ^{− 2} yr ⁻¹) is similar to other arctic study sites, but a short water residence time results in the poor retention of DRS (∼17%) within the lake. Biogenic SiO₂ concentrations in surficial sediments average 123 mg g⁻¹ with the highest concentrations arising from the production of benthic diatoms in the littoral zone. Lake water DRS concentrations are highest prior to spring flow and photic zone concentrations exceed 23 μmol 1⁻¹ at the time of greatest primary production. Wintertime increases in watercolumn DRS concentrations and spring and summer pore water DRS flux estimates indicate that internal cycling within the lake may supply an amount of DRS equivalent to that which biologically utilized.