Diffusive emissions of methane and nitrous oxide from a cascade of tropical hydropower reservoirs in Kenya

The present study investigated diffusive emissions of methane (CH₄) and nitrous oxide (N₂O) to the atmosphere from three relatively small (3–120 km²) reservoirs (Masinga, Kamburu and Gitaru) on the Tana River (Kenya). Sampling was conducted biweekly in 2011, 2012 and 2013, at sampling sites upstream and downstream of these reservoirs while five sampling campaigns were carried out in 2011, 2012 and 2013 for different sites within each of the reservoirs. The dissolved CH₄ (range: 19–2101 nmol/L) and N₂O (range: 6.2–11.5 nmol/L) concentrations in the surface waters were generally very low in the three reservoirs, compared with other reservoirs globally. The lower diffusive emissions of CH₄ (20–216 µmol/m² day^?1) and N₂O (1.0–1.6 µmol/m² day^?1) from these reservoirs, compared with other tropical reservoirs, are probably related to their age (30–40 years), and lower vegetation biomass (savannah) originally present and submerged during their commissioning. The reservoirs with longer water residence times were characterized by higher diffusive CH₄ fluxes (216 ± 666 µmol/m² day^?1) and slightly lower N₂O fluxes (1.0 ± 1.5 µmol/m² day^?1). The relative contribution of turbine fluxes of CH₄ and N₂O, compared to diffusive fluxes, was also highly variable among the three dams, being lower in Masinga Reservoir and higher in Gitaru Reservoir.     

Groundwater discharge drives water quality and greenhouse gas emissions in a tidal wetland

Wetlands play an important role in the global carbon cycle as they can be sources or sinks for greenhouse gases. Groundwater discharge into wetlands can affect the water chemistry and act as a source of dissolved greenhouse gases, including CO₂ and CH₄. In this study, surface water quality parameters and CO₂ and CH₄ concentrations were evaluated in a tidal wetland (Hunter Wetlands National Park, Australia) using time series measurements. Radon (²²²Rn), a natural groundwater tracer, was used to investigate the role of groundwater as a pathway for transporting dissolved CO₂ and CH₄ into the wetland. In addition, water-to-air CO₂ and CH₄ fluxes from the wetland were also estimated. The results showed a high concentration of radon in wetland surface water, indicating the occurrence of groundwater discharge. Radon concentration had a strong negative relationship with water depth with a determination coefficient (R²) of 0.7, indicating that tidal pumping was the main driver of groundwater discharge to the wetland. Radon concentration also showed a positive relationship with CO₂ and CH₄ concentrations (R² = 0.4 and 0.5, respectively), while the time series data revealed that radon, CO₂, and CH₄ concentrations peaked concurrently during low tides. This implied that groundwater discharge was a source of CO₂ and CH₄ to the wetland. The wetland had an average water-to-air CO₂ flux of 99.1 mmol/(m²·d), twice higher than the global average CO₂ flux from wetlands. The average CH₄ flux from the wetland was estimated to be 0.3 mmol/(m²·d), which is at the higher end of the global CH₄ flux range for wetlands. The results showed that groundwater discharge could be an important, yet unaccounted source of CO₂ and CH₄ to tidal wetlands. This work has implications for tidal wetland carbon budgets and emphasizes the role of groundwater as a subsurface pathway for carbon transport.     

Sediment greenhouse gases (methane and carbon dioxide) in the Lobo-Broa Reservoir, São Paulo State, Brazil: Concentrations and diffuse emission fluxes for carbon budget considerations

Carbon gases (methane, CH₄, and carbon dioxide, CO₂) were measured for the first time in sediments of the Lobo‐Broa Reservoir, near São Carlos in São Paulo State, Brazil. It is believed these are the first measurements of this kind in any of the many reservoirs located in Brazil. Even though the Lobo‐Broa Reservoir is classified as oligotrophic, the sediment gas concentrations were exceedingly high, ranging from 0.4–3 mmol L^?1 for CH₄ and 1–9 mmol L^?1 for CO₂. Both gases exceeded their in situ gas saturation values at these shallow water depths (7 m in central basin; 11 m at dam), resulting in numerous sediment bubbles. Organic matter was highly concentrated in the reservoir sediments, averaging 25.5% loss on ignition (LOI) (dam) to 26.9% LOI (central basin) for the 0–12 cm depth interval, with values as high as 29–30% LOI (12% organic carbon) in the surface 0–5 mm layer. The theoretical flux of dissolved pore water carbon gases to the sediment–water interface (SWI) averaged 3.4 mmol L^?1 m^?2 day^?1 CH₄ and 7.3 mmol L^?1 m^?2 day^?1 CO₂ for the surface 0–10 mm. From gas emission measurements at the water surface, it was calculated that 90% of CH₄ is consumed either at the SWI or in the water column, resulting in a loss of 0.31 mmol L^?1 m^?2 day^?1 of CH₄ to the atmosphere. However, only 20% of the total CO₂ gas transported across the water–atmosphere interface (36.3 mmol L^?1 m^?2 day^?1, or 1600 mg CO₂ m^?2 day^?1) was produced in the sediments. The remaining 80% of CO₂ probably comes from other carbon sources. With CH₄ oxidation in the aerobic water column, close to 30% of the carbon gas flux to the atmosphere could be accounted for by gas production of CO₂ and CH₄ in the sediments and their diffuse transport to the water column.     

First estimation of the diffusive methane flux and concentrations from Lake Winnipeg,a large,shallow and eutrophic lake

Freshwater lakes are increasingly recognized as significant sources of atmospheric methane (CH₄), potentially offsetting the terrestrial carbon sink. We present the first study of dissolved CH₄ distributions and lake-air flux from Lake Winnipeg, based on two-years of observations collected during all seasons. Methane concentrations across two years had a median of value of 24.6 nmol L^-1 (mean: 41.6 ± 68.2 nmol L^-1) and ranged between 5.0 and 733.8 nmol L^-1, with a 2018 annual median of 24.4 nmol L^-1 (mean: 46.8 ± 99.3 nmol L^-1) and 25.1 nmol L^-1 (mean: 38.8 ± 45.2 nmol L^-1) in 2019. The median lake-air flux was 1.1 µmol m^?2 h^?1 (range: 0.46–70.1 µmol m^?2h^?1, mean: 2.9 ± 10.2 µmol m^?2 h^?1) in 2018, and 5.5 µmol m^?2h^?1 (range: 0.0–78.4 µmol m^?2 h^?1, mean: 2.7 ± 8.5 µmol m^?2 h^?1) in 2019, for a total diffusive emission of 0.001 Tg of CH₄-C yr^?1. We found evidence of consistent spatial variability, with higher concentrations near river inflows. Significant seasonal trends in CH₄ concentrations were not observed, though fluxes were highest during the fall season due to strong winds. Our findings suggest Lake Winnipeg is a CH₄ source of similar mean magnitude to Lake Erie, with lower concentrations and fluxes per unit area than smaller mid- to high-latitude lakes. Additional work is needed to understand the factors underlying observed spatial variability in dissolved gas concentration, including estimations of production and consumption rates in the water column and sediments.     

Emissions of porewater compounds and gases from the subaquatic sediment disposal site “Rodewischhafen”, Hamburg harbour

In the year 1993 a confined and unused harbour basin was used to store 290,000 m³ of fine-grained dredged material from Hamburg harbour. About 70% of the deposit surface was water covered. The edge areas were above the water table and covered with reed. Emissions of dissolved compounds into the groundwater, as well as surface gas emissions were measured from 1994 to 1996. As indicators for water fluxes from the deposit we used NH₄⁺ and HCO₃⁻ because of their high concentrations in mud porewater in comparison to groundwater. The average concentrations of NH₄⁺ and HCO₃⁻ in the porewater increased during 2 years from 85 to 250 mg NH₄⁺ 1⁻¹ and from 2.0 to 3.1 g HCO₃⁻ 1⁻, while the groundwater samples showed constant values of 8 mg NH₄⁺ 1⁻¹ and 0.7 g HCO₃ 1⁻¹. Furthermore, the average gas emissions over the water surface were 3.2 g CH₄ m⁻² d⁻¹ and 0.8 g CO₂ m⁻² d⁻¹. In contrast, no methane and 3.0 g CO₂ m⁻² d⁻¹ were emitted from land areas. The results indicated, that there were no significant emissions of mud porewater compounds into the groundwater but high CH₄-emissions over the water covered surface of the mud deposit.     

Methane and nitrous oxide measured throughout Lake Erie over all seasons indicate highest emissions from the eutrophic Western Basin

Eutrophication has been linked to increased greenhouse gas emissions from inland waters. Phytoplankton blooms in Lake Erie have increased since the 1990s, although its greenhouse gas emissions are not well characterized. We measured CH₄ and N₂O concentrations and diffusive fluxes in four seasons around the entire lake, and CO₂ fluxes in one summer season. Lake Erie is a source of CH₄ all year across the lake, concentrated in spring and summer in the Western Basin. Methane emissions ranged from 0.03 to 14.87 mg C m⁻² d^-1. Methane is predominantly biogenic, and natural gas leaks are an insignificant source. While Lake Erie is an overall N₂O source, it is an N₂O sink in winter and occasionally during summer. Emissions of N₂O ranged from −0.08 to 1.22 mg N m⁻² d^-1. We also measured CO₂ fluxes in summer only, when Lake Erie is a small atmospheric CO₂ sink. While areal fluxes of CH₄ and N₂O are similar to those observed elsewhere, total fluxes from Lake Erie are higher due to its surface area. Lake Erie emits ~ 6300 (±19%) metric tons of CH₄-C yr⁻¹ and ~600 (±37%) metric tons N₂O-N yr⁻¹: almost 500,000 metric tons CO₂-eq yr⁻¹ total. This is the first comprehensive dataset of CH₄ and N₂O concentrations and diffusive emissions in a very large lake. More measurements and monitoring are needed to determine whether increased eutrophication in the Great Lakes is tied to increased emissions of these powerful climate forcers in a possible positive feedback to climate warming.     

Comparison of Carbon Sequestration Ability and Effect of Elevation in Fenced Wetland Plant Communities of the Xilin River Floodplains: A Model Case Study

Floodplain habitats of the Xilin River in Inner Mongolia, China, were overgrazed by sheep and cattle until fencing of the floodplains was implemented in 2000. Carbon cycling of three plant communities of differing floodplain elevation after fencing showed that biomass in low‐elevation wetlands increased fastest until reaching its maximum at 20 years in the future, while a slower increase in biomass existed in high‐elevation and ‘hummock’ wetlands. Modelling and field experiments revealed differences between the three plant communities that were primarily attributed to different elevation levels and inundation periods. This study also determined the carbon sequestration capacity of the three floodplain wetland types (0.18 kg C m^?2 year^?1 in low‐elevation wetlands, 0.09 kg C m^?2 year^?1 in high‐elevation wetlands, and 0.05 kg C m^?2 year^?1 in hummock wetlands). Copyright © 2014 John Wiley & Sons, Ltd.     

Deoxygenation potential of the Richmond River Estuary floodplain,northern NSW,Australia

Periodic deoxygenation events (DO < 1 mg/L) occur in the Richmond River Estuary on the east coast of Australia following flooding and these events may be accompanied by total fish mortality. This study describes the deoxygenation potential of different types of floodplain vegetation in the lower Richmond River catchment and provides a catchment scale estimate of the relative contribution of floodplain vegetation decomposition to deoxygenation of floodwaters. Of the major vegetation types on the floodplain slashed pasture was initially (first 5 to 7 h) the most oxygen demanding vegetation type after inundation (268 ± mg O₂ m^?2 h^?1), followed by dropped tea tree cuttings (195 ± 18 mg O₂ m^?2 h^?1) and harvested cane trash (110 ± 8 mg O₂ m^?2 h^?1). However, 10 h after inundation the oxygen consumption rates of slashed pasture (105 ± 5 mg O₂ m^?2 h^?1) and tea tree cuttings (59 ± 7 mg O₂ m^?2 h^?1) had decreased to a rate less than the harvested cane trash (110 ± 8 mg O₂ m^?2 h^?1). The oxygen demands of the different floodplain vegetation types when inundated were highly correlated with their nitrogen content (r² = 0.77) and molar C:N ratio (r² = 0.82) reflecting the dependence of oxygen demand of vegetation types on their labile carbon content. The floodplain of the lower Richmond River (as flooded in February 2001) has the potential to deoxygenate about 12.5 × 10³ mL of saturated freshwater at 25°C per day which is sufficient to completely deoxygenate floodwater stored on the floodplain with 3 to 4 days. In addition, oxidation of Fe²⁺ mobilized during the decomposition of floodplain vegetation via iron reduction and discharged from groundwater and surface runoff in acid sulfate soil environments could account for about 10% of the deoxygenation of floodwater stored on the floodplain. Management options to reduce floodplain deoxygenation include removing cuttings from slashed pasture and transporting off‐site, reducing slashed pasture windrow loads by using comb‐type mowers, returning areas of the floodplain to wetlands to allow the establishment of inundation tolerant vegetation and retaining deoxygenated floodwaters in low lying areas of the floodplain to allow oxygen consumption process to be completed before releasing this water back to the estuary. Copyright © 2006 John Wiley & Sons, Ltd.     

Environmental characteristics and phytoplankton productivity of a shallow Ramsar‐site floodplain in the western Himalaya

Hokersar wetland (altitude of 1584 masl), a shallow (0.5 m) floodplain waterfowl habitat in Kashmir, India, gained international importance in 2005 with its declaration as a Ramsar site. Although isolated in the western Himalayan mountains, it is being impacted by a silt‐laden river (Doodhganga). Human activities, not the least of which is affected by the closeness of the wetland to suburban areas, have gradually altered its trophic state. Its alkaline‐ and calcium‐rich waters contain many planktonic diatoms, green algae and cyanobacteria. Nitrate–nitrogen and total phosphorus concentrations are high (370–4750 and 101–968 μg L^?1, respectively). A pronounced seasonal cycle in phytoplankton production (g C m^?2 day^?1) was evident in a minimum value of 0.38 (Jan–Feb, 2002) and a maximum value of 4.02 (July, 2002), closely paralleling the seasonal cycles of temperature and light. The photosynthetic efficiency of the wetland was highest (1.97%) during the summer. The annual phytoplankton production of this wetland ecosystem was 210 × 10² KJ m^?2, suggestive of a meso‐eutrophic status.     

Assessing the Impacts of Four Land Use Types on the Water Quality of Wetlands in Japan

This study examined how changes in the composition of land use can affect wetland water quality. Twenty-four wetlands located in Hiroshima prefecture in the western part of Japan were selected for this purpose. The water quality parameters that were explored include: pH, electrical conductivity, turbidity, dissolved oxygen, total dissolved solid, temperature and different forms of nitrogen. These important indicators of the water quality in the study area were measured from December 2005 to December 2006. The composition of land uses was determined for the catchments of the wetlands. They were then categorized into three classes, including non-disturbed, moderately-disturbed and highly-disturbed wetlands, based on the extent of urban area (as the most disruptive land use type within the catchment of the wetlands). The relationship between land use types and water quality parameters for the wetlands was statistically examined. The findings indicated that there were significant positive relationships between the proportion (%) of urban areas within catchments of the wetlands and EC (r?=?0.67, p?<?0.01), TDS (r?=?0.69, p?<?0.01), TN (r?=?0.92, p?<?0.01), DON (r?=?0.6, p?<?0.01), NH₄ ⁺(r?=?0.47, p?<?0.05), NO₂ ^? (r?=?0.50, p?<?0.05), while negative relationships were observed between the proportion (%) of forest area in these wetlands and EC (r?=??0.62, p?<?0.01), TDS (r?=??0.68, p?<?0.01), TN (r?=??0.68, p?<?0.01), DON (r?=?-0.43, p?<?0.05), and NH₄ ⁺ (r?=??0.55, p?<?0.01). Analysis of the variance also revealed significant differences within the wetland groups in terms of the annual mean of electrical conductivity, total dissolved solids, total nitrogen, nitrite, dissolved inorganic nitrogen and dissolved organic nitrogen in the study area. Moreover, the study also indicated that the forest area plays a significant role in withholding nutrient loads from the wetlands, and hence, it can act as a sink for surface/subsurface nutrient inputs flowing into such water bodies from the watersheds.     

Anaerobic methane oxidation and aerobic methane production in an east African great lake (Lake Kivu)

We investigated CH₄ oxidation in the water column of Lake Kivu, a deep meromictic tropical lake with CH₄-rich anoxic deep waters. Depth profiles of dissolved gases (CH₄ and N₂O) and a diversity of potential electron acceptors for anaerobic CH₄ oxidation (NO₃^?, SO₄^2?, Fe and Mn oxides) were determined during six field campaigns between June 2011 and August 2014. Denitrification measurements based on stable isotope labelling experiments were performed twice. In addition, we quantified aerobic and anaerobic CH₄ oxidation, NO₃^? and SO₄^2? consumption rates, with and without the presence of an inhibitor of SO₄^2?-reducing bacteria activity. Aerobic CH₄ production was also measured in parallel incubations with the addition of an inhibitor of aerobic CH₄ oxidation. The maximum aerobic and anaerobic CH₄ oxidation rates were estimated to be 27?±?2 and 16?±?8?μmol/L/d, respectively. We observed a difference in the relative importance of aerobic and anaerobic CH₄ oxidation during the rainy and the dry season, with a greater role for aerobic oxidation during the dry season. Lower anaerobic CH₄ oxidation rates were measured in presence of molybdate in half of the measurements, suggesting the occurrence of linkage between SO₄^2? reduction and anaerobic CH₄ oxidation. NO₃^? consumption and dissolved Mn production rates were never high enough to sustain the measured anaerobic CH₄ oxidation, reinforcing the idea of a coupling between SO₄^2? reduction and CH₄ oxidation in the anoxic waters of Lake Kivu. Finally, significant rates (up to 0.37?μmol/L/d) of pelagic CH₄ production were also measured in oxygenated waters.     

Effect of mixing patterns and light dose on growth of Anabaena circinalis in a turbid,lowland river

Anabaena circinalis is common in the lower Murray River, Australia, and may compromise water quality due to the release of toxins. The water is turbid and thermal structure may significantly affect light availability. An in situ experiment was designed to represent complete mixing, diurnal stratification and persistent stratification and test the effect on growth of A. circinalis. To represent the mixing treatments, cells were incubated in diffusion chambers that were adjusted to different positions in the water‐column throughout the day. Populations exposed to persistent stratification over six days grew significantly faster than the other treatments at a rate of 0.65 day^?1. However, growth of the diurnally stratified populations was slower than (0.28 day^?1), or similar to (0.40 day^?1) the mixed population (0.40 day^?1). Therefore, the growth of the subpopulations exposed to the euphotic zone was insufficient to counteract the slow growth of the majority that were confined to darkness during the stratified period. A relationship between growth rate (G) and average daily light dose (I) was constructed and growth rate at optimal light dose (G_max), slope of linear section of G–I curve (α), and light dose where lines of G_max and light‐limited portion of G–I curve intersect (I_k) were solved as 0.66 day^?1, 0.12 day^?1 (mol^?1 m^?2 day^?1)^?1 and 5.4 mol m^?2 day^?1, respectively. Using these parameters, a model was developed to predict possible differences in growth between diurnal and mixed populations under varying conditions of vertical light attenuation, mixed depth and incubation time. Copyright © 2003 John Wiley & Sons, Ltd.     

First assessment of methane and carbon dioxide emissions from shallow and deep zones of boreal reservoirs upon ice break-up

Most studies dealing with greenhouse gas (GHG) emissions from large boreal reservoirs were conducted during the ice‐free period. In this paper, the potential methane (CH₄) and carbon dioxide emissions are estimated for two hydroelectric reservoirs, as well as for a small experimental reservoir from boreal latitudes (northern Quebec, Canada) at the ice break‐up event through diffusion (diffusive fluxes) and release of bubbles (bubbling fluxes). The results of this preliminary study suggest that the winter diffusive fluxes at the air–water interface of the sampled reservoirs represent < 7% of their cumulative carbon emissions during the ice‐free period. Furthermore, the release upon ice‐break of CH₄ bubbles accumulated under the ice cover during the winter could represent  2% of the summer carbon emissions from hydroelectric reservoirs in northern Quebec. The results presented herein suggest that the GHG emissions upon ice break‐up from the boreal reservoirs investigated are a small, but non‐negligible, component of their annual GHG emissions.     

Modeling Anthropogenic Impacts and Hydrological Processes on a Wetland in China

In many rapidly urbanizing countries like China, wetlands are constantly affected by anthropogenic factors such as landscaping, additional abstractions, reduction in catchment perviousness, etc. Thus, modeling of such anthropogenic factors should be explicitly considered when simulating wetlands. In this paper, the wetland module in a distributed hydrological model, SWAT, is modified to simulate the artificial water input to the designated wetlands. Local river runoff is used as the water sources to study the wetland restoration potential and to analyze the effects on local hydrological cycle and sea outflow. The QingDianWa depression, near Tianjin city, China, is used as a case study to study the restoration impact. Results showed that after restoration, the QinDianWa depression can reduce the potential impact of flooding by an average of 61 million m³ per year, increase the annual surface runoff by about 32 million m³ in non-flood seasons, and increase groundwater recharge by 9.4 million m³. This illustrated the importance of wetland restoration on flood control, river flow increase, groundwater recharge, and flood reclamation. But with local water resources is far from meeting the demands of wetland restoration, the actual restored water surface area is only 21.5 and 40.9 km² for the designated surface areas of 60 and 150 km² respectively. Compared with nature wetlands without human disturbance, the anthropogenic effect of reduction of runoff and groundwater recharge may be attributed to large amount of human consumption of local water. However, the results showed that the aims of restoring Tianjin wetlands cannot be achieved fully by relying solely on local water resources. It is necessary to consider a combination of external sources of water and using artificial recharge from reclaimed water.     

Effects of residence time on summer nitrate uptake in mississippi river flow‐regulated backwaters

Nitrate uptake may be improved in regulated floodplain rivers by increasing hydrological connectivity to backwaters. We examined summer nitrate uptake in a series of morphologically similar backwaters on the Upper Mississippi River receiving flow‐regulated nitrate loads via gated culverts. Flows into individual backwaters were held constant over a summer period but varied in the summers of 2003 and 2004 to provide a range of hydraulic loads and residence times (τ). The objectives were to determine optimum loading and τ for maximum summer uptake. Higher flow adjustment led to increased loading but lower τ and contact time for uptake. For highest flows, τ was less than 1 day resulting in lower uptake rates (U_net < 300 mg m^?2 day^?1), low uptake efficiency (U% < 20%) and a long uptake length (S_net > 4000 m). For low flows, τ was greater than 5 days and U% approached 100%, but U_net was 200 mg m^?2 day^?1. S_net was < half the length of the backwaters under these conditions indicating that most of the load was assimilated in the upper reaches, leading to limited delivery to lower portions. U_net was maximal (384–629 mg m^?2 day^?1) for intermediate flows and τ ranging between 1 and 1.5 days. Longer S_net (2000–4000 m) and lower U% (20–40%) reflected limitation of uptake in upper reaches by contact time, leading to transport to lower reaches for additional uptake. Uptake by ～10 000 ha of reconnected backwaters along the Upper Mississippi River (13% of the total backwater surface area) at a U_net of ～630 mg m^?2 day^?1 would be the equivalent of ～40% of the summer nitrate load (155 mg day^?1) discharged from Lock and Dam 4. These results indicate that backwater nitrate uptake can play an important role in reducing nitrate loading to the Gulf of Mexico. Copyright © 2008 John Wiley & Sons, Ltd.     

淡水湿地生态系统中微生物驱动氮转化过程研究进展

淡水湿地在全球氮循环中发挥着重要作用,微生物驱动氮转化过程对于淡水湿地的自然净化功能及水体富营养化控制具有重要意义。近年来,随着分子生物学和生物信息学等技术的快速发展,湿地系统的氮转化功能菌群和微生物多样性等方面的研究取得了突破性进展。与此同时,受气候条件变化及人类活动影响,淡水湿地系统生境因子出现了复杂变化,进而影响了功能微生物及其氮转化途径。氮循环过程中产生的一氧化二氮(N_2O)是仅次于二氧化碳(CO_2)、甲烷(CH_4)的第三大温室气体,与淡水湿地系统微生态及功能密切相关,其产生机理和影响因素也受到广泛关注。由于参与氮循环的微生物复杂多样、功能菌群协同作用机制不明,以及其受气候条件变化和人类活动影响显著,仍有必要深入研究淡水湿地系统的微生物驱动氮转化过程及机制。     

Anaerobic microbial metabolism in hyporheic sediment of a gravel bar in a small lowland stream

Distribution of dissolved oxygen, nitrate, sulphate, carbon dioxide and dissolved organic carbon (DOC), acetate and lactate was studied in the stream and interstitial water along the subsurface flowpath in the hyporheic zone of a small lowland stream. Sediments were found to act as a source of nitrous oxide and methane. Interstitial methane concentrations were significantly much higher in comparison to those from surface water, and were significantly lower in the relatively well oxygenated downwelling zone than in the rather anoxic upwelling zone. The interstitial concentrations of O₂, NO₃^?1 and SO₄^?2 showed significant decline along the subsurface flowpath, while concentrations of CO₂, N₂O, DOC, acetate and lactate remained unchanged. In addition to field measurements, ex situ incubation of sediments was carried out in the laboratory. Maximal methane production was found in the incubation assay using acetate (mean value 380 µg CH₄ kg DW^?1 d^?1). Mean value of the denitrification potential was 1.1 mg N₂O kg DW^?1 d^?1. Nitrous oxide production potential reached 71–100% of denitrification potential. Our results demonstrate that respiration of oxygen, nitrate, sulphate and methanogenesis may coexist within the hyporheic zone and that anaerobic metabolism is an important pathway in organic carbon cycling in the Sitka stream sediments. Copyright © 2005 John Wiley & Sons, Ltd.     

Transport of Fine Sediment Through a Wetland Using Radionuclide Tracers: Old Woman Creek,OH

The Old Woman Creek estuary (OWC), a coastal wetland in Ohio, traps 47% of incoming suspended sediment and has a sedimentation rate of ∼1 cm/yr. Persistence of the OWC wetland and other coastal wetlands with high sedimentation rates seems problematic unless some previously trapped sediment is exported from the wetland.Suspended sediment, ⁷Be, and ²¹⁰Pb_xs budgets for a single runoff event in the OWC wetland were developed to understand short-term sediment dynamics. The budgets were balanced by subtracting the sum of the imports from the sum of the exports and attributing the difference to either deposition on, or resuspension from, the wetland bed. The wetland exported 118 ± 2%, 93 ± 1%, 74 ± 2% of the delivered sediment, ²¹⁰Pb_xs, and ⁷Be, respectively, during the studied event. The ⁷Be/²¹⁰Pb_xs ratios of the total suspended solids and bed sediment were distinct from one anther and used to quantify the relative proportions of recently delivered and resuspended bed material in the sediment efflux from the wetland. The ⁷Be/²¹⁰Pb_xs ratios suggest that 26 ± 20% of the sediment efflux was resuspended from the bed. While the wetland trapped 13 ± 3% of the sediment it received during the runoff event, resuspension and removal of previously deposited sediment in the wetland was sufficiently large to result in a net loss of sediment from the wetland during the event. Thus, the Old Woman Creek wetland is a sediment sink over the long-term, but can be a net exporter of sediment during single events.     

Temperature,redox, and amendments alter wetland soil inorganic phosphorus retention dynamics in a Laurentian Great Lakes priority watershed

Surface water phosphorus loading must be reduced to improve water quality and decrease harmful algal blooms. Many wetlands have a natural capacity to retain inorganic reactive PO₄^3? via soil sorption. However, soil PO₄^3? retention capacity is finite and may be limited by soil legacy phosphorus effects in agricultural and urban areas. This study evaluated soil PO₄^3? retention in soils from a wetland constructed on former agricultural land in the Lake Erie, Maumee River watershed targeted for nutrient load reduction. Soil PO₄^3? sorption isotherms were evaluated under cool (10 °C), warm (22°), aerobic, and anaerobic treatments to determine changes in PO₄^3? retention due to environmental conditions and estimate seasonal changes in PO₄^3? sorption. The soils displayed a strong capacity for PO₄^3? retention by sorption. However, results indicate that cooler temperatures and anaerobic conditions decreased PO₄^3? sorption and lowered retention rates at PO₄^3? concentrations observed in the region. Soil amendment experiments investigated opportunities to increase PO₄^3? retention because many soils display elevated phosphorus concentrations due to historic land use, limiting their ability to adsorb additional PO₄^3?. Amendments increased PO₄^3? retention capacity compared to unamended soils in the presence of high PO₄^3? concentrations, suggesting soil PO₄^3? retention can be improved in areas where natural storage capacity has been exhausted. Results from this study can inform natural resources managers in the Laurentian Great Lakes and elsewhere when identifying potential nutrient reduction wetland locations and assist with developing operational guidelines to optimize PO₄^3? retention and water quality improvements using wetlands.     

Nitrogen Dynamics in Sandy Freshwater Sediments (Saginaw Bay,Lake Huron)

Sediment-water nitrogen fluxes and transformations were examined at two sites in Saginaw Bay, Lake Huron, as a model for sandy freshwater sediments. Substantial ammonium release rates (74 to 350 μmole NH₄⁺/m²/h¹) were observed in flow-through cores and in situ benthic chamber experiments. Sediment-water ammonium fluxes were similar at the inner and outer bay stations even though inner bay waters are enriched with nutrients from the Saginaw River. The high net flux of remineralized ammonium into the overlying water from these sandy sediments resembles typical data for marine systems (11 to 470 μmole NH₄⁺/m²/h¹) but were higher than those reported for depositional freshwater sediments (0 to 15 μmole NH₄⁺/m²/h¹; Seitzinger 1988). Addition of montmorillonite clay (ca. 1 kg dry weight/m²) to the top of the sandy cores reduced ammonium flux. Mean “steady-state” ammonium flux following clay addition was 46 ± 2 (SE) % of the initial rates as compared to 81 ± 8% of the initial rates without clay addition. Zebra mussel excretion dominanted ammonium regeneration in the inner bay where the bivalve was abundant, but addition of zebra mussel feces/psuedofeces (3.0 g dw/m²) to sediments did not increase ammonium or nitrate flux. Partial nitrification of ammonium at the sediment-water interface was suggested by removal of added ¹⁵NH₄⁺ from lake water passing over dark sediment cores. Sediment-water fluxes of nitrogen obtained from flow-through sediment cores resembled those from in situ benthic chambers. However, extended static incubations in gas-tight denitrification chambers caused more of the regenerated nitrogen to be nitrified and denitrified than occurred with the other two measurement systems.