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.     

Do Alluvial Sand Dunes Create Energetic Refugia for Benthic Fishes? An Experimental Test with the Endangered Pallid Sturgeon

River modifications have altered critical habitats for fishes at a variety of spatial scales and caused global declines of many fluvial species. At small spatial scales (<1 m²), alluvial sand dunes, a ubiquitous habitat in highly modified rivers, are thought to provide energetic relief for benthic fishes in energetically costly riverine landscapes created by water flow. However, use of alluvial dune habitat is not well understood, and it is unclear whether dunes provide refuge that effectively reduces energetic costs. We designed a scale‐relevant experiment to examine the energetic responses associated with sand dune habitat in rivers. We tested whether the US federally endangered pallid sturgeon (Scaphirhynchus albus ), a benthic fish commonly associated with sand dunes, experienced reduced energetic costs with different configurations of simulated sand dune habitat. We quantified mass specific oxygen consumption (M O₂; mg O₂ kg^?1 h^?1) using intermittent flow‐through respirometry for age‐0 sturgeon (140–170 mm) in front of a sand dune, behind a sand dune and in the absence of a sand dune at two velocities (25 and 50 cm s^?1) commonly observed in field studies of sturgeon habitat use. Sturgeon displayed distinct station holding behaviours for each habitat configuration. Dune location did not affect energy expenditure, but sturgeon M O₂ was on average 16–20% higher in the absence of a sand dune depending on dune configuration. M O₂ was on average 14% higher at 50 cm s^?1 compared with 25 cm s^?1. Our results provide a potential mechanism for over two decades of research on why sturgeon and other benthic fishes exhibit selection for sand dune habitat in large rivers. Fishes that select main channel habitats may depend on energetic relief provided by sand dunes, especially when other forms of structure are not available. For this reason, alluvial sand dune habitat may be important to the persistence of benthic fishes in high flow environments. Copyright © 2017 John Wiley & Sons, Ltd.     

An eutrophication index for lowland sandy rivers in Mediterranean coastal climatic regions of Southern Africa

The eutrophication of waterways has become an endemic global problem. Nutrient enrichment from agriculture activities and waste water treatment plants are major drivers, but it remains unclear how lowland sandy rivers respond to eutrophication. The objective of this study was the development and verification of eutrophication index for sandy rivers (EISR) to prioritize nutrient enrichment river stretches caused by different land use activities that include point and nonpoint sources of nutrient enrich water. The Berg River drainage system in South Africa served as a case study area for this purpose during the dry seasons (December and January) of 2015 and 2016. In the initial EISR development phase, periphyton, benthic biomass (chl‐a mg m^?2), and macroinvertebrate families were employed as benthic bioindicators of river bedforms, whereas in the second phase, physicochemical and abiotic variables were used as target indicator. Using a weight of support approach, the site receiving sewage effluent was categorize as heavily polluted whereas sites impacted by agriculture land use activities were polluted. The EISR that focuses strongly on benthic bioindicators, which are close to the transfer of nutrients and energy in the food web, showed a distinct difference between river bedform impacted by sewage effluent and agriculture none point source. A maximum benthic algae biomass of 110 mg m^?2 chl‐a was recorded with higher sediment orthophosphate concentration at sewage‐impacted sites. The outcome of the proposed EISR showed that it can be employed as a decision support tool for eutrophication management of sandy rivers.     

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.     

A Comparison of Metabolic Rates in Off‐Channel Habitats of the Middle Mississippi River

Autochthonous material has been found to be an important base in large river food webs. However, a spatial understanding of primary production in large rivers is lacking. We modeled primary productivity and community respiration (CR) during a low water period in two types of off‐channel habitat present in the Middle Mississippi River, side channels and wing dike fields. Wing dike fields are constantly connected to the main channel and are well mixed along most of their length, while side channels are typically connected only at the top and mouth. Gross primary production (GPP) in wing dike fields ranged from 0.0 to 8.9 g O₂ m^?2 D^?1 and in side channels GPP ranged from 0.4 to 33.5 g O₂ m^?2 D^?1. Both habitat types experienced periods of positive net ecosystem production (NEP) especially in the late summer and early fall. Correlations between metabolic rates and ecosystem characteristics differed between habitat types. Discharge was negatively correlated to NEP in wing dike fields but was not associated with metabolic rates in side channels. Light was positively correlated with GPP and CR at both site types and with NEP in side channels. These areas are protected from high velocity and likely experience greater light penetration, allowing more photosynthesis to take place especially during low water periods. This study demonstrates the potential for high productivity in off‐channel habitats that are permanently connected to the main channel. Copyright © 2016 John Wiley & Sons, Ltd.     

Patterns of primary and heterotrophic productivity in an arid lowland river

Three river conceptual models make differing predictions about the major source of primary production in lowland rivers, acknowledging the importance of primary productivity in the ecology and management of lowland rivers. Patterns of primary production in lowland rivers are still an area of considerable uncertainty. The objective of this study was to examine the major sources and transformations of organic matter in an Australian lowland river and compare them to the predictions of existing models. The broad approach adopted was to quantify the contribution from the major ecosystem components and compare these with estimates of system metabolism determined using open water measures of diel oxygen change. Three 4‐km river reaches were selected to represent the extent of variation found along the free‐flowing lowland sections of the Murray River, one of Australia's largest and most regulated rivers. Annual open water gross primary production (GPP) estimates for the Murray R. during this study ranged from 221 to 376 gC m^?2 y^?1 and were similar to other large rivers. Examination of the net contribution of organic matter to the channel indicates that primary productivity in the Murray R. is derived from a combination of phytoplankton, riparian vegetation and macrophytes, but that the major source varies both spatially and temporally. The present study confirms that the River Continuum Concept (RCC), the Flood Pulse Concept (FPC) and Riverine Productivity Model (RPM) all have some application to Australian lowland rivers, but that synthesis of the models will be difficult until we can incorporate the extent, causes and consequences of primary production variability. This study also highlights the importance of the microbial loop and macrophytes in the ecology of the Murray R. Copyright © 2007 John Wiley & Sons, Ltd.     

Diffusive fluxes and equilibrium processes in relation to phosphorus dynamics in the Upper Mississippi River

We examined total suspended solids (TSS) and phosphorus (P) dynamics in the Upper Mississippi River (UMR) above naturally impounded Lake Pepin between 1994 and 1996, with attention to P loadings which may be contributing to impaired water quality conditions. The Minnesota River, located 97 km upstream of Lake Pepin, accounted for most of the annual and summer TSS and total P load while the Metropolitan Wastewater Treatment Plant, located 80 km upstream of Lake Pepin, accounted for much of the annual soluble reactive P (SRP) loading to the UMR. Lake Pepin retained 80% and 13% of the TSS and total P load, respectively, during the summer period. However, the lake was a source of SRP during the same period, exporting up to 130% of the summer SRP load it received. Diffusive P flux from profundal sediments averaged 7.5 mg m^?2 d^?1 during the summer, accounting for 41% of the net SRP export from the lake. Recently deposited sediments in Lake Pepin (originating primarily from the Minnesota River) also exhibited a high equilibrium P concentration (EPC=0.155 mg l^?1) and linear adsorption coefficient (K_d=1043 l kg^?1). Application of these P equilibrium characteristics to TSS loads entering the system resulted in a calculated potential P desorption flux from TSS of 2.0 mg m^?2 d^?1 during the summer. Potential P desorption flux to the system was driven by dilution of SRP concentrations in the Mississippi River below the EPC by SRP‐deficient inflows of the St Croix River, located 16 km upstream of the lake. P desorption was, thus, an important additional internal P flux that is not commonly included in P budgets of riverine systems. Published in 2004 by John Wiley & Sons, Ltd.     

The Interplay between Environmental Conditions and Filamentous Algae Mat Formation in Two Agricultural Influenced South African Rivers

The regulation of nutrient inputs into rivers dominated by agriculture land use activities is an important aspect of ecological resilience of aquatic systems and the management of river eutrophication. The overabundance of benthic filamentous algae mats in river systems due to nutrient enrichment can modify the habitats of macroinvertebrate and fish communities as well as clogging irrigation crop sprayers of downstream water users. The current study examined over a period of 2 years (2013–2014) the interplay between physical and chemical river characteristics and epilithic filamentous algae biomass in two South African agricultural influenced rivers. The study area consisted of the Touw and Duiwe Rivers, which run into a proclaimed Ramsar site, namely, the Wilderness Lake System. A strong positive correlation was observed between the maximum filamentous algae biomass (97 chl‐a mg m^?2) observed during the dry season and the average water column alkalinity >30 mg l^?1. The benthic trophic status of the nine sampling sites during the dry seasons indicated the highest benthic algae biomass with mesotrophic (1.7–21 chl‐a mg m^?2) to hypertrophic (>84 chl‐a mg m^?2) conditions. During the dry season, only three sampling sites were below the suggested guideline value (35 µg l^?1) for total phosphorus (TP), while four sampling sites were below the total nitrogen guideline of 252 µg l^?1. In the wet season, two sites were below TP values with five sites below total nitrogen guideline values. From the data gathered, it was evident that water column alkalinity and hardness were the main drivers for the formation or absence of benthic filamentous algae mats in the two river systems and that nitrogen and/or phosphorus concentrations was overshadowed by the physical and chemical characteristics of the river systems at certain sites. Nutrient results for the river bottom sediments revealed that the sediment qualities were variable at the different sampling sites, but more specifically along the longitudinal paths of flow. It was apparent that the high TP concentrations in the water column and bottom sediment, which were lowest during the dry season, were associated with the highest epilithic filamentous algae mat formation. The outcome of the current study shows that a more holistic approach must be followed for the development of future eutrophication guidelines and nutrient thresholds in South African rivers influenced by agriculture land use activities. Copyright © 2016 John Wiley & Sons, Ltd.     

Long-term nitrate trends in the river trent and four major tributaries

Nitrate levels which have been rising in many British rivers for at least thirty years have become of particular interest since Britain adopted the EC Drinking Water Directive. The literature examining nitrate trends in the rivers of the U.K. and Eire is reviewed. Long-term nitrate concentration trends are derived for eight sites in the River Trent drainage system. These include sites on two rural tributaries, two urbanized tributaries, and the main river. Currently, mean annual concentrations for these sites range from about 4.0 mg1^?1N in the rural catchments, to 12.4 mg 1^?1 N in the urbanized tributaries, and to between 8.7 mg 1^?1N and 11.0 mg 1^?1 N along the mainstream. Moreover rates of increase of nitrate concentration of approximately 0.06 mg 1^?1 N yr ^?1 in the rural catchments, contrast with values up to 0.20 mg 1^?1 N yr ^?1 in the two urbanized tributaries and River Trent. Long term nitrate loads have been calculated for the River Trent at Nottingham using a discharge weighted mean interval method. This method probably provides the best available estimate of nitrate loads calculable from irregular data of variable sampling frequency for lowland rivers. Annual nitrate loads varied between 10700t yr ^?1 and 29 800t yr ^?1 over the period of record. A rate of increase in nitrate load of 455 t yr ^?1 was obtained. The increases in nitrate concentrations and loads within the Trent basin are matched by similar rates of increase in other European catchments.     

Changes in benthic animal production of a weir basin after eight years of succession

Benthic animal production was studied in a weir basin area of western Norway in 1984, eleven years after construction of the basin and eight years after the first benthic study. Compared to estimates from 1976, a marked change in production and species composition had occurred. Benthic animal production at two stations inside the basin was 199 and 169 KJ m^?2 yr^?1, an increase of 440 and 270 per cent respectively compared to 1976. In a riffle upstream of the basin, the production estimate was 50 KJ m^?2 yr^?1, a decrease of 40 per cent since 1976. Chironomids formed the main contribution to the increased production in the weir basin, mainly through increased abundance of larger species like Stictochironomus pictulus and Micropsectra spp. Also other species more adapted to lentic water, like the ephemeropterans Siphlonurus aestivalis and Ameletus inopinatus, showed increased density. The fauna succession and increased production in the weir basin was mainly due to improved habitats and food conditions, as the amount of organic sediments had increased since the first production study.     

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.     

Nitrogen processing by biofilms along a lowland river continuum

While numerous studies have examined N dynamics along a river continuum, few have specifically examined the role of biofilms. Nitrogen dynamics and microbial community structure were determined on biofilms at six sites along a 120 km stretch of the lowland Ovens River, South Eastern Australia using artificial substrates. Terminal restriction fragment length polymorphism (T‐RFLP), chlorophyll a and protein analyses were used to assess biofilm microbial community composition. N dynamics was determined on the biofilms using the acetylene (C₂H₂) block technique and assessing changes in NH, NO_x and N₂O. Unlike microbial community structure, N dynamics were spatially heterogeneous. Nitrification, determined from the difference in accumulation of NH before and after addition of C₂H₂, occurred mostly in the upper sites with rates up to 1.4 × 10^?5 mol m^?2 h^?1. The highest rates of denitrification occurred in the mid‐reaches of the river (with rates up to 1 × 10^?5 mol m^?2 h^?1) but denitrification was not detected in the lower reaches. At the very most, only 50% of the observed uptake of NO_x by the biofilms following addition of C₂H₂ could be accounted for by denitrification. Copyright © 2005 John Wiley & Sons, Ltd.     

A review of the effects of water transfers in the La Grande Hydroelectric Complex (Québec,Canada)

Construction of the La Grande Complex in Eastern Canada called for complete or partial diversion of six rivers, the mean annual discharge of which varied from 60 to 1600 m³ s^?1, and the transfer of water from two adjoining watersheds (1586m³ s^?1) into La Grande Rivière. Three of these rivers with reduced flow, Eastmain (from 700 to 95m³ s^?1), Opinaca (from 260 to 35m³ s^?1), and Caniapiscau (from 1708 to 960m³ s^?1) and the waterways with increased flows, Boyd-Sakami (from 1 to 811m³ s^?1) and La Grande Rivière (from 1760 to 3400m³ s^?1) were intensively monitored from 1978 to 1988, while the others were surveyed sporadically. Very few modifications were observed, compared to previous conditions, in banks stability, water quality, and aquatic fauna of the affected rivers: those which appeared can be explained by the water level fluctuations, the water quality of the tributaries of the residual drainage basin, and/or by the direct influence of the reservoirs. In James Bay, the areal extent of the Eastmain River and La Grand Rivière plumes varied with discharge in an exponential relation, mostly under ice cover; resources from the coastal environment had not yet undergone any major changes. Because of the very small population dwelling in the area, less than 3000 inhabitants, modifications in the hydrological regime of these rivers did cause relatively few impacts, the main being greater banks instability in specific locations, reduction of the thickness and ice cover on La Grande Rivière, enhancement of navigation constraints on Eastmain River, and higher mercury levels in fish related to operation of the reservoirs.     

Production and Respiration in Lake Erie Plankton Communities

Planktonic community metabolism (photosynthesis and respiration) was assessed in the oligotrophic east basin of Lake Erie, from November 2001 to October 2002 using O₂ and ¹⁴C methods. Areal gross production (AGP; mmol O₂ m⁻² d⁻¹) exceeded areal respiration (AR; mmol O₂ m⁻² d⁻¹) in the surface mixed layer for 69% of the observations during the study period. The median AGP:AR for the entire study period was 1.32. A significant positive relationship between AGP and AR existed, but AGP explained only 25% of the variation in AR. AGP:AR varied seasonally, being below 1.0 in the fall/winter of 2001 and in early spring 2002. High (>> 1.0) AGP:AR was observed in late spring (May) and AGP:AR remained > 1.0 for most of the summer stratified period (July–September). AGP:AR was > 1.0 in the fall of 2002, but the magnitude was less than observed during spring. The results supported traditional concepts of the seasonal production and consumption cycles in planktonic communities of large oligotrophic lakes. Parallel incubations of ¹⁴C uptake and gross O₂ production determined with the light and dark bottle method revealed a mean experimental gross photosynthetic quotient (PQ_G) of 1.29 ± 0.48, indicating that the ¹⁴C method used here had a variable but, on average, close relationship to gross production as it is commonly measured.     

An Investigation of the Impacts of Agricultural Runoff on the Water Quality and Aquatic Organisms in a Lowveld Sand River System in Southeast Zimbabwe

In this research we examined the hypothesis that agricultural pollution is a key determinant of variability in nutrients concentrations and benthic fauna in a semi-arid tropical lowveld region of southeast Zimbabwe. Water quality was monitored in the river water column and river bottom sediments at a time when dissolved oxygen concentration was thought to be very low during the winter period in the rivers passing through low input agricultural sections and intensive commercial agricultural sections. The surveys used established chemical methods and biological methods. Benthic fauna assemblages were used to complement chemical cases of nutrient loading at localities chosen for sampling. Unpolluted control sites were not significantly different (t test, p < 0.05) from polluted sites in levels in mean values of dissolved oxygen, conductivity, total dissolved solids and mean density (no.m²) of benthic invertebrates in May. Significant differences (t test, p < 0.05) were not found in mean values of calcium, magnesium, potassium, total nitrogen, nitrate nitrogen, ammonia nitrogen and total phosphorous in river bottom sediments in May and August. These data certainly do not support the notion that the Runde River is severely polluted by the upstream agricultural activities and the hypothesis that agricultural runoff is a key determinant of water quality is rejected. As the data suggests the Runde River may be receiving moderate nutrient pollution. The positive effects of moderate eutrophication on fish catch and the trade-off in pollution implied here needs to be addressed by appropriate agricultural and environmental policies that relate to water pollution and land use.     

Nitrogen cycling in large temperate floodplain rivers of contrasting nutrient regimes and management

Hydraulic connection between channels and floodplains (“connectivity”) is a fundamental determinant of ecosystem function in large floodplain rivers. Factors controlling material processing in these rivers depend not only on the degree of connectivity but also on the sediment conditions, nutrient loads, and source. Nutrient cycling in the nutrient‐rich upper Mississippi River (MISS) is relatively well studied, whereas that of less eutrophic tributaries is not (e.g., St Croix River; SACN). We examined components of nitrogen cycling in 2 floodplain rivers of contrasting nutrient enrichment and catchment land use to test the hypothesis that N‐cycling rates will be greater in the MISS with elevated nutrient loads and productivity in contrast to the relatively nutrient‐poor SACN. Nitrate (NO₃^?‐N) concentrations were greatest in flowing habitats in the MISS and often undetectable in isolated backwaters except where groundwater inputs occurred. In the SACN, NO₃^?‐N concentrations were greatest in the flowing backwater where groundwater inputs were high. Ambient nitrification in the MISS was twice that in the SACN and tended to be lowest in the main channel. Denitrification was 3× greater in the MISS than that in the SACN, N‐limited in both rivers. Community production/respiration was >1 in the MISS and likely provisioned labile C to fuel microbial metabolism and dissimilatory NO₃^?‐N reduction, whereas the heterotrophic (production/respiration < 1) nature of the SACN likely limited microbial metabolism and NO₃^?‐N dissimilation. It appears that N‐cycling in the SACN was driven by groundwater, whereas that in the MISS was supported mainly by water column N‐sources.     

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.     

Littoral and shoreline wood in mid‐continent great rivers (USA)

Woody debris has several important roles in running water. Less is known about the ecology of wood in great rivers than in smaller rivers and streams. We used a probability survey to estimate the abundance of littoral and shoreline wood along the following mid‐continent great rivers of the United States in summer 2004–2006: the Missouri River, Upper Mississippi River, and the Ohio River. We counted wood pieces >0.3 m in diameter from a zone between the bank full level out into the river 10 m. We categorized wood according to its origin and function as “beached” (transported from upriver but not providing aquatic habitat), “wet” (origin unknown and providing aquatic habitat; includes snags), or “anchored” (attached to the bank at its current location and providing aquatic habitat). We counted 5900 pieces of wood at 447 sites across rivers. Approximately 56 percent of pieces were beached, 30 percent were wet, and 14 percent were anchored. Overall, mean abundance of wood was 2.6 pieces of wood 100 m^?1 of shoreline (approximately 3.0 m³ 100 m^?1). Abundance of wood (pieces per unit distance of river) was much lower than has been reported for many smaller streams and rivers. There was more wood along the Upper Mississippi River (3.3 pieces 100 m^?1) than elsewhere (≤2.4 pieces 100 m^?1). The mean abundance of wood on the Ohio River decreased significantly between the 2004 and 2005 survey periods due to high flows. Longitudinal patterns in wood abundance were weak. There was less anchored and wet wood along shorelines protected by revetment (e.g., rip rap). There was generally more wood along shorelines where the riparian land use was characterized as forest rather than agriculture or developed. Mean abundance of wood along forested, un‐revetted shorelines was approximately four pieces 100 m^?1 of shoreline (= 80 pieces km^?1 of river). This estimate of mean wood abundance for what amounts to least disturbed riparian and shoreline conditions is relevant for great river bioassessment and management. Published in 2009 John Wiley & Sons, Ltd.     

Sediment geochemistry and contributions to carbon and nutrient cycling in a deep meromictic tropical lake: Lake Malawi (East Africa)

The biogeochemical functioning of large tropical lakes differs substantially from temperate lakes, yet remains poorly understood. We characterized the carbon, nitrogen, and phosphorus cycling in the water column and sediments of a deep meromictic tropical Lake Malawi (East Africa) by measuring geochemical distributions and compiling whole-lake geochemical budgets. Four locations (100 to 650 m water depth) were characterized. The results reveal that sediments contribute significantly to lake-wide biogeochemical budgets. Sedimentation rates have significantly increased in recent decades. While the export efficiency of organic matter from photic zone to deep sediments is low (14%), organic carbon is buried in the anoxic sediments with high efficiency (27–46%). Area-specific rates of carbon mineralization (4.1 mmol m^? 2 d^? 1) are similar to those in temperate well-oxygenated large lakes and marine sediments in similar water depths. Ammonium effluxes from sediments (0.44 mmol m^? 2 d^? 1) contribute 29% to the total nitrogen inputs into the water column, while sediment denitrification (0.035 mmol m^? 2 d^? 1) and burial of organic nitrogen (0.27 mmol m^? 2 d^? 1) remove 28% of total inputs in the lake. The recycling efficiency of phosphorus in anoxic sediments is high (73%). P effluxes average 0.037 mmol m^? 2 d^? 1, suggesting a large and previously unquantified contribution (42%) to water column P inputs. The results underscore the importance of sediments in the geochemical budgets of even large lakes and suggest trends in lacustrine carbon cycling that hold across a wide range of environments.     

A case study of water retention and ecosystem functioning restoration in Wenying Lake (China)

Wenying Lake in Datong, China, has been drying for the last several years. Analysis of standard penetration test data revealed that a missing waterproof layer of silt soil and silty clay in the south part of the lake, resulting from frequent flood flushing, was responsible for the water loss from the lake. Accordingly, 6.7 × 10⁵ m² of geosynthetic clay liner (GCL) was used in May 2010 to repair the area of sediment exhibiting the water leakage, equivalent to 15.6% of the total lake bottom area. Approximately 4.1 × 10⁶ m³ of floodwater was then diverted from upstream rivers to the lake. Eight months later, about 0.9 × 10⁶ m³ of the water was retained, with the water loss being reduced from 4.0 to 1.1 m³ m^?2 year^?1. Ecologically safe and cost‐effective clay materials also can be used to furthermore improve the water retention characteristics. Modelling results indicated that capping the whole lake with a 2 cm layer of bentonite with a permeability coefficient of 5.0 × 10^?9 cm s^?1 could reduce the sediment water leakage to 0.04 m³ m^?2 year^?1. The quality of the retained water after GCL treatment project was poor, with a total nitrogen concentration of 11.0 mg L^?1 and sulphate 307.0 mg L^?1, which were 5 and 300 times higher, respectively, than the Chinese class V surface water standard. Restoration of aquatic vegetation in the lake and constructed wetlands near the lake inlets could be helpful to improve the lake’s water quality over the long term.