Headwater reservoirs weaken terrestrial‐aquatic linkage by slowing leaf‐litter processing in downstream regulated reaches

Leaf litter breakdown is a key process, providing matter and energy to communities inhabiting many headwater streams that flow through forests. This detrital pathway is affected by many human landscape transformations; but it is little known about the impact of small headwater reservoirs on leaf litter decay in streams. Alder leaf litter breakdown rates and associated fauna were studied upstream and downstream of five small water supply reservoirs (surface‐release in rainy autumn‐winters), in the Nerbioi‐Ibaizabal drainage basin (Basque Country, Spain), to assess the effect of impoundment on headwater streams function. Breakdown rates were significantly lower below the dams, mainly associated with a reduction of the density and the biomass of shredders. Among the shredders, Nemouridae and especially Protonemura were less abundant downstream of the dam. Alterations in the physicochemical characteristics of the water due to the reservoirs were negligible throughout our study, and temperature showed only slight variations that could not explain the reduction of the rates. The effect on shredders is likely to be related to differences in the riparian environment and flow regulation by the dams. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of Land Use on the Composition and Structure of Aquatic Invertebrate Community and Leaf Breakdown Process in Neotropical Streams

Different land uses directly affect the characteristics of a river basin and influence the aquatic biota and ecosystem processes. This study aimed to analyse the community structure and composition of aquatic invertebrates and the role of these organisms in the process of leaf litter breakdown in streams with different land uses. The study was conducted from September to December 2013 in five streams in the Neotropical region. At each stream, we placed 18 bags containing litter for colonization by aquatic invertebrates, and the bags were collected at different exposure times (5, 30, 45, 60, 75 and 90 days). We registered spatial differences in the aquatic invertebrate community structure and composition. There were no significant differences in the activity of invertebrates in the leaf litter breakdown process among streams with different land uses. However, the variability in mass decay rate was lower for the reference stream. This result may have been influenced by habitat quality, availability of organic matter and the structure and composition of benthic community present in the reference stream, which differs significantly among locations with different types of land use. The results of this study shows that human activities, particularly agriculture and urbanization, modify the structure and composition of the benthic community and acts on ecosystem processes, especially in the variability of the processing of allochthonous material invertebrates. However, we reject the hypothesis that land use negatively influences the decomposition of litter, measured by weight loss. Copyright © 2016 John Wiley & Sons, Ltd.     

Response of Bed Sediments on the Grade‐Control Structure Management of a Small Piedmont Stream

The grain sizes of 53 small lateral and mid‐channel deposits were evaluated on the 2.7 km long channel‐reach of a headwater piedmont stream that has been affected by check‐dams and bed sills constructions (ca. 14 constructions per km), riprap bank stabilisations and the removal of instream wood. Such interventions have led to the loss of natural vertically oscillating bedforms, and pool‐riffles have been replaced by plane beds. The evaluated sediment deposits were divided based on their positions relative to grade‐control structures on downstream, intermediate and upstream types. The individual types of deposits produced different downstream trends in the grain sizes that reflected their positions with respect to the grade‐control structures (GCSs). The downstream deposits generally produced slow downstream coarsening, which may point to the increased efficiency of the energy dissipation of the GCSs under a gradual downstream decrease in the channel slopes. However, they were generally not coarser than the other two types. The upstream deposits did not indicate any notable downstream grain‐size trend. Despite the significant downstream decrease in the channel slopes and related bankfull shear stresses at the positions of the intermediate deposits and the lack of a lateral sediment supply in the studied longitudinal profile, the sediment calibre of the deposits did not produce any systematic downstream fining. Thus, the observed grain sizes most likely pointed to the selective scouring of the fine sediments, which was caused by the presence of the GCSs and their effects on the decreasing sediment connectivity. Copyright © 2016 John Wiley & Sons, Ltd.     

The effect of a headwater dam on the use of leaf bags by invertebrate communities

Breakdown rates and colonisation of leaves from four tree or scrub species differing in quality are studied upstream and downstream of the Canales reservoir, a dam located in the headwater of the River Genil, Sierra Nevada, in southern Spain. This dam, with hypolimnetic release, displays short‐term fluctuations of discharge and nutrient enrichment in the tailwater during the study period. Breakdown rates of the four leaf species studied do not differ between sites, despite the higher dissolved nutrient concentration in the tailwater. This lack of differences is attributed to the potentially high physical breakage of leaves during peak flows that are of higher magnitude at the upstream site. The invertebrate density in leaf bags does not differ between sites, and Chironomidae and Ephemeroptera are the numerically dominant taxa at both sites. With regard to functional feeding groups, the scarcity and lack of significant differences between sites for shredders do not match the trend predicted by the Serial Discontinuity Concept in relation to the effect of a headwater dam. Possibly, the discharge fluctuations at both sites causes excessive instability of the natural substrate (leaf litter) for the shredder guild. However, as expected, the biomass of collectors colonising leaf bags is significantly higher at the tailwater, which might be explained not in terms of quantity, but as a consequence of the higher nutritional quality of the fine particulate organic matter (FPOM) accumulated in leaf bags at this site, owing to the eutrophication caused by the dam. Despite the scarcity of functional shredders at both sites, at the community level, the leaf material is significantly more ingested at the upstream site, suggesting the importance of this source of nutrition for the trophic web at this site in contrast with the tailwater, as predicted by the Serial Discontinuity Concept. This also suggests that caution is needed in using functional feeding groups as trophic guilds to infer system‐level trophic dynamics in streams, given the prevalence of generalist feeders among benthic macroinvertebrates in these environments. Copyright © 2000 John Wiley & Sons, Ltd.     

Evaluating the Effects of Culvert Designs on Ecosystem Processes in Northern Wisconsin Streams

Culvert replacements are commonly undertaken to restore aquatic organism passage and stream hydrologic and geomorphic conditions, but their effects on ecosystem processes are rarely quantified. The objective of this study was to investigate the effects of two culvert replacement designs on stream ecosystem processes. The stream simulation design, where culverts accommodate bankfull width and streambeds are reconstructed through the culvert, was compared with the bankfull and backwater design, where streambeds were left to fill naturally, as well as to non‐replaced culverts. We predicted that stream simulation culverts would best preserve water velocity and coarse particulate organic matter (CPOM) retention within the culvert relative to upstream reaches, and that both replaced culvert styles would exhibit rates closer to upstream reaches than non‐replaced culverts. In addition, we predicted that ecosystem processes (CPOM retention, transient storage and nutrient uptake) would be similar in reaches upstream and downstream of both replaced culvert styles, because both designs are constructed to maintain stream slopes and bankfull widths through the structure. We found that stream simulation design better maintained CPOM retention through culverts compared with non‐replaced and bankfull and backwater design culverts, but observed no differences in ecosystem processes between reaches located upstream or downstream of replaced culverts. Although the stream simulation design requires additional streambed construction relative to the bankfull and backwater design, this step may lead to additional improvement if maintaining ecological conditions through the culvert is an important restoration goal. Copyright © 2017 John Wiley & Sons, Ltd.     

Effects of Oil Palm Plantations on the Habitat Structure and Biota of Streams in Eastern Amazon

Oil palm plantations have expanded around the world, increasing concern about its pressure on deforestation rates and the homogenization of the landscape. In this context, the present study aimed to evaluate the impact of oil palm plantations on the physical characteristics of streams in Amazonia and their effects on Heteroptera, Odonata and Fish assemblages. A total of 23 streams were sampled, eight within fragments of primary forest, while 15 were in oil palm plantations. Data were collected on characteristics of the channel morphology, substrate, hydraulics, instream cover for aquatic organisms, riparian vegetation, human impacts and woody debris. Instream cover and proximity of human impacts were the variables that had the greatest effects on the physical structure of the streams, showing variation between streams of forest in pristine areas and oil palm plantations. Of the analyzed parameters, substrate, instream cover and woody debris influence the richness of Heteroptera, Odonata and Fish. The impact of oil palm plantations on local streams depended on the size of the plantations, and they cannot be considered an adequate substitute for lost habitats in efforts to preserve the physical habitat of Amazonian streams. Any type of conversion of the natural forest can have direct or indirect impacts on the dynamics and structure of these environments, with potentially negative consequences for their biodiversity. The maintenance of an adequate buffer zone of native riparian vegetation adjacent to the streams that flow through the plantations recommended, because this appears to be the principal factor determining the physical conditions of these streams. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of bank slope on the flow patterns in river intakes

In this work, we studied the dimensions of stream tube in the vertical as well as inclined bank conditions. Data were collected from both a physical model and a 3-D numerical model（SSIIM 2）. Equations for predicting stream tube dimensions were presented and compared with existing formulae. In comparison with vertical bank, it is found that inclining bank causes the bottom stream tube width to be greater than at the surface. The strength of secondary current formed at the entrance of branch channel is reduced. These changes in flow pattern can reduce the amount of sediment delivery into the intake.     

分汊河道分流比对河道水流流态影响分析

结合曹继文关于明渠岸边横向取水的水槽试验成果,应用RNG紊流模型,采用两种自由表面处理方法即刚盖假定和VOF法,对分汉河道不同分流比工况下的水流运动进行三维数值模拟。通过数值模拟计算,得出不同表面处理方法下分汉河道不同分流比工况下的流速分布、水流特性等结果,分流比对分流宽度影响较为明显。     

人工阶梯 深潭改善下切河流水生栖息地及生态的作用

本文通过野外试验研究了人工阶梯深潭对下切河流水生栖息地及生态的作用，试验河段位于西南山区的吊嘎河上150m侵蚀下切严重的河段，布置15级人工阶梯，对水流 (水深、流速、水面宽、流量)、河床底质、河床微地貌和水生底栖动物物种及数量变化进行了5个月监测。试     

不同灌水量对绿洲玉米叶面积动态及产量的影响

以河西绿洲春玉米为研究对象,分析了不同灌水量对玉米全生育期叶面积动态、产量及产量构成要素的影响。结果表明:不同灌水处理玉米单株叶面积在灌浆初至灌浆中期达到高峰,且稳定期持续至灌浆末期,此后单株叶面积衰减较快。进入灌浆期以后,不同灌水处理玉米叶面积相对生长率已开始出现负值,叶面积已有开始衰减迹象。与充分灌水相比,有限灌水不仅显著节水31.1%,还可使玉米产量提高2.9%,而有限灌水粒重、行粒数和穗粒数的增加则是玉米节水高产的主要原因。因此,不同灌水量显著影响玉米叶面积生长和产量形成过程,生产中应侧重通过前期有限水分亏缺(拔节期中度水分调亏而大喇叭口-孕穗期轻度调亏)产生的后补偿效应促进生育中后期叶面积的持续稳定发展,从而实现玉米高产和水分高效利用的双赢。     

Effect of flood regime on tree growth in the floodplain and surrounding uplands of the Wisconsin River

Flood regime and vegetation flood tolerance interact to influence tree growth in riverine landscapes. We studied tree growth in floodplain and upland forests of the Wisconsin River. About a century ago, levees set back from the river were constructed on this floodplain. The levee restricts some floodplain area from overbank flood events, but leaves a portion of active floodplain still inundated by floods. We addressed two questions: (1) how do growth rates of flood‐tolerant and flood‐intolerant tree species in the floodplain differ with flood regime? (2) At the stand level, how does growth rate differ with flood regime and between floodplain and upland areas? Annual tree growth rates from 1991 to 2000 were determined from tree increment cores for both individual species and stands. Tree growth rates of individual species varied between flood regimes. The most flood‐tolerant species (Betula nigra and Fraxinus pennsylvanica) grew faster in areas with active flooding, while the growth of less flood‐tolerant species (Quercus velutina and Q. ellipsoidalis) was depressed in swales and active floodplain. However, stand‐level tree growth did not differ between the floodplain and upland, or between flood regimes within the floodplain. Therefore, variation in the growth of individual species may not scale up to create differences in stand‐level tree growth because forest community composition varies spatially with flood regime. We suggest that growth rates are similar among sites because each community comprises of species adapted to their current flood regime. Copyright © 2008 John Wiley & Sons, Ltd.