Functioning Of River Ecosystems Research Articles

Effects of hydromorphological impacts on river ecosystem functioning: a review and suggestions for assessing ecological impacts

Because of the serious effects of pollution on water supply much closer attention has been paid to water quality than to other aspects of river integrity. However, channel form and water flow are relevant components of river health, and recent evidences show that their impairment threatens the services derived from them. In this article, we review the literature on the effects of common hydromorphological impacts (channel modification and flow modification) on the functioning of river ecosystems. There are evidences that even light hydromorphological impacts can have deep effects on ecosystem functioning, and that different functional variables differ in their responses. Three criteria (relevance, scale and sensitivity) in the selection of functional variables are suggested as a guide for the river scientists and managers to assess the ecological impacts of hydromorphological modifications.

Ecosystem Function of Sand Bar Segment in Sandy River with Particular Reference to Denitrification Potential

River is an artery and simultaneously a vein of human activity of river basin, and we have to conserve river ecosystem to expect its ecological function for sustainable human life there. However, we have degraded river environment through rapid development in floodplain. In order to make our efforts vital, it is important to understand the structure and function of river ecosystem. In this paper, we focused on denitrification as one of the ecosystem function in the sand bar landscape possesses and its temporal change with landscape transition and change of human activity along a river. The objective of this study was to develop a model that could be used to examine how the nitrogen is trapped and retained by several elements of a sandbar in an alternate bar reach. We also developed a framework to analyze the temporal change in the denitrification by using a numerical simulation, aerial photos, and a water quality information system. The main results of this study are that the temporal change in the denitrification ecosystem function in a sandbar reach can be quantified using the proposed model and the denitrification activity has increased over the past 34 years.

Preface: Assessment of physical habitat characteristics in rivers, implications for river ecology and management

Preface: Assessment of physical habitat characteristics in rivers, implications for river ecology and management Physical habitat characteristics are an extremely important factors determining the structure and composition of fluvial biological communities and fluvial ecosystem functioning. Existing methods for characterising the physical realm of river habitats are increasingly important, not only for monitoring river ecosystem health and the success of river restoration projects but also for increasing the understanding of river ecosystem functioning and improving the efficiency of management actions. However, there is no scientific consensus on which methods to use for long-term monitoring and which river features to monitor because many fundamental questions relating hydrological, geomorphological and biological characteristics remain unanswered and because river habitats are monitored to cover a wide variety of objectives. Several initiatives worldwide have demonstrated the importance of analysing and discussing the application of different methods to assess river habitat characteristics for different objectives. It would be interesting to achieve a common approach for river habitat characterization for different objectives, especially for mid- to long- term monitoring programs and different river management issues (e.g., monitoring river restoration projects). More elusive but no less important is the contribution of these methods to understanding the interactions between river habitat architecture and river functioning. In this regard, the Physical Habitat Assessment Methods in Rivers (PHAM) seminar was organised in Santander, northern Spain, in November 2009 to review current practices on river habitat assessment. The contributions presented in that seminar and some later contributions form the basis of this special volume. These papers identify the main factors creating and maintaining river habitats, review physical habitat assessment methods and propose improvements and adaptations, apply different methods in distinctive geographical areas and characterise river habitats for estimating habitat availability of different fish species.

Analysis of Bed Load Equations and River Bed Level Variations Using Basin-Scale Process-Based Modelling Approach

Bed load transport is a key process in maintaining the dynamically stable channel geometry for restoring the form and function of river ecosystems. Bed load consists of relatively large sediment particles that are moved along the streambed by rolling, sliding or saltation. Currently, various empirical correlations are used to estimate bed load transport rates since no single procedure, whether theoretical or empirical, has yet to be universally accepted as completely satisfactory in this aspect. Bed load particles are primarily sourced from river bed materials or banks. The amount of bed load and its spatial distribution contributes significantly to river bed level changes. Hillslope sediment contribution, mostly available to the river in the form of suspended load, also plays an important role in river bed level changes. This study aims to analyse different bed load equations and the resultant computations of river bed level variations using a process-based sediment dynamic model. Analyses have revealed that different bed load equations were mainly deduced from the concept of relating bed shear stresses to their critical values which are highly factored by the slope gradient, water discharge and particle sizes. In this study, river bed level variations are calculated by estimating total surplus or deficit sediment loads (suspended loads and bed loads) in a channel section. This paper describes the application of different widely used bed load equations, and evaluation of their various parameters and relative performances for a case study area (Abukuma River Basin, Japan) using a basin-scale process-based modelling approach. Relative performances of river bed level simulations obtained by using different bed load equations are also presented. This paper elaborates on the modelling approaches for river bed load and bed level simulations. Although verifications were not done due to unavailability of field data for bed load, qualitative evaluations were conducted vis-a-vis field data on flow and suspended sediment loads as well as the bed loads presented in different past studies.

Temporal variations in organic carbon utilization by consumers in a lowland river

ABSTRACTUnderstanding temporal trophic interactions in riverine food webs is essential for predicting river ecosystem function and improving management of these ecosystems. Temporal changes in energy flows through riverine food webs are readily assumed but are rarely tested. Temporal variability in food webs from two reaches of a lowland river (Ovens River, south‐eastern Australia) with differing levels of floodplain connectivity were examined over 12 months. We investigated how seasonal changes, flow variability and floodplain connectivity influence (i) stable isotope signatures of basal organic carbon sources (terrestrial sources: trees and grasses; aquatic sources: seston, biofilm and filamentous algae) and consumers (macroinvertebrates and fish) and (ii) the relative proportions of organic carbon sources contributing to consumer biomass using mixing models. We hypothesized (i) that during high flows, increased floodplain connectivity would increase the lateral exchange of terrestrial carbon subsidies to main channel consumers and (ii) that during low flow periods, main channel consumers would derive the majority of their carbon from aquatic benthic sources. Results indicated that isotope signatures for basal sources and for most of the consumers varied temporally and spatially. Mixing models indicated that increased floodplain connectivity did not increase terrestrial subsidies to consumers during high flows. Seston was the primary source during high flows whilst terrestrial vegetation increased in importance during low flows. Filamentous algae was also important during low flows for some consumers. These findings indicate that it is essential to include temporal variability in order to understand energy flows in lowland rivers, thus allowing for the dynamic nature of these ecosystems. Copyright © 2010 John Wiley & Sons, Ltd.

Foreword: Global change and river ecosystems—implications for structure, function, and ecosystem services

Foreword: Global change and river ecosystems-implications for structure, function, and ecosystem services.- Understanding effects of global change on river ecosystems: science to support policy in a changing world.- Biogeochemical implications of climate change for tropical rivers and floodplains.- Dynamics of a benthic microbial community in a riverine environment subject to hydrological fluctuations (Mulargia River, Italy).- Global changes in pampean lowland streams (Argentina): implications for biodiversity and functioning.- Factors regulating epilithic biofilm carbon cycling and release with nutrient enrichment in headwater streams.- Periphyton biomass and ecological stoichiometry in streams within an urban to rural land-use gradient.- The physico-chemical habitat template for periphyton in alpine glacial streams under a changing climate.- The periphyton as a multimetric bioindicator for assessing the impact of land use on rivers: an overview of the Ardieres-Morcille experimental watershed (France).- Discharge and the response of biofilms to metal exposure in Mediterranean rivers.- Effects of eutrophication on the interaction between algae and grazers in an Andean stream.- Comparing fish assemblages and trophic ecology of permanent and intermittent reaches in a Mediterranean stream.- Global change and food webs in running waters.- Effects of hydromorphological integrity on biodiversity and functioning of river ecosystems.- Organic matter availability during pre- and post-drought periods in a Mediterranean stream.- Flow regime alteration effects on the organic C dynamics in semiarid stream ecosystems.- A multi-modeling approach to evaluating climate and land use change impacts in a Great Lakes River Basin.- Implications of global change for the maintenance of water quality and ecological integrity in the context of current water laws and environmental policies.

Evaluation of side-effects of glyphosate mediated control of giant reed (Arundo donax) on the structure and function of a nearby Mediterranean river ecosystem

The aim of this study was to evaluate the effect of the application of the herbicide Herbolex (Aragonesas Agro, S.A., Madrid, Spain) to control giant reed (Arundo donax), which has glyphosate as active ingredient, on the structure and function of a nearby river ecosystem. Specifically, we assessed glyphosate environmental fate in the surrounding water and its effects on transplanted Daphnia magna, field collected caddisfly (Hydropsyche exocellata) and on benthic macroinvertebrate structure assemblages. Investigations were conducted in the industrialized and urbanized Mediterranean river Llobregat (NE Spain) before and after a terrestrial spray of glyphosate. Four locations were selected to include an upstream site and three affected ones. Measured glyphosate levels in river water following herbicide application were quite high (20-60 microg/l) with peak values of 137 microg/l after three days. After 12 days of its application, leaching of glyphosate from sprayed riverbanks was quite high in pore water (20-85 microg/l) but not in the river. Closely linked with the measured poor habitat and water physico-chemical conditions, macroinvertebrate communities were dominated by taxa tolerant to pollution and herbicide application did not affect the abundance or number of taxa in any location. Nevertheless, significant specific toxic effects on transplanted D. magna and field collected H. exocellata were observed. Effects included D. magna feeding inhibition and oxidative stress related responses such as increased antioxidant enzyme activities related with the metabolism of glutathione and increased levels of lipid peroxidation. These results emphasize the importance of combined chemical, ecological and specific biological responses to identify ecological effects of pesticides in the field.

Synergistic effects of regional climate patterns and local water management on freshwater mussel communities

Human alterations to aquatic ecosystems are leading to decreases in species richness and biomass and subsequent changes in community composition. In many cases species losses are non-random: species with traits poorly adapted to the new environmental conditions suffer greater losses. We used long-term data from a southern US river, the Kiamichi River, to evaluate the synergistic effects of regional climate patterns coupled with water management practices on freshwater mussel communities. Mussel communities in the river changed over the 15 year period of this study, with overall densities and species richness decreasing and community structure shifting from assemblages dominated by thermally sensitive to thermally tolerant species. These changes corresponded with a period of very low flows in the river caused by a combination of climate patterns (a regional drought) and local water management practices (decreased reservoir releases). These low flows, coupled with high summer air temperatures, changed the river in many locations from a continuously flowing river to a series of shallow, isolated pools where water temperatures sometimes exceeded 40 °C. Altered conditions led to higher mortality rates of thermally sensitive compared to tolerant species. We predict future shifts in river ecosystem function as mussel communities change in response to changing climate and water management.

Effects of hydromorphological integrity on biodiversity and functioning of river ecosystems

River channels tend to a dynamic equilibrium driven by the dynamics of water and sediment discharge. The resulting fluctuating pattern of channel form is affected by the slope, the substrate erodibility, and the vegetation in the river corridor and in the catchment. Geomorphology is basic to river biodiversity and ecosystem functioning since the channel pattern provides habitat for the biota and physical framework for ecosystem processes. Human activities increasingly change the natural drivers of channel morphology on a global scale (e.g. urbanization increases hydrological extremes, and clearing of forests for agriculture increases sediment yield). In addition, human actions common along world rivers impact channel dynamics directly, e.g. river regulation simplifies and fossilizes channel form. River conservation and restoration must incorporate mechanisms of channel formation and ecological consequences of channel form and dynamics. This article (1) summarizes the role of channel form on biodiversity and functioning of river ecosystems, (2) describes spatial complexity, connectivity and dynamism as three key hydromorphological attributes, (3) identifies prevalent human activities that impact these key components and (4) analyzes gaps in current knowledge and identifies future research topics.

Food webs and the Upper Mississippi River: contributions to our understanding of ecosystem function in large rivers

Studies on the Upper Mississippi River, particularly over the last 15 years, have contributed to our understanding of trophic processes in large rivers. The framework established by earlier population-specific studies, however, cannot be overlooked. Examination of the feeding habits of fish ranging from planktivores to piscivores gave the first indication that trophic processes were influenced by the spatial complexity and annual hydrological patterns of river-floodplain ecosystems. Experimental studies, which have often been considered impossible or impractical in large rivers, demonstrated the potential for biotic controls of system dynamics through predator–prey and competitive interactions. Such studies have been particularly helpful in understanding the potential impact of non-native species, including zebra mussels and Asian carp, to biodiversity and secondary production. Our understanding of riverine ecosystem function expanded greatly as food web studies began the application of a new tool—natural stable isotopes. Studies employing stable isotopes illustrated how food webs in a number of large rivers throughout the world are supported by the autochthonous production of microalgae. This study, coupled with other studies testing the prevailing models of riverine ecosystem function, has brought us to a point of better understanding the nature of river ecosystem functions. It is through looking back at the earlier studies of fish diet that we should realize that the temporal and spatial complexities of river ecosystem function must still be addressed more fully. This and a better grasp of the significance of the arrangement of patches within the riverine landscape will prove beneficial, as we assess the appropriate scale of river rehabilitation with an eye on how rehabilitation promotes productivity within complex ecosystems, including the Upper Mississippi River.

Flood mitigation designs with respect to river ecosystem functions—A problem oriented conceptual approach

We formulated five proposals to improve specifications of flood hazard reduction projects and five proposals to preserve the river ecosystem functions, based on literature and authors’ experience. Starting from these, we established a conceptual approach using the notion of riverscape, with related function richness and bioassessment tools. We defined four riverscape types based on the gradient of artificialness, from type I (impervious bed) to type IV (less than 30% or no artificial bed). To apply our approach to dry dam designs, we individualized different parts of dry dams and characterized them by an ideal riverscape type, according to the hydraulic constraints of each part. Type I is unavoidable in the bottom outlet, and types I or II at the foot of the dam. Types III (30–50% of artificial bed) and IV were recommended at upper and lower parts of the dam, according to the risk of bank erosion and need for bank reinforcements. The approach appeared efficient to help biologists and hydraulic practitioners work together, and find technical solutions complying with both flood protection and biodiversity preservation requirement.

Comparison of several methods to calculate reaeration in streams, and their effects on estimation of metabolism

Metabolism is an integrative measurement of stream and river ecosystem functioning, and thus, could be used to assess impairment. Stream metabolism is measured by different methods which often yield contrasting results. Furthermore, open-channel measurements of metabolism, which offer the best potential for continuous monitoring of stream functioning, rely on calculations of gas exchange with the atmosphere, for which a plethora of methods exists. Therefore, to incorporate metabolism in stream monitoring programs, it is necessary to determine which methods yield comparable results under a given set of environmental conditions. We studied 21 streams in the Basque Country (northern Spain), ranging widely in physical characteristics and water quality. We calculated reaeration during summer baseflows using three different approaches: the night-time drop in oxygen, the lag between noon and peak oxygen concentration, and ten empirical equations relating depth and velocity with reaeration coefficients obtained from the literature. Differences among methods were very large, especially at the shallower sites. The results obtained with most empirical equations were highly correlated, but showed little agreement with the night-time and peak lag methods. We then analyzed the response of reaeration rate to river stage: reaeration calculated by the night-time method during 1 year of continuous monitoring was regressed against discharge at each site, and the resulting model was compared to the results of empirical equations, using software HecRas 2.2 to model hydraulic conditions at different river stages. The shape of reaeration-discharge plots differed greatly and in a site-dependent manner, and there was little agreement between methods. Finally, we investigated the effects of reaeration rate on estimates of metabolism. The choice of method greatly affected the estimates of both primary production and respiration. The empirical equations, except E7 and E10, yielded the most unrealistic estimations of stream metabolism. Overall, the night-time method, especially when regressed against discharge, seems to be the most robust and reliable among those tested, with the energy dissipation method (E10) appearing to be a viable alternative when the night-time method does not work.

Ecological and environmental water requirement in the middle Yellow River

As a significant factor for maintaining river ecosystem function, ecological and environmental water requirement of riv- ers is a key issue of hydrological research. Based on real conditions of the middle Yellow River, a few methods (Montana method, minimum monthly mean discharge method, etc.) were adopted to calculate ecological and environmental water requirement of the river. The results show that in the middle Yellow River the environment water requirement is 47.356×108 m3, the water requirement for sediment delivery is 40.666×108 m3, and the water surface evaporation is 10.314×108 m3. Total tributary ecological and envi- ronmental water requirement from Kuye River, Wuding River, Fen River, Jing River, Beiluo River, Wei River, Yiluo River, and Qin River is respectively 2.071×108 m3, 3.393×108 m3, 10.143×108 m3, 8.699×108 m3, 3.575×108 m3, 34.601×108 m3, 6.481×108 m3 and 2.505×108 m3. Due to the non-independence of the middle Yellow River, further ecological and environmental water requirement research is recommended.

Treating causes not symptoms: restoration of surface - groundwater interactions in rivers

Many river restoration projects seek to address issues associated with impaired hydrological and ecological connectivity in longitudinal (e.g. effects of dams, weirs) or lateral (e.g. alienated floodplain) dimensions. Efforts to restore the vertical dimension of impaired stream–groundwater exchange are rare, hampered by limited understanding of the factors controlling this linkage in natural alluvial rivers. We propose a simplified two-axis model of the ‘primary drivers’ (sediment structure and vertical hydraulic gradient) of stream–groundwater exchange that acknowledges their interaction and provides a practical template to help researchers and river managers pose hypothesis-driven solutions to restoration of damaged or lost vertical connectivity. Many human activities impact on one or both of these drivers, and we review some of the tools available for treating the causes (rather than symptoms) in impacted stream reaches. For example, creating riffle-pool sequences along stream reaches will enhance vertical hydraulic gradient, whereas flushing flows can remove clogging layers and sustain sediment permeability. Our model is a first step to specifying mechanisms for recovery of lost vertical connectivity. Assessing results of river restoration using this approach at reach to catchment scales will provide scientific insights into the interplay of hydrology, fluvial geomorphology and river ecosystem function at appropriately broad scales.

Bank Erosion as a Desirable Attribute of Rivers

ABSTRACT Bank erosion is integral to the functioning of river ecosystems. It is a geomorphic process that promotes riparian vegetation succession and creates dynamic habitats crucial for aquatic and riparian plants and animals. River managers and policymakers, however, generally regard bank erosion as a process to be halted or minimized in order to create landscape and economic stability. Here, we recognize bank erosion as a desirable attribute of rivers. Recent advances in our understanding of bank erosion processes and of associated ecological functions, as well as of the effects and failure of channel bank infrastructure for erosion control, suggest that alternatives to current management approaches are greatly needed. In this article, we develop a conceptual framework for alternatives that address bank erosion issues. The alternatives conserve riparian linkages at appropriate temporal and spatial scales, consider integral relationships between physical bank processes and ecological functions, and avoi...

수문변화 지표법에 의한 영천댐이 하류하천에 미치는 유황변화 분석

하천의 유황특성을 평가 하는 것은 하천생태계의 인위적인 변형을 이해하고 예측하는데 중요한 역할을 한다. 본 연구에서는 댐 건설에 따른 하류하천의 영향을 분석하고자 수문변화 지표모형을 이용하여 용수 전용댐인 영천댐을 대상으로 댐 건설전 후의 유황변동을 분석하였다. 분석결과는 다음과 같다. (1) 월 평균유량은 갈수기와 홍수기 모두 댐건설 후 감소하였다. (2) 연 극치유량의 크기와 기간 분석결과, 댐 건설전 후의 1일 최소유량은 <TEX>$3.48m^3/sec$</TEX>와 <TEX>$0.89m^3/sec$</TEX> 이였으며, 1일 최대유량은 <TEX>$833.1m^3/sec$</TEX>와 <TEX>$672.1m^3/sec$</TEX> 이었다. (3) 연극치 유량의 발생시기 분석결과, 최소유량의 Julian Day는 댐 건설전 180 일(6월), 댐 건설 후 257 일(9월)이었으며, 최대유량의 Julian Day는 댐 건설 전 209 일(7월), 댐 건설 후는 217 일(8월)에 발생하였다. (4) 홍수맥파의 빈도와 기간의 분석결과, 저맥파(Low Pulse)의 발생횟수는 댐 건설전 3회, 지속기간은 23 일, 댐 건설 후에는 7회, 지속기간 61 일이었으며, 고맥파(High Pulse)의 발생횟수는 댐 건설 전 4회 지속기간은 2 일, 댐 건설후에는 2회 1.2 일로 분석되었다. (5) 변화율과 빈도의 분석결과, 상승율은 댐 건설 전, 후의 각각 39.27 %와 19.36 %로 댐 건설 전에 수문변동이 크게 발생하였으며, 감소율은 각각 -15.85 %와 -8.16 %로 분석되었다. (6) 분산정도를 변동계수로 분석하였으며, 1일 최소 최대유량은 0.9054에서 0.6314와 1.0440에서 0.9617로 모두 감소하였으며, 연 극치유량 발생시기는 댐 건설전 후 최소유량은 0.269에서 0.282, 최대유량은 0.069에서 0.153으로 댐건설이후 변동계수가 증가하였다. Hydrologic regimes play a major role in determining the biotic composition, structure, and function of river ecosystem. In this study, hydrologic regimes were analyzed on down stream effects of the Young-Chun dam construction using the Indicators of Hydrologic Alterations(IHA). The analysis results were as follows ; (1) Monthly mean flows were decreased during drought and flood season on the pre and post dam, (2) Magnitude and Duration of Annual Exterm Conditions, annual minima 1-day means was <TEX>$3.48m^3/sec$</TEX>, <TEX>$0.89m^3/sec$</TEX> and annual maxima 1-day mean was <TEX>$833.1m^3/sec$</TEX>, <TEX>$672.1m^3/sec$</TEX> on the pre and post dam (3) Timing of Annual Exterm conditions, Julian date of the annual minima 1-day means was 180th(June) in the pre dam, 257th(September) in the post dam, Julian date of the annual maxima 1-day means was 209th(July) in the pre dam, 217th(August) in the post dam, (4) Frequency and Duration of High and Low Pulse, Low Puls counts and duration were 3 times and 23 days in the pre dam, High Pulse counts and duration were 4 times and 2 days in the pre dam. (5) Rate and Frequency of Water Condition Changes, rise rates was 39.27 %, 19.36 % and fall rates -15.85 %, -8.16 % in the pre and post dam, respectively (6) Coefficient of Variation, annual exteram water conditions were decreased from 0.9054 to 0.6314 and from 1.0440 to 0.9617, Timing of Annual Exterm conditions were incereased for minima flow from 0.269 to 0.282, for maxima form 0.069 to 0.153.

RIVERMET ©: An Excel-based tool to calculate river metabolism from diel oxygen–concentration curves

There is increasing awareness of the need to include ecosystem function into river assessment protocols. Metabolism, one of the most integrative ecosystem functions, is relevant across all sizes and types of rivers, and highly sensitive to many stressors. Continuous monitoring allows using Odum's classic method, but the mathematical analysis of long data series is arduous. Here we present RIVERMET ©, a flexible, user-friendly software to assist in these calculations, that can be freely downloaded from the Internet. It consists in an Access database plus an Excel file to help import and store data, and in an Excel file with macros to make calculations. RIVERMET © can calculate the reaeration coefficient directly from night-time oxygen concentration, indirectly from empirical formulae, or can accept values obtained with any other method. It corrects the reaeration coefficient values so obtained for diel changes in temperature, and can automatically analyze the relationship between reaeration and discharge. All results can be graphed, outliers easily purged and results recalculated. Continuous monitoring of river discharge, temperature and oxygen concentration, combined with RIVERMET © allows to easily measure river metabolism, to monitor the function of river ecosystems, to assess the effect of natural or human disturbances, and to analyze post-disturbance recovery trajectories.

Bank stabilization, riparian land use and the distribution of large woody debris in a regulated reach of the upper Missouri River, North Dakota, USA

AbstractLarge woody debris (LWD) is an important component of ecosystem structure and function in large floodplain rivers. We examined associations between LWD distribution and riparian land use, bank stabilization (e.g. riprap revetment), local channel geomorphology, and distance downriver from the dam in the Garrison Reach, a regulated reach of the upper Missouri River in North Dakota, USA. We conducted a survey of shoreline‐associated LWD in the reach during typical summer flow conditions. Reach‐wide LWD density was 21.3 pieces km−1 of shoreline, of which most pieces (39% ) were ‘beached’ between the waterline and the bankfull level, 31% of pieces had evidence of originating at their current location (anchored), 18% of pieces were in deep water (&gt;1 m), and 13% were in shallow water. LWD density along unstabilized alluvial (sand/silt) shorelines (27.3 pieces km−1) was much higher than along stabilized shorelines (7.2 pieces km−1). LWD density along forested shorelines (40.1 pieces km−1) was higher than along open (e.g. rangeland, crop land; 9.2 pieces km−1) or developed (e.g. residential, industrial; 7.8 pieces km−1) shorelines. LWD density was highest overall along unstabilized, forested shorelines (45 pieces km−1) and lowest along open or developed shorelines stabilized with a blanket‐rock revetment (5.5 pieces km−1). Bank stabilization nearly eliminated the positive effect of riparian forest on LWD density. A predicted longitudinal increase in LWD density with distance from the dam was detected only for deep LWD (including snags) along unstabilized alluvial shorelines. Partial resurvey in the summer following the initial survey revealed a reduction in total LWD density in the reach that we attribute to an increase in summer flow between years. Changes in riparian management and land use could slow the loss of LWD‐related ecosystem services. However, restoration of a natural LWD regime in the Missouri River would require naturalization of the hydrograph and modification of existing bank stabilization and channel engineering structures. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Exotic snails dominate nitrogen and carbon cycling in a highly productive stream

Individual animal species can impact ecosystem processes, but few exotic invaders have demonstrated ecosystem-scale impacts, even when population sizes are large. We combined whole-stream measures of carbon and nitrogen fluxes with rates of consumption and ammonium excretion to show that an exotic freshwater snail, Potamopyrgus antipodarum, dominated these fluxes in a highly productive stream. The snails consumed 75% of gross primary productivity, and their excretion accounted for two-thirds of ammonium demand. Such large fluxes were due to high snail biomass rather than high rates of excretion or consumption. This exotic species may dramatically alter ecosystem function in rivers, with potential consequences for food web structure and element transport.

Planktic community assembly in flowing water and the ecosystem health of rivers

Some of the modern criteria for assessing ecosystem health are compared with current understanding of ecosystem function in rivers. Owing to the predominance of catchment imports over autochthonous primary production, most rivers are naturally heterotrophic. This does not make them unhealthy but the pristine condition is that much harder to determine. The case is put for an index of ecosystem health and sustainability that takes into account the system's capacity for processing its resources, the species richness and its interdependence and its resilience to external forcing. Although these are not easily quantified, the qualitative indicators of healthy ecosystem function are easily checked. The sensitivity of organisms in suspension to fluvial flow may seem to counter the suitability of plankton as a reliable state indicator of river health. On the other hand, the rules governing the assembly of planktic communities in rivers are often strict and quantifiable: this makes them attractive candidates to act as indicators of the ecological condition of rivers.