Influence of water abstraction on the macroinvertebrate community gradient within a glacial stream system: La Borgne d'Arolla, Valais, Switzerland

1. Water abstraction from glacial rivers is an important characteristic of hydroelectric power schemes in Alpine regions. Streams in the Valais region of Switzerland have been particularly affected. 2. Invertebrate distributions are described in La Borgne d'Arolla, a glacial stream with icemelt-, snowmelt- and groundwater-dominated tributaries. The icemelt-dominated streams have been affected by abstractions for more than 30 years. 3. The glacial streams contain only Chironomidae (Diamesa), and are devoid of fauna for between 200 and 500 m below the glacier snouts. 4. Immediately below the water intakes the streams are intermittent, flowing only during system purges and high floods, and are devoid of fauna for short distances (<1.5km). 5. Further downstream, abstraction of glacial meltwater increases the importance of snowmelt and groundwater, increasing water temperatures, improving water clarity and increasing the length of krenal/rhithral streams at the expense of kryal streams. 6. A community including Chironomidae, Simuliidae, Baetidae, Nemouridae, Limnephilidae and Chloroperlidae occurs as soon as a permanent flow is maintained by tributary runoff, and the channel becomes stable. 7. A wide range of taxa inhabit snowmelt- and groundwater-dominated tributary streams with stable channels, often at much higher altitudes than the main river. The tributaries provide sources for rapid colonization of the main channel following ice retreat or physical disturbance. 8. Purges and high floods are important disturbances within the main channel. Recovery may be rapid because of drift from tributaries, but sites influenced by frequent disturbances have reduced faunas in comparison to stable channel sites. 9. This study supports the model proposed by Milner & Petts (1994) and shows that deterministic responses of macroinvertebrate communities may be observed to changes of temperature, turbidity, flow regime and channel stability.     

Tributary and mainstem benthic macroinvertebrate communities linked by direct dispersal and indirect habitat alteration

Benthic communities in tributary–mainstem networks might interact via downstream drift of invertebrates or material from tributaries and adult dispersal from the mainstem. Depending on the strength of these interactions, mainstem downstream communities are expected to be more similar to tributary communities due to drift or habitat alteration. Communities not connected by flow are expected to be similar due to adult dispersal but decreasing in similarity with distance from the mainstem. We investigated interactions between invertebrate communities of a 7th order river and 5th order tributary by comparing benthic community structure in the river upstream and downstream of the tributary confluence and upstream in the tributary. Non-metric multidimensional scaling showed invertebrate communities and habitat traits from river locations directly downstream of the tributary clustered tightly, intermediate between tributary and mid-channel river locations. In addition, Bray–Curtis dissimilarity increased between the mainstem and tributary with distance upstream in the tributary. Our results indicate that similarities between mainstem and tributary communities are potentially caused by direct mass effects from tributary to downstream mainstem communities by invertebrate drift and indirect mass effects by habitat restructuring via material delivery from the tributary, as well as potential effects of adult dispersal from the river on proximal tributary communities.     

Ichthyoplankton transport in relation to floodplain width and inundation and tributary creek discharge in the lower Savannah River of Georgia and South Carolina

We report the results of a 3-year study of ichthyoplankton in the lower Savannah River and its coastal plain tributaries. Sampling was weekly from February through July in 1983, 1984 and 1985. Ichthyoplankton transport was used as an estimate of ichthyoplankton production. Ichthyoplankton transport in the river, for both total and most common taxa, was highest in 1983 and lowest in 1985. Ichthyoplankton transport into the river from tributary streams was also highest in 1983 and lowest in 1985. Ichthyoplankton transported from these tributaries sometimes comprised a significant percent of the larval transport at the next river station downstream from the tributary mouth. The highest larval transport occurred when the spring flood pulse was most elevated for the longest time, and larval transport at particular river stations or creeks was correlated with floodplain width in 1983 and 1984, years when flood pulses were high enough so that the adjacent floodplain was inundated during or following spawning. In 1985 the flood pulse was brief, inundation levels were low, and inundation occurred in February before most fish had spawned. Except for American shad and sunfishes, larval transport for all common taxa was greatly reduced in 1985 compared to 1983.     

浑太流域洪涝灾害及其治理方略

抚顺、鞍山、辽阳等在内的中部城市群提供年用水量７．０×１０９ｍ３的７０％［５］,对辽宁经济发展与生态环境建设具有至关重要的作用．本文试图剖析浑河、太子河洪涝灾害问题,找出其成因与特点,从生态与工程结合角度提出治理方略,为防洪减灾,振兴经济服务．２　洪涝灾害成因分析２．１　洪涝灾害的一般分析　　洪涝灾害是天降暴雨和下垫面综合作用的结果．它通常依赖气象、水文地理和生态环境三方面因素．气象因素主要指暴雨,包括雨量、雨强和降雨落区．暴雨主要受大气环境制约,目前人类难以左右．然而,通过人工控制增减局地降雨…     

Persistence and stability of lotic invertebrate communities in New Zealand

1. Persistence and stability of lotic invertebrate communities were determined at an annual time scale over a 9‐year period (1990–98) at 26 river sites over the northern half of New Zealand. In addition, a number of water quality variables were measured monthly and flow information collected continuously over the same period at each site. 2. The aim of the study was to describe the levels of interannual variability in invertebrate communities, and relate community changes to variability in environmental conditions. The consequences of this temporal variability for the sensitivity of predictive models were also investigated. 3. Levels of change in environmental conditions varied significantly between years, but were relatively similar across sites. In contrast, community persistence (similarity between years in species assemblage composition), and stability (similarity between years with respect to relative abundance of species in the community) both varied significantly between sites, but changes between years were similar. Community stability was highest at sites with relatively harsh flow conditions (high coefficient of variation, high relative size of floods), and was also greater in communities dominated by Ephemeroptera. 4. Relationships between change in environmental conditions and changes in community composition and structure were relatively weak for most individual sites. However, when average levels of change for each of the 26 sites were used, communities showed greater persistence under conditions where flow conditions remained relatively constant. Water quality changes had no significant effect on community persistence when assessed for all 26 sites combined. 5. Results from this study suggest that lotic invertebrate communities fluctuate around a relatively stable state, at least over a 9‐year period. However, the extent of interannual variation in community composition and structure observed, along with the relatively low degree of cluster fidelity observed within a single region, suggests that predictive models based on reference site conditions extrapolated over several years should be applied with caution in New Zealand streams.     

The response of hyporheic invertebrate communities to a large flood in the Hunter River, New South Wales

Previous studies on recovery in hyporheic communities have found that communities rapidly return to pre-disturbance levels. However, most of these studies have concentrated on small floods or ones with short return periods. I studied the impact of a large 1 in 6 year flood on the hyporheic community at 2 sites in the Hunter River, a large coastal river in New South Wales with a mean daily flow of 15 m³ s⁻¹. The flood peaked at 1270 m³ s⁻¹ and afterwards invertebrate densities at the 2 sites were 83 and 67% less than they were before. Recovery to pre-flood densities was slow but was aided by increases in the oligochaete and cyclopoid populations. At Site 1, there was a boom in oligochaete and cyclopoid numbers 61 d after the flood, but the communities resumed their pre-flood densities by Day 139. Recovery at Site 2 took 139 d. Most groundwater taxa (stygobites) living in the hyporheic zone did not recover from the disturbance when compared to non-stygobites. Apart from Microturbellaria and the harpacticoid Parastenocaris sp., numbers of all stygobite taxa continued to decline after the flood, becoming absent after 61 d. The poor recovery of stygobites is probably due to their adaptations for survival in the relatively stable groundwater environment. This study shows that hyporheic communities are sensitive to large bed-moving floods and supports the hypothesis that ecotonal species with a strong affinity to one ecosystem can be poor at recovering from disturbances that occur in an adjacent ecosystem.     

Effects of floods versus low flows on invertebrates in a New Zealand gravel-bed river

1. Floods and low flows are hydrological events that influence river ecosystems, but few studies have compared their relative importance in structuring invertebrate communities. Invertebrates were sampled in riffles and runs at eight sites along 40 km of a New Zealand gravel‐bed river every 1–3 months over 2.5 years, during which time a number of large flood and low flow events occurred. Flows were high in winter and spring, and low in summer and autumn. Four flow‐related variables were calculated from hydrological data: flow on the day of sampling (Q_sample), maximum and minimum flow between successive samples (Q_max and Q_min, respectively), and the number of days since the last bed‐moving flood (N_days). 2. The invertebrate community was summarised by relative densities of the 19 most abundant taxa and four biotic metrics [total abundance, taxon richness, the number of Ephemeroptera, Plecoptera and Trichoptera taxa (i.e. EPT richness), and per cent EPT]. Invertebrate density fluctuated greatly, and was high in summer and autumn, and low during winter and spring. Stepwise multiple regression (SMR) analysis was used to investigate relationships between the invertebrate community and season, flow, habitat and water temperature. 3. Seasonal variables were included in almost 50% of the SMR models, while flow‐related variables were included in >75% of models. Densities of many taxa were negatively correlated to Q_min and Q_max, and positively correlated to N_days, suggesting that while high flows reduced invertebrate densities, densities recovered with increasing time following a flood. Although season and flow were confounded in this study, many of the taxa analysed display little seasonal variation in abundance, suggesting that flow‐related variables were more important in structuring communities than seasonal changes in density associated with life‐cycles. 4. Five discrete flood and low flow events were identified and changes to invertebrate communities before and after these events examined. Invertebrate densities decreased more commonly after floods than after low flows, and there was a significant positive relationship between the number of taxa showing reductions in density and flood magnitude. Densities of most invertebrates either remained unchanged, or increased after low flow events, except for four taxa whose densities declined after a very long period (up to 9 months) of low flow. This decline was attributed to autogenic sloughing of thick periphyton communities and subsequent loss of habitat for these taxa. 5. Invertebrate communities changed more after floods and the degree of change was proportional to flood magnitude. Community similarity increased with increasing time since the last disturbance, suggesting that the longer stable flows lasted, the less the community changed. These results suggest that invertebrate communities in the Waipara River were controlled by both floods and low flows, but that the relative effects of floods were greater than even extended periods of extreme low flow. 6. Hydraulic conditions in riffles and runs were measured throughout the study. Riffles had consistently faster velocities, but were shallower and narrower than runs at all measured flows. Invertebrate density in riffles was expressed as a percentage of total density and regressed against the flow‐related variables to see whether invertebrate locations changed according to flow. Significant negative relationships were observed between the per cent density of common taxa in riffles and Q_sample, Q_max and Q_min. This result suggests either that these animals actively drifted into areas of faster velocity during low flows, or that their densities within riffles increased as the width of these habitats declined.     

Impacts of environmental changes on the hydrology and sedimentary processes at the confluence of St. Lawrence tributaries: potential effects on fluvial ecosystems

This study examines the effects of changes in St. Lawrence water levels and in hydrological regimes of two tributaries of the St. Lawrence, the Richelieu and Saint-François Rivers, on the sedimentary dynamics of their confluence in an historical context and in response to projected climate changes. Analysis of historical data indicates that alteration to St. Lawrence mean water levels and spring flood characteristics (duration, timing, and magnitude) have already impacted flow and sediment dynamics at tributary confluences. The major progradation of more than 1.5 km of the sedimentary front of the Saint-François delta into the shallow water of the fluvial Lake Saint-Pierre between 1859 and 2001 is associated with the combined effect of water level reduction in the lake, as a consequence of dredging of the navigation channel, and discharge regulation. High tributary discharges during periods of low water levels in the St. Lawrence have also contributed to accelerated delta front progression. Increases in temperature and changes in the precipitation regime projected for the current century were used into the HSAMI model to obtain hydrological simulations for the tributaries. Results indicate that the greatest hydrological changes will be observed during the winter and spring seasons, a time that is critical for many ecosystems. The mean spring discharge is projected to decrease by 17% for the Richelieu and 40% for the Saint-François by the end of the century. During this season, the frequency of discharge higher than the sediment transport threshold will decrease by 63% for the Saint-François and by 17% for Richelieu when compared to the reference period. However, during winter, the frequency of these high discharges will be four times higher for both rivers. These changes are expected to occur simultaneously with a decrease of the St. Lawrence water levels, a situation that will have significant impacts on sedimentary processes. A hydro-sedimentological modeling approach based on SEDROUT was used to investigate the morphological response of the river beds to the projected changes on the hydrology of St. Lawrence tributaries and to three base level scenarios. Results show that the combined hydrology and base level effects will lead to an increase in sediment supply compared to the current state, and to the extension of accumulation zones. While the effects will vary among the tributaries according to their particular characteristics, the projected increase in sediment supply will modify the extent of freshwater wetlands at the mouth of the St. Lawrence tributaries with feedback effects on local flow and sediment distributions.     

The relationship between land use and physicochemistry, food resources and macroinvertebrate communities in tributaries of the Taieri River, New Zealand: a hierarchically scaled approach

1. The relationship between land use and stream conditions was investigated, including physicochemistry, the availability of primary food resources and species richness, species composition and trophic structure of stream macroinvertebrate communities. The survey involved eight subcatchments of the Taieri River (New Zealand) encompassing reasonably homogeneous examples of four major land uses: native forest, native tussock grassland, plantations of introduced pine and agricultural pasture.
2. Each land use was represented by two subcatchments, each subcatchment by two to four tributaries, and each tributary by two to three sampling sites. These three sampling scales each represent typical designs for stream community studies. By recording responses at all scales, it can be determined explicitly whether the scale of sampling influences interpretation of community structure.
3. Elevation, riffle length, proportion of large substrata in the bed, total phosphorus and alkalinity were significantly related to land use, as were canopy cover and the relative abundance of leaves and wood in the streams. Principal components analysis of invertebrate density data identified nine orthogonal community types, the distributions of two of which were significantly related to land use. The role played by browsers and shredders in the stream community depended on land use.
4. Primary analysis was at the level of the tributary. When it focused on sites within tributaries, more variables were related to land use and at a higher level of significance. This was largely a result of enhanced statistical power due to increased replication. When whole subcatchments were the focus of attention, statistical power was so low, even with six to eleven subsamples to generate overall means, that few significant patterns could be identified. However, the community patterns that were revealed were similar whatever the scale of sampling.     

The relationship between land use and physicochemistry, food resources and macroinvertebrate communities in tributaries of the Taieri River, New Zealand: a hierarchically scaled approach

1. The relationship between land use and stream conditions was investigated, including physicochemistry, the availability of primary food resources and species richness, species composition and trophic structure of stream macroinvertebrate communities. The survey involved eight subcatchments of the Taieri River (New Zealand) encompassing reasonably homogeneous examples of four major land uses: native forest, native tussock grassland, plantations of introduced pine and agricultural pasture.
2. Each land use was represented by two subcatchments, each subcatchment by two to four tributaries, and each tributary by two to three sampling sites. These three sampling scales each represent typical designs for stream community studies. By recording responses at all scales, it can be determined explicitly whether the scale of sampling influences interpretation of community structure.
3. Elevation, riffle length, proportion of large substrata in the bed, total phosphorus and alkalinity were significantly related to land use, as were canopy cover and the relative abundance of leaves and wood in the streams. Principal components analysis of invertebrate density data identified nine orthogonal community types, the distributions of two of which were significantly related to land use. The role played by browsers and shredders in the stream community depended on land use.
4. Primary analysis was at the level of the tributary. When it focused on sites within tributaries, more variables were related to land use and at a higher level of significance. This was largely a result of enhanced statistical power due to increased replication. When whole subcatchments were the focus of attention, statistical power was so low, even with six to eleven subsamples to generate overall means, that few significant patterns could be identified. However, the community patterns that were revealed were similar whatever the scale of sampling.     

Effects of upland clearcutting and riparian partial harvesting on leaf pack breakdown and aquatic invertebrates in boreal forest streams

1. Leaf litter breakdown and associated invertebrates were compared among three logged and three reference stream reaches 2–3 years before and 3–4 years after logging to assess the environmental impacts of partial‐harvest logging as a novel riparian management strategy for boreal forest streams. 2. Partial‐harvest logging at three sites resulted in 10, 21 and 28% average basal area removal from riparian buffers adjacent to upland clear‐cut areas. 3. Leaf litter breakdown rates were not significantly different between reference and logged sites after logging, but litter breakdown was significantly different from year to year at all sites. 4. Significant post‐logging differences in aquatic invertebrate communities were detected at only one of the three logged sites. These differences were largely the result of increases in some leaf‐shredding stoneflies and a detritivorous mayfly and a decrease in a chironomid group 2–4 years after logging. This site where significant change was detected had the lowest intensity of riparian logging (average 10% removal) but the highest proportion of the catchment area that was clear cut (85%). 5.The post‐logging differences in invertebrate communities at this site were more related to catchment‐wide influences (e.g. weather patterns, water yield, possibly upland clearcutting) than to reach‐level disturbances from riparian logging. 6.The study indicates that partial‐harvest logging in riparian buffers at up to 50% removal should pose little risk of harm to leaf litter breakdown processes or aquatic invertebrate communities beyond any impacts that might arise from upland logging disturbance or catchment‐wide influences. However, the results should be viewed in the context of the natural disturbance (summer drought conditions) through the post‐logging assessment period of this study. Post‐logging summer drought conditions may have masked or confounded logging impacts on streams.     

Individual and combined effects of fish predation and bed disturbance on stream benthic communities: a streamside channel experiment

1. The composition and spatiotemporal dynamics of biological communities are influenced by biotic processes, such as predation and competition, but also by physical disturbances, such as floods in running waters. However, the interplay of disturbance with predation is still poorly understood, especially in frequently disturbed streams. Further, different predator species can affect prey communities in different ways depending on their feeding mode and efficiency. 2. We investigated the individual and combined effects of flood‐induced bed disturbance and fish predation on the benthos for 4 weeks in 18 streamside channels fed by a flood‐prone New Zealand river. Bed movements caused by floods were simulated by tumbling the substratum in half the channels. Six channels each were stocked with introduced brown trout (Salmo trutta) or native upland bully (Gobiomorphus breviceps) or had fish excluded. We studied algal biomass and both invertebrate density and daytime activity on surface stones on several dates after the disturbance, invertebrate community composition in the substrata of the entire channels on day 28 and leaf decomposition rates over the 28‐day period. 3. Disturbance affected algal biomass and density, richness and activity of surface stone invertebrates, and overall density and richness of channel invertebrates. Presence or absence of fish, by contrast, did not influence overall invertebrate standing stocks when subsurface substrata were included but did affect invertebrate densities on surface stones in 45% of all analysed cases and invertebrate activity on surface stones in all cases. Leaf decomposition rates were not influenced at all by the experimental manipulations. 4. Native upland bullies featured more often than exotic brown trout in causing invertebrate density changes and equally often in causing changes to grazer behaviour. Overall, our results imply that fish predation can have strong effects on the benthic invertebrate community in frequently disturbed streams, especially via behavioural changes.     

Relationships between benthic biota and hydrological indices in New Zealand streams

1. The objective of this study was to identify the most ecologically relevant hydrological indices for characterizing hydrological regimes in New Zealand streams. To do this we related measures of periphyton chlorophyll a, ash-free dry mass (AFDM), species richness, and diversity and invertebrate density, species richness and diversity, to thirty-four hydrological variables derived from daily flow records at eighty-three sites. The hydrological variables included some describing average flow conditions, flow variability, floods, and low-flow characteristics. 2. A principal components analysis showed that the interrelationship between many of the hydrological variables was high, and most variables correlated significantly with Principal Component 1 (PC1). The flood frequency variables formed a distinct component of the flow regime and were the main contributor to PC2. 3. We found that both the average flow conditions and some measure of variability were significantly related to most of the biological variables, and these individual hydrological variables were more strongly correlated to the biological measures than the composite principal components. Only four of the thirty-four flow variables were significantly correlated (P < 0.05) with measures of periphyton biomass (chlorophyll a and AFDM), whereas twenty-four variables were correlated with periphyton diversity. Conversely, thirty-one of the thirty-four flow variables were correlated with total invertebrate density, whereas only four variables correlated with diversity. 4. We selected the flood frequency (FRE₃), where a flood is defined as flows higher than three times the median flow, as the most ecological useful overall flow variable in New Zealand streams because it explained a significant amount of the variance in four out of the six main benthic community measures, and it had a clear mechanism of control of the biota which was commensurate with current stream ecosystem theory. Periphyton biomass decreased with increasing FRE₃, whereas invertebrate density had an increasing/curvilinear relationship with FRE₃. Periphyton species richness and diversity decreased with increasing FRE₃.     

Comparison of landscape diversity in forests of the upper Rhine and the middle Loire floodplains (France)

Traditional measures of biotic indices (, , ) from phytosociological relevés are used to compare diversity at three scale-levels in the upper Rhine valley (Alsace, northeastern region) and the middle Loire (Bourgogne, central region), France. Both study areas included a tributary. The Ill, which is the Rhine tributary in this sector, has no forest communities in common with the main river because of opposite hydrological and edaphic characteristics. This is not the case in the Loire valley, in which the fluvial characteristics are close to those of its tributary, the Allier.Nearly all forest communities (12/14) were shown to be species-rich, due to the role of natural disturbances. Species richness peaks in late-successional forests of the Rhine valley (for example, up to 50 woody species in Querco-Ulmetum). Sixty per cent of the forest-communities are comon to the two floodplains, which explains why 85% of the families, 65% of the genera and 55% of the species are in common. Compared to the Loire valley, the Rhine valleys shows higher biotic diversity. This was related first to its geographical situation, and second to higher landscape heterogeneity. Human disturbance through river management is responsible for changes in species composition as shown by floristic comparison of flooded and unflooded sites. Insularity of natural forests among man-made landscapes is involved in species diversity. In order to preserve the natural diversity of such rich landscapes, floodable areas should be recreated and the attitudes of foresters should evolve accordingly.     

Longitudinal variation in recolonization rates of macroinvertebrates along an upland river in south-eastern Australia

SUMMARY. 1. Recolonization rates of benthic invertebrates were estimated at five sites in the catchment of the Acheron River, in spring (October) and in summer (January), The sites ranged from those that experience short floods and high shear stress at the streambed (upstream sites) to those that experience prolonged floods and low shear stress (downstream sites). We hypothesized that these differences should affect recolonization rate.
2. In October, absolute rates of recolonization of taxa (number of taxa 0.05 m⁻² d⁻¹) onto 1-m² patches of substratum, which had been raked to remove fauna, did not vary between the three sites studied, nor did the relative rates of recolonization of taxa (absolute rate/mean number of taxa in control samples, which were taken from adjacent undisturbed patches of substratum). Absolute rates of recolonization of individuals (number of individuals 0.05 m⁻² d⁻¹) were proportional to the mean number of individuals in control samples; relative rates of recolonization of individuals (absolute rate/mean number of individuals in control samples) did not vary between sites.
3. In January, absolute rates of recolonization either of taxa or individuals were positively correlated with the mean densities of taxa or individuals in control samples; relative rates did not vary between the four sites studied.
4. We conclude that the benthic invertebrate communities at the various sites do not adapt to variations in flooding regime by altering relative recolonization rates. Absolute recolonization rates are directly proportional to the prevailing number of taxa or individuals at a site.     

The use of Percent Model Affinity to assess the effects of agriculture on benthic invertebrate communities in headwater streams of southern Ontario, Canada

1. Benthic invertebrates were surveyed in 213 headwater streams in southern Ontario to test the utility of Percent Model Affinity (PMA) for assessing the relative impact of agriculture on invertebrate communities. Qualitative samples were collected from reference (= forested, sixty-nine samples) and agricultural (195 samples) streams between May 1990 and August 1993. The percentage composition of samples from reference streams was averaged by season and type of substratum, and the most homogeneous groupings were used as expected communities for the calculation of PMA. 2. The lower confidence limit about the mean PMA for each expected community was calculated as for a one-tailed t-test comparing a single observation with the mean of a sample. Samples from agricultural streams whose PMA scores were outside these confidence limits were judged to indicate significant impact. 3. The sensitivity of PMA to differences in land use increased with taxonomic resolution: 40% of agricultural sites were significantly different from reference communities at the ordinal level, 79.5% were significantly different at the lowest practical taxonomic level. Most of the increase in discrimination between reference and agricultural sites was due to Chironomidae. Identification to species, rather than genus, yielded only a small increase in discrimination between reference and agricultural sites, largely because of taxonomic difficulties and because most members of several diverse genera were very similar in their distributions. 4. All of the samples from streams which drained orchards and vinyards (n = 7) were significantly different from the expected communities, as were 89% of those from streams draining corn (n = 94). A larger percentage of streams draining hay (78%, n = 26) were impacted than were streams bordered by pasture (66%, n = 51), but the average magnitude of impact was greater in pasture streams. Only six of eleven samples from streams draining tobacco indicated significant impact. 5. The effect of land use adjacent to the sampling site was greatest early in the growing season. Comparison of natural channels and ditches draining agricultural land suggests that channelization and subsurface tile drainage eliminate most of the benefits of riparian buffer strips and magnify the effects of farming practices on the benthic fauna.     

Impact of river management history on the community structure,species composition and nutrient status in the Rhine alluvial hardwood forest

The present-day Rhine alluvial hardwood forest (Querco-Ulmetum minoris, Issler 24) in the upper Rhine valley (France/Germany) is comprised of three vegetation units, one still flooded by calm waters (F) and the two others unflooded, one for 30 years (UF30) (after the river canalisation) and the other for about 130 years (UF130) (after river straightening and embankment work in the middle nineteenth century). In the three stands, species composition, structure and diversity of vegetation and nutrient content of mature leaf, leaf litter and soil have been studied. Fungi (Macromycetae) were only studied in two stands (F and UF130). The intensity of nutrient recycling was exemplified by comparing the chemical composition of rainwater, flood, throughfall, mature leaf, leaf litter, soil and groundwater in two of these stands (F and UF30).The elimination of floods has caused a change in floristic composition, tree density and plant diversity. Tree density was higher in the two unflooded stands and was related to a large increase in sapling (< 6 cm dbh) density more than to a change of stem (> 6 cm dbh) density. Sapling density increased 2 times and three times in the UF30 and the UF130 respectively, whereas the stem density increased only 12% in the first stand and decreased 29% in the second one. The saprophytic macromycete communities have been supplemented with mycorrhizal species. Leaf litter production was slightly greater in the flooded (4.44 T ha^-1 yr^-1) than in the two unflooded stands ( 3.72 T ha^-1 yr^-1). Foliar N level is twice as high in the flooded stands in spite of an opposite soil status. P level decreased in soil and leaves with the duration of isolation and was at the same level in the groundwater in two stands (F and UF30). K, Mg and Ca contents of green leaf and leaf litter were high due to the geochemistry of the Rhine substrate (calcareous gravels and pebbles) and similar in all the stands studied, even though there are large inputs of these three elements by floods. Moreover we showed that the groundwater chemistry reflected the variations of nutrient inputs and thus could be a good indicator of the functioning of an alluvial ecosystem and of its change as a result of human activities. The restoration of floods in hardwood forest contributes to the preservation of alluvial vegetational structure and composition, the stimulation of biological processes and a better plant mineral nutrition and water supply.     

Small-scale variation in growing season length affects size structure of scarlet monkeyflower

Growing season length can control plant size over altitudinal and biogeographic scales, but its effect at the scale of meters is largely unexplored. Within the riparian zone of a northern California river, scarlet monkeyflower, Mimulus cardinalis , grows significantly larger at sites high in the channel as compared to sites low in the channel, and even larger where tributaries meet the main stem of the river. We explored the hypothesis that markedly different growing season length controls this size variation. Due to the very gradual retreat of the water level following winter flooding, emergence time is three months longer for plants growing at tributary confluences than for plants growing at low elevations in the channel. Consistent with the growing season length hypothesis, we found no difference in transplant growth between river and tributary confluence sites in an experiment where we equalized growing season length at these locations. Moreover, a second experiment showed that individuals planted earlier in the year gain a distinct size advantage over those planted later, even though growing conditions are less ideal. These results suggest that emergence time may be a key determinant of plant size structure along rivers, an important result considering forecasted variation in water flows with climate change.     

Ecology of leaf pack macroinvertebrate communities in streams of the Fraser River Basin, British Columbia

1. To characterise geographic and small scale variation in the structure of macroinvertebrate communities in stream leaf packs, we collected one to three natural leaf pack communities from 119 reference streams in the Fraser River Basin and quantified their variability and correlation with aspects of the stream environment at several scales. We also sampled leaf packs in 19 test streams in the same geographic area exposed to stressors (nine logged, seven farmed, three mined catchments) to evaluate the leaf pack community as a tool for bioassessment. 2. There was substantial variation in the composition of invertebrate communities in leaf packs among reference streams of the Fraser River Basin. Capnia and Zapada (stoneflies), Baetis and Ephemerella (mayflies) and Tvetnia (midge) were the most common taxa found in the leaf packs. There were three types of assemblages identified by non‐metric multidimensional scaling; Capnia, Baetis and Ephemerella communities. 3. Leaf pack communities from the 19 test streams were plotted on a non‐metric multidimensional scaling ordination of the reference communities, and 14 of 19 sites fell outside the 80% confidence ellipse of the reference sites, including eight of nine logged, four of seven farmed and one of three mined catchments. Most of these streams plotted on the ordination near the Ephemerella reference communities. Reference stream communities had a similar number of genera per leaf pack (12.0) and genera per site (18.7) as the test streams (12.6 genera per leaf pack and 18.7 genera per site). Among the test sites, the farmed catchments had higher genera per leaf pack (17.8) and genera per site (21.9) than either the logged (11.5 genera per leaf pack; 19.9 genera per site) or mined (3.4 genera per leaf pack; 7.7 genera per site) catchments. 4. Heterogeneity of leaf pack communities within a site decreased as the number of genera found at the site increased. This was determined by allometric regression of the number of genera found at a site on the maximum number of genera possible, given the average number found per leaf pack. 5. There was a significant relationship between the composition of the leaf pack invertebrate community and stream geography (latitude, longitude, altitude, stream order). Canonical correspondence analysis showed differences among ‘big river’, ‘mountain stream’ and ‘southern’ communities. 6. There was no relationship between the composition of the leaf pack invertebrate community and stream channel and flow characteristics (bank dimensions, flow, slope). There was a significant relationship between the composition of the leaf pack invertebrate community and water quality of the stream (oxygen, nitrogen, phosphorus, conductivity, pH, temperature). ‘Cold, oxygen rich water’ communities were distinguishable from communities in streams with warmer, lower oxygen concentration. ‘High nutrient water’ communities were also distinct from communities in low nutrient streams. There was no relationship between the composition of the leaf pack invertebrate community and the nature of the leaf pack itself (i.e. morphology, decomposition, coniferous needle content). 7. Invertebrate communities in leaf packs show substantial, interpretable variation among reference streams. They are sensitive to human stressors at a landscape scale such as forestry and agriculture. Their diversity and composition varies at different spatial scales in a way that is at least partially explained by the environment of the stream and its catchment area.     

Post-Flood Recovery of a Macroinvertebrate Community in a Regulated River: Resilience of an Anthropogenically Altered Ecosystem

Preservation of biodiversity depends on restoring the full range of historic environmental variation to which organisms have evolved, including natural disturbances. Lotic ecosystems have been fragmented by dams causing a reduction in natural levels of environmental variation (flow and temperature) and consequently a reduction of biodiversity in downstream communities. We conducted a long‐term study of the macroinvertebrate communities before and after natural flood disturbances in an unregulated reference site (natural flows and temperatures), a regulated site (regulated flows and temperatures), and a partially regulated reference site (regulated flows and natural temperatures) on the upper Colorado River downstream from a deep‐release storage reservoir. We aimed to test the hypothesis that floods and temperature restoration would cause an increase in macroinvertebrate diversity at the regulated site. Over the short term, macroinvertebrate richness decreased at the regulated site when compared to pre‐flood levels, whereas total macroinvertebrate density remained unchanged. Over the long term (1 and 10 years after the floods), macroinvertebrate diversity and community structure at the regulated site returned to pre‐flood levels without increasing to reference conditions. Occasional floods did not restore biodiversity in this system. As long as the physical state variables remain altered beyond a threshold, the community will return to its altered regulated condition. However, temperature restoration at the partially regulated site resulted in an increase in macroinvertebrate diversity. Our results indicate that restoration of the natural temperature regime will have a stronger effect on restoring biodiversity than occasional channel‐forming floods.