Analysis of benthic macroinvertebrate communities from the Lower sections of Large river in relation to different environmental factors

The aim of this study was to make a comparative analysis of macrozoobenthos composition at different sites at selected sections of lower the Odra River with relation to different physicochemical factors. The observations were made on the lower section of Odra River at five study sites: two of them were localized in the main channel, one in the left branch of river, another one in the channel carrying post-cooling water from Dolna Odra power plant, and the last one was in the channel connecting both branches of Odra River. At all sites, 26 taxa were found representing by: Bivalvia, Gastropoda, Oligochaeta, Hirudinea, Malacostraca and Insecta. The greatest biodiversity and the highest abundance of zoobenthos organisms were noted in the channel joining the two branches of the river, site 5, characterised by the lowest water flow rates and the densest coverage of the macrophytes at the bottom. Temperature was the sole parameter to be studied that affected the composition of benthic invertebrates. A clearly negative impact of temperature on the diversity of invertebrates was observed only in the channel with post-cooling water discharged from the power plant.     

Variability of zooplankton community along a section of the Upper Victoria Nile, Uganda

Zooplankton community along a stretch of Upper Victoria Nile was investigated at selected sites between Kalange and Namasagali during April, August and October 2000 as part of a wider ecological study to generate biological baseline data prior to construction of a hydropower plant at Dumbbell Island. The study provided an opportunity to investigate a riverine zooplankton community for comparison with the more studied lake communities. Field sampling was made with a plankton net of 25 cm mouth opening and 60 μm nitex mesh; fitted with sandbags for ease of operation in water currents. Zooplankton subsamples were examined under an inverted microscope at ×100 magnification. The 27 species encountered belong to three broad taxonomic groups: Copepoda, Cladocera and Rotifera. Rotifers had the highest number of species (sixteen) followed by copepods (six). Species richness and total numerical abundance were high at the two upstream sites and decreased significantly downstream. High zooplankton densities at upstream sites during April plummeted to much lower levels in August and October. By comparison, species richness and numerical abundance were much lower than those of Lake Victoria; the ultimate source of the Upper Victoria Nile water. This may be due to differences between lotic and lentic habitats as both physical and biological processes are known to be limited under flowing water conditions. The high species richness and numerical abundance at the two upstream sites may be due to positioning of sampling points downstream of islands and/or rock outcrops for ease of net sampling as well as a richer food environment observed in a parallel study of algal biomass. The type and level of impacts of the proposed hydropower station on biological communities will be determined by site location: either upstream or downstream of the plant. Monitoring surveys of both environment and biological parameters during and after construction of the power plant will be necessary to keep track of ecosystem changes and their implications.     

Seasonal zooplankton dynamics in main channel and backwater habitats of the Upper Mississippi River

This study used stratified random sampling to examine the spatial and temporal distribution of zooplankton communities in a large floodplain river (Mississippi River, USA). Potential mechanisms controlling zooplankton abundance and community structure were considered. Main channel and backwater habitats included in this study differed between a turbid upper pool reach where aquatic macrophytes were sparse and a lower pool reach which was considerably less turbid and had extensive aquatic macrophyte coverage. Samples were collected monthly during the summer over a 2-year period and multivariate analysis was used to examine the spatial and temporal distribution of zooplankton. Significant differences were found in zooplankton density and community composition among habitats and reaches within the pool. Rotifers were the dominant taxa and seasonality was pronounced, with peak densities often occurring in late-spring. Community structure varied by habitat and reach, which suggests that water quality, physical habitat characteristics, presence of aquatic macrophytes, and zooplankton sources can all influence the zooplankton communities of the Upper Mississippi River. Characterization of the zooplankton communities provides a basis for understanding changes in the river ecosystem and examination of zooplankton communities among habitats provides insight into the mechanisms affecting zooplankton dynamics.     

Population genetic structure,parasite infection and somatic condition of pumpkinseed Lepomis gibbosus (Actinopterygii: Centrarchidae) in the Oder river basin

In Poland, distribution of non-native pumpkinseed Lepomis gibbosus (Centrarchidae) is strictly limited to the Oder river basin, where it was introduced in the early 20th century. Recently, several populations have been found in waterbodies adjacent to the Oder, particularly in its lower reaches. In this study, we compare the genetic relatedness of populations in the Oder basin with other European populations using nuclear (microsatellite) and mitochondrial (partial cytochrome c oxidase subunit I; cox1) markers. Microsatellite analysis indicated that four populations in the lower Oder form a separate cluster, while one in the middle Oder clustered with Danubian populations, from where probably having been introduced. Microsatellite data suggested that the lower Oder populations differ from other non-native European populations, making it impossible to estimate the source of introduction. Nevertheless, analysis of cox1 indicated that Oder pumpkinseeds belong to the same haplotype as the vast majority of European populations. Parasitological examination confirmed the presence of two North American species, the monogenean Onchocleidus dispar and trematode Posthodiplostomum centrarchi, in the lower Oder, both previously unknown in the region. Fifteen other parasite species were acquired, including glochidia of invasive Sinanodonta woodiana. In the middle Oder, parasite infection was more limited. Fish from the Gryfino Canal, considered one of the most invasive populations in Europe, showed the highest parasite abundance and diversity, and the highest somatic condition and growth rate due to warm water released from the Dolna Odra power plant. Our results highlight significant differences in somatic condition and parasite infection in long-established non-native pumpkinseed populations in the same river system, reflecting mainly environmental conditions.     

The diversity and longitudinal changes of zooplankton in the lower course of a large, regulated European river (the lower Vistula River, Poland)

The diversity and longitudinal variation of zooplankton in the lower Vistula River were analyzed. Samples were taken from 40 stations located along a 272-km long section of the lower river course. During the study the unique technique of taking samples from “the same water” was used. The zooplankton community was dominated by rotifers and nauplii — larval stages of copepods. The most abundant species were: Brachionus angularis, Brachionus calyciflorus and Brachionus budapestiensis. The zooplankton species diversity in the main channel of the lower Vistula River was similar to other large European rivers; however, its abundance was lower. The diversity, abundance and biomass of potamoplankton steadily decreased downstream. This could be related both to scarcity of storage zones for potamoplankton development in the river due to the extensive regulation processes, and changes in hydrological conditions of the main channel (by the straightening of riverbed) where the samples were collected.     

Spatio-temporal Investigations on the Planktonic Organisms of the Middle Loire (France), During the Low Water Period: Biodiversity and Community Dynamics

The objective of this work was to analyse the distribution of the planktonic communities involved in the functioning of a 255-km river stretch and to get a better understanding of the influence of the river morphology on the diversity and dynamics of the micro-organisms. The planktonic communities (phytoplankton, bacterioplankton, proto- and metazooplankton) scarcely considered together in fluvial systems, were analysed at four sites of the Middle Loire during the low water period, in parallel to physical and chemical analyses. Physical and chemical variables such as turbidity, pH, suspended matter and chlorophyll a concentration were high, illustrating the classical, productive summer period. The algae played a major role in the water oxygenation until end-summer, then the algal drop concomitant to the bacterial sustained abundance appeared responsible for oxygen depletion. The downstream site enriched by nutrients inputs of two tributaries, carried the highest algal and bacterial densities. Situated in a meanders zone, the Middle Loire is characterised by a high habitat heterogeneity, the up- and downstream sites were wide and spread of vast standing zones and vegetated islands, whereas the two intermediate ones were narrower and more uniform. This morphological variability strongly impacted the micro-organisms diversity and distribution. Indeed, the algae and zooplankton composition were clearly influenced by the physical habitats of the river, the Cyanophyta were favoured by the lentic conditions and the Bacillariophyta by the turbulent ones, while the young stage of copepod and the large rotifer predators were indicator of a lentic origin. Thereafter, the river heterogeneity interfered with the zooplankton dynamics, the standing conditions enhancing the rotifer predation. In that way, we hypothesise that two opposite patterns characterised the wide sites spread of lentic water and the more uniform channels. In the first case, the zooplankton could prey on the ciliates protozoan, which in return favoured the flagellate ones; conversely in the second situation the zooplankton limited by the physical constraints did not impact the ciliates which could depress the flagellates. Thus, the similar geomorphology of the distant upstream and downstream sites (255-km apart) induced relatively close organisms distribution. Hence, disagreeing with the river continuum concept, this assertion shows the strong influence played by the local morphological characteristics of the Middle Loire in potamoplankton composition and dynamics.     

Influence of Intermittent Hydropeaking on the Longitudinal Zonation Patterns of Benthic Invertebrates in a Mountain Stream

The longitudinal zonation patterns of invertebrate species were studied in the River Oriège (Pyrénées, France) from 920 to 819 m a.s.l. At 912 m a.s.l., the river receives hypolimnetic water diverted from a nearby reservoir lake, and the natural flow may be enhanced several times a day from 1 to 11 m³ s^—1 in summer and winter, and from 5 to 15 m³ s^—1 during spring spates. During hydropeaking, the water was cooled in summer and slightly warmed in winter, but this was attenuated 3500 m downstream from the plant. Invertebrate densities were recorded in July (end of spates) and October (low flow period) at ten sampling sites : a reference site upstream of the hydrostation's discharge point (site 1), and nine sites each 400 m below the outlet (sites 2—10). The longitudinal distribution of invertebrate taxa was studied using Factorial Analyses, taxa and sites were clustered using Analysis of Dynamic Cluster. The hydroelectric facility did not clearly modify the qualitative composition of the benthic fauna, but clearly affected the longitudinal zonation of several populations. The low abundance of several species below the outlet reflected the impact of both hydropeaking and zonation. These species were high mountain species, the density of which decreases towards downstream sites in the Pyrenees, and low mountain species, the density of which naturally decreases towards upstream sites. Some taxa disappeared in the regulated section, so their distribution in the River Oriège was primarily influenced by hydropeaking. Finally, some taxa preferentially lived in the lower section of the river where the impact of peaking flows was strongly attenuated, so their longitudinal distribution was first governed by their natural longitudinal zonation. Under this kind of river regulation (natural discharge and temperature except during periods of power generation, intermittent hydropeaking from a separate reservoir) modifications of the thermal regime had a minor effect on the population dynamics, unlike hydraulic disturbances which strongly influenced the structural attributes of benthic communities.     

The longitudinal distribution and community dynamics of zooplankton in a regulated large river: a case study of the Nakdong River (Korea)

The longitudinal distribution and seasonal dynamics of zooplankton were examined along a 200-km section of the middle to lower Nakdong River, Korea. Zooplankton was sampled twice a month from January 1995 to December 1997 at five sites in the main river channel. There was considerable longitudinal variation in total zooplankton abundance (ANOVA, p < 0.001). All major zooplankton groups (rotifers, cladocerans, copepodids and nauplii) increased significantly with distance downstream along the river. There also were statistically significant seasonal differences in zooplankton abundance at the sampling sites (ANOVA, p < 0.01). Zooplankton abundance was high in spring and fall and low in summer and winter. The seasonal pattern of rotifers was similar to that of total zooplankton. This reflected the fact that rotifers (Brachionus calyciflorus, B. rubens, Keratella cochlearis and Polyarthra spp.) strongly dominated the zooplankton community at all locations. Among the macrozooplankton, small-bodied cladocerans (e.g. Bosmina spp.) dominated; the abundance of large-bodied cladocerans (e.g. Daphnia) was negligible (0–5 ind. l^–1). Among the environmental variables considered, partial residence time seemed to play the most important role in determining characteristics of the river zooplankton community.     

Quantitative development of zooplankton in waterbodies and watercourses of the Great Lakes Depression (Mongolia)

The indices of the zooplankton of various waterbodies and watercourses of the Great Lakes Depression (Mongolia) are analyzed. The maximum quantity of zooplankton is registered in freshwater bodies. Irrespective of the waterbody type, the greatest number and biomass of zooplankton are recorded in the littoral zone, where communities are characterized by a high abundance of rotifers, including the indicator species of eutrophic waters. This indicates a high degree of organic load from the catchment area. In comparison with the data obtained by the end of 1970s, the zooplankton biomass decreased with a concomitant increases in the shares of rotifers and cladocerans. These changes are most pronounced in the freshwater ecosystem. The quantitative development of the zooplankton of watercourses depends on the character of the river head, the location of sampling stations along the watercourse’s channel, and the water current velocity.     

The Reference Index Method for the Macrophyte‐BasedAssessment of Rivers – a Contribution to the Implementationof the European Water Framework Directive in Germany

The European Water Framework Directive requires ecological status classification and monitoring of surface and ground waters using biological indicators. To act as a component of the “Macrophytes and Phytobenthos” biological quality element, as demanded by the Directive, a macrophyte‐based assessment system was developed for application in river site types in Germany. Macrophyte abundance data were collected from 262 sites in 202 rivers. Seven biocoenotic river site types were established using differences in characteristic macrophyte communities reflecting ecoregion, channel width, water depth, current velocity, water hardness, and ground water influence. For four of these river site types, a macrophyte assessment system was developed, for the remaining three river site types data were insufficient for developing an assessment system. Ecological status classification of river sites is based on the calculation of a Reference Index value, in some cases supplemented by additional vegetation criteria. The Reference Index quantifies the deviation of species composition and abundance from reference conditions and classifies sites as one of the five possible ecological quality classes specified in the Directive. The assessment of long river stretches with changing river site types along its course is discussed based on an example from the Forstinninger Sempt River, southeast Germany. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparisons of zooplankton and phytoplankton in created shallow water habitats of the lower Missouri River: implications for native fish

Shallow water habitat (SWH) is important for riverine fish and their invertebrate prey, yet the availability of SWH has declined in many systems due to human impacts. We evaluated the potential ecological benefits of restoring SWH by comparing zooplankton and phytoplankton from created backwaters (a floodplain feature connected to the river on the downstream end but disconnected at the upstream end) and chutes (a side channel of the river that diverts flow from the main channel through the chute and back into the main channel) on the lower Missouri River. We tested the hypothesis that backwaters support higher abundances of zooplankton and phytoplankton than chutes using data that were collected during the summer of 2010. As predicted, backwaters had more diverse cladoceran communities and greater abundances of rotifers, copepod nauplii, adult copepods, and cladocerans than chutes. Total algal biovolume was the same in chutes and backwaters; however, phytoplankton taxa richness was higher in backwaters, and there was a greater biovolume of green algae (Chlorophyta), Crypotophyta, cyanobacteria, and Euglenophyta in backwaters than in chutes. Differences in zooplankton and phytoplankton between backwaters and chutes appeared to be related to slower current velocities, longer retention times, and lower levels of turbidity and total suspended solids in backwaters. While chutes have the potential to provide greater habitat diversity than the mainstem, there were no differences in water quality or phytoplankton abundance, community structure, or diversity between these two habitats. Combined, our results suggest that created backwaters initially provide a greater potential food resource for native fishes. However, additional research is needed to determine whether chutes can also develop beneficial shallow water features over a longer period of time.     

Spawning tactics of summer chum salmon Oncorhynchus keta in relation to channel complexity and hyporheic exchange

Vertical hydraulic gradient, channel hydraulic, and substratum data were collected at active chum salmon redds to evaluate their influence on redd site selection in the Kwethluk River, a wandering gravel-bedded tributary of Alaska’s Lower Kuskokwim River. Two distinct summer chum salmon spawning tacitcs: primary- and off-channel groups spawning in different hydraulic and thermal environments. Channel hydraulics, ground-surface water exchanges, and bed sediment data were collected throughout the range of topographic conditions at sites occupied and unoccupied by spawners. Analyses of these data showed that redd site selection was incoherent with channel hydraulics and substratum size, when all channel types were considered. Rather, spawning was associated with the exchange of river and groundwater at four spatial scales. In primary channels, chum salmon spawned only in reaches where the regional hydraulic gradient (regional scale) promoted the penetration of river water, or downwelling, into the bed sediments. Within these reaches, primary-channel spawning was influenced by bed topography (fluvial scale) that promoted strong localized downwelling; a logit model, fitted to constituent hydraulics, discriminated between selected and unselected sites with 89 % accuracy. In contrast, spawners in off-channel habitats (flood and spring channels) selected areas of upwelling groundwater, regardless of channel hydraulics and the regional hydraulic gradient. In parafluvial flood channels, spawners selected localized upwelling associated with short groundwater flow paths (parafluvial scale). In spring channels, within the floodplain’s forested riparia (orthofluvial scale), spawners eschewed areas of localized downwelling and uniformly selected upwelling zones feeding these channels. Thus, redd selectivity differentiated two spawning tactics (groups) in this complex channel network where ground and surface water exchange is an important predictor of spawning habitat.     

Response of stream macroinvertebrate assemblages to erosion control structures in a wastewater dominated urban stream in the southwestern U.S.

Effects of stream erosion control structures on aquatic macroinvertebrates were studied (2000–2009) in a wastewater dominated drainage (Wash) in Las Vegas, Nevada. Mainstem sites with and without structures, wastewater treatment plant outfalls, a reference site above treatment plant inputs, and tributary sites were sampled. Ordination suggested hydrology and channel characteristics (current velocity, stream depth, and width), and water quality (conductivity) were primary factors in organizing macroinvertebrate communities, with some variables altered at structures. Treatment plant inputs changed hydrology (increased flows), water chemistry (conductivity decreased below treatment plants), and temperature. Assemblages differed between site types, with midges and damselflies important at tributary sites and Fallceon mayflies and Smicridea caddisflies common at erosion control structures. Locally unique communities developed at structures which also may have facilitated exotic species invasions. Analyses showed that taxa richness increased over time at these sites and differed significantly from richness at sites without structures. Structures appeared important in retaining organic matter and, among mainstem sites, coarse particulate organic matter was highest, but variable, at structures and at wetlands above the structures. Erosion control structures, coupled with warm effluent, high baseflows, and altered water quality resulted in development of a macroinvertebrate community that did not trend towards reference or tributary sites. In this case, ecological communities at structures used for river restoration were not on a continuum between disturbed and reference sites. Goal setting of community responses at these structures would have required insight beyond the simple use of reference site attributes.     

The influence of leaf litter on zooplankton in floodplain wetlands: changes resulting from river regulation

1. The loss of input of leaf litter through clearing of riparian vegetation may result in significant changes to aquatic ecosystems. River red gums (Eucalyptus camaldulensis) surrounding floodplain wetlands in the Murray–Darling Basin, Australia, contribute large quantities of leaf litter, but the quality of this resource may change depending on the timing of inundation. 2. We used experimental mesocosms to test the hypotheses that zooplankton would have a greater abundance with an input of leaf litter and that fewer zooplankton would emerge from egg banks in cleared than forested wetlands. The experiment was carried out in summer/autumn and in spring to test a third hypothesis that zooplankton would respond to changes in the timing of wetland inundation as a result of river regulation. 3. In summer/autumn, leaf litter reduced zooplankton abundance by 89% at the beginning of the experiment through its influence on water quality. Only a few taxa (Polyarthra spp., Colurella spp. and the cladoceran Family Moinidae) responded positively to leaf litter when water quality improved later in the experiment, indicating a switch in the role of leaf litter from a non‐trophic to a trophic pathway. 4. In spring, microcrustaceans emerged in smaller numbers from sediment sourced from cleared compared to forested wetlands, reflecting different communities in these two wetland types and/or disturbances to the sediment that interfere with emergence. 5. Although leaf litter appears not to be an important resource for zooplankton in floodplain wetlands, riparian clearing may have lasting effects on future emerging zooplankton communities. Additionally, river regulation may have considerable impacts on the influence of leaf litter on zooplankton, which has implications for the management of floodplain river systems.     

Evolution saisonniere du zooplancton dans trois sites peu profonds de Thau,une lagune Nord-Mediterraneenne

A monthly zooplankton survey has been conducted from March 1982 to March 1983 at three shallow stations of Thau, a northern-mediterranean lagoon. These stations differ by their distance from the entry of sea-water in the lagoon. Parameters such as pH, temperature, dissolved oxygen, conductivity and turbidity have been measured simultaneously with the sampling in the same water layer. The zooplankton collected corresponds to species usually found in marine neritic communities. In the spring, species richness is maximum. During the summer, meroplankton forms dominant populations in the communities. Autumn is marked by a reduction of the number of species and of the abundance in both components, holoplankton and meroplankton. Few species are present in the winter holoplankton while meroplankton disappears practically from the samples. The sites under observation are subjected to unpredictable changes, in water movement as well as in the organization of the zooplankton communities. These modifications are strongly influenced by weather conditions.     

Colonization,variability, and the use of substratum-filled trays for biomonitoring benthic communities

Sampling variability and colonization rate of introduced substrates (plastic trays filled with pebble and cobble) in two southwestern Virginia streams are described. Substrates were rapidly colonized by aquatic macroinvertebrates, but colonization rates differed between years, possibly due to annual variability in macroinvertebrate abundance. To examine the applicability of using these substrates for biomonitoring benthic communities, trays were placed at several locations in a river receiving power plant discharges. Only six samples were necessary to detect a 15%reduction in macroinvertebrate density and a 12% reduction in number of taxa at effluent sites. Benthic communities established on rock-filled trays and multiplate samplers collected from the same stations during the same period were compared. Although multiplate samplers were more variable than rock trays and were selective for different taxa, both substrate types showed significant differences in community parameters among locations. Rock trays at all sites were dominated by Cheumatopsyche sp., whereas chironomids were more abundant on multiplate samplers. The relative abundance of mayflies was reduced at the effluent site on both substrate types.     

Invertebrate community response to fire and rodent activity in the Mojave and Great Basin Deserts

Recent increases in the frequency and size of desert wildfires bring into question the impacts of fire on desert invertebrate communities. Furthermore, consumer communities can strongly impact invertebrates through predation and top‐down effects on plant community assembly. We experimentally applied burn and rodent exclusion treatments in a full factorial design at sites in both the Mojave and Great Basin deserts to examine the impact that fire and rodent consumers have on invertebrate communities. Pitfall traps were used to survey invertebrates from April through September 2016 to determine changes in abundance, richness, and diversity of invertebrate communities in response to fire and rodent treatments. Generally speaking, rodent exclusion had very little effect on invertebrate abundance or ant abundance, richness or diversity. The one exception was ant abundance, which was higher in rodent access plots than in rodent exclusion plots in June 2016, but only at the Great Basin site. Fire had little effect on the abundances of invertebrate groups at either desert site, with the exception of a negative effect on flying‐forager abundance at our Great Basin site. However, fire reduced ant species richness and Shannon's diversity at both desert sites. Fire did appear to indirectly affect ant community composition by altering plant community composition. Structural equation models suggest that fire increased invasive plant cover, which negatively impacted ant species richness and Shannon's diversity, a pattern that was consistent at both desert sites. These results suggest that invertebrate communities demonstrate some resilience to fire and invasions but increasing fire and spread of invasive due to invasive grass fire cycles may put increasing pressure on the stability of invertebrate communities.     

甬江干流浮游动物群落结构季节动态与水环境的关系

对采自2011年3月-2012年2月间甬江干流13个断面的水样, 按照国家标准测试了9项理化指标, 并应用浮游动物定性和定量方法, 研究了浮游物群落结构特点和季节变化. 研究共记录浮游动物95种, 其中轮虫24属72种、枝角类5属10种、桡足类11属13种和4种其他门类无脊椎动物幼虫; 优势种主要来自于轮虫动物的臂尾轮虫属(Brachionus)和龟甲轮虫属(Keratella), 前者占了浮游动物总丰度的22.1%, 后者占总丰度的20.2%. 浮游动物的丰度值在各断面均呈明显的季节变化, 从断面1至断面6, 最大峰值出现在春季, 从断面7至断面13, 最大峰值多出现在夏季; 全干流最大峰值出现在断面5 (3160 ind./L). 浮游动物群落生物多样性指数呈上游段低, 中、下游河段高的现象; 应用中度干扰假说(Intermediate disturbance hypothesis)对此结果给予了解释. Two-way ANOSIM相似性分析和Bray-Curtis相似性等级聚类分析显示: 不同断面上不同季节的浮游动物群落间均存在显著差异, 分别为R=0.264、P=0.1%和R=0.234、P=0.1%, 且季节性差异大于断面间差异. 在季节性差异中以春季和夏季间的差异性最大. 主成分分析(PCA)表明: 对不同断面浮游动物群落起作用的理化因子依次为CODCr、TN、DO和BOD5; 对不同季节浮游动物群落起作用的理化因子依次为pH、TN/TP、盐度和水温.       

Patterns of fish community composition along a river affected by agricultural and urban disturbance in south-central Chile

Patterns of fish community composition in a south-central Chile river were investigated along the altitudinal-spatial and environmental gradient and as a function of anthropogenic factors. The spatial pattern of fish communities in different biocoenotic zones of the Chillan River is influenced by both natural factors such a hydrologic features, habitat, and feeding types, and also by water quality variables which can reduce the diversity and abundance of sensitive species. A principal component analysis incorporating both water quality parameters and biomarker responses of representative fish species was used to evaluate the status of fish communities along the spatial gradient of the stream. The abundance and diversity of the fish community changed from a low in the upper reaches where the low pollution-tolerant species such as salmonid dominated, to a reduced diversity in the lower reaches of the river where tolerant browser species such as cypriniformes dominated. Even though the spatial pattern of fish community structure is similar to that found for the Chilean Rivers, the structure of these communities is highly influenced by human disturbance, particularly along the lower reaches of the river. Handling editor: C. Sturmbauer     

Longitudinal changes in zooplankton distribution below a reservoir outfall with reference to river planktivory

The fate and interactions with river organisms of zooplankton as they drift downriver from a reservoir on a fourth-order mountain stream (Hiji River, Japan) were investigated. Monthly samples were collected at the reservoir and six river sites, simultaneously, from May 2005 to May 2006. Aquatic macroinvertebrates and fish were colleted, and their stomach contents were analyzed in April and May, 2006, respectively. Drift from the reservoir was the primary source for the river plankton community; the abundance of zooplankton, particularly those of cladocerans and large rotifer, rapidly decreased within several kilometers of the dam. Analysis of the contents of fish stomachs showed that drifting zooplankton was the main food for fish, with strong food selectivity for cladocerans and large rotifers. However, fish and insect planktivores showed longitudinally different stomach contents, with progressively fewer zooplankton found in the stomachs at the downriver sites. The results suggest that the outflow of zooplankton from the reservoir is an important food source for the downstream predators, especially fish, but the drift of zooplankton and consequent food availability for the predators at lower sites are strongly limited by concentrated fish predation just below the reservoir dam.