Phases in the development of riverine plankton: Examples from the rivers Rhine and Meuse

Observations on phyto and zooplankton in two hydrographically different rivers were compared in order to discriminate phases in plankton development. Along the longitudinal axis of the River Rhine a gradual increase in the development of phytoplankton was observed, which reached its maximumca. 100 km before the river flows into its artificial sedimentation area. The development of rotifer populations was slightly retarded as compared with that of phytoplankton and highest population densities were only reached in the sedimentation area. Crustaceans developed in significant numbers, not until the river water had entered the sedimentation area. Development of zooplankton coincided here with a strong decrease in the density of phytoplankton. A similar trend in plankton development was observed in the River Meuse, although in this river the highest densities of phyto and zooplankton already occurred in its middle reaches. The differences in the timing of plankton growth in the two rivers are probably caused by differences in flow regime between both rivers. The River Rhine, which is fed by rainwater and melting of glaciers in the Alps, has a relatively constant discharge. On the other hand, the low discharge of the rain-fed River Meuse combined with an increased residence time of the water as a consequence of large numbers of weirs, allows a full cycle of plankton development long before its discharge into the sea. This phenomenon was also reflected in the silicate cycle in the Meuse, where the consumption by planktonic diatoms and the regeneration of silicate of deposits seem to be important. In contrast, in the main branches of the River Rhine only the effects of silicate consumption were detectable.     

Zooplankton distribution in flowing waters and its implications for sampling: case studies in the River Meuse (Belgium) and the River Moselle (France, Luxembourg)

Zooplankton spatial distribution was studied on four occasions by transect sampling on the rivers Moselle and Meuse in July 1996. Sampling sites were selected for their variety in morphology. A pseudoreplicate-based sampling scheme was adopted that allowed small-scale longitudinal variations in density to be tested along with transversal position and depth. In the Moselle, zooplankton were unevenly distributed transversally, an observation tentatively linked with the complexity of the river channel and the influence of tributaries. In the Meuse, spatial heterogeneity was stronger, and depth also played a key role. Its effect was different for rotifers and microcrustaceans. The occurrence of zooplankton patches, similar to those commonly reported from lakes, was noted. The factors leading to the establishment of such distribution patterns, and their relevance to routine sampling, are briefly discussed; a short list of recommendations for sampling in large rivers is proposed.      

Zooplankton abundance in the River Danube, Austria: the significance of inshore retention

1. Zooplankton density was examined in a free-flowing section of the River Danube, Austria. Spatial and temporal patterns were used to identify mechanisms regulating zooplankton dynamics. 2. Zooplankton abundance differed significantly between sampling sites. These differences were the result of variations in water residence time within the habitats. Inshore habitats with low flushing rates act as storage zones for zooplankton. 3. The hydrological conditions govern the number and size of storage zones. In the study area, the highest availability of storage zones was observed at medium water level. 4. Total zooplankton numbers, total rotifer numbers and the densities of the most abundant rotifer taxa were significantly positively related to the availability of adjacent storage zones. 5. The present authors propose that inshore retention capacity, and the hydrological processes and mechanisms controlling the extent of inshore retention may be of major significance in regulated rivers with high water velocities, preventing substantial zooplankton growth in the main channel.     

Patterns of population genetic diversity in riparian and aquatic plant species along rivers

Aim The downstream hydrochoric spread of seeds of aquatic and riparian plant species, without upstream compensation, can be expected to result in downstream accumulation of population genetic diversity. This idea has been termed the ‘unidirectional dispersal hypothesis’ and is the genetic equivalent of the more generally known ‘drift paradox’. Our aim was to test this unidirectional diversity hypothesis, and to present a general synthesis of the patterns of population genetic variation across different riparian and aquatic plant species along rivers. Location The Meuse River (Belgium) and rivers world‐wide. Methods First, we used amplified fragment length polymorphism markers to compare patterns of within‐ and between‐population genetic diversity among three riparian plant species (Sisymbrium austriacum, Erysimum cheiranthoides and Rorippa sylvestris), typically occurring in different habitats along a gradient perpendicular to the Meuse River. Second, we performed a meta‐analysis on studies reporting on the population genetic structure of riparian and aquatic plant species along rivers. Results Along the Meuse River, we found significant genetic differentiation among populations of all three riparian species, and significant isolation by distance for one of them (R. sylvestris). There was no clear association between the typical habitat of a species and its population genetic structure. None of the three species provided evidence for the unidirectional dispersal hypothesis. The meta‐analysis, based on 21 data records, did not support the unidirectional dispersal hypothesis either. Average weighted population genetic differentiation across species was significant. Main conclusions Important mechanisms of upstream seed dispersal, probably through zoochory, together with higher seed recruitment opportunities in upstream habitats due to density dependence of recruitment, may explain the absence of downstream accumulation of genetic diversity. Also, it seems difficult to find consistent patterns in genetic variation in species from aquatic and riparian habitats. We argue that this is due to the recurrent extinctions and colonizations characteristic of these habitats, resulting in complex genetic patterns. Our results strongly support previous suggestions that stream ecology should consistently embrace metapopulation theory to be able to understand patterns of genetic diversity, as well as species diversity.     

Phytoplankton dynamics in the River Meuse as affected by pollution

The temporal and spatial dynamics of phytoplankton in the River Meuse and a number of physical and chemical parameters were analyzed to gain insight into the pollution level in the river affecting the phytoplankton community. During 1993, samples were taken every two months at eight stations along the River Meuse for physical, chemical and biological characterization of the water. In February, water samples were also taken for laboratory incubations of the natural plankton community. The algae showed a rapid successive development with highest densities (up to 77 mg chlorophylla l^–1) in the middle reach of the river in spring and summer. During winter algal biomass remained very low. A substantial input of nutrients (nitrogen, phosphorus) was observed in the middle reach of the river. Concurrently, an increase in temperature and a reduction in pH was observed. Furthermore, the toxicity of polar organic compounds in the Microtox test showed a strong increase up to the city of Liège and the Belgian/Dutch border; the water quality improved further downstream. The relatively high toxicity at the location Liège was reflected in relatively low growth rates of phytoplankton in the laboratory experiments using water from these locations.     

Monitoring of algae in Dutch rivers: does it meet its goals?

Observations on phytoplankton in the lower reaches of the rivers Rhine and Meuse were carried out in the framework of the national monitoring programme of the main water systems in The Netherlands. Seasonal changes in density and species composition were analysed to detect the major variables and to discuss whether this monitoring meets its goals. Phytoplankton reached peak densities of 140 and 65 μg L^-1 Chl a in the Rhine and Meuse, respectively. Overall density was correlated with seasonal variation in water discharge rather than with nutrient concentrations, that are high in both rivers. The position of sampling sites in relation to the downstream development of the plankton was very important, especially in the regulated Meuse. Despite hydrographic and chemical differences between the two rivers, many species, predominantly diatoms and green algae, were shared. The occurrence of the diatom Skeletonema subsalsum in the Rhine and the green algae Neodesmus danubialis, Micractinium pusillum and Pseudotetrastrum punctatum in the Meuse (1992, but not 1996) was interpreted as a feature related, respectively, to the high salinity of the Rhine and specific riverine conditions of the Meuse. In general the potamoplankton was characterised as an opportunistic assemblage exploiting the high nutrient contents and disturbed hydrography of both rivers. Nevertheless, the phytoplankton contributed to the characterisation of the present river communities, ones that differ rather strongly in historic records. Although year to year variation in phytoplankton density is mainly related to variation in water discharge, phytoplankton biomass did increase as a result of eutrophication over a period of decades. The widespread nature of many of today's potamoplankton species in the two rivers render these organisms less useful for indicating short-term changes in water quality. However the importance of phytoplankton in trophic relationships merits its inclusion in the monitoring of riverine ecosystems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Variaciones temporales y distribución horizontal del zooplancton en algunos cauces secundarios del rio Paraná Medio

The object of this work was to determine the principal characteristics of the Zooplankton of several secondary rivers of the Middle Paraná River. Special emphasis was given to the influence of the most important physical factors relation to the hydrologic cycle.

Transparency, depth, pH, temperature, and speed stream data were obtained in the field. Data on hydrometric level and discharge of the Santa Fe and Correntoso rivers were also available. A summary of physical and chemical data is shown.

With respect to the Zooplankton the following aspects were analyzed: density; biomass; specific diversity; equitability, faunistic distance.     

The Mayfly,Ephoron Virgo (Olivier), back in the Dutch parts of the rivers Rhine and Meuse

Due to severe water pollution, the mayflyEphoron virgo (Olivier) disappeared from the Dutch parts of the rivers Rhine and Meuse in the first half of this century. In August 1991, however, larvae were found in the littoral zone of the Rhine near the Dutch-German border. Later, other stages were found along the Rhine branches and a small section of the Meuse. The occurrence in The Netherlands is the result of further expansion from the upstream regions of the Rhine.     

Fish impingement at power stations situated along the rivers Rhine and Meuse in The Netherlands

Summary Substantial numbers of predominantly O⁺ fish were impinged on the cooling water screens of Harculo, Gelderland, Merwedehaven and Amer power stations. Low numbers were impinged at the two stations located on the river Meuse (Maas and Claus). Two possible reasons for this difference are suggested,viz. a low fish density in the river Meuse and the deep position of the cooling water intakes. In comparison with Bergum power station situated on a lake the impingement rate at the river stations was more than ten times as low.Most fish were impinged during the summer months when large numbers of juvenile fish are present in the rivers. Dominating species impinged were roach, pikeperch, bream and perch. The abundance of pikeperch in the screensamples, points to a recent increase in the population of pikeperch in the rivers. The growth pattern of O⁺ pikeperch and O⁺ roach during summer seemed very similar for power stations located at different rivers. Thirty fish species and four crustacean species were collected, among whichOrconectus limosus was quite abundant at Maas and Claus power stations.     

Rapid range expansion of the invasive quagga mussel in relation to zebra mussel presence in The Netherlands and Western Europe

Since its appearance in 2006 in a freshwater section of the Rhine–Meuse estuary (Hollandsch Diep, The Netherlands), the non-indigenous quagga mussel has displayed a rapid range expansion in Western Europe. However, an overview characterising the spread and impacts of the quagga mussel in this area is currently lacking. A literature study, supplemented with field data, was performed to gather all available data and information relating to quagga mussel dispersal. Dispersal characteristics were analysed for rate and direction and in relation to hydrological connectivity and dispersal vectors. To determine ranges of conditions suitable for quagga mussel colonisation, physico-chemical characteristics of their habitats were analysed. After its initial arrival in the freshwater section of the Rhine-Meuse estuary and River Danube, the quagga mussel demonstrated a rapid and continued range expansion in Western Europe. Quagga mussels have extended their non-native range to the network of major waterways in The Netherlands and in an upstream direction in the River Rhine (Germany), its tributaries (rivers Main and Moselle) and the River Meuse (Belgium and France). The calculated average quagga mussel dispersal rate in Europe was 120 km year^?1 (range 23–383 km year^?1). Hydrological connectivity is important in determining the speed with which colonisation occurs. Dispersal to water bodies disconnected from the freshwater network requires the presence of a suitable vector e.g. pleasure boats transferred over land. Upstream dispersal is primarily human mediated through the attachment of mussels to watercraft. The relative abundance of quagga mussel to zebra mussel has greatly increased in a number of areas sampled in the major Dutch rivers and lakes and the rivers Main and Rhine and the Rhine–Danube Canal leading to a dominance shift from zebra mussels to quagga mussels. However, evidence for displacement of the zebra mussel is limited due to the lack of temporal trends relating to the overall density of zebra and quagga mussel.     

Impact of turbulence on riverine zooplankton: a mesocosm experiment

1. With increases in river discharge over time and space, zooplankton generally encounter increased turbulence, turbidity, hydraulic forces, downstream advection and food limitations, all of which should affect species diversity and densities. Of these factors, the role of turbulence on the distribution of zooplankton is least known along longitudinal and lateral dimensions in river networks. 2. We tested the factorial effects of turbulence and grazing level on Ohio River potamoplankton in spring and summer using twelve 1600‐L, outdoor mesocosms. Turbulence was calculated using the Froude number for equal depths but with current velocities of 0.064 and 0.32 m s^?1. Grazing levels corresponded to a high density treatment (=ambient river densities of rotifers, copepods and cladocerans) and a low density treatment (initially no zooplankton >64 μm). All tanks had the same water residence time, and hydraulic stress was minimized by circular flow patterns. 3. Zooplankton densities and population growth rates were significantly affected by turbulence level and season. In general, rotifer populations grew faster in high turbulence tanks (though Keratella and Brachionus populations flourished in both treatments in summer) and microcrustaceans thrived better in low turbulence environments. The larger, calanoid copepods handled more turbulent conditions much better than cyclopoids or nauplii. Zooplankton had no detectable effects on particulate organic carbon concentrations in either month (values were higher in spring), but rotifers reduced chlorophyll concentrations in both months. 4. The relative importance of turbulence in controlling potamoplankton is probably to vary not only on a longitudinal basis in river networks but also with both the hydrogeomorphic complexity of river reaches and the type and amount of river regulation. Plans for river rehabilitation and management should incorporate non‐turbulent habitats in large rivers as a means of enhancing zooplankton populations and providing an important food web component for planktivores.     

Zooplankton of turbid and hydrologically dynamic prairie rivers

1. Compared with rivers in more humid, forested ecoregions of eastern and midwestern U.S.A., rivers in semi‐arid grassland of the U.S. Great Plains tend to be relatively shallow, more variable in discharge, and characterised by high suspended sediment loads. Although critical life stages of fish in prairie rivers probably depend at least partially on zooplanktonic food, data on community and distributional patterns of potamoplankton in these widespread ecosystems are almost entirely absent. 2. We examined summer zooplankton distribution in five prairie rivers (Arkansas, Kansas, Platte, Elkhorn, and Niobrara Rivers) spread over six degrees of latitude during 2003–2004. We compared our results from 126 samples with previously collected data from the Ohio and St Lawrence Rivers in forested ecoregions and correlated differences with abiotic environmental conditions. 3. The importance of hydrological retention zones to stream biota has been recently demonstrated for rivers with quasi‐permanent islands and slackwater regions, but the importance of slackwaters formed by ephemeral sandbar islands in prairie rivers is unknown. We evaluated the role of hydrological retention for planktonic rotifers, cladocera, and copepods in the Kansas River during the summer of 2004. 4. Zooplankton assemblages were extremely similar among prairie rivers (Sorensen Dissimilarity Index: mean = 0.07) but moderately disparate for comparisons of prairie versus forested‐basin rivers (mean = 0.50). 5. Total zooplankton densities in prairie rivers (approximately 81 L^?1) were intermediate between the Ohio (approximately 92 L^?1) and St Lawrence Rivers (approximately 43 L^?1), but relative abundances were significantly different. Rotifers represented >99% of zooplankton individuals in grassland rivers, but only approximately 37–68% in other rivers. Rotifer species richness was lower in prairie rivers, but relative abundances of common genera were much less skewed compared with eastern rivers where Polyarthra dominated rotifer assemblages (41–73%). 6. For comparisons among rivers, rotifers were significantly more abundant in turbid rivers, while microcrustaceans were less dense. However, for comparisons within the Kansas River over time, rotifer densities were inversely related to turbidity. We hypothesise that rotifers indirectly benefit from river turbidity because their food competitors (cladocera) and predators (e.g. cyclopoid copepods and visually feeding fish) are relatively more susceptible to suspended sediments. 7. Crustacean densities were positively related to the degree of hydrological retention (negatively to current velocities) throughout the study, but rotifer densities were significantly depressed by current velocities only when river discharge was high, making slackwaters that much more valuable. Ephemeral sandbars may not provide sufficient hydrological retention in time and space to sustain viable crustacean populations, but they are adequate to help sustain growth of rotifer populations.     

晋江流域水质污染与浮游动物四季群落结构的关系

报道了2003年2、4、6、8、10和12月在福建晋江流域15个样点进行浮游动物群落生态学调查研究的结果。采用种类组成、种群数量、优势种群、Margalef和Shannon-Wiener多样性指数等参数,初步比较了流域中各不同河段的水质状况,结果显示,流域受污染程度的顺序为东、西溪〈晋江干流〈南、北干渠。东溪、西溪上过度开发的梯级水电站对浮游动物群落结构和水质有明显的影响,加速了水体富营养化进程。     

Impact of a diazinon calamity in 1996 on the aquatic macroinvertebrates in the River Meuse, The Netherlands

A more or less continuous load of pesticides has been noted in the River Meuse in recent years. In April 1996, when high concentrations (up toca. 1 g 1^–1) of the insecticide diazinon were measured in the River Meuse at the Belgian-Dutch border, the maximum concentration for drinking water production was exceeded. This was alerted after activity changes of fish and daphnids in the biological monitoring systems (RIZA). These observations were compared with literature toxicity data of diazinon, in order to determine the ecological impact of this diazinon discharge on the aquatic macroinvertebrates. LC₅₀ values of several aquatic macroinvertebrate species were exceeded. In addition, a high mortality was observed in bioassays with the midgeChironomus riparius and the caddisflyHydropsyche angustipennis. It is inferred that the species composition of the macrofauna community in the River Meuse is likely to be reduced by such an accident. Recolonization of the River Meuse by sensitive macrofauna species may therefore be prevented by incidental pesticide discharges.     

The filtration rate of the zebra mussel Dreissena polymorpha used for water quality assessment in Dutch rivers

The effects of contaminated river water on the filtration rate of zebra mussels from a clean reference site were studied. After a 48-h exposure period to filtered water from the rivers Rhine, Meuse and Amstel (The Netherlands), the filtration rate was measured. It was demonstrated that water from contaminated locations inhibited the filtration rate. Inhibition was higher during low water levels in the rivers Rhine and Meuse than during high water levels, suggesting that contaminants are diluted during high water levels. It is concluded that the shortterm filtration assay with D. polymorpha can be used for assessing water quality.     

Freshwater zooplankton in the Hawkesbury-Nepean River: comparison of community structure with other rivers

The Hawkesbury-Nepean River is a regulated coastal river in New South Wales, Australia. Between March 1992 and March 1993, the freshwater portion of the river was inhabited by a taxonomically rich (total: 116 taxa) and dense (annual mean community density: up to 1024 animals l-1) microzooplankton community, comparable to that in some of the large regulated temperate rivers in the Northern Hemisphere. The common zooplankton taxa in the river were similar to those observed in other rivers at the genus or species level, with a characteristic increase in protists ( Vorticella spp.) towards the downstream reaches of the river. Zooplankton community density in the Hawkesbury-Nepean River was, to some degree, predictable from river environmental variables: density was significantly negatively correlated with river flow rate but positively correlated with temperature, turbidity, conductivity, total phosphorus and chlorophyll a. The results of the present study generally conform to the hypothesis of similar structure of zooplankton communities in rivers, relative to that in lakes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reproduction, Growth, and Migration of Fishes in a Regulated Lowland Tributary: Potential Recruitment to the River Meuse

Many European rivers are characterized by a canalized main channel, steep stony embankments, the absence of aquatic vegetation, regulated flow dynamics, reduced hydrological connectivity to the floodplains and a lack of spawning and nursery areas for many fish species. In such regulated rivers, tributaries may be particularly important for recruitment of fish populations in the main channel. This paper describes the reproduction, growth and migration of fishes in the Everlose Beek, a regulated lowland tributary stream of the river Meuse (The Netherlands), using bi-weekly sampling from January to December 2002. A total of 8615 fishes were caught, belonging to 13 different species. The fish species were classified into three groups, viz., residents, migrants and transients, based on the presence of various life-stages in the tributary. Size-frequency data suggest that each group uses the Everlose Beek differently: (i) Stone loach (Barbatula barbatula), Gudgeon (Gobio gobio) and Three-spined stickleback (Gasterosteus aculeatus) were resident species using the tributary as a spawning, nursery and adult habitat; (ii) Bream (Abramis brama), Roach (Rutilus rutilus), Rudd (R. erythrophthalmus), Tench (Tinca tinca), and Pike (Esox lucius) were migratory species, using the tributary as a spawning area, as well as a nursery habitat during their first year of growth, but migrating towards the river Meuse typically at a length of 5–15 cm; and (iii) Bleak (Alburnus alburnus), Sunbleak (Leucaspius delineatus), Carp (Cyprinus carpio), Crucian carp (Carassius carassius), and Perch (Perca _,fluviatilis) were transient species, characterized by an absence of reproduction, ^.and the occurrence in very low densities of >age-1 juveniles and adults only. Lowland tributaries, such as the Everlose Beek, can contribute to the recruitment of particularly migrant species, hence contributing to fish populations of the regulated river Meuse.     

Distribution and abundance of zooplankton in the Waikato River,New Zealand

Potamoplankton is often a well developed component in large lowland rivers, yet little is known about its structure in New Zealand's longest river, the Waikato River. To redress this gap we sampled bimonthly at seven sites along the length of the river over 12 months. Rotifers were the dominant zooplankton in the Waikato River making up 85% of the total densities. Cladocerans represented 9% and copepods only 6%. Rotifers were also the most taxonomically rich group with 41 species in 20 genera identified throughout the study. Thirty rotifer species and nine genera represent new records for the river – two cladoceran species were also recorded for the first time. The highest densities of crustaceans and rotifers were found in the hydro lakes. Densities of crustaceans decreased with increasing distance downstream and densities of rotifers were on average 15 times greater than crustaceans in the lower river. The seasonality of Crustacea was similar to that in New Zealand lakes and rivers with high densities in summer and minimum densities over the winter period. Total rotifer densities showed a similar trend although there were marked seasonal differences between individual species.     

Order and disorder in the river continuum: the contribution of continuity and connectivity to floodplain meadow biodiversity

Aim Aspects of connectivity and continuity operating in the River Meuse were analysed for their contribution to the biodiversity of the floodplain. From this analysis of the diversity and composition of the meadow communities, we aimed to derive effective biodiversity conservation strategies. Location The River Meuse is one of the larger rivers in the European Western Plains ecoregion. The alluvial plains of the river have a long history of cultivation, and for these plains the floodplain meadow vegetation is a highly appreciated and valuable nature conservation asset. Method We sampled floodplain meadows from 400 km of the six geomorphic reaches of the middle to lower course of the River Meuse. For each, 50 vascular plant relevés were recorded, representing the spectrum of floodplain meadow communities of that reach. Beta diversity was calculated to quantify similarity in species pools between the reaches. A dissimilarity formula was used to determine the turnover between the reaches, and these dissimilarities were compared with a Mantel test to detect whether species composition of the floodplain meadows exhibited connectivity and continuity between the reaches. Species richness for the floodplain vegetation data of the reaches was compared with data for riparian invertebrate communities. The vegetation data sets were ordinated using detrended correspondence analysis (DCA) to reveal patterns in the floodplain meadow species composition, and the DCA axes were related to plant functional groups and population strategies. The axis scores of the species and plots were linked to river and plant species traits. Results We did not observe an overall continuity trend in similarity, nor one in diversity in a downstream direction. Lateral connectivity was highlighted by the dissimilarity between the reaches and in the influxes of species from adjacent ecoregions. The DCA ordination showed statistically significant separations between reaches and between the plant functional groups. The second DCA axis related to the longitudinal gradient of the river, whereas the first axis showed stronger correlations with river and plant species traits. We termed this axis the ‘disorder axis’. Plant invaders and avoiders are located at the left side of the disorder axis, whereas the true river‐adapted categories of resisters and endurers are at the right extremity. Contributions to the disorder were identified in terms of connectivity with adjacent ecoregions and the physical disturbance regime of natural and anthropogenic perturbations, resulting in community changes between the reaches. Main conclusions We concluded that a single overall strategy for biodiversity conservation of the river or its floodplains is not feasible. Strategies can, however, be derived for separate river reaches based on functional groups in the communities, the disorder characteristics of the reach, and the influence of surrounding ecoregions.     

Seasonal dynamics and larval drift of Chironomus riparius (Diptera) in a metal contaminated lowland river

Chironomus riparius is one of the insect species which inhabit polluted rivers in large densities, indicating a high adaptive capacity. Previous studies showed that this capacity is expressed by the occurrence of adapted strains in metal polluted rivers. Differences in life history between metal-exposed and non-exposed midges have been demonstrated in laboratory experiments, and therefore a comparative field study of seasonal dynamics was carried out at two metal polluted sites and one reference site. Just downstream from a massive metal discharge, seasonal dynamics were almost identical to the upstream reference site. Circa four generations per year were produced. Further downstream, lower larval densities were recorded, especially during the second half of the sampling period. The influx of upstream C. riparius larvae into polluted sites was estimated by measuring larval drift just upstream from the point source of metal contamination and indicated a massive input to the standing stock downstream. It is concluded that drift of non-tolerant larvae is dominating the seasonal dynamics of midges downstream. Accordingly, genetic uniformity of chironomids inhabiting upstream and downstream sites is expected most of the time. However, research performed during the last decade, demonstrated that genetically adapted strains of C. riparius may develop at certain stages in the seasonal cycle. However, a stable metal-adapted C. riparius population at the first downstream site, is most likely present on rare occasions only.