Shifts in macrophyte species composition as a result of eutrophication and pollution in Dutch transboundary streams over the past decades

The catchment areas of transboundary streams in the Netherlands have been subject to increasing agricultural and industrial activities over the past decades. To evaluate the effects of these activities on the aquatic vegetation, a study has been carried out in 28 Dutch transboundary lowland streams. Recent data on distribution of 58 aquatic plant species and their growth forms were compared with historical data and were correlated with abiotic variables. Most of these streams lost species that are characteristic for streams and are sensitive to turbidity, eutrophication and pollution (e.g. Potamogeton alpinus, P. polygonifolius, P. densus, Ranunculus peltatus ssp. heterophyllus, Callitriche stagnalis and Myriophyllum alterniflorum.) Species, not common in streams but tolerant to turbidity, eutrophication or pollution (e.g. Potamogeton trichoides, Elodea nuttallii) appeared in many streams or increased in abundance. There was also a shift in growth forms: submerged species decreased or were replaced by emergent/floating-leaved species. Correspondence analysis was carried out to study the relation between the observed changes and the abiotic characteristic of the streams. The magnitude of the shift in species composition was positively correlated with the PO₄ ^3- concentration and pH (which was highly correlated with Cd²⁺) of the water. This leads to the hypothesis that increased input of sewage, agricultural and industrial water causes a change in species composition and main growth forms of aquatic plant species in lowland streams.     

Diatom Assemblages and their Associations with Environmental Variables in Oregon Coast Range Streams, USA

The Oregon Coast Range, rich in natural resources, is under increasing pressure from rapid development. The purpose of this study was to examine diatom species patterns in relation to environmental variables in streams of this region. Diatoms, water quality, physical habitat and watershed characteristics were assessed for 33 randomly selected stream sites. Watershed size, elevation, geology, vegetation and stream morphology varied substantially among sites. Streams were characterized by dilute water chemistry and a low percent of fine substrate. A total of 80 diatom taxa were identified. Taxa richness was low throughout the region (median 15, range 10–26). Assemblages were dominated by two adnate species, Achnanthidium minutissimum and Achnanthes pyrenaicum. Diatoms sensitive to organic pollution dominated the assemblages at all sites (median 85%). Non-metric multidimensional scaling (NMDS) and correlational analysis showed quantitative relationships between diatom assemblages and environmental variables. NMDS axes were significantly correlated with watershed area, watershed geology, conductivity, total nitrogen, total solids and stream width. Diatom-based site classification (Two-way Indicators Species Analysis, (TWINSPAN)) yielded 4 discrete groups that displayed weak correlations with environmental variables. When stream sites were classified by dominant watershed geology, overall diatom assemblages between groups were significantly different (Analysis of Similarity (ANOSIM) global R = 0.19, p < 0.05). Our results suggest that streams in the coastal region are in relatively good condition. High natural variability in stream conditions in the Oregon Coast Range ecoregion may obscure quantitative relationships between environmental variables and diatom assemblages. A bioassessment protocol that classifies sites by major landscape variables and selects streams along the major human disturbance gradient might allow for detection of early signs of human disturbance in environmentally heterogeneous regions, such as the Pacific Northwest.     

Abundance of Three Bacterial Populations in Selected Streams

The population sizes of three bacterial species, Acinetobacter calcoaceticus, Burkholderia cepacia, and Pseudomonas putida, were examined in water and sediment from nine streams in different parts of the United States using fluorescent in situ hybridization (FISH). Population sizes were determined from three sites (upstream, midstream, and downstream) in each stream to compare differences in the occurrence and distribution of the species within each stream and among streams. Physical and chemical variables measured reflected differences in environmental conditions among the streams. In the water, B. cepacia numbers were highest in the agricultural, Iowa stream. P. putida numbers were highest in the southern coastal plain streams, Black Creek (GA) and Meyers Branch (SC). Compared to the other two species, the abundance of A. calcoaceticus was similar in all the streams. In sediment, the greatest abundance of all three species was found in the Iowa stream, while the lowest was in Hugh White Creek (NC). Detrended correspondence analysis (DCA) explained 95.8% and 83.9% of the total variation in bacterial numbers in water and sediment of the streams, respectively. In sediments and water, B. cepacia numbers were related to nitrate concentrations. A. calcoaceticus in water clustered with several environmental variables (i.e., SRP, pH, and conductivity) but benthic populations were less well correlated with these variables. This study reveals the potential influence of various environmental conditions on different bacterial populations in stream communities.     

Limnological Features of Different Acidic Drinking Water Reservoirs in the Erzgebirge (Germany)

The biological and chemical investigations of three drinking water reservoirs differing in their acidic and trophic conditions (neutral/meso-eutrophic; circumneutral/oligotrophic; acid/oligomesotrophic) and of some of their tributaries in the Erzgebirge (Germany) are presented. It is shown that the chemistry of acidic waters is very different from circumneutral ones, particularly with respect to hardness, acid neutralizing capacity, pH and aluminium. As acidity in running and stagnant waters increases, the filtrable aluminium concentration also rises. A consequence of the changed chemical and biological conditions and their direct and indirect effects on the organisms is an overall decrease in the number of species (despite the fact that also some species newly appear in the acidified waters). Furthermore, a loss of food web complexity is observed and the trophic structure changes, e.g. fish have disappeared and invertebrate predators have become more numerous. In the acidic reservoir, species well adapted to waters low in pH and hardness (e.g. Synura sphagnicola) are more abundant and commonly benthic animals (e.g. Chydorus sphaericus) are able to extend their habitat to the open waters.     

The Influence of Flowing Water on the Resource Pursuit-Risk Avoidance Tradeoff in the Crayfish Orconectes virilis

The influence of hydrodynamics on chemically mediated behavioral tradeoffs has received little attention. We tested the hypothesis that individuals of the crayfish Orconectes virilis would be more sensitive to chemical cues in flowing water than in still water. Orconectes virilis is a good subject for this test, because it is found in both still water (e.g. ponds), and flowing water (e.g. rivers). A factorial design was used, with two stimulus treatments and two habitat types. Crayfish were exposed to either food cue or food + alarm cue in either still water or flowing water in an artificial stream arena. Habitat use and activity were significantly influenced by stimulus treatment, with more time spent away from the stimulus source and less activity in the food + alarm treatment than in the food treatment. Neither habitat type nor the interaction of stimulus treatment and habitat type had a significant effect on the response variables. Given the natural history of O. virilis, we suggest that selection has favored the ability to equally utilize chemical cues in both still and flowing water. We acknowledge that different flow conditions may influence chemical ecology in this species and caution against the view that tests in flowing waters necessarily provide a more accurate approximation of natural responses.     

Macroinvertebrate stream communities along regional and physico-chemical gradients in Western Greenland

1. Macroinvertebrates were collected and physico‐chemical variables measured at 16 stream sites in Western Greenland during July 1999. Eight sites were located on Disko Island in an arctic oceanic climate and eight sites in the Kangerlussuaq area close to the icecap where the climate is arctic continental. The streams had different water sources (glacial, groundwater, snowmelt and lake water). 2. The streams showed pronounced differences in water temperature (2.2–17.3 °C), concentrations of suspended solids (0–2400 mg L^?1), and conductivity (10–109 μS cm^?1). Principal component analysis (PCA) analysis of the physico‐chemical variables separated the Disko Island sites into a distinct group, whereas the sites in the Kangerlussuaq area were more dispersed. 3. A total of 56 macroinvertebrate species were found, including 31 species of Chironomidae, the most abundant of which was Orthocladius thienemanni. Diamesa sp. was only the sixth most abundant chironomid taxon. Species composition varied between sites, and abundance varied from about 20 individuals m^?2 in a glacier fed stream to more than 16 000 m^?2 in a lake outlet. 4. The macroinvertebrate communities of the 16 streams were separated into five TWINSPAN groups reflecting water source, irrespective of region. Lake outlets and ground‐water‐fed streams had the highest species richness and abundance, temperature and bed stability, while glacier‐fed streams were characterized by low species richness, abundance, temperature, bed stability and high concentrations of suspended solids. Macroinvertebrate species richness was positively correlated with water temperature and negatively with bed stability. Conductivity was positively correlated with invertebrate abundance. 5. The results of this study suggest that the source of stream water can be used to predict invertebrate community composition in Greenlandic streams and thus the effects of changes in water balance and flow regime, and to identify sites of special conservation interest.     

Global climate change in large European rivers: long‐term effects on macroinvertebrate communities and potential local confounding factors

Aquatic species living in running waters are widely acknowledged to be vulnerable to climate‐induced, thermal and hydrological fluctuations. Climate changes can interact with other environmental changes to determine structural and functional attributes of communities. Although such complex interactions are most likely to occur in a multiple‐stressor context as frequently encountered in large rivers, they have received little attention in such ecosystems. In this study, we aimed at specifically addressing the issue of relative long‐term effects of global and local changes on benthic macroinvertebrate communities in multistressed large rivers. We assessed effects of hydroclimatic vs. water quality factors on invertebrate community structure and composition over 30 years (1979–2008) in the Middle Loire River, France. As observed in other large European rivers, water warming over the three decades (+0.9 °C between 1979–1988 and 1999–2008) and to a lesser extent discharge reduction (?80 m³ s^?1) were significantly involved in the disappearance or decrease in taxa typical from fast running, cold waters (e.g. Chloroperlidae and Potamanthidae). They explained also a major part of the appearance and increase of taxa typical from slow flowing or standing waters and warmer temperatures, including invasive species (e.g. Corbicula sp. and Atyaephyra desmarestii). However, this shift towards a generalist and pollution tolerant assemblage was partially confounded by local improvement in water quality (i.e. phosphate input reduction by about two thirds and eutrophication limitation by almost one half), explaining a significant part of the settlement of new pollution‐sensitive taxa (e.g. the caddisfly Brachycentridae and Philopotamidae families) during the last years of the study period. The regain in such taxa allowed maintaining a certain level of specialization in the invertebrate community despite climate change effects.     

The role of oligochaetes in the management of waters

To better understand the role of an aquatic oligochaete in the management of water quality, it was determined if oligochaetes were representative of water quality, if indices based on oligochaetes were widely applicable to access water quality, and if oligochaete assemblages were related to environmental factors other than organic pollution. It was found that if only one monitoring sample a year was taken in December, about 73% of the more common species were collected. Seasonality did not seem to affect the taxa collected except for forms in low presence, e.g., Nais. Several indices we used were not applicable to regulated streams. Oligochaete ordination procedures revealed only a partial picture of the environmental conditions of regulated streams.     

The endemic headwater stream amphibians of the American Northwest: associations with environmental gradients in a large forested preserve

We used a large forested preserve (Olympic National Park, USA) to examine the habitat associations of a unique and environmentally sensitive stream amphibian fauna: Ascaphus truei Stegneger, Rhyacotriton olympicus (Gaige) and Dicamptodon copei Nussbaum. We quantified the relative abundance of stream amphibians and compared them to physical, topographic, climatic and landscape variables. All three species were associated with the south‐west to north‐east climate gradient, tending to be most abundant in the south‐west. Although a habitat generalist relative to the other two species, Dicamptodon copei was absent from the north‐eastern portion of the park. Ascaphus truei and Rhyacotriton olympicus were both associated with coarse substrates and steep gradients. Unlike studies in harvested forests, all stream amphibians were common in waters with unconsolidated surface geology (e.g. marine sediments that erode easily). Studies of ecological preserves can provide an important baseline for evaluating species associations with environmental gradients and can reveal patterns not evident in more disturbed landscapes.     

黄山陈村水库上游河源溪流的鱼类群落及其纵向梯度格局

确定鱼类群落的分布格局及其对人类活动的响应,是合理保护、恢复和管理鱼类多样性的基础。基于2011年5月和10月自黄山陈村水库上游3条河源溪流共39个样点的调查数据,比较研究了溪流间鱼类群落及其纵向梯度格局的异同,着重探讨了人类活动对溪流鱼类群落纵向梯度格局的影响。研究结果显示,同人为干扰较轻的舒溪相比,人为干扰严重的浦溪和麻溪中水宽、底质和植被覆盖率等局域栖息地条件显著变化,这造成了后者的鱼类多样性显著下降及物种组成的显著变化,主要表现为敏感性的地方物种(如宽鳍鱲、光唇鱼、原缨口鳅等)数量减少、耐受性的广布物种(如泥鳅、麦穗鱼、高体鰟鲏等)数量增多。舒溪的鱼类物种数及其组成均与海拔显著相关,但这种"海拔-鱼类群落"关系在麻溪和浦溪中削弱甚至消失。底质、植被覆盖率对舒溪鱼类群落具有重要影响,但对浦溪和麻溪鱼类群落却无显著影响。研究结果表明,在子流域空间尺度上,诸如城镇化发展、土地利用、河道治理等人类活动可通过对局域栖息地条件的影响,导致溪流鱼类多样性下降及其物种组成的变化,破坏鱼类群落的纵向梯度格局,并改变栖息地与鱼类群落之间的联系。     

Do Contaminants Originating from State-of-the-Art Treated Wastewater Impact the Ecological Quality of Surface Waters?

Since the 1980s, advances in wastewater treatment technology have led to considerably improved surface water quality in the urban areas of many high income countries. However, trace concentrations of organic wastewater-associated contaminants may still pose a key environmental hazard impairing the ecological quality of surface waters. To identify key impact factors, we analyzed the effects of a wide range of anthropogenic and environmental variables on the aquatic macroinvertebrate community. We assessed ecological water quality at 26 sampling sites in four urban German lowland river systems with a 0–100% load of state-of-the-art biological activated sludge treated wastewater. The chemical analysis suite comprised 12 organic contaminants (five phosphor organic flame retardants, two musk fragrances, bisphenol A, nonylphenol, octylphenol, diethyltoluamide, terbutryn), 16 polycyclic aromatic hydrocarbons, and 12 heavy metals. Non-metric multidimensional scaling identified organic contaminants that are mainly wastewater-associated (i.e., phosphor organic flame retardants, musk fragrances, and diethyltoluamide) as a major impact variable on macroinvertebrate species composition. The structural degradation of streams was also identified as a significant factor. Multiple linear regression models revealed a significant impact of organic contaminants on invertebrate populations, in particular on Ephemeroptera, Plecoptera, and Trichoptera species. Spearman rank correlation analyses confirmed wastewater-associated organic contaminants as the most significant variable negatively impacting the biodiversity of sensitive macroinvertebrate species. In addition to increased aquatic pollution with organic contaminants, a greater wastewater fraction was accompanied by a slight decrease in oxygen concentration and an increase in salinity. This study highlights the importance of reducing the wastewater-associated impact on surface waters. For aquatic ecosystems in urban areas this would lead to: (i) improvement of the ecological integrity, (ii) reduction of biodiversity loss, and (iii) faster achievement of objectives of legislative requirements, e.g., the European Water Framework Directive.     

Variation in responses to spawning Pacific salmon among three south-eastern Alaska streams

1. Pacific salmon are thought to stimulate the productivity of the fresh waters in which they spawn by fertilising them with marine‐derived nutrients (MDN). We compared the influence of salmon spawners on surface streamwater chemistry and benthic biota among three south‐eastern Alaska streams. Within each stream, reaches up‐ and downstream of barriers to salmon migration were sampled during or soon after spawners entered the streams. 2. Within streams, concentrations of dissolved ammonium and soluble reactive phosphorus (SRP), abundance of epilithon (chlorophyll a and ash‐free dry mass) and biomass of chironomids were significantly higher in reaches with salmon spawners. In contrast, biomass of the mayflies Epeorus spp. and Rhithrogena spp. was significantly higher in reaches lacking spawners. 3. Among streams, significant differences were found in concentrations of dissolved ammonium, dissolved organic carbon, nitrate and SRP, abundance of epilithon, and the biomass of chironomids and Rhithrogena. These differences did not appear to reflect differences among streams in spawner density, nor the changes in water chemistry resulting from salmon spawners. 4. Our results suggest that the ‘enrichment’ effect of salmon spawners (e.g. increased streamwater nutrient concentrations) was balanced by other concurrent effects of spawners on streams (e.g. sediment disturbance). Furthermore, the collective effect of spawners on lotic ecosystems is likely to be constrained by conditions unique to individual streams, such as temperature, background water chemistry and light attenuation.     

Diatoms and water quality in lowland streams in the province of northern Brabant (The Netherlands)

Summary The diatoms of two Dutch lowland streams were studied. The stream Beerze is heavily polluted by the effluent of a sewage treatment plant, causing a change in the pH from less than 4 to more than 6. The acidobiontic diatomEunotia exigua is replaced by more alkaline and eutraphentic species such asGomphonema parvulum, Cyclotelia meneghiniana andFragilaria capucina. The Simpson diversity index rises from 0.2 in the upper course to 0.9 in the middle course and maintains this level in the lower course.From stream Rosep samples of 1919–1930 and recent ones were studied. This stream is polluted now by agriculture. In the earlier samples oligo-mesotraphentic species dominated (Eunotia spp.,Melosira distans). In 1976 the species of organically polluted waters were most abundant. The oligo-mesotraphentic species were hardly recovered.In spite of the severe pollution the number of rare species in both streams is still high, compared with that in stagnant waters. Only in mesotrophic moorland pools and mesotrophic quivering bogs, which are situated in trophic gradients just like both streams, a comparable number of rare species may be found.     

Niche partitioning and the effect of interspecific competition on microhabitat use by two sympatric galaxiid stream fishes

1. Numerous interacting abiotic and biotic factors influence niche use and assemblage structure of freshwater fishes, but the strength of each factor changes with spatial scale. Few studies have examined the role of interspecific competition in structuring stream fish assemblages across spatial scales. We used field and laboratory approaches to examine microhabitat partitioning and the effect of interspecific competition on microhabitat use in two sympatric stream fishes (Galaxias‘southern’ and Galaxias gollumoides) at large (among streams and among sites within streams) and small (within artificial stream channels) spatial scales. 2. Diurnal microhabitat partitioning and interspecific competition at large spatial scales were analysed among three sympatry streams (streams with allotopic and syntopic sites; three separate catchments) and four allopatry streams (streams with only allotopic sites; two separate catchments). Electro‐fishing was used to sample habitat use of fishes at 30 random points within each site by quantifying four variables for each individual: water velocity, depth, distance to nearest cover and substratum size. Habitat availability was then quantified for each site by measuring those variables at each of 50 random points. Diet and stable isotope partitioning was analysed from syntopic sites only. Diel cycles of microhabitat use and interspecific competition at small spatial scales were examined by monitoring water velocity use over 48 h in artificial stream channels for three treatments: (i) allopatric G. ‘southern’ (10 G. ‘southern’); (ii) allopatric G. gollumoides (10 G. gollumoides) and (iii) sympatry (five individuals of each species). 3. One hundred and ninety‐four G. ‘southern’ and 239 G. gollumoides were sampled across all seven streams, and habitat availability between the two species was similar among all sites. Galaxias‘southern’ utilised faster water velocities than G. gollumoides in both the field and in channel experiments. Both species utilised faster water velocities in channels at night than during the day. Diet differences were observed and were supported by isotopic differences (two of three sites). No interspecific differences were observed for the other three microhabitat variables in the field, and multivariate habitat selection did not differ between species. Interspecific competition had no effect on microhabitat use of either species against any variable either in the field (large scale) or in channels (small scale). 4. The results suggest that niche partitioning occurs along a subset of microhabitat variables (water velocity use and diet). Interspecific competition does not appear to be a major biotic factor controlling microhabitat use by these sympatric taxa at any spatial scale. The results further suggest that stream fish assemblages are not primarily structured by biotic factors, reinforcing other studies de‐emphasising interspecific competition.     

Diversity of aquatic hyphomycetes in the aquatic ecosystems of the Western Ghats of India

Relationships between physico-chemical factors of waters, riparian vegetation, altitude and species richness of the hyphomycete communities in six rivers and a sulfur spring in the Western Ghats of Karnataka were analysed statistically. Linear regression and correlation between log fungal species and riparian vegetation was highly significant. The similarity in the fungal diversity between different rivers and the sulphur spring was evaluated by Sorensen's index. Generally, the indices of similarity in the mycoflora between the streams were high. However, in the streams of Lakya, Bhadra, Ranganathittu bird sanctuary and the sulfur spring the indices of similarity were low compared with the rest of the water systems. The low pH, low oxygen concentration, hardness, high iron, high temperature and sulfide contents of the waters of these streams are thought to be the reasons for the differences in these streams. Anguillospora longissima, Helicomyces roseus, Lunulospora curvula, Triscelophorus monosporus and Wiesneriomyces laurinus were found to be the most abundant species.     

Demonstration of migration of juvenile cyprinid fish using parasite biomarkers

Migration of juvenile (0+) cyprinid fish from the Kiso River to experimental streams in the Aqua Restoration Research Center was demonstrated by the presence of parasites on fish. The experimental streams were located a minimum of 0.7 km from the main reach of the Kiso River, and they were connected via a tributary (Shinsakai River). The experimental streams were used as a spawning site and a nursery for juveniles, similar to the use of a temporary water body such as a paddy field. A digenean trematode (Centrocestus spp.) and the glochidia of unionid bivalves, both of which did not infect fish in the experimental streams or in the Shinsakai River, were used as marker parasites. Their presence indicated that the fish moved in the experimental streams from the Kiso River. For both fish species (Gnathopogon elongatus and Zacco platypus) assessed in this study, juveniles greater than 20–30 mm standard length could migrate from the Kiso River. This study showed that temporary waters are important as a nursery for juvenile fish, for whether or not a species spawns there. For the conservation of fish populations, it is important that temporary waters that are used as a nursery are connected with a permanent water area by a channel, through which not only adults, but also juvenile fish can migrate.     

Shifts in leaf litter breakdown along a forest–pasture–urban gradient in Andean streams

Tropical montane ecosystems of the Andes are critically threatened by a rapid land‐use change which can potentially affect stream variables, aquatic communities, and ecosystem processes such as leaf litter breakdown. However, these effects have not been sufficiently investigated in the Andean region and at high altitude locations in general. Here, we studied the influence of land use (forest–pasture–urban) on stream physico‐chemical variables (e.g., water temperature, nutrient concentration, and pH), aquatic communities (macroinvertebrates and aquatic fungi) and leaf litter breakdown rates in Andean streams (southern Ecuador), and how variation in those stream physico‐chemical variables affect macroinvertebrates and fungi related to leaf litter breakdown. We found that pH, water temperature, and nutrient concentration increased along the land‐use gradient. Macroinvertebrate communities were significantly different between land uses. Shredder richness and abundance were lower in pasture than forest sites and totally absent in urban sites, and fungal richness and biomass were higher in forest sites than in pasture and urban sites. Leaf litter breakdown rates became slower as riparian land use changed from natural to anthropogenically disturbed conditions and were largely determined by pH, water temperature, phosphate concentration, fungal activity, and single species of leaf‐shredding invertebrates. Our findings provide evidence that leaf litter breakdown in Andean streams is sensitive to riparian land‐use change, with urban streams being the most affected. In addition, this study highlights the role of fungal biomass and shredder species (Phylloicus; Trichoptera and Anchytarsus; Coleoptera) on leaf litter breakdown in Andean streams and the contribution of aquatic fungi in supporting this ecosystem process when shredders are absent or present low abundance in streams affected by urbanization. Finally, we summarize important implications in terms of managing of native vegetation and riparian buffers to promote ecological integrity and functioning of tropical Andean stream ecosystems.     

The ecology of Potamogeton species in central North America

The distribution of 17 Potamogeton species was examined at 430 sites in central North America with respect to water body type, bottom substrate and 8 water chemistry parameters (pH, total dissolved solids, total alkalinity, chloride, sulphate, phosphorus, combined nitrate and nitrite, and dissolved organic matter). All except 2 of the species showed statistically significant distributions with respect to some of the environmental parameters examined. Each species occupied a different combination of chemical parameter ranges. The species formed a distribution spectrum with respect to water chemistry, with P. epihydrus and P. obtusifolius frequenting low inorganic concentrations at one end of the scale, and P. filiformis, P. pectinatus and P. vaginatus at the other end, these species characterized by high mean inorganic values and occurring most often in saline or alkaline habitats. While 41 significant positive interspecific associations were found within the genus in lentic waters, only 4 were apparent in lotic habitats. Potamogeton species richness (PR) was related to water body and bottom type, with lakes and sand bottoms showing the highest mean PR values. Stepwise multiple regression analysis showed that, in the area as a whole, the water chemistry parameters examined accounted for <25% of the observed variability in PR. The relative importance of individual chemical parameters in relation to PR varied in different water body types.     

Half a century of oligochaete research in Estonian running waters

About 51500 specimens from 1542 samples, collected over the years 1954–1975 and 1986–1999 in different running water bodies throughout Estonia, were identified. Tubificidae prevailed in the material, with Limnodrilus hoffmeisteri forming about 40%. This species was followed by the tubificids Tubifex tubifex, Potamothrix hammoniensis, Psammoryctides barbatus, L. udekemianus and Spirosperma ferox, the naidid Stylaria lacustris, and the lumbriculid Stylodrilus heringianus. Two main ecological assemblages were distinguished: the pelophilous assemblage, dominated by L. hoffmeisteri, and the psammophilous one, where usually P. barbatus was dominant. The relationships between different species and the chemical parameters of water were usually weak but in contrast, correlated well with sediment preferences. In organically enriched reaches, L. hoffmeisteri usually dominated. The fauna of the streams of the islands was poorer in species due to their small size rather than geographical isolation. Some recent antropochorous Ponto-Caspian invaders have only reached the lowermost reaches of the two largest rivers. Some brackish water species were found in the mouth of the Pärnu River. No essential differences were found between the comparable sets of oligochaete samples collected in 1954–1975 and 1987–1997 in the Estonian running waters.     

The influence of predatory fish on mayfly drift: extrapolating from experiments to nature

1. A knowledge of how individual behaviour affects populations in nature is needed to understand many ecologically important processes, such as the dispersal of larval insects in streams. The influence of chemical cues from drift‐feeding fish on the drift dispersal of mayflies has been documented in small experimental channels (i.e. < 3 m), but their influence on dispersal in natural systems (e.g. 30 m stream reaches) is unclear. 2. Using surveys in 10 Rocky Mountain streams in Western Colorado we examined whether the effects of predatory brook trout (Salvelinus fontinalis) on mayfly drift, that were apparent in stream‐side channels, could also be detected in natural streams. 3. In channel experiments, the drift of Baetis bicaudatus (Baetidae) was more responsive to variation in the concentration of chemical cues from brook trout than that of another mayfly, Epeorus deceptivus (Heptageniidae). The rate of brook trout predation on drifting mayflies of both species in a 2‐m long observation tank was higher during the day (60–75%) but still measurable at night (5–10%). Epeorus individuals released into the water column were more vulnerable to trout predation by both day and night than were Baetis larvae treated similarly. 4. Drift of all mayfly taxa in five fishless streams was aperiodic, whereas their drift was nocturnal in five trout streams. The propensity of mayflies to drift was decreased during the day and increased during the night in trout streams compared with fishless streams. In contrast to the channel experiments, fish biomass and density did not alter the nocturnal nature nor magnitude of mayfly drift in natural streams. 5. In combination, these results indicate that mayflies respond to subtle differences in concentration of fish cues in experimental channels. However, temporal and spatial variation in fish cues available to mayflies in natural streams may have obscured our ability to detect responses at larger scales.