Influence of side-arms on aquatic macroinvertebrate drift in the main channel of a large river

1. Drift from lotic and lentic side-arms to the main channel was studied over 2 years in different sections of the Upper Rhône River (France). The hypothesis of an influence of drift on the main channel was tested by measuring drift in the main river at sites downstream and upstream from the confluences with two side-arms, as well as in the two side-arms themselves.
2. Side-arms were differentiated from the main channel by having a typical composition of macrofauna with a high propensity to drift, particularly in spring–summer and during spates. Drift densities in side-arms averaged more than twice those measured in main channel sites, but these inputs did not significantly affect taxon richness and total drift density in the main channel.
3. Nevertheless, some taxa found in the more lotic of the two side-arms were more abundant downstream of the confluence than upstream, indicating that they had drifted into the main channel; no such direct effect was found throughout the sampling period for the more lentic side-arm. The densities of two competitive filter feeders ( Hydropsyche and Simuliidae) were significantly higher below the confluences than above, suggesting that side-arms were also contributing substantial amounts of seston to the main channel.
4. Therefore, side-arms influence the community structure of drifting macroinvertebrates in the Upper Rhône River, both directly through immigration—depending on the degree of connectivity to the river—and indirectly via food supply.     

Recognition of cohorts and growth of larval and juvenile roach Rutilus rutilus (L.), using size-class ordination of developmental steps

Roach larvae and 0+ juveniles were sampled weekly in three different floodplain channels (stagnant, semi-lotic, lotic) of the Upper Rhône River, France, from spring to autumn 1985 using Point Abundance Sampling by electrofishing. Length-frequency analysis by size-class ordination of developmental steps successfully distinguished both overlapping and non-overlapping histograms within the data matrix and revealed the temporal and spatial segregation of a number of cohorts, individuals born at a common site on the same day, which in some cases presented significant differences in standard length within a given developmental step. Despite these initial differences, the various cohorts converged to form one relatively homogeneous cohort around the moment of transition to the juvenile period. Significant differences in standard length were found between populations in the open lotic channel (relatively cooler) and those in the closed, stagnant (relatively warmer) former channel, suggesting that fish growth in isolated ecosystems is influenced by higher or periodically elevated temperatures and by greater levels of resource competition and predation. A similar pattern of differences in length was also observed in 0+ juvenile roach collected from 18 other floodplain channels of the Upper Rhône.     

Disruption of a longitudinal pattern in environmental factors and benthic fauna by a glacial tributary

SUMMARY 1. In the upper Rhône catchment (Swiss Alps), modifications in the longitudinal pattern of environmental conditions and the benthic macroinvertebrate fauna were investigated in a glacier-fed stream (Rhône) at its confluence with a smaller glacier-fed tributary (Mutt) in June, August and September 1998. The distance to the source glacier was greater for the Mutt than for the Rhône.
2. Environmental conditions were harsher for the biota in the main stream upstream of the confluence than in the tributary. The tributary upstream of the confluence was characterised by higher taxonomic richness and abundance of the zoobenthos than the Rhône upstream.
3. Although environmental conditions in the main stream were little modified by the tributary, the fauna was richer and more diverse below the confluence. During the period of ice melt, colonisation from the Mutt led to the occurrence of faunal elements atypical of glacial streams in the main glacial stream upstream of the confluence, where water temperature remains below 4 °C.
4. Although contributing an average of only 10% to the Rhône discharge, the Mutt tributary is suggested to be the faunal driver of the system.     

Relative abundance of macroinvertebrates found in habitats associated with backwater area confluences in Pool 13 of the Upper Mississippi River

Ephemeroptera showed the greatest diversity and Diptera (due mainly to Chaoboridae) the greatest absolute abundance among the macroinvertebrate taxa collected in and near three backwater areas of the Upper Mississippi River from 24 April to 24 August 1983. Of the 32 taxa identified, all were insect larvae, pupae, or adults except for one amphipod and one copepod. Three peaks in total abundance were observed — a major one in late July and August and smaller ones in May and June. These peaks corresponded to periods of high temperatures and low flows. When the macroinvertebrates were divided into groups, depending on whether they were found predominantly in the backwater areas, in the main channel, or in a combination of the two, total densities were greater in the backwater areas than in either of the main-channel habitats. In the two main-channel habitats, densities were slightly greater downstream than upstream from the confluences of the backwaters with the main channel, possibly indicating that macroinvertebrates were drifting out of the backwater areas or that nutrients and zooplankton were drifting out of the backwater areas, increasing productivity in downstream areas.     

Field experiments on the relationship between drift and henthic densities of aquatic insects in tropical streams (Ivory Coast). III Trichoptera

SUMMARY. 1. Based on in situ gutter trials we related the drift of caddis flies to their benthic densities and to various abiotic factors in streams in the Ivory Coast (West Africa). Members of the families Hydropsychidae, Philopotamidae. Hydroptilidae and Leptoceridae were considered in detail.
2. The drift of larvae peaked at night in both early and late larval instars.
3. Drift of a larval group (a certain instar, species or higher taxon) was more often related to the benthic density of other larval groups than to its own benthic density.
4. Self-regulation of an upper benthic density of a larval group by emigration through drift was not statistically evident.
5. There was no straightforward relationship between drift and abiotic factors.
6. Drift rates differed between taxa as well as between larval instars (size groups) within a taxon. Newly hatched larvae had very high drift rates, whereas the last larval instar usually had the lowest drift rate.
7. We related these results to the violently fluctuating discharge of the streams in the study area and the consequent variability of space for lotic insects.
8. Drift estimates, made at the same time as a monitoring programme on possible side-effects of insecticides (Onchocerciasis Control Programme), failed to reflect benthic densities except in the night drift of Hydropsychidae.     

Macroinvertebrate movements in a large European river*

SUMMARY. 1. Quantitative variations in downstream movements of benthic macroinvertebrates were studied in a large European river, the Rhône, upstream from Lyon. Artificial substrates were suspended at three depths in the water column, both near a bunk and in mid-channel, monthly from December 1978 to March 1980. Drift nets were used to determine the diurnal rhythm in drift and to investigate the efficiency of our suspended artificial substrates in capturing the drifting macrofauna. 2. Drift densities (number and biomass) reached a maximum during summer, especially near the river bank, and at night. Mean individual weight of organisms was higher close to the bottom and at night. 3. Artificial substrates were reliable, but underestimated drift by about a quarter in number and a sixth in biomass. compared with drift nets. Two detailed analyses of the drift distribution across the width of the river revealed similar densities along both banks, and uniformity in the channel as a whole. 4. Mean annual drift densities estimated for the section of river were 100 individuals, per 100 m³ and 60 mg dry weight per 100 m³. These densities are similar to those obtained from other temperate rivers.     

Partial migration in a landlocked brown trout population

Population densities of landlocked lake‐migratory brown trout Salmo trutta were estimated in two distinct lotic sections, separated by a lentic segment, in the Greåna River, Sweden, and individual growth and habitat use were monitored for 835 tagged brown trout from September 1998 to June 2000. Residency dominated in the upstream section where density of 0+ and 1+ year brown trout was low and growth rate high. In contrast, >90% of the brown trout that migrated to the lake originated from the downstream section, where density was high and growth rate low. For ≥2+ year individuals, growth rate was similar between the two stream sections, but densities were higher in the upstream than in the downstream section. Lake‐migrants had higher growth rates than non‐migrants (residents) during the autumn of both years. From September to May, migrants increased their body mass by >35%, whereas non‐migrants increased by <5%. Approximately 70% of the brown trout moved <10 m and <2% moved between the two stream sections, indicating that the lentic habitat might function as a barrier for juveniles. Differences in migratory behaviour, density and growth between the upstream and the downstream section might indicate that environmental factors influence the decision to migrate. It cannot be excluded, however, that the observed differences are genetically programmed, selected by migration costs that favour migratory behaviour downstream and residency upstream.     

Ecology of fish spawning and nursery zones in the flood plain,using a new sampling approach

Horizontal zonation of fish reproduction, a lotic-to-lentic succession similar to that seen with increasing stream order, was evident from the relative abundance of larval and 0 + juvenile fishes in three floodplain spawning and nursery areas (lotic, semi-lotic, lentic) of the Upper Rhône River, France. Although the lotic and lentic ecosystems provided similar estimates of standing crop (0 + juveniles), differences were apparent in the reproductive and trophic guild structure of the YOY taxocoenoses at the three sites. A new sampling approach (Random Point-Abundance Sampling and modified electrofishing) is described for early-life fish ecology. The electrofishing method employed is mobile, effective for all sizes of larvae and 0 + juveniles of most species, quantitative, and applicable to a number of freshwater situations; and the punctual data resulting from this sampling approach are comparable both spatially and temporally.     

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.     

The effect of the acanthocephalan parasite (Pomphorhynchus laevis) on the drift of its intermediate host (Gammarus pulex)

SUMMARY 1. The drift of Gammarus pulex in a population in which approximately 20% of adults are infected with the acanthocephalan parasite Pomphorhynchus laevis was monitored at margin and mid-river sites in the River Teme, England over a 24-h period in mid-summer.
2. Drift densities showed a diurnal pattern with a large increase at night, independent of parasite burden.
3. There was no significant effect of site (margin or mid-river) on the proportion of parasitized and unparasitized G. pulex found in the drift or the benthos.
4. The drift of parasitized G. pulex was significantly greater than unparasitized animals. Gammarids harbouring only one parasite were found in significantly higher proportions in the drift than those with two or more parasites.
5. At both sites (margin and mid-river) the proportion of unparasitized adult G. pulex in the drift was significantly lower than that in the benthos. However, there was a significantly higher proportion of parasitized animals in the drift than in the benthos.     

Multi-scale approach to species–habitat relationships: juvenile fish in a large river section

1. Juvenile fish were sampled by electrofishing using the point abundance sampling method from August to November 1991 in the Lower Rhône River. Variations in fish abundance between point samples were partitioned according to three spatial scales of habitat heterogeneity.
2. The compartment scale, differentiating channel and 'dike fields' (an area marginal to the main channel partially enclosed by an artificial embankment), was associated with variation in water current, temperature and biological productivity. Every juvenile fish species studied was more abundant in the dike fields, using them as a nursery.
3. Within both compartments (i.e. main channel and dike fields), the same five bank types were represented, defining the intermediate scale of habitat heterogeneity. In the dike fields, beaches were selected by most of the species, whereas steep silt banks were more often selected in the main channel.
4. After having removed the variation explained at the two larger habitat scales, only depth and shelter were found to have a significant effect on fish abundance at the microhabitat scale.
5. When compared with a classical single-scale approach, the multi-scale approach was found to explain a greater amount of fish abundance variations and to facilitate the interpretation of observed patterns by scaling the species' responses to habitat heterogeneity. The difference between the amounts of variation explained was proposed as a measure of how much habitat selection is hierarchical.     

Multi-scale approach to species–habitat relationships: juvenile fish in a large river section

1. Juvenile fish were sampled by electrofishing using the point abundance sampling method from August to November 1991 in the Lower Rhône River. Variations in fish abundance between point samples were partitioned according to three spatial scales of habitat heterogeneity.
2. The compartment scale, differentiating channel and 'dike fields' (an area marginal to the main channel partially enclosed by an artificial embankment), was associated with variation in water current, temperature and biological productivity. Every juvenile fish species studied was more abundant in the dike fields, using them as a nursery.
3. Within both compartments (i.e. main channel and dike fields), the same five bank types were represented, defining the intermediate scale of habitat heterogeneity. In the dike fields, beaches were selected by most of the species, whereas steep silt banks were more often selected in the main channel.
4. After having removed the variation explained at the two larger habitat scales, only depth and shelter were found to have a significant effect on fish abundance at the microhabitat scale.
5. When compared with a classical single-scale approach, the multi-scale approach was found to explain a greater amount of fish abundance variations and to facilitate the interpretation of observed patterns by scaling the species' responses to habitat heterogeneity. The difference between the amounts of variation explained was proposed as a measure of how much habitat selection is hierarchical.     

Diet partitioning in summer of two syntopic neogobiids from two different habitats of the lower Vistula River, Poland

Monkey goby Neogobius fluviatilis and racer goby N. gymnotrachelus, both native to the Ponto‐Caspian region, were discovered in Polish rivers during the mid‐1990s and currently they are invading the River Vistula. Preliminary studies were carried out on the diet of both species at two sites in the lower River Vistula, one lotic (main channel of the river downstream of the W?oc?awek river reservoir) and the other lentic (left bank, middle section of the W?oc?awek Reservoir). Significant differences were noted in diet between the two species and within species according to habitat, in particular the types of chironomid taxa taken. Over a single diel cycle, monkey goby were captured between 07:00 and 19:00 hours only, feeding more intensively on chironomid larvae than the racer goby, which was captured at most times of day. The wide diet plasticity of these non‐native fish species favours their expansion.     

Using spatial, seasonal, and diel drift patterns of larval Lost River suckers Deltistes luxatus (Cypriniformes: Catostomidae) and shortnose suckers Chasmistes brevirostris (Cypriniformes: Catostomidae) to help identify a site for a water withdrawal structure on the Williamson River, Oregon

A small irrigation diversion dam near Chiloquin, Oregon, was removed and replaced with a pump station to improve fish passage for Lost River suckers (Deltistes luxatus) and shortnose suckers (Chasmistes brevirostris) entering the Sprague River on their spawning migrations. During the developmental phase of the pump station, a need was identified to better understand the larval drift characteristics of these endangered catostomids in order to reduce entrainment into the irrigation system. The spatial, seasonal, and diel distribution of drifting larvae was measured during the 2004 spawning season at two proposed sites on the Williamson River where the pump station could be located. Larval drift for both species coincided with the irrigation season making them subject to entrainment into the irrigation system. Drift occurred almost exclusively at night with larvae entering the drift at sunset and exiting the drift at sunrise. Nighttime larval densities were concentrated near the surface and at midchannel at both sites. Densities were generally greater on the side of mid-channel with greater flow. During early morning sampling we detected a general shift in larval drift from surface to subsurface drift. We also observed an increase in larval densities towards the shore opposite from the proposed pump station at the upper site whereas larval densities remained high at midchannel at the lower site. During daytime sampling, the few larvae that were collected were distributed throughout the water column at both pump sites. This study found that larvae drifting during all time periods were generally distributed further across the cross section, deeper in the water column, and closer to where the proposed water withdrawal structure would be built at the downstream site when compared to the upstream site. Recommendations were provided to locate the withdrawal facility at the upstream site and operate it in a manner such that larval entrainment would likely be minimized.     

Latitudinal variation of diversity in European freshwater animals is not concordant across habitat types

Aim   We analysed the variation of species richness in the European freshwater fauna across latitude. In particular, we compared latitudinal patterns in species richness and β-diversity among species adapted to different habitat types.
Location   Europe.
Methods   We compiled data on occurrence for 14,020 animal species across 25 pre-defined biogeographical regions of European freshwaters from the Limnofauna Europaea . Furthermore, we extracted information on the habitat preferences of species. We assigned species to three habitat types: species adapted to groundwater, lotic (running water) and lentic (standing water) habitats. We analysed latitudinal patterns of species richness, the proportion of lentic species and β-diversity.
Results   Only lentic species showed a significant species–area relationship. We found a monotonic decline of species richness with latitude for groundwater and lotic habitats, but a hump-shaped relationship for lentic habitats. The proportion of lentic species increased from southern to northern latitudes. β-Diversity declined from groundwater to lentic habitats and from southern to northern latitudes.
Main conclusions   The differences in the latitudinal variation of species richness among species adapted to different habitat types are in part due to differences in the propensity for dispersal. Since lentic habitats are less persistent than lotic or groundwater habitats, lentic species evolved more efficient strategies for dispersal. The dispersal propensity of lentic species facilitated the recolonization of central Europe after the last glaciation. Overall, we stress the importance of considering the history of regions and lineages as well as the ecological traits of species for understanding patterns of biodiversity.     

Upstream movements of benthic macroinvertebrates in two different types of fishways in southwestern Germany

Upstream movements and migrations of benthic macroinvertebrates are an important component of running water dynamics. Many lotic organisms perform upstream movements at some stage in their life-cycle. We investigated whether a boulder ramp and a concrete bypass fishway on two streams in Baden-Württemberg, Germany, are successfully used by benthic invertebrates to reach upstream areas. We compared macroinvertebrate distribution at sites upstream and downstream of the fishways, rated the fishways according to their constructional features and conducted migration experiments with marked Gammaridae in the passages. The number of taxa and densities of macroinvertebrates did not significantly differ between upstream and downstream sites in both rivers. Constructional features and the results of the mark-recapture experiments indicated that the boulder ramp allows upstream migrations in principal, whereas the bypass is more difficult for macroinvertebrates to ascend. Both fishways immediately exit into an impoundment, which is a potential barrier for migrating invertebrates. High average current velocities, lack of lenitic banks and sheltered interstices, as well as problems caused by the operation of a power plant are factors explaining why the Enz River bypass is only marginally functional for invertebrates. The investigation concludes that in constructing fishways suitable for upstream movements of lotic invertebrates, more attention should be paid to their specific requirements.     

Quantifying phenotypic gradients in freshwater snails: a case study in Lithasia (Gastropoda: Pleuroceridae)

Many authors have described a pattern of morphological variation in freshwater bivalves where shells taken from lentic and lotic environments, or headwaters and main stem reaches, appear to exhibit phenotypic gradients in size and shape. For example, mussels taken from headwater reaches tend to possess smooth, less inflated shells compared to the more obese, sculptured individuals downstream. Others observed similar relationships in certain freshwater gastropods, but this variation has not been quantified nor its existence explained in an ecological or evolutionary context. Geometric morphometrics indicated freshwater snails shells from the pleurocerid genus Lithasia from the Duck River, Tennessee, USA, show phenotypic gradients similar to those in freshwater mussels. Shells from upstream areas were narrow and less sculptured on the posterior portions of their body whorls, while downstream shells were more inflated and possessed significantly more sculpture. This phenotypic variation may reduce predation or damage due to dislodging. The nature of the observed plasticity suggests an unidirectional environment similar to that proposed by the river continuum concept. Handling editor: K. Martens     

Macroinvertebrate community structure in relation to environmental variables in a Swiss glacial stream

1. Benthic macroinvertebrate distribution was examined in relation to channel characteristics (including stability), substratum, hydraulic variables, primary production (chlorophyll a ) and coarse particular organic matter (CPOM) in an alpine glacial stream, the Mutt (Upper Rhône valley, Switzerland). Co-inertia analysis and canonical correspondence analysis were used to identify the major environmental gradients influencing community variations.
2. The Mutt (length: 3.6 km, altitudinal range: 1800–3099 m a.s.l.) exhibited typical characteristics of a kryal stream. Average summer temperature remained below 2 °C immediately downstream from the snout but was on average 5 °C higher 1700 m downstream. Seasonal variations in water sources were evidenced by the high late-summer (September) contribution of groundwater with increased conductivity.
3. Sixty-six taxa were recorded from the five reaches sampled at three periods (snowmelt, ice melt and low water in late summer) in 1996 and 1997, of which 29 were Chironomidae. Three taxa of Diamesinae were the first colonizers of the stream below the glacier, but 16 taxa, including Ephemeroptera, Plecoptera and Trichoptera, were already recorded 200 m downstream. Water depth, channel slope and Pfankuch's Index of channel stability were strongly correlated with the longitudinal faunal gradient. Maximum temperature, current velocity and water conductivity were also correlated, but to a lesser extent.
4. The rapid incorporation of non-chironomid taxa into the stream community represented a departure from Milner & Petts's (1994) conceptual model of invertebrate succession downstream of glacial margins. The results confirmed that glacial stream communities are primarily driven by physical determinants.     

Geometry of biodiversity patterning: assemblages of benthic macroinvertebrates at tributary confluences

We assessed whether tributaries in upland catchments (=watersheds) affected assemblages of benthic macroinvertebrates in mainstems, as has been reported in northern hemisphere systems. Eight confluences of small to medium streams (stream orders 1–4, 2.2–10.8 m wide) were studied in the Acheron River basin in Victoria, Australia. For each confluence, two transects were sampled at each of five zones relative to the confluence: two zones upstream in the mainstem, one zone upstream in the tributary, one zone at the confluence and one zone downstream in the mainstem. Surveys were conducted in both high-flow and low-flow conditions. In mainstems, there was no change in macroinvertebrate density, taxonomic richness or functional feeding group composition downstream relative to upstream of the confluences. While tributaries statistically had distinctive benthic macroinvertebrate assemblages compared to mainstems, these distinctions were small. In low flows, densities in tributaries were substantially lower than in mainstems, but densities during high flows were more similar (albeit only about one-third as high as in low flow) in tributaries and mainstems. An inverse pattern was evident for taxonomic richness, where richness in tributaries and mainstems was similar in low flows but was greater in mainstems than in tributaries in high flows. We found little evidence of tributary effects in macroinvertebrate assemblages in this basin, which is at odds with some previous results from other continents. To explain this divergence, we suggest a conceptual model outlining factors that control variation in effects of tributaries on assemblages of benthic macroinvertebrates in mainstems.     

Dispersal in drift-prone macroinvertebrates: a case for density-independence

1. Studies of dispersal of macroinvertebrates in streams and rivers tend to be focused on drift, whilst benthic movements are usually considered to be less important.
2. Field-enclosure experiments with the mayfly Baetis rhodani indicate that net dispersal in this species is simply a proportional loss of individuals from the benthos.
3. Neither net upstream or downstream movements exhibited evidence of density-dependence in the form of curvilinear relationships between initial and final densities.
4. The net number of animals moving upstream did not differ significantly from the net number moving downstream.
5. The probable mechanisms behind density-independent dispersal are discussed, as are the implications for our understanding of population dynamics in relation to invertebrate drift.