The influence of two differently sized dams on mussel assemblages and growth

Dams have been shown to impact freshwater mussels. We examined how mussels respond to differently sized dams (18 vs. 4 m) on the St. Croix River and its tributary, the Sunrise River. We hypothesized that: mussel density and growth rate would be greater downstream of the smaller dam due to the relatively greater food subsidies and temperature effects of the reservoir above it; and the effects of the small dam would moderate downstream as the localized impacts of the dam were reduced. We quantitatively sampled mussels upstream and downstream of the dams. For a common species, Actinonaias ligamentina, we ascertained growth rates by measuring successive growth rings. The highest mussel richness and diversity were upstream and downstream of the large dam. Higher mussel density was found immediately below the small dam but declined downstream. A. ligamentina downstream of the small dam grew faster and were larger than individuals in other reaches. Food availability and temperature appeared to influence mussel density and growth rate for A. ligamentina downstream of the small dam. This study provides information that may help managers decide whether to remove small dams or to maintain them because of the unique mussel habitats below these structures.     

Response of Unionid Mussels to Dam Removal in Koshkonong Creek, Wisconsin (USA)

Dam removal is a potentially powerful tool for restoring riverine habitats and communities. However, the effectiveness of this tool is unknown because published data on the effects of dam removal on in-stream biota are lacking. We investigated the effects of a small dam removal on unionid mussels in Koshkonong Creek, Wisconsin (USA). Removal of the dam led to mortality both within the former impoundment and in downstream reaches. Within the former reservoir, mortality rates were extremely high (95&p e r c n t;) due to desiccation and exposure. Mussel densities in a bed 0.5km downstream from the dam declined from 3.80±0.56 musselsm^?2 in fall 2000 immediately after dam removal to 2.60±0.48 musselsm^?2 by summer 2003. One rare species, Quadrula pustulosa, was lost from community. Mortality of mussels buried in deposited silt was also observed at a site 1.7km below the dam. Silt and sand increased from 16.8 and 1.1% of total area sampled in fall 2000 to 30.4 and 15.9%, respectively, in summer 2003. Total suspended sediment concentrations in the water column were always higher downstream from the reservoir than upstream, suggesting that transport and deposition of reservoir sediments likely contributed to downstream mussel mortality. Thus, while benefits of the dam removal included fish passage and restoration of lotic habitats in the former millpond, these changes were brought about at some cost to the local mussel community. Pre-removal assessments of potential ecological impacts of dam removal and appropriate mitigation efforts should be included in the dam removal process to reduce short-term negative ecological effects of this restoration action.     

Patternizing of impoundment impact (1985–2002) on fish assemblages in a lowland river using the Kohonen algorithm

Impoundment impact on fish assemblage structure was investigated in the dammed middle course of the Warta River. A backwater site (AB) was located 2 km upstream of the Jeziorsko Reservoir, and a tailwater site (CD) 1.5 km downstream of the dam. Both sites were studied for 3 years in the pre‐impoundment period (1985–1987) and 15 years after damming (1988–2002). Quantitative electrofishing in spring and autumn assured obtaining yearly average biomass for each population. Most of the data analysis aimed to assess the dam impact on the fish assemblage structure but other accompanying impacts such as discharge manipulations, revetment, different forms of engineering, and water quality improvement in the tailwater and backwater reaches were also discussed. The Kohonen algorithm (self‐organizing map, SOM) was used for the analysis, and perfectly separated AB and CD samples into two clusters. Samples from the backwater (AB) proved that this reach of the Warta River had maintained its almost natural character and that fish assemblages had changed moderately, now occupying only five neighbouring hexagons out of a total of 16. In the tailwater (CD), however, because of considerable fluctuations in fish assemblages the SOM produced three subclusters, which engaged nine hexagons: (i) the pre‐impoundment period (1985–1987, two hexagons); (ii) 7 years after the definite closure of dam sluices (1988–1994, five hexagons); and (iii) the past 8 years of sampling (1995–2002, two hexagons), when stabilization in the assemblage was observed. The SOM also definitely proved profound changes in fish assemblage composition: most lithophilous species declined and many phytolithophilous and phytophilous species became dominants, particularly in the tailwater site where downstream migration of 0+ of successfully spawned species from the reservoir took place.     

Influences of low-head dams on the fish assemblages in the headwater streams of the Qingyi watershed, China

The influences of low-head dams on the fish assemblages were examined in this study, using fish data collected in six treatment and five reference sites at three low-head dams in the headwater streams of the Qingyi watershed, China. Comparing with those in the reference sites, local habitat variables were significantly altered by low-head dams in the treatment sites, involving wider channel (only in the impoundment area), deeper water and slower flow. Fish species richness varied significantly across seasons, not across site categories, suggesting that these low-head dams did not alter species richness. However, significant decreases in fish abundance and density were observed in the impoundment areas immediately upstream of dams, but not in the plunge areas downstream. Fish assemblage structures kept relative stability across seasons, and their significant difference between-site was only observed between the impoundment areas and the sites far from dams upstream. This variation in assemblage structures was due to the differing relative abundance of some co-occurring species; more lentic but less lotic fish was observed in the impoundment areas. The spatial and temporal patterns of fish assemblages were correlated with local habitat in this study area. Wetted width had negative correlation with fish species richness, abundance and density, respectively. Water temperature also positively affected species richness. In addition, wetted width, water depth, current velocity and substrate were the important habitat variables influencing assemblage structures. Our results suggested that, by modifying local habitat characteristics, low-head dams altered fish abundance and density in the impoundment areas immediately upstream of dam, not in the plunge areas immediately downstream, and thereby influenced fish assemblage structures in these stream segments.     

Benthic invertebrate assemblage change following dam removal in a Wisconsin stream

Small dams (height <10 m) have transformed stream networks across the United States. Shopiere Dam was removed from Turtle Creek, a fourth order stream in Southeastern Wisconsin in the fall of 1999. We sampled three sites (upstream of the impoundment, immediately below the dam, and farther downstream) before and after dam removal to identify changes in the invertebrate assemblage following removal. Prior to removal, upstream and downstream sites had similar taxonomic composition. In contrast the dam site had more taxonomic variation. The upstream, dam and downstream sites responded differently to dam removal in analyses of diversity, functional feeding groups, and invertebrate composition. Upstream at the reference site, changes in functional feeding group composition appeared to be associated with a decrease in silt coverage. At the dam site, taxonomic composition changed following dam removal, however diversity and functional feeding groups remained similar. At the downstream site, the invertebrate assemblage remained similar in all analyses. Our observations indicate that the effects of dam removal were not uniform through the stream, rather each site responded in a different way.     

Evaluating the influence of land use,drought and reach isolation on the occurrence of freshwater mussel species in the lower Flint River Basin,Georgia (U.S.A.)

1. North American freshwater mussels have been subjected to multiple stressors in recent decades that have contributed to declines in the status and distribution of many species. However, considerable uncertainty exists regarding the relative influence of these factors on observed population declines. 2. We used an occupancy modelling approach to quantify relationships between mussel species occurrence and various site‐ and catchment‐level factors, including land cover, stream size, the occurrence of drought and reach isolation due to impoundment for 21 mussel species native to the lower Flint River Basin, Georgia, U.S.A. 3. Our modelling approach accounted for potential biases associated with both incomplete detection and misidentification of species, which are frequently not accommodated as sources of bias in freshwater mussel studies. 4. Modelling results suggested that mussel species were, on average, four times less likely to be present following severe drought, but the negative effects of drought declined rapidly with increasing stream size. Similarly, mussel species were 15 times less likely to occupy small streams that were isolated from mainstem tributaries by impoundments. 5. This study provides insight into the effects of natural and anthropogenic factors on freshwater mussel species. Our findings add to a growing body of literature aimed at improving understanding of the predominant factors influencing freshwater mussel populations and fostering the development of more informed and effective conservation strategies.     

Analysis of Naturalization Alternatives for the Recovery of Moist-soil Plants in the Floodplain of the Illinois River

The hydrologic regime of the Illinois River has been substantially altered by floodplain levees, navigation dams, and water diversion. Unnaturally frequent and untimely water level fluctuations, large and small, have decreased the productivity of many floodplain vegetation communities that provide important ecological services, including the moist-soil plant community. We simulated three scenarios, including two that were expected to benefit moist-soil plants: (1) existing conditions, with levees and navigation dams closed during the summer growing season; (2) levees opened to reconnect the river and its floodplain during the growing season; and (3) both the downstream navigation dam and the levees opened during the growing season. A 1-dimensional hydraulic model generated daily hydrographs of the river at three positions in the 135 km study reach: (1) near the downstream dam, (2) in the middle of the reach, and (3) near the upstream dam. These hydrographs then were used to run a model that predicts the growth of moist-soil plants at a range of floodplain elevations. As expected, the model predicted that plants would grow over a larger area with levees open during the growing season than under the existing conditions, but the outcomes showed a strong location dependency. Moist-soil plant production would increase in the upper and mid-reach locations, but there would be no change near the downstream dam despite opening the levees. Modelling revealed that the existing operation of the navigation dam permanently floods most of the floodplain zone where moist soil plants might grow for at least 15 km upstream of the dam. Trees currently grow all the way to the low water line and are likely to exclude moist soil plants from any restored portion of the floodplain. Sites for reconnecting the river with its floodplain should be carefully chosen to maximize the chances of recovering the important moist-soil plant community in this regulated river.     

Coping with warmer, large rivers: a field experiment on potential range expansion of northern quagga mussels (Dreissena bugensis)

SUMMARY 1. Zebra mussels ( Dreissena polymorpha ) have established a much greater range in North America and Europe than quagga mussels ( D. bugensis ), which occupy a very similar niche.
2. We hypothesised that quaggas are physiologically capable of sustaining populations in warmer rivers currently occupied only by zebra mussels and that unidentified, non-physiological factors account for their more limited distribution.
3. Growth and survival of individually tagged mussels (976 D. bugensis from Lake Erie; 2625 D. polymorpha from Lake Erie and the Ohio River) were recorded monthly for up to 15 months in an outdoor stream mesocosm receiving unfiltered water from the Ohio River.
4. Extreme temperatures stressed both species; but in contrast to several previous laboratory studies, quaggas survived high temperatures better than zebra mussels. We suspect this was the result of species-specific differences in their ability to obtain, assimilate and/or catabolise food at high, sublethal temperatures.
5. A unimodal growth pattern was observed in both species, with the highest growth rates from late spring to early autumn.
6. Our survival and growth data suggest that quaggas are not physiologically limited from expanding southward.
7. While lacking definitive proof that dreissenid populations in rivers are ecologically sustainable without upstream lentic ecosystems and/or unintended human intervention, we suggest that complex river currents and upstream retentive and highly productive slackwater habitats in rivers may help sustain downstream populations of these meroplanktonic, dreissenid mussels.     

Effects of damming on the biological integrity of fish assemblages in the middle Lancang-Mekong River basin

The Lancang-Mekong River basin contains a diverse assemblage of freshwater fish species; however, their populations are threatened by current and planned dam construction along the river. Fish assemblages are sensitive indicators of environmental degradation and can be used to assess aquatic ecosystem health. This research compared the fish fauna at the Xiaowan hydropower dam located on the middle reaches of the Lancang-Mekong River at three time periods: in 2008 (before impoundment), 2010 (water storage) and 2011 (full operation). A modified fish index of biological integrity (modified F-IBI) was developed and it synthesized information on the taxonomic composition, trophic guilds, and tolerance levels of the fish and habitat diversity to quantitatively assess the condition of fish populations before and after damming. This index also was used to assess the longitudinal diversity of the fish fauna along the river channel and could assess the barrier effect associated with the dam. Jaccard's index of similarity was used as a feasible tool to assess fish diversity loss and biotic homogenization. The analysis clearly showed a homogenization of the fish communities after damming, and the reservoir impoundment region showed much more serious homogenization than the downstream region. The Xiaowan dam had an immediate and profound effect on the fish fauna in this region of the Lancang-Mekong River. A total of eight cascading dams are planned for development in this region, and, unless conservation mitigation efforts are considered, the results could be devastating on the native fish populations of middle reaches of the Lancang-Mekong River basin.     

Reach- and catchment-scale determinants of the distribution of freshwater mussels (Bivalvia: Unionidae) in south-eastern Michigan, U.S.A.

1. We investigated the diversity and distribution of freshwater mussels at 40 sites in an agricultural catchment, the River Raisin in south‐eastern Michigan, to relate mussel assemblages and individual taxa to reach and catchment‐scale variables. Unionids were surveyed by timed searches in 100‐m reaches, and in‐stream and riparian habitat were quantified as well as flow, water chemistry and channel morphology. Land use/cover and surficial geology were determined for site subcatchments and riparian buffers. 2. Some 21 mussel species were found overall; richness ranged from 0 to 12 living species per site. From the upper to middle to lower catchment, the number of individuals, number of species, Shannon–Weaver diversity and relative abundance of intolerant unionids all declined significantly. 3. Four groupings based on overall mussel diversity and abundance were significantly related to reach‐scale habitat variables. The richest mussel assemblages were associated with sites with higher overall habitat quality, greater flow stability, less fine substratum, and lower specific conductance. 4. Stepwise multiple regressions revealed that the distribution and abundance of the total mussel assemblage, as well as the most common species, could be predicted from a combination of reach‐ and catchment‐scale variables (R² = 0.63 for total mussels, R² = 0.51–0.86 for individual species). 5. Flow stability, substratum composition and overall reach habitat quality were the most commonly identified reach‐scale variables, and measures of surficial geology were the most effective catchment‐scale variables. The spatial pattern of geology is likely to be responsible for the diversity gradient from the upper to the lower catchment. 6. Prior studies, attempting to explain mussel distributions from local habitat features alone, have found relatively weak relationships. By employing a combination of reach‐ and catchment‐scale habitat variables, this study was able to account for a substantial amount of the spatial variability in mussel distributions.     

The influence of Dworshak Dam on epilithic community metabolism in the Clearwater River, U.S.A.

Epilithic community metabolism was determined on a seasonal basis over two years in nonregulated and regulated reaches of the Clearwater River in northern Idaho, U.S.A. Metabolism was estimated using three, 12-liter recirculating chambers and the dissolved oxygen method, with parameters expressed as g O₂ m^?2 d^?1. In the nonregulated reach above the reservoir, gross community productivity (GCP) ranged from 0.8 to 3.2, community respiration (CR₂₄) from 0.3 to 1.2, and production/respiration (P/R) ratios from 1.2 to 3.3. Epilithic metabolism in the regulated reach immediately below the dam increased sharply; GCP ranged from 4.2 to 25.5, CR₂₄ from 1.9 to 9.7, and P/R ratios from 1.4 to 5.7. Increased primary production and respiration in the regulated reach was a result of extensive growth of an aquatic moss (Fontanalis neo-mexicanus). The influence of the dam on epilithic community metabolism was mitigated 2.5 km downstream of the dam due to the regulated North Fork of the Clearwater River (NFCR) merging with the larger, nonregulated Clearwater River. While the regulated Clearwater River below the confluence was somewhat affected by the regulated NFCR flows upstream, metabolism was similar to that found above the reservoir (GCP = 1.2 – 2.6, CR₂₄ = 0.6 – 1.3, and P/R = 1.4 – 2.2). This study demonstrates that while Dworshak Dam has altered both primary production and respiration directly below the dam, the placement of the dam only 2.5 km upstream from a nonregulated reach greatly mitigates its effects on stream metabolism downstream. %     

Modelling and mapping the distribution,diversity and abundance of freshwater mussels (Family Unionidae) in wadeable streams of Illinois,U.S.A.

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Drought‐induced changes in flow regimes lead to long‐term losses in mussel‐provided ecosystem services

Extreme hydro‐meteorological events such as droughts are becoming more frequent, intense, and persistent. This is particularly true in the south central USA, where rapidly growing urban areas are running out of water and human‐engineered water storage and management are leading to broad‐scale changes in flow regimes. The Kiamichi River in southeastern Oklahoma, USA, has high fish and freshwater mussel biodiversity. However, water from this rural river is desired by multiple urban areas and other entities. Freshwater mussels are large, long‐lived filter feeders that provide important ecosystem services. We ask how observed changes in mussel biomass and community composition resulting from drought‐induced changes in flow regimes might lead to changes in river ecosystem services. We sampled mussel communities in this river over a 20‐year period that included two severe droughts. We then used laboratory‐derived physiological rates and river‐wide estimates of species‐specific mussel biomass to estimate three aggregate ecosystem services provided by mussels over this time period: biofiltration, nutrient recycling (nitrogen and phosphorus), and nutrient storage (nitrogen, phosphorus, and carbon). Mussel populations declined over 60%, and declines were directly linked to drought‐induced changes in flow regimes. All ecosystem services declined over time and mirrored biomass losses. Mussel declines were exacerbated by human water management, which has increased the magnitude and frequency of hydrologic drought in downstream reaches of the river. Freshwater mussels are globally imperiled and declining around the world. Summed across multiple streams and rivers, mussel losses similar to those we document here could have considerable consequences for downstream water quality although lost biofiltration and nutrient retention. While we cannot control the frequency and severity of climatological droughts, water releases from reservoirs could be used to augment stream flows and prevent compounded anthropogenic stressors.     

Short-term effects of dam removal on macroinvertebrates in a Taiwan stream

Dam removal is an approach for restoring rivers. However, there are increasing concerns about the impact of removal on downstream biota. We examined the short-term responses of benthic macroinvertebrates and their avian predator (Brown Dipper, Cinclus pallasii Temminck) in reaches downstream of a check dam after it was removed from a mountain stream in central Taiwan. The density and taxonomic richness of downstream macroinvertebrates decreased immediately after dam removal. The decreases were associated with scouring or burial by sediments from the upstream impoundment. Ten weeks post-removal, downstream macroinvertebrate densities, although marginally recovering, remained lower than both pre-removal and upstream densities. Substantial changes in community structure were not significantly associated with an increase in the proportion of taxa with short life spans. However, this small-scale disturbance had no strong effect on the abundance of their very mobile, avian predator. This study and other studies of dam removal have found that downstream sedimentation following dam removal can reduce macroinvertebrate densities and that they may recover over time. Thus, timescale must be considered when interpreting the consequences of dam removal, especially when the long-term goal is stream restoration.     

Role of habitat in determining macroinvertebrate production in an intermittent-stream system

1. The effect of channel drying on macroinvertebrate production was studied at the habitat and reach scale in a catchment drained by intermittent streams in Maine, U.S.A. The catchment includes two first‐order streams and their second‐order confluence. Six reaches were selected for study based on differences in channel slope and habitat cover (bedrock, riffle/run, debris dam and pool). Stream water in each reach was acidic and oligotrophic. 2. The study reaches had different degrees of channel drying. In the first‐order reaches, surface flow ceased earlier in the season and for longer periods than second‐order reaches. Regardless of reach, pool and debris dam habitats retained water longer than riffle/runs and bedrock. Unlike other habitats, debris dams retained moisture for relatively long periods following cessation of surface flow. 3. Reach‐specific macroinvertebrate production ranged from approximately 1.7 to 2.9 g AFDM m⁻² year⁻¹ which are among the lowest values ever reported. Habitat‐specific production ranged from approximately 0.5 to 5.0 g AFDM m⁻² year⁻¹ (bedrock and debris dams, respectively). 4. At the reach scale, quantities of stored benthic organic matter (range approximately 200–600 g AFDM⁻²) decreased in a downstream direction. 5. A combination of differences in the timing and duration of channel drying, habitat structure and detritus standing stocks appeared to influence levels of invertebrate production among the study reaches. 6. Our interpretation of a canonical correspondence analysis indicates that drying is more important than habitat in affecting macroinvertebrate production in this intermittent stream system.     

Retention and supply of zebra mussel larvae in a large river system: importance of an upstream lake

1. Persistence of zebra mussel populations in river systems probably depends upon the presence of upriver sites capable of hosting self‐recruiting adult populations that act as sources of larvae. In this paper we examine the importance of Lake Pepin, a natural riverine lake in the Upper Mississippi River, as an upriver source of larvae to the downstream populations of zebra mussels. 2. Field studies and modelling suggest that Lake Pepin plays a major role in maintaining zebra mussel populations in the Upper Mississippi River. Long water residence times in Lake Pepin allow for self‐recruitment under the right hydraulic conditions. Larval abundance was low to absent upstream of the lake but increased dramatically downriver in all 3 years of the study. Travel time estimates in the Upper Mississippi River show that newly fertilised larvae drifting out of Lake Pepin can contribute substantially to the major downstream peak in larval abundance. In contrast, backwater and other off‐channel sites are unlikely to drive main‐channel abundance patterns. Larval abundances in off‐channel sites were less than or equal to those in the main channel. 3. A key factor in assessing the importance of Lake Pepin as a source population was the abundance of early stage, unshelled larvae. Studies that consider only abundances of older shelled stages (visible by cross polarised lighting) may yield misleading results. Results of this study suggest that efforts to control zebra mussels in the Upper Mississippi River should focus on controlling adult populations within Lake Pepin and reducing or eliminating larvae exiting the lake.     

Downstream dispersal of zebra mussels (<Emphasis Type="Italic">Dreissena polymorpha</Emphasis>) under different flow conditions in a coupled lake-stream ecosystem

Dispersal, establishment, and spread of aquatic invasive species such as the zebra mussel (Dreissena polymorpha) can be influenced by riverine velocities and volumetric flows in invaded lake-stream ecosystems. Zebra mussels, which have a planktonic larval form (veliger), disperse rapidly downstream from a source population. Concentrations, dispersal, and body conditions of zebra mussel veligers were studied under different volumetric flow, or discharge, conditions in a coupled lake-stream ecosystem in northern Texas, USA. Veliger densities in lotic environments were strongly related to population dynamics in upstream lentic source populations. A strong exponential decrease in veliger density was observed through a 28-km downstream study reach. Increased water releases from the source reservoir increased veliger flux and dispersal potential, concomitantly increasing veliger flux and decreasing transit time. However, passage through release gates in the dam and increased turbulence in the river during high-discharge events could negatively affect body condition of veligers, and veliger body condition generally decreased from the source population to the farthest downstream site and was lower for veligers during periods of high discharge. Thus increased discharge appears to reduce the proportion of viable veligers because of increased turbulence-induced mortality. Colonization of distant downstream reservoirs can occur if discharge and propagule pressure are sufficient or if interim habitats are suitable for establishment of in-stream populations.     

Wetlands as barriers: effects of vegetated waterways on downstream dispersal of zebra mussels

1. Stream flow is a major vector for zebra mussel spread among inland lakes. Veligers have been found tens to hundreds of km from upstream source lakes in unvegetated stream and river systems. It has been suggested, however, that the downstream transport of zebra mussels is restricted by wetland ecosystems. We hypothesized that vegetated waterways, (i.e. wetland streams) would hinder the downstream dispersal of zebra mussels in connected inland lake systems. 2. Veliger abundance, recruitment and adult mussels were surveyed in four lake‐wetland systems in southeastern Michigan, U.S.A. from May to August 2006. Sampling was conducted downstream of the lakes invaded by zebra mussels, beginning at the upstream edge of aquatic vegetation and continuing downstream through the wetland streams. 3. Veliger abundance decreased rapidly in vegetated waterways compared to previously reported rates of decrease in non‐vegetated streams. Veligers were rarely found more than 1 km downstream from where vegetation began. Newly recruited individuals and adults were extremely rare beyond open water in the wetland systems. 4. Densely vegetated aquatic ecosystems limit the dispersal of zebra mussels downstream from invaded sources. Natural, remediated and constructed wetlands may therefore serve as a protective barrier to help prevent the spread of zebra mussels and other aquatic invasive species to other lakes and ecosystems.     

How Big of an Effect Do Small Dams Have? Using Geomorphological Footprints to Quantify Spatial Impact of Low-Head Dams and Identify Patterns of Across-Dam Variation

Longitudinal connectivity is a fundamental characteristic of rivers that can be disrupted by natural and anthropogenic processes. Dams are significant disruptions to streams. Over 2,000,000 low-head dams (<7.6 m high) fragment United States rivers. Despite potential adverse impacts of these ubiquitous disturbances, the spatial impacts of low-head dams on geomorphology and ecology are largely untested. Progress for research and conservation is impaired by not knowing the magnitude of low-head dam impacts. Based on the geomorphic literature, we refined a methodology that allowed us to quantify the spatial extent of low-head dam impacts (herein dam footprint), assessed variation in dam footprints across low-head dams within a river network, and identified select aspects of the context of this variation. Wetted width, depth, and substrate size distributions upstream and downstream of six low-head dams within the Upper Neosho River, Kansas, United States of America were measured. Total dam footprints averaged 7.9 km (3.0–15.3 km) or 287 wetted widths (136–437 wetted widths). Estimates included both upstream (mean: 6.7 km or 243 wetted widths) and downstream footprints (mean: 1.2 km or 44 wetted widths). Altogether the six low-head dams impacted 47.3 km (about 17%) of the mainstem in the river network. Despite differences in age, size, location, and primary function, the sizes of geomorphic footprints of individual low-head dams in the Upper Neosho river network were relatively similar. The number of upstream dams and distance to upstream dams, but not dam height, affected the spatial extent of dam footprints. In summary, ubiquitous low-head dams individually and cumulatively altered lotic ecosystems. Both characteristics of individual dams and the context of neighboring dams affected low-head dam impacts within the river network. For these reasons, low-head dams require a different, more integrative, approach for research and management than the individualistic approach that has been applied to larger dams.     

Recruitment and distribution of Dreissena polymorpha (Bivalvia) on substrates of different shape and orientation

Dreissena polymorpha recruitment on artificial substrates was studied in the Włocławek Dam Reservoir (the Vistula River, Poland). Densities on downstream and upstream vertical surfaces of plastic plates differed significantly from each other, with the former settled by more individuals. Vertical and horizontal plates, as well as upper and lower horizontal surfaces were settled similarly. In another experiment mussels settled on flat, convex and concave glass surfaces, directed upstream or downstream. Among the upstream surfaces, the concave ones were the most densely settled. No significant differences in mussel recruitment on various downstream surfaces were found. Thus, substrate shape influenced mussels only when they were exposed to water flow. Mussels were aggregated (Lloyd index > 1) along all the edges of the horizontal plates and along the upper edge of the vertical ones. Such distribution was probably caused by the post‐settlement movement of metamorphosed individuals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)