Effects of a small dam on freshwater mussel growth in an Alabama (U.S.A.) stream

1. Dams, ubiquitous features in many lotic ecosystems, are believed to have many broad‐ranging and predominantly negative effects on stream biota. Whereas the impacts of larger dams are well studied, few studies have quantified effects of small dams on streams. 2. Recent surveys found numerous locations where mussels were abundant and larger in reaches immediately downstream from small dams. We examined mussel shell growth and resource conditions in Sandy Creek, a small (third‐order) tributary of the Tallapoosa River in east‐central Alabama (U.S.A.), to determine whether larger populations and individuals result from more rapid growth or longer lifespans of mussels downstream from the dam. 3. Growth rates for populations occurring immediately downstream from the dam (mill reach), c. 5 km downstream from the dam (downstream reach) and upstream from the impoundment (upstream reach) were compared with environmental conditions (seasonal measures of nutrient concentrations and water chemistry) and food availability [total suspended solids (TSS)]. Water temperature was continuously monitored using data loggers. 4. Analysis of length‐at‐age data using multiple growth models found that mill reach mussels grew faster than both up‐ and downstream populations. This dam appears to substantially increase water temperatures and may extend the shell growth period in the mill reach. TSS quantity varied seasonally between sites but was generally highest in the impoundment and mill reach during spring and autumn. TSS quality was highest in the upstream reach from spring through autumn but was highest in the impoundment and mill reach during winter. 5. Our data suggest that some small impoundments enhance conditions for freshwater mussel growth in downstream reaches. However, we do not know how far downstream this subsidy extends or how different species respond to mill dam augmentation. Regardless, mounting evidence suggests that this phenomenon is geographically and taxonomically widespread in eastern North America. Heretofore, undocumented positive effects of small dams suggest that some older dams may warrant protection or restoration if downstream reaches support imperilled mussel populations. Further, some small dams may prove useful conservation tools for natural resource managers attempting to identify sites for mussel culture facilities or translocation refugia.     

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.     

Impact of dams on distribution,population structure,and hybridization of two species of California freshwater sculpin (Cottus)

Dams represent a beneficial way to maximize riverine potential, though the benefits often come with costs. Modified conditions to rivers downstream of dams (release temperature, flow, barriers to migration) can lead to changes in species compositions. In California, these effects are amplified, as limited water resources lead to extensive anthropogenic changes. Our study examined the role of seven western Sierra Nevada river dams on localized distribution and population structure of native riffle sculpin (Cottus gulosus) and their role in potential hybridization with native prickly sculpin (C. asper). Individuals were collected above and below dams, genotyped with 10 microsatellite loci, and analyzed for possible hybridization. Three downstream locations (American, Tuolumne, and Kings River) support populations of both species whereas the remaining downstream sites supported only prickly sculpin. River specific genetic population structure was found for both species but was more extensive in riffle sculpin. Hybridization was limited to the Kings River, and represented less than 3 % of individuals sampled. Comparisons between dams including elevation above sea level, type of dam, distance from dam to sampling location, and average released water temperature showed no correlation with riffle sculpin presence below a dam. Expanded sampling within the Kings River found no association with distance and riffle sculpin or hybrid presence, although both were limited to recent trout restoration areas. Therefore, despite initial inclinations, dams show no direct correlation with sculpin distributions or hybridization in the Great Central Valley of California.     

The effect of cascaded huge dams on the downstream movement of <Emphasis Type="Italic">Coreius guichenoti</Emphasis> (Sauvage &amp; Dabry de Thiersant, 1874) in the upper Yangtze River

Huge dams (installed capacity > 100 MKW) are generally built on large rivers that display high biodiversity and include major migration routes for many diadromous and potamodromous fish. As a result, these dams lead to severe ecological impacts and receive more attention than smaller dams. Most previous work on the impact of huge dams on fish downstream movement has focused on a single dam and reservoir, so little is known about the effect of cascaded huge dams and reservoirs on downstream movement. During the period 2012–2014, two huge dams (the Xiangjiaba and the Xiluodu) were constructed on the upper Yangtze River and the reservoir impoundments began, respectively in October 2012 and May 2013. These cascaded hydroelectric projects could have a large adverse effect on the downstream movement of an important potamodromous fish species, Coreius guichenoti. To study the effect of cascaded impact of sequential huge dams and reservoirs on passive and active downstream movement of C. guichenoti, eggs and larvae were collected in the Yibin section during 2012–2014 and fish were collected monthly at a site in the Hejiang section from June 2012 to July 2014. Our results showed that, compared to one huge reservoir and dam, cascaded dams exert a more serious effect and obstruct downstream movement of eggs, larvae and young fish (particularly the yearlings and two-year-olds) of C. guichenoti. Individual C. guichenoti were able to pass with relative ease through one reservoir and dam, but passing through both reservoirs and dams was very difficult. To allow access to the spawning grounds upstream, a fish passage should be built on the Xiluodu dam. However, due to the hydropower development in the whole upper basin, captive breeding and maintaining at least 60 km of riverine habitat upstream of a spawning ground could be a more cost-effective approach to maintaining C. guichenoti populations in the Upper Yangtze River.     

In situ experiments on predatory regulation of a bivalve mollusc (Dreissena polymorpha) in the Mississippi and Ohio Rivers

1. In situ exclosure experiments in the Mississippi and Ohio Rivers determined the importance of fish predation in regulating zebra mussels (Dreissena polymorpha), an increasingly important constituent of the benthic invertebrate assemblages in both rivers. 2. We evaluated the effects of predatory fish on the density, biomass and size distribution of zebra mussels in a floodplain reach of the upper Mississippi River and in a naturally constrained reach of the Ohio River. Fifty, six-sided, predator-exclusion cages and fifty ‘partial’ cages (mesh at the upstream end only) were deployed, with half the cages containing willow snags and half clay tiles suspended 12–16 cm above the bottom. A single snag or tile sample unit was removed from each cage at approximately monthly intervals from July to October 1994. Types and relative abundances of molluscivorous fish were evaluated by electrofishing near the cages in both rivers. Actual and potential recruitment of young zebra mussels on to the substrata were measured using benthic samples in both rivers and estimated (Ohio River only) from counts of planktonic veligers. 3. Zebra mussels were consumed by at least three fish species in the upper Mississippi River (mostly carp, Cyprinus carpio, and redhorse suckers, Moxostoma sp.) and five species in the Ohio River (primarily smallmouth buffalo, Ictiobus bubalus, and channel catfish, Ictalurus punctatus), but potential recruitment seemed adequate to replace consumed mussels, at least in the Ohio River. The number of juvenile benthic mussels showed no apparent link with the density of veligers soon after initiation of reproduction. Recruitment of juveniles on snags and tiles was not affected by cage type (thus eliminating a potentially confounding ‘cage effect’). 4. Fish significantly influenced mussel populations, but the impact was often greatest among low density populations in the upper Mississippi. Density and biomass differed in both rivers for cage type (higher inside cages), substratum (greater on tiles), and date (increased over time). Presumed size-selective predation was present in the Mississippi (greater on larger size classes) but was not evident in the Ohio. We hypothesize that fish in the Mississippi can more easily select larger prey from the low density populations; whereas size-selective predation on tightly packed zebra mussels in the Ohio would be difficult. 5. Although fish can reduce numbers of Dreissena polymorpha in the two rivers, current levels of fish predation seem insufficient to regulate zebra mussel densities because of its great reproductive capacity. The recent invasion of zebra mussels, however, could lead to larger fish populations while promoting greater carbon retention and overall ecosystem secondary production.     

Looking to the past and the future: were the Madeira River rapids a geographical barrier to the boto (Cetacea: Iniidae)?

In the present study we tested if a series of 18 rapids on the upper Madeira River form an effective barrier to gene flow, and in particular if they delimit the distribution of the boto Inia boliviensis—which it is believed to occurs only in the Bolivian sub-basin, above the rapids—and I. geoffrensis, which occurs throughout the Amazon basin and below the upper Madeira River rapids. We analyzed 125 individuals from the Madeira River basin sampled from upstream and downstream of the rapids. As the two species are morphologically similar, we used diagnostic molecular characters from known reference specimens to assign individuals to species. We observed that all individuals of Inia from the Bolivian sub-basin up to almost the mouth of the Madeira River belong to the species I. boliviensis. Therefore we concluded that the rapids do not delimit the distribution of I. boliviensis upstream and I. geoffrensis downstream of the rapids as previously hypothesized. Since we registered I. boliviensis along almost the entire length of the Madeira River, we estimated gene flow, time of divergence and effective population sizes of the upstream (Bolivian) and downstream (Madeira River) groups of I. boliviensis using IMa2. We concluded that gene flow is uni-directional from the upstream to the downstream group. Divergence time between the two groups was estimated to have occurred ~122 thousand years ago. The coalescent effective population size for the upstream group was estimated at ~131 thousand individuals, while for the downstream group it was estimated at ~102 thousand individuals. Recently two dams have been constructed in the region of the rapids; neither has a mechanism that will maintain connectivity between the upstream and downstream regions, and together with anthropogenic alterations to the hydrodynamic regime and ecology of the river will likely pose serious long-term and short-term consequences for I. boliviensis and other aquatic taxa.     

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. %     

Hierarchical spatial structure in soft-bottom mussel beds

Mussels (Mytilus edulis L.) are unusual because they thrive in both rocky shore and soft-bottom habitats. Despite their ecological and economic importance, little is known about their spatial structure. Mussels do not generally recruit to bare soft substrate because larvae and postlarvae cannot attach to a bottom of small sediment particles. They attach to hard objects on the sediment surface (especially other mussels), so soft-bottom mussel beds may be spatially organized in ways that are fundamentally different from those on rocky shores. The purpose of our study was to characterize the scales of spatial variability for several mussel abundance parameters in soft-bottom, intertidal M. edulis beds in coastal Maine. We used a random factor nested-ANOVA design of 200 cm² Cores within 1 m² Quadrats within 6 m Transects within Positions within bed Sites along 70 km (euclidean distance) of the Maine coast. Based on the literature and our field observations, we hypothesized that Sites and Positions account for most of the spatial variance in soft-botttom mussel beds. We rejected this hypothesis. Sites and Positions were not important in explaining variation in total mussel density, density of new recruits, or density of larger mussels. Although most of the variance in surface silt-clay fraction did occur at these levels, most mussel variation occurred at smaller spatial scales, specifically at the Quadrat scale for new recruits and total mussels and at the Transect scale for larger mussels. Variance in mussel parameters was not closely linked to the silt-clay fraction of surface sediment or to Site rankings of wind exposure and tidal flow. Variance in total mussel density was due primarily to variance in recruitment. No single scale explained more than about half the mussel variance, and no single scale was best at explaining all the mussel parameters. Greater knowledge about mussel bed spatial variability would be useful because it can help direct scale-dependent sampling regimes, field experiments, and coastal management practices.     

Asymmetric dispersal structures a riverine metapopulation of the freshwater pearl mussel Margaritifera laevis

Unidirectional water flow results in the downstream‐biased, asymmetric dispersal of many riverine organisms. However, little is known of how asymmetric dispersal influences riverine population structure and dynamics, limiting our ability to properly manage riverine organisms. A metapopulation of the freshwater pearl mussel Margaritifera laevis may be sensitive to river currents because mussels are repeatedly exposed to downstream drift during floods—a parasitic life stage is the only, limited period (~40 days) during which larvae (glochidia) can move upstream with the aid of host fish. We hypothesized that water‐mediated dispersal would overwhelm upstream dispersal via host fish, and therefore, that upstream subpopulations play a critical role as immigrant sources. To test this hypothesis, we examined the effects of both up‐ and downstream immigrant sources on the size of target subpopulations in the Shubuto River system, Hokkaido, Japan. We found that target subpopulation size was dependent on the upstream distribution range of reproductive subpopulations and the number of upstream tributaries, which are proxies for the number of potential immigrants moving downstream. In contrast, little influence was observed of downstream immigrant sources (proximity to downstream reproductive subpopulations). These results were consistent even after accounting for local environments and stream size. Our finding suggests that upstream subpopulations can be disproportionately important as immigrant sources when dispersal is strongly asymmetric.     

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.     

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

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

Turbidity-mediated interactions between invasive filter-feeding mussels and native bioturbating mayflies

1. Invasive dreissenid mussels are known to cause large ecosystem changes because of their high filter‐feeding capacity, while native bioturbators may interfere with the mussels filter feeding. In this experiment, we investigated indirect environmental interactions between invasive filter‐feeding dreissenid mussels (zebra and quagga mussels) and native recolonizing bioturbating hexagenid mayflies (Hexagenia) at two mussel densities and two Hexagenia densities in a 2‐month long laboratory experiment. 2. Mean turbidity increased with increasing density of Hexagenia and decreased with increasing density of mussels. Turbidity showed the fastest decline at the highest mussel density, and no decline or a lower rate of decline at the low mussel density, dependent on Hexagenia density. 3. Mussel growth decreased with increasing Hexagenia density at low but not at high mussel density. Moreover, growth of mussels decreased as a function of increased mean turbidity at low mussel density but not at high mussel density. Filtering activity at the highest mussel density increased after introduction of food at the lower two densities of Hexagenia, but was constantly high at the highest Hexagenia density. 4. There was no difference in emergence of Hexagenia among the treatments, but mortality of Hexagenia was higher in the presence of mussels than in their absence. 5. Our results indicate that interactions between dreissenids and hexagenids are mediated through the sediment, and depend on density of both dreissenids and hexagenids. As the natural densities of these animals vary considerably within lakes, their growth and survival because of indirect environmental interactions is expected to vary spatially.     

Are dams hotspots for Didymosphenia geminata blooms?

1. The diatom Didymosphenia geminata has emerged in recent years as a globally invasive species. Although considered native to North America, reports of nuisance blooms have increased over the last decade.
2. Previously, we determined that D. geminata was ubiquitous in two major headwaters of the South Saskatchewan River Basin (SSRB), Alberta, Canada, but found it only bloomed at certain sites, including those immediately downstream from dam outfalls. To evaluate the role of dams in the abundance and blooming of D. geminata , we compared sites just below dams to unregulated upstream reference sites in six dammed rivers of the SSRB.
3. There was a high degree of seasonal variability in D. geminata abundance among sites, but statistical analyses showed a significant propensity for the diatom to have higher cell densities and an increased frequency of blooms at dam sites.
4. Important predictor variables of D. geminata abundance included dam presence, water clarity and total phosphorus concentration. When data from dam sites were analysed, a multiple regression model using mean discharge and pH as independent predictors explained 73% of the variation in D. geminata cell density.
5. Analysis of 3 years of data from one study river (Red Deer River) revealed consistently higher D. geminata cell densities at the dam site compared to the upstream reference. This analysis also showed that average cell density fluctuated by orders of magnitude from year-to-year.
6. Due to the ecological and aesthetic concerns regarding the global spread and blooming of D. geminata , we recommend that dams be viewed as key candidates for mitigating blooms in flow regulated systems.     

Identifying potential drivers of distribution patterns of invasive Corbicula fluminea relative to native freshwater mussels (Unionidae) across spatial scales

This study aimed to identify the importance of ecological factors to distribution patterns of the invasive Clam (Corbicula fluminea) relative to native mussels (family: Unionidae) across seven rivers within the Mobile and Tennessee basins, Southeast United States. We quantitatively surveyed dense, diverse native mussel aggregations across 20 river reaches and estimated mussel density, biomass, and species richness along with density of invasive C. fluminea (hereafter Corbicula). We measured substrate particle size, velocity, and depth in quadrats where animals were collected. Additionally, we characterized reach scale environmental parameters including seston quantity and quality (% Carbon, % Nitrogen, % Phosphorous), water chemistry (ammonium [NH4+], soluble reactive phosphorous [SRP]), and watershed area and land cover. Using model selection, logistic regression, and multivariate analysis, we characterized habitat features and their association to invasive Corbicula within mussel beds. We found that Corbicula were more likely to occur and more abundant in quadrats with greater mussel biomass, larger substrate size, faster water velocity, and shallower water depth. At the reach scale, Corbicula densities increased where particle sizes were larger. Mussel richness, density, and biomass increased with watershed area. Water column NH4+ increased at reaches with more urban land cover. No land cover variables influenced Corbicula populations or mussel communities. The strong overlapping distribution of Corbicula and mussels support the hypothesis that Corbicula are not necessarily limited by habitat factors and may be passengers of change in rivers where mussels have declined due to habitat degradation. Whether Corbicula is facilitated by mussels or negatively interacts with mussels in these systems remains to be seen. Focused experiments that manipulate patch scale variables would improve our understanding of the role of species interactions (e.g., competition, predation, facilitation) or physical habitat factors in influencing spatial overlap between Corbicula and native mussels.     

Effects ofFucus vesiculosus covering intertidal mussel beds in the Wadden Sea

The brown algaFucus vesiculosus formamytili (Nienburg) Nienhuis covered about 70% of mussel bed (Mytilus edulis) surface area in the lower intertidal zone of Königshafen, a sheltered sandy bay near the island of Sylt in the North Sea. Mean biomass in dense patches was 584 g ash-free dry weight m^?2 in summer. On experimental mussel beds, fucoid cover enhanced mud accumulation and decreased mussel density. The position of mussels underneath algal canopy was mainly endobenthic (87% of mussels with >1/3 of shell sunk into mud). In the absence of fucoids, mussels generated epibenthic garlands (81% of mussels with <1/3 of shell buried in mud). Mussel density underneath fucoid cover was 40 to 73% of mussel density without algae. On natural beds, barnacles (Balanidae), periwinkles (Littorina littorea) and crabs (particularly juveniles ofCarcinus maenas) were significantly less abundant in the presence of fucoids, presumably because most of the mussels were covered with sediment, whereas in the absence of fucoids, epibenthic mussel clumps provided substratum as well as interstitial hiding places. The endobenthic macrofauna showed little difference between covered and uncovered mussel beds. On the other hand, grazing herbivores — the flat periwinkleLittorina mariae, the isopodJaera albifrons and the amphipodsGammarus spp. — were more abundant at equivalent sites with fucoid cover. The patchy growth ofFucus vesiculosus on mussel beds in the intertidal Wadden Sea affects mussels and their epibionts negatively, but supports various herbivores and increases overall benthic diversity.     

Novel insights into habitat suitability for Amazonian freshwater mussels linked with hydraulic and landscape drivers

Novel insights into habitat suitability for two Unionida freshwater mussels, Castalia ambigua Lamarck, 1819 (Hyriidae) and Anodontites elongatus (Swainson, 1823) (Mycetopodidae), are presented on the basis of hydraulic variables linked with the riverbed in six 500‐m reaches in an eastern Amazonian river basin. Within the reaches, there was strong habitat heterogeneity in hydrodynamics and substrate composition. In addition, we investigated stressors based on landscape modification that are associated with declines in mussel density. We measured hydraulic variables for each 500‐m reach, and landscape stressors at two spatial scales (subcatchment and riparian buffer forest). We used the Random Forest algorithm, a tree‐based model, to predict the hydraulic variables linked with habitat suitability for mussels, and to predict which landscape stressors were most associated with mussel density declines. Both mussel species were linked with low substrate heterogeneity and greater riverbed stability (low Froude and Reynolds numbers), especially at high flow (low stream power). Different sediment grain size preferences were observed between mussel species: Castalia ambigua was associated with medium sand and Anodontites elongatus with medium and fine sand. Declines in mussel density were associated with modifications linked to urbanization at small scales (riparian buffer forest), especially with percent of and distance from rural settlements, distance to the nearest street, and road density. In summary, the high variance explained in both hydraulic and landscape models indicated high predictive power, suggesting that our findings may be extrapolated and used as a baseline to test hypotheses of habitat suitability in other Amazonian rivers for Castalia ambigua and Anodontites elongatus and also for other freshwater mussel species. Our results highlight the urgent need for aquatic habitat conservation to maintain sheltered habitats during high flow as well as mitigate the effects of landscape modifications at the riparian buffer scale, both of which are important for maintaining dense mussel populations and habitat quality.     

Grazing effects of a freshwater bivalve (Corbicula leana Prime) and large zooplankton on phytoplankton communities in two Korean lakes

This study examined the effects of a freshwater filter feeding bivalve (Corbicula leana Prime) and large zooplankton (>200 μm, mostly cladocerans and copepods) on the phytoplankton communities in two lakes with contrasting trophic conditions. A controlled experiment was conducted with four treatments (control, zooplankton addition, mussel addition, and both zooplankton and mussel addition), and each established in duplicate 10-l chambers. In both lakes there were significant effects of mussel grazing on phytoplankton density and biomass. The effects were greater in mesotrophic Lake Soyang than in hypertrophic Lake Ilgam. Effects of zooplankton grazing did not differ between these lakes, and zooplankton effects on phytoplankton were much less than the effects of mussels. Although mussels exerted a varying effect on phytoplankton according to their size, mussels reduced densities of almost all phytoplankton taxa. Total mean filtering rate (FR) of mussels in Lake Soyang was significantly greater than that in Lake Ilgam (p=0.002, n=5). Carbon fluxes from phytoplankton to mussels (977–2,379 μgC l^?1d^?1) and to zooplankton (76–264 μgC l^?1 d^?1) were always greater in Lake Ilgam due to the greater phytoplankton biomass (p<0.01, n=6). Based on the C-flux to biomass ratios, the mussels consumed 170–754% (avg. 412%) of phytoplankton standing stock in Lake Soyang, and 38–164% (avg. 106%) in Lake Ilgam per day. The C-flux to biomass ratio for mussels within each lake was much greater than for large zooplankton. Mussels reduced total phosphorus concentration by 5–34%, while increasing phosphate by 30–55% relative to the control. Total nitrogen also was reduced (by 9–25%), but there was no noticeable change in nitrate among treatments. The high consumption rate of phytoplankton by Corbicula leana even in a very eutrophic lake suggests that this mussel could affect planktonic and benthic food web structure and function by preferential feeding on small seston and by nutrient recycling. Control of mussel biomass therefore might be an effective tool for management of water quality in shallow eutrophic lakes and reservoirs in Korea.     

Habitat traits, population structure and host specificity of the freshwater pearl mussel Margaritifera margaritifera in the Waldaist River (Upper Austria)

In European streams and rivers, the freshwater pearl mussel (Margaritifera margaritifera L., 1758) faces extinction. This is also true for the Waldaist River, with 20,000 specimens recorded in the early 1990s then Austria’s most important pearl mussel river. Nowadays, there is only a single 320 m stretch with noteworthy mussel densities. During an in-depth survey of this river stretch in 2010, we detected a total of 2,774 specimens. Mussel microhabitats were confined to patches of sand and fine gravel (psammal and akal) at run sections of the river, stabilized by large boulders. Pearl mussels avoided large accumulations of fine sediments. Typically situated at undercut slopes, preferred microhabitats were 0.25–0.50 m deep at baseflow with current velocities (at 40% depth) of 0.2–0.6 m s^?1. A comparison of the present stock with data from 1997 revealed a rapid decline in mussel density down to 27%. We also noticed strongly reduced growth and a high mortality of medium age classes. Juvenile mussels were completely lacking. With respect to host specificity in terms of glochidia survival, the brook char Salvelinus fontinalis (Mitchill, 1814), a suitable host in North America, shed glochidia within eight days. In the brown trout Salmo trutta L., 1758, two strains were investigated. Glochidia survival, growth and prevalence were significantly higher in the Danish than in the Austrian hatchery strain.     

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)     

Context-dependent effects of freshwater mussels on stream benthic communities

1. We asked whether unionid mussels influence the distribution and abundance of co‐occurring benthic algae and invertebrates. In a yearlong field enclosure experiment in a south‐central U.S. river, we examined the effects of living mussels versus sham mussels (shells filled with sand) on periphyton and invertebrates in both the surrounding sediment and on mussel shells. We also examined differences between two common unionid species, Actinonaias ligamentina (Lamarck 1819) and Amblema plicata (Say 1817). 2. Organic matter concentrations and invertebrate densities in the sediment surrounding mussels were significantly higher in treatments with live mussels than treatments with sham mussels or sediment alone. Organic matter was significantly higher in the sediment surrounding Actinonaias than that surrounding Amblema. Actinonaias was more active than Amblema and may have increased benthic organic matter through bioturbation. 3. Living mussels increased the abundance of periphyton on shells and the abundance and richness of invertebrates on shells, whereas effects of sham mussels were similar to sediment alone. Differences in the amount of periphyton growing on the shells of the two mussel species reflected differences in mussel activity and shell morphology. 4. Differences between living and sham mussel treatments indicate that biological activities of mussels provide ecosystem services to the benthic community beyond the physical habitat provided by shells alone. In treatments containing live mussels we found significant correlations between organic matter and chlorophyll a concentrations in the sediment, organic matter concentrations and invertebrate abundance in the sediment and the amount of chlorophyll a on the sediment and invertebrate abundance. There were no significant correlations among these response variables in control treatments. Thus, in addition to providing biogenic structure as habitat, mussels likely facilitate benthic invertebrates by altering the availability of resources (algae and organic matter) through nutrient excretion and biodeposition. 5. Effects of mussels on sediment and shell periphyton concentrations, organic matter concentrations and invertebrate abundance, varied seasonally, and were strongest in late summer during periods of low water volume, low flow, and high water temperature. 6. Our study demonstrates that freshwater mussels can strongly influence the co‐occurring benthic community, but that effects of mussels are context‐dependent and may vary among species.