Genetic diversity and divergence in the fountain darter (<Emphasis Type="Italic">Etheostoma fonticola</Emphasis>): implications for conservation of an endangered species

The endangered fountain darter Etheostoma fonticola is found only in the Comal and San Marcos rivers in the Guadalupe River basin in central Texas, USA. Comal River fountain darters were believed to be extirpated following a severe drought in the 1950s and were reintroduced in the early 1970s using 457 darters from the San Marcos River. In this study we used 23 microsatellite loci to describe and evaluate the genetic diversity, population structure and effective population size (N _e) of fountain darters. We also evaluated the genetic effect of the Comal River reintroduction and the influence of low-head dams (dams) on dispersal in both rivers. Bayesian analysis of individual genotypes and Analysis of Molecular Variation supported two distinct populations concordant with the two rivers. Estimates of N _e were much smaller (<10 %) than census size (N _c) in both rivers but did not indicate the populations are at risk of an immediate and rapid loss of genetic diversity. Coalescent-based estimates of the genetically effective number of founders (Nf) for the Comal River averaged about 49 darters and, together with the indices of genetic diversity and the bottleneck test (heterozygosity excess) results, were consistent with a founder event following the reintroduction in the Comal River. Finally, our results regarding the influence of dams on fountain darter dispersal were equivocal and did not support a conclusion. We recommend this issue be examined further as part of the fountain darter recovery program.     

Effects of dispersed recreational activities on the microbiological quality of forest surface water.

The microbiological quality of forest surface waters in the Greenwater River watershed was examined to investigate the influence of heavy motorized camping in an area with no sanitary facilities. Indicator densities increased during weekend human-use periods when compared to weekdays. Increases in indicator densities were also noted downstream from heavily used camping areas when compared to upstream sites. Seasonal, weekly, and diurnal fluctuations in indicator densities were observed. This study suggests that potential health hazards exist in this watershed during periods of human use.     

太浦河大型底栖动物群落组成及时空分布特征

太浦河连接东太湖和黄浦江,是东太湖向下游行洪的主河道,也是黄浦江上游重要的引水水源。分别于2017年10月、12月和2018年3月、6月在太浦河沿程设置的5个站点开展了大型底栖动物季节频次的取样调研,分析了太浦河大型底栖动物群落的结构特征及其主要影响因子。调查期间共采集到大型底栖动物25种,隶属于3门6纲14科,其中以河蚬（Corbicula fluminea）为代表的双壳类和以铜锈环棱螺（Bellamya aeruginosa）为代表的腹足类处优势地位。太浦河大型底栖动物群落的密度、生物量及多样性特征在不同站点间均存在显著差异（P<0.05）;而在不同季节间均不存在显著差异（P>0.05）。聚类排序分析表明,太浦河大型底栖动物群落在空间上的差异较季节间的变化更为明显。太浦河大型底栖动物群落的物种组成特征与太浦河、东太湖的水文连通格局密切相关;而沿程不同程度人为干扰下的水体营养水平以及底质类型与生境异质性等生境条件对太浦河大型底栖动物群落的分布特征具有重要影响,其综合作用机理有待进一步研究。     

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.     

Influence of Intermittent Hydropeaking on the Longitudinal Zonation Patterns of Benthic Invertebrates in a Mountain Stream

The longitudinal zonation patterns of invertebrate species were studied in the River Oriège (Pyrénées, France) from 920 to 819 m a.s.l. At 912 m a.s.l., the river receives hypolimnetic water diverted from a nearby reservoir lake, and the natural flow may be enhanced several times a day from 1 to 11 m³ s^—1 in summer and winter, and from 5 to 15 m³ s^—1 during spring spates. During hydropeaking, the water was cooled in summer and slightly warmed in winter, but this was attenuated 3500 m downstream from the plant. Invertebrate densities were recorded in July (end of spates) and October (low flow period) at ten sampling sites : a reference site upstream of the hydrostation's discharge point (site 1), and nine sites each 400 m below the outlet (sites 2—10). The longitudinal distribution of invertebrate taxa was studied using Factorial Analyses, taxa and sites were clustered using Analysis of Dynamic Cluster. The hydroelectric facility did not clearly modify the qualitative composition of the benthic fauna, but clearly affected the longitudinal zonation of several populations. The low abundance of several species below the outlet reflected the impact of both hydropeaking and zonation. These species were high mountain species, the density of which decreases towards downstream sites in the Pyrenees, and low mountain species, the density of which naturally decreases towards upstream sites. Some taxa disappeared in the regulated section, so their distribution in the River Oriège was primarily influenced by hydropeaking. Finally, some taxa preferentially lived in the lower section of the river where the impact of peaking flows was strongly attenuated, so their longitudinal distribution was first governed by their natural longitudinal zonation. Under this kind of river regulation (natural discharge and temperature except during periods of power generation, intermittent hydropeaking from a separate reservoir) modifications of the thermal regime had a minor effect on the population dynamics, unlike hydraulic disturbances which strongly influenced the structural attributes of benthic communities.     

Spatially variable response of native fish assemblages to discharge,predators and habitat characteristics in an arid‐land river

1. Fish assemblages and habitats were sampled annually at fixed sites in three tributaries of the Gila River catchment over a 21‐year span that included prolonged low‐ and high‐flow periods. Model selection was used to evaluate responses of seven native fishes with variable ecological traits (four small‐bodied cyprinids, one large‐bodied cyprinid, and two large‐bodied catostomids) to mean annual discharge and predacious non‐native fishes across the three sites. We also compared habitat use and overlap of native and non‐native fishes to identify potential for negative interactions among species. 2. Assemblage structure (species abundance and richness) and recruitment of native species was strongly and primarily affected by mean annual discharge and secondarily by location and densities of non‐native predators (mainly the centrarchid Micropterus dolomieui). 3. Densities of age‐0 catostomids and small‐bodied cyprinids were positively associated with discharge, and this pattern was strongest in the tributary with the lowest densities of non‐native predators. Absence or extreme low abundance of natives during low‐flow years was most pronounced at the sites where non‐native predators were comparatively common. Densities of adults of large‐bodied native species also varied by site, but often were positively associated with densities of non‐native predators. 4. Spatially variable responses of native fish assemblages indicated that the persistence of native fishes could be jeopardized if key habitats were lost or flow regimes unnaturally altered, particularly during low‐flow conditions when recruitment of native fishes is low and predation by non‐natives is high. Large‐bodied species may be less vulnerable to multiple years of poor conditions because adults are able to avoid predation by non‐natives and thus can rely on occasional high discharge years for successful recruitment. 5. As in other arid‐land streams, native fish assemblages of the Gila River Basin continue to decline. Our results indicate that conservation requires specific knowledge and consideration of physical influences as well as life‐history attributes of native and non‐native fishes.     

Impacts of dam-regulated flows on channel morphology and riparian vegetation: a longitudinal analysis of Satsunai River, Japan

We examined the impacts of the Satsunai River Dam on the hydrology and development of riparian vegetation along the upper and lower reaches of the Satsunai River downstream from the dam. We estimated frequency curves of the flood discharge during the pre-dam (1976–1996) and post-dam (1997–2006) periods and simulated the flood frequency at sampling points within sites under pre-dam, post-dam and dam-removal (using the pre-dam flood discharge and post-dam cross-sections) scenarios. Changes in channel morphology and land cover were investigated by analyzing aerial photographs. Our results indicate that the 20-year flood at the upper site decreased substantially (from 599 to 271 m³/s) after dam operation, while that of the lower site decreased slightly (from 1025 to 977 m³/s). Within the upper site, the proportion of >20-year return periods increased considerably (from 31.0 to 48.6%) while the proportion of 1- to 20-year return periods decreased (from 30.5 to 8.9%) after dam operation. Flood frequency results for the dam-removal scenario were similar to those for the pre-dam period, suggesting that a return to pre-dam discharge rates would restore the pre-dam distribution of flood frequency at the upper site. Within the lower site, however, the distribution of flood frequency varied little between the pre- and post-dam scenarios, because tributary inflows between the sites mitigated the impacts of dam-regulated flows. Land cover types were associated with flood frequency at both sites. The reduced flood frequency of the upper site resulted in increased area of riparian vegetation and decreased area of active channel.     

Seasonality of macroalgae and epilithic diatoms in spring-fed streams in Texas, USA

A seasonal study of two spring-fed stream systems in south-central Texas was undertaken over a 15-month period from June 1996 to September 1997. Relative abundance of the epilithic diatom flora, percent cover of macroalgae, and several physical and chemical conditions were monitored in one 20-m stretch in each of the streams at approximately two-month intervals. Six additional spring-fed stream segments were sampled in May 1996 for macroalgae and measured for the same conditions, in order to determine how representative the macroalgal floras of the study sites were of south-central and western Texas. Conditions in the two stream segments were relatively constant over the 15 months, except for maximum depth; this finding is similar those reported for other spring-fed systems worldwide. Sixty-eight diatom taxa and nine macroalgae taxa were identified from the San Marcos River, which compares to 46 diatom taxa and six macroalgae taxa from the Comal River. Several diatom and macroalgae species were significantly correlated to stream conditions (e.g. Dichotomosiphon tuberosus and Hildenbrandia angolensis with daylength, and Amphora pediculus with maximum depth). Twenty-one macroalgal taxa were identified from the six additional spring-fed steam segments. Conditions in these streams were very similar to those measured in the San Marcos River and Comal River, except that they had a higher average pH _x= 8.1 versus _x= 7.6). The epilithic diatom flora of the San Marcos River and the Comal River contains many of the diatom and some macroalgal taxa commonly reported from springs globally. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of headwater impoundment and channelization on invertebrate drift

The construction of a flood control impoundment on Twitty's Creek added large numbers of organisms of limnetic origin to the stream ecosystem. However, the number of limnetic organisms per unit volume of water decreased rapidly as the distance downstream from the reservoir increased and, during most sampling periods, made up an insignificant portion of the total drift biomass at 7.2 km downstream. Factors favoring the extended downstream drift of limnetic organisms were high stream discharge and low water temperature.Several taxa of benthic organisms had much lower drift rates in the station immediately below the dam than at other stations and several taxa commonly taken at other stations were not captured immediately below the reservoir outfall. One possible explanation is that these organisms may have longer drift recruitment distances than the distance from the reservoir outfall to the sample location.A comparison of drift densities of organisms of benthic origin and benthic standing crop densities in channeled and unchanneled streams revealed that drift densities were higher in channeled streams than in unchanneled streams for most taxa of invertebrates. In addition, channeled streams appeared to have lower benthic standing crops than unchanneled streams for most taxa of invertebrates.In stream sections impacted by either channelization or the Twitty Lake outfall, the energy dynamics of the stream ecosystems were altered by increased density of drifting invertebrates. From the standpoint of increasing food availability to the fish fauna of the stream, these changes would appear to benefit drift feeding species and negatively impact bottom feeding species.     

Effects of experimental flow regulation on invertebrate drift and stranding in the Flathead and Kootenai Rivers,Montana, USA

Studies were conducted to determine the effects of experimental manipulations of discharge on invertebrate drift in two regulated rivers in northwestern Montana, USA. During these studies the discharge regime in the Flathead River was characterized by frequent flow fluctuations, while in the Kootenai River high discharge was maintained for much longer periods before flow was reduced to minimum discharge. The magnitude of the response of invertebrates to disturbance was different in the two rivers, in part because of the different frequencies of flow changes. Midstream invertebrate drift increased an order of magnitude during increasing discharges in the Flathead River but was not substantially increased during decreasing discharges. When the prior discharge regime had been sustained at high levels in the Kootenai River, invertebrate drift densities as high as 300 000/100 m³ were measured along the shoreline following reductions in discharge, both immediately after flow began to decrease and after dark on the same day. There was also more recolonization of shoreline areas and more stranding of insects following dewatering of nearshore regions when there had been sustained high discharge levels prior to the flow reduction. More insect stranding occurred during a faster rate of decrease in discharge (50 000 to 100 000 organisms m⁻²).     

Seahorse (Hippocampinae) population fluctuations in the Ria Formosa Lagoon,south Portugal

Comparisons of three sets of surveys in the Ria Formosa Lagoon, Portugal, over a 13 year period (2001–2002, 2008–2009 and 2010–2013) revealed significant population fluctuations in at least one of the two seahorse (Hippocampinae) species living there, and that those fluctuations were potentially associated with habitat changes in the lagoon. After a significant decline between the first two survey periods (2001–2002 v. 2008–2009), long‐snouted seahorse Hippocampus guttulatus populations increased significantly between 2008–2009 surveys and new 2010–2013 surveys. There were no significant differences in H. guttulatus populations between the 2001–2002 and 2010–2013 surveys. In contrast, there were no significant differences in short‐snouted seahorse Hippocampus hippocampus densities among the 16 sites surveyed throughout the three sampling periods, although the ability to detect any change was hampered by the low densities of this species in all time periods. Fluctuations in H. guttulatus densities were positively correlated with the percentage of holdfast coverage, but with none of the other environmental variables tested. These results highlight the importance of holdfast availability in maintaining stable seahorse populations. While population fluctuations are certainly more promising than a consistent downward decline, such extreme fluctuations observed for seahorses in the Ria Formosa Lagoon could still leave these two species vulnerable to any additional stressors, particularly during low density periods.     

Abundance and movement patterns of age-0 juvenile lake sturgeon in the Peshtigo River,Wisconsin

In Great Lakes tributaries, age-0 juvenile lake sturgeon Acipenser fulvescens use riverine nursery habitats during their first summer of life and migrate to adjacent lakes after a seasonal decline in water temperature. We used mark-recapture data collected during this river-residency phase to monitor patterns in juvenile abundance and movement in the Peshtigo River, Wisconsin, during 2006 and 2007. Jolly-Seber and multistate models were used to estimate abundance and describe the probability of movement between river sections. Juvenile abundance was higher in 2007 than in 2006; however, in both years, abundance at the end of the sampling season was lower than at the beginning. Downstream movements were observed more frequently than upstream movements, and the probability of a downstream movement was higher than the probability of an upstream movement. The lower abundance later in the sampling season and propensity for downstream movements suggests that some age-0 juvenile lake sturgeon may leave the natal river before a seasonal decline in water temperature.     

Spawning lake sturgeon (Acipenser fulvescens Rafinesque, 1817) and their habitat characteristics in Rainy River,Ontario and Minnesota

Lake Sturgeon (Acipenser fulvescens) spawning habitat in the Rainy River was studied in light of the ongoing review of the rules governing water levels upstream that affect discharge rates through the International Falls dam. The objectives of this study were to assess the current status of spawning Lake Sturgeon below the dam by: (i) evaluating weight–length relationships, condition, age and growth; (ii) identifying spawning locations and characterizing its physical attributes; and, (iii) evaluating the effects of water surface elevation on the availability of spawning habitat. Spawning was confirmed with use of egg mats, and targeted sampling of spawning individuals was completed using gillnets and electrofishing. Physical attributes of spawning locations, including temperature, depth and water velocity were collected at a range of water levels using an Acoustic Doppler Current Profiler and representative sediment sampling. Biological characteristics and timing of spawning events were consistent with those previously reported for Lake Sturgeon. However, a wider range of depths (0.3–10.5 m) and velocities (0.01–1.9 m³/s) than reported in most single site studies was found here, with the ranges encompassing what is reported in the literature as a whole. Lake Sturgeon were found to prefer coarse spawning substrates dominated by bedrock, boulder or cobble. The availability of preferred spawning habitat varied significantly with the changing water levels observed during the study and was eventually limited at higher water levels by river channel form. Because of the implications for dewatering of nest sites, keeping flows constant during spawning is critical to Rainy River Lake Sturgeon spawning success.     

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.     

Movement and dispersal of the invasive signal crayfish Pacifastacus leniusculus in upland rivers

1. The American signal crayfish Pacifastacus leniusculus, an invasive species widely introduced throughout Europe, is a major threat to native European crayfish species and is causing increasing concern because of its wide impact on aquatic ecosystems. 2. Whilst various control and management methods have been proposed, very little is known about the factors influencing dispersal and movements of signal crayfish. 3. Sixty‐four adult signal crayfish (carapace length 31.9–63.8 mm) were radiotagged in upland rivers in northern England, during four periods. Tracking was carried out at two sites, a low‐density establishing population and a high‐density established population. Tracking was carried out at both sites concurrently during midsummer (June to August 2002), during late summer (August to September 2001) at the low‐density population site and during autumn to winter (October to February 2000/01) at the high‐density population site. 4. Maximum movement occurred during midsummer. Temperature appeared to be a major factor influencing the timing and extent of movements between tracking periods. 5. The frequency distribution of the maximum distance moved upstream and downstream by radiotagged crayfish showed an inverse power relationship. The median maximal upstream and downstream distances moved were 13.5 m (range 0–283 m) and 15 m (range 0–417 m), respectively. There was a significant difference between the distributions of upstream and downstream ranges, with greater distances moved downstream. 6. All downstream movements made by crayfish appeared to be active movements and not the result of passive movement during periods of high discharge. There was no apparent influence of size, sex or density on the amount of movement recorded. 7. The study provides important information on the spatial and temporal behaviour of introduced crayfish in upland lotic systems. In contrast to lowland rivers, our results suggest that flow or gradient may influence the invasive potential of signal crayfish in an upstream direction in upland rivers.     

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

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

Occurrence,abundance and distribution of benthic macroinvertebrates in the Nyando River catchment,Kenya

A baseline study was conducted of the occurrence of macroinvertebrates at 26 sites in the Nyando River catchment in 2005–2006. A total of 13 orders and 16 families of Arthropoda, Mollusca, Platyhelminthes and Annelida were collected, with the order Ephemeroptera being most abundant in the up- and mid-stream reaches, followed by Hemiptera and Plecoptera respectively. The downstream sections of the river were dominated by Hirudinea and tubificids, as the water quality deteriorated mainly due to local land use, raw sewage effluent discharge and annual floods. Insects and annelids were the main invertebrates found and the extent of pollution increased from mid-section (Site 15) downwards as the river flowed into the Winam Gulf. Stringent management measures are required to safeguard the environment and ecosystems of Lake Victoria.     

Decline in zoobenthos densities in the profundal sediments of Lake Mendota (Wisconsin,USA)

High densities of zoobenthos inhabited Lake Mendota's profundal zone in the early 1900s through the mid-1940s. Chaoborus punctipennis was the most abundant organism during the winter, along with moderate densities of Chironomus spp., Pisidium sp., oligochaetes, and Procladius sp. By the early 1950s, Chaoborus punctipennis densities had declined to 10% of former levels, while Chironomus increased significantly. However, by the mid-1960s, Chaoborus, Chironomus, and Pisidium densities had decreased to very low population levels. By 1987–89, Pisidium was no longer found. Zoobenthos that had not decreased from earlier surveys were oligochaetes and Procladius, although further sampling of oligochaetes is needed to confirm current densities. These organisms are the most tolerant of severe anoxia.Four possible reasons for this decline were evaluated: (a) decline in food availability, (b) increase in fish predation, (c) use of toxic insecticides in the drainage basin, and (d) changes in the profundal sediment environment. Based on literature information and long-term data for Lake Mendota, a change in the profundal sediment environment is the most likely explanation for the decline in the less-tolerant zoobenthos species. Although the duration and extent of anoxia in the hypolimnion have not changed since the early 1900s, hypolimnetic ammonia and hydrogen sulfide concentrations apparently have increased as Mendota became more eutrophic after the mid-1940s. However, further study is needed to determine if these higher concentrations or other factors were responsible for the dramatic decline in lake Mendota's profundal zoobenthos.     

Fish parasite communities in tropical reservoirs along the Perak River, Malaysia

The parasite communities of fish in three reservoirs (Temengor, Kenering and Cenderuh) along the Perak River, as well as in the lower Perak, are compared. A total of 36 species, 51 species, 61 species and 58 species of metazoan parasites from six types of fishes common in all studied locations, were obtained in Temengor, Kenering, Cenderuh Reservoirs, and downstream Perak River, respectively. The various criteria (prevalence, mean intensity, diversity index, similarity index and evenness) of the parasite community were highest at the Cenderuh Reservoir, followed by the downstream river, Kenering, and Temengor Reservoirs. Monogeneans were the most common parasites in all species of fish examined from all study sites. Fish of the same species showed a high degree of similarity of its parasite fauna regardless of study site, but different hosts had a low similarity of their parasite fauna, even within the same study site. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.