Hypolimnetic Release Research Articles

Phosphorus retention and transformation in a dammed reservoir of the Thames River, Ontario: Impacts on phosphorus load and speciation

Extensive efforts are underway to reduce phosphorus (P) export from the Lake Erie watershed. On the Canadian side, the Thames River is the largest tributary source of P to Lake Erie’s western basin. However, the role of dams in retaining and modifying riverine P loading to the lake has not been comprehensively evaluated. We assessed whether Fanshawe Reservoir, the largest dam reservoir on the Thames River, acts as a source or sink of P, using year-round discharge and water chemistry data collected in 2018 and 2019. We also determined how in-reservoir processes alter P speciation by comparing the dissolved reactive P to total P ratio (DRP:TP) in upstream and downstream loads. Annually, Fanshawe Reservoir was a net sink for P, retaining 25% (36 tonnes) and 47% (91 tonnes) of TP in 2018 and 2019, respectively. Seasonally, the reservoir oscillated between a source and sink of P. Net P release occurred during the spring of 2018 and the summers of 2018 and 2019, driven by internal P loading and hypolimnetic discharge from the dam. The reservoir did not exert a strong influence on DRP:TP annually, but ratio increases occurred during both summers, concurrent with water column stratification. Our analysis demonstrates that Fanshawe Reservoir is not only an important P sink on the Thames River, but also modulates the timing and speciation of P loads. We therefore propose that the potential of using existing dam reservoirs to attenuate downstream P loads should be more thoroughly explored alongside source based P mitigation strategies.

Evaluation of the influences of dam release types, land use, and habitat affecting abundance, richness, diversity, and community structure of larval and juvenile fish

We systematically evaluated the influence of dams on downstream aquatic habitat and early life stage (ELS) fish at two spatial scales from epilimnetic and hypolimnetic discharges and made comparisons with a control stream. ELS fish exhibited a general increase in abundance, richness, and Shannon diversity with increasing distance from dams in both epilimnetic and hypolimnetic release types. Nonmetric multidimensional scaling (NMDS) indicated larval and juvenile fish communities were structured differently between epilimnetic and hypolimnetic releases, and habitat variables structuring those communities were more variable in epilimnetic releases than in hypolimnetic releases. Rapid changes occurred within the first 1100 m reach and a second more gradual gradient extended beyond the 5100 m sample reach. Generally, our findings agreed with that of the Serial Discontinuity Concept (SDC), but we believe future studies should be explicit in the systematic evaluation of the SDC, and further studies are required to understand the two recovery gradients that exist below impoundments.

Content of highly unsaturated fatty acids in fish from rivers of contrasting temperature

AbstractContents of highly unsaturated long‐chain fatty acids of omega‐3 family (HUFA); eicosapentaenoic acid (EPA; 20:5n‐3); and docosahexaenoic acid (DHA; 22:6n‐3) are the principal indicators of the nutritive quality of fish for humans. Effects of environmental factors, first of all water temperature, on EPA and DHA contents in fish tissue are not currently completely understood. To reveal the putative effect of water temperature, fatty acid composition and contents were studied for 6 fish species, inhabiting the cold waters of the Yenisei River (Siberia, Russia) downstream of a dam (hypolimnetic release from reservoir) and its adjacent warm water tributaries (the Mana River and the Kacha River). It was hypothesized that (a) fish species from the cold river would have higher HUFA contents than fish from the warm rivers and (b) temperature would be negatively correlated with HUFA content in fish species. Using gas chromatography–mass spectrometry, distinct species‐specific fatty acid profiles were observed, whereas contents of the essential fatty acids, EPA and DHA, in fish species from the cold Yenisei River were in general similar to that from warm tributaries. Thus, in contrast to the first hypothesis, phylogenetic factors overweighed the effect of water temperature and food (benthic invertebrate) composition, on fatty acid composition and HUFA content in fish. For the second hypothesis, for the 2 species inhabiting both cold and warm rivers, only one had higher EPA and DHA content at lower temperatures. Consequently, the response of EPA and DHA content in fish tissue to temperature variations may be species‐specific.

Downstream effects of the Three Gorges Dam on larval dispersal, spatial distribution, and growth of the four major Chinese carps call for reprioritizing conservation measures

Larval drift and dispersal are critical processes that affect recruitment success for many riverine fishes. Hypolimnetic discharge from the Three Gorges Dam (TGD) lowers river temperature and reduces downstream nutrients, inducing distinct shifts in habitat conditions downstream of the dam. The inflow of major tributaries buffers these influences and creates physiochemical gradients according to the distance from the dam. We investigated the abundance, feeding, and growth of larvae of four major Chinese carps in three sections of the middle Yangtze River. Water temperature and transparency showed clear spatial gradients. Larvae in the river section closest to the dam tended to be lower in abundance and temporally delayed peak abundance and showed lower feeding intensity, poorer condition, and slower growth than those further from the dam. Our results demonstrate that physiochemical gradients influenced by the TGD have strong effects on abundance, feeding, and growth of the drifting larvae. We recommend that river sections farther from the TGD, particularly around the mouth of Poyang Lake, should become high-priority conservation areas to enhance protection of critical aquatic species.

Potential effects of dam cascade on fish: lessons from the Yangtze River

Construction of hydroelectric dams affect river ecosystems, fish diversity, and fisheries yields. However, there are no studies assessing the combined effects on fish caused by several adjacent dams and their reservoirs, as in a 'dam cascade'. This study predicts the potential effects that a cascade of ten dams currently under construction in the upper Yangtze River in China will have on local fishes, and uses such predictions to assess the effectiveness of possible fish conservation measures. We found that the dam cascade will have serious combined effects on fishes mainly due to impoundment, habitat fragmentation and blocking, flow regime modification, and hypolimnetic discharges. The impoundments will cause loss of critical habitats for 46 endemic species. The dams will fragment the populations of 134 species and will block migration routes for 35 potamodromous fishes. Corieus guichenoti will have a high risk of extinction due to the combined effects of impoundment and blocking. Modification of the flow regime will adversely affect the recruitment of 26 species that produce drifting eggs. The start of annual spawning for 13 fishes will be postponed by more than 1 month, and fish spawning and growth opportunities will be reduced due to low water temperatures associated with hypolimnetic discharges. Combined dam effects will further reduce the likelihood of successful recruitment of some endangered species, such as Acipenser dabryanus and Psephurus gladius. Three countermeasures hold promise to mitigate the near-term effects of the dam cascade, including preservation and rehabilitation of critical habitat, restoration of a semi-natural flow regime, and stock enhancement that respects genetic integrity. These conclusions can guide the development of protection plans for fishes in the upper Yangtze River. The approach undertaken in this study-by which the known and likely effects of present and future dams were simultaneously considered in light of the biology of the species-highlights the usefulness of ichthyology for fish conservation.

Understanding the role of induced mixing for management of nuisance algal blooms in an urbanized reservoir

Mixed layer dynamics were studied during June to August in 2008 to 2012 in Ford Lake, Michigan, using real-time data acquisition from in situ water temperature, dissolved oxygen, and chlorophyll sensors plus a shore-side weather station. The purpose of the study was to examine the effectiveness of managed hypolimnetic discharge for inducing vertical mixing, preventing hypolimnetic anoxia, and reducing the incidences of cyanobacterial blooms. Maximum mixed layer thickness each day was deduced from vertical density profiles measured at 30-minute intervals plus estimated kinetic energy supplied by wind stress and convection. Surface heat flux and resulting convection was more important to mixing dynamics than was wind stress. Selective withdrawal of hypolimnetic water was practiced in 2008 to 2011 and resulted in enhanced vertical mixing, hypolimnetic oxygenation, and absence of nuisance cyanobacterial blooms. In 2012, drought conditions restricted upstream water availability to such an extent that selective hypolimnetic withdrawal could not be used that year. The result was more than a month-long period of stable thermal stratification during which hypolimnetic oxygen concentrations became depleted and phosphate was released from anoxic sediments. A subsequent cold weather event induced temporary destratification, and a bloom of Aphanizomenon quickly ensued, despite reduced external loading of P from the influent river. Results of this study point to mixing dynamics and internal nutrient loading as the master variables responsible for nuisance algal blooms in this system.

Dynamics of Aphanizomenon and Microcystis (cyanobacteria) during experimental manipulation of an urban impoundment

Ford Lake in southeastern Michigan has a history of midsummer cyanobacterial blooms marked by a regular succession from heterocystous Aphanizomenon flos-aquae to nonheterocystous Microcystis aeruginosa. Nitrogen (N)-fixing Aphanizomenon has an advantage following seasonal thermal stratification because nitrate is consumed by assimilative uptake in the epilimnion and by anaerobic respiration in the anoxic hypolimnion. If nitrate levels increase, however, Microcystis succeeds Aphanizomenon. Bioassays corroborated the importance of nitrate to Microcystis success in late summer. Experimental releases of hypolimnetic water from the hydroelectric outlet dam before anoxia developed in 2006 destabilized the water column without altering lake volume and promoted a diatom bloom rather than cyanobacteria. To establish that the experimental manipulation was reproducible, we followed 2006 with a control year and then a second experimental year. In 2007, experimental manipulation was not performed, water was withdrawn from the epilimnion alone, and the lake reverted to historical dominance by first Aphanizomenon and then Microcystis. In 2008 we again commenced hypolimnetic discharge at onset of thermal stratification, and a sequence of diatom blooms resulted, demonstrating that community transformation first produced in 2006 was reproducible. The algal community again became transformed from cyanobacteria to diatoms, but overall biomass did not change. Bioassays demonstrated that Aphanizomenon became P limited during diatom dominance. Cyanobacterial blooms were postponed until after termination of the whole lake experiment and subsequent diatom decline.

Fish recruitment in rivers with modified discharge depends on the interacting effects of flow and thermal regimes

Summary Alteration of flow and thermal regimes is a key consequence of human use of river and floodplain ecosystems, and these impacts result from a range of interacting ecological mechanisms. Environmental flow regimes are a management strategy to restore or maintain ecologically important aspects of river hydrology. However, inadequate understanding of the processes that determine the effects of flow on population dynamics of biota hinders the maximum benefits of environmental flows. Spawning and recruitment of riverine fish is mediated by temperature, access to nursery habitat and the timing and availability of resources. Differences in population sizes between regulated and unregulated rivers are often attributed to the effect of flow and thermal regime change on population dynamics. Dams and extraction of water have altered the flow and thermal regimes of rivers in the northern Murray–Darling Basin, Australia. Combining three studies, we (i) examined differences in the recruitment of fish and patterns of potential prey availability between a regulated and unregulated river, (ii) studied the relationships between patterns of flow and temperature on spawning of fish in two rivers with contrasting regulated flow regimes and (iii) experimentally tested the effects of environmental flow releases in a regulated river on fish spawning and recruitment by comparing patterns over time against two unregulated rivers. Comparisons between regulated and unregulated rivers indicated that the significantly lower recruitment of Macquaria ambigua in the regulated river was linked with large differences in available prey. Species‐specific differences in the abundance of fish larvae between two regulated rivers indicated that different reproductive strategies determine population‐level responses to flow regulation. Finally, the experimental flow releases in a regulated reach during late spring did not result in a change in spawning and recruitment of fish, although influences of antecedent flow and temperature on larval and juvenile assemblage composition were significant. These findings indicate that the responses of fish spawning and recruitment to flow regime change and restoration are dependent on the conditions that determine the success of these critical life‐history processes. Specifically, key effects of flow regulation are reduced summer water temperatures due to hypolimnetic discharge from dams, and the reduced frequency and duration of low flows. Reduced water temperatures limit opportunities for fish to spawn, and sustained base flows were associated with lower densities of potential prey sources in conjunction with potential flushing of larvae and juveniles, thereby influencing recruitment. The benefits of environmental flow programmes can be enhanced if flow regimes are modified to ensure that abiotic and biotic conditions that support persistence of biota are provided. In conjunction with flow pulses and floods, low flows are a natural and ecologically important component of flow regimes, particularly in dryland rivers. Understanding the interactive and mechanistic effects of flow and thermal regimes on population viability and patterns of biodiversity is critical to maximising the value of environmental flows.

Estimation of hypolimnetic discharge on improvement of water quality and control of phytoplankton blooms in yamaguchi reservoir

Sakai water purification plant (Sakai WPP) and Higashi-murayama water purification plant (Higashi-murayama WPP) take water from Yamaguchi reservoir (20 m-effective depth) by Intake tower No.1 and No. 2. Yamaguchi reservoir, refilled with water after seismic reinforcement to the dam body, started its operation again in 2002. In the spring of 2004 and 2005, massive blooms of Asterionella spp. and Fragilaria crotonensis (diatoms) occurred, which resulted in sharp decreases in filtration capacity of Sakai WPP, taking slow sand filtration. Results of water analysis for the reservoir between 2003 and 2004 indicated that nutrients eluted from sediments during stratification periods were utilized for the blooms in the following springs. Since intake gates at five different depths enable us to conduct selective intake, we conducted hypolimnestic discharge; while Sakai WPP continued taking water with low turbidity and phytoplankton production, the lowest gates of Intake tower No.1 were also slightly opened in the middle of 2005. In April 2006, the lowest gate of Intake tower No. 2 was fully opened instead of the upper ones. Consequently, the scale of spring diatom blooms reduced after 2006. Nevertheless, rise in turbidity increased T-P, which resulted in massive blooms of Synedra spp. (diatoms) in 2009 when the water level was lowered to 8 m. This illustrated that this operation of the selective intake was not effective for conservation of the reservoir at low water levels.

Effect of small reservoirs on leaf litter decomposition in Mediterranean headwater streams

Leaf litter decomposition is a crucial process providing matter and energy to communities inhabiting headwater streams. This process could be affected by many man-made landscape transformations and its response can vary depending on the climate setting. In this study, we test the hypothesis that the presence of small headwater reservoirs decreases litter decomposition downstream, as reported for temperate Oceanic climatic regions, and that this effect is more accentuated in the Mediterranean. The effect of small dams on the decomposition of alder (Alnus glutinosa) leaves was studied in four headwater streams in Catalonia (NE Spain). The presence of a dam affected litter decomposition rates in three of the four streams studied, and this depended on reservoir typology. In those with seasonal surface release, decomposition rates were slower downstream from the dams, but in the case of a continuous hypolimnetic release, it was faster, with higher DIN and temperature and abundance of shredders. Alder litter decomposition rates were twice those reported for Oceanic climatic conditions. In Mediterranean headwaters, the effect of small dams will even be more evident at an annual scale due to the diminished flow rates in summer and this effect will be more pronounced than in the more Oceanic.

Discharge pulses of hydroelectric dams and their effects in the downstream limnological conditions: a case study in a large tropical river (SE Brazil)

AbstractTo address daily fluctuations in electricity demands, the quantities of water passing through the turbines of hydropower plants can vary significantly (up to fourfold) during a 24‐h cycle. This study evaluates the effects of hourly variations in water discharges on the limnological conditions observed in two below‐dam river stretches. The study reservoirs, Capivara and Taquaruçu, are the 9th and 10th reservoirs in a cascade of dams in the Paranapanema River in south‐east Brazil. The reservoirs exhibit different trophic conditions, water retention times, thermal regimes and spillway positions. Capivara Reservoir is deeper, meso‐eutrophic, with a high water retention time and hypolimnetic discharges (32 m) varying between 500 and 1400 m3 s−1. In contrast, Taquaruçu Reservoir is relatively shallow, oligo‐mesotrophic, and has a low retention time, with water discharges varying between 500 and 2000 m3 s−1. Its turbine water intake zone also is more superficial (7 m). For two periods of the year, winter and summer, profiles of limnological measurements were developed in the lacustrine (above‐dam site) zones of the reservoirs, as well as in the downstream river stretches (below‐dam site). In both cases, the sampling was carried out at 4‐h intervals over a complete nictemeral cycle. The results demonstrated that the reservoir operating regime (water discharge variations) promoted significant differences in the conditions of the river below the dams, especially for water velocity, turbidity, and nutrient and suspended solids concentrations. The reservoir physical characteristics, including depth, thermal stratification and outlet structure, are also key factors influencing the limnology and water quality at the below‐dam sampling sites. In the case of Capivara Reservoir, for example, the low dissolved oxygen concentration (<5.0 mg L−1) in its bottom water layer was transferred to the downstream river stretches during the summer. These study results demonstrated that it is important to continue such investigations as a means of verifying whether or not these high‐amplitude/low‐frequency variations could negatively affect the downstream river biota.

Impact of hydropeaking on hyporheic invertebrates in an Alpine stream (Trentino, Italy)

The impact of repeated hydropeaking events was assessed in the Alpine stream Noce Bianco (Trentino, NE Italy). Three stations were selected, one upstream and two at 0.25 km and 6 km downstream from a hydropower plant which causes 7-fold discharge increases. We collected hyporheic invertebrates for two years. Taxa diversity and abundance were reduced in the impacted sites, especially affecting the stygobites, which were significantly less abundant at the impacted sites, whereas stygoxene invertebrates increased exponentially. Repeated hydropeaking events alter the physical-chemical characteristics of the hyporheic habitat, resulting in the recorded faunistic pattern. The deposition of the fine sediment transported by the turbinated water downstream of the power plant and the absence of natural peak floods which remove fine sediments, probably cause a reduction of the interstitial space interstitial habitat available to stygobitic taxa. Surface water natural thermal regime is altered by the hypolimnetic discharges, and such alterations propagate into the hyporheic, affecting the stenothermic taxa. Some benthic taxa used the hyporheic habitat as a refuge to avoid catastrophic drift during hydropower production operation. The main trophic roles represented in the hyporheic assemblages do not support an important role for biological interactions such as predation in determining the fine-scale patchiness of the hyporheos.

Shifts in sauger spawning locations after 40 years of reservoir ageing: influence of a novel delta ecosystem in the Missouri River, USA

AbstractTo determine how reservoir ageing affects sauger Sander canadense spawning locations in Lewis and Clark Reservoir, we quantified riverine habitat and determined sauger spawning locations during recent years (2003–2004) and compared these results to historic (1950–1966) habitat conditions and spawning locations. The riverine habitat above the reservoir, which was historically homogeneous, has developed into two distinct areas: the upstream remnant river reach, characterized by a distinct main channel, and cold and clear water from the hypolimnetic discharge from the upstream reservoir, and the downstream delta section, which has abundant side channels and backwater areas and is warmer and more turbid water than the remnant reach. Sauger spawning locations have shifted markedly in the last 50 years. Sauger historically spawned in the upper reaches of this system, near Fort Randall Dam. However, in 2003, we found that sauger spawned in the lower delta reach and avoided the remnant reach. This shift likely occurred concomitantly with the development of the novel delta habitat. The delta likely functions more similarly to the historic remnant reach of this system (warmer temperature, turbidity, active meandering, complex habitats, etc.), indicating that sauger prefer to spawn in areas with historic riverine function. Thus, future management activities intended to enhance sauger populations should focus on the restoration of riverine function, such as that provided by emerging reservoir deltas, which may mimic pre‐impoundment conditions on the Missouri River. Copyright © 2008 John Wiley & Sons, Ltd.

Meta-analysis of the impacts of water management on aquatic communities

Systematic meta-analyses were conducted on the ecological impacts of water management, including effects of (i) dewatering on macroinvertebrates, (ii) a hypolimnetic release on downstream aquatic fish and macro invertebrate communities, and (iii) flow modification on fluvial and habitat generalists. Our meta-analysis indicates, in general, that (i) macroinvertebrate abundance is lower in zones or areas that have been dewatered as a result of water fluctuations or low flows (overall effect size, –1.64; 95% confidence intervals (CIs), –2.51, –0.77), (ii) hypolimnetic draws are associated with reduced abundance of aquatic (fish and macroinvertebrates) communities (overall effect size, –0.84; 95% CIs, –1.38, –0.33) and macroinvertebrates (overall effect size, –0.73; 95% CIs, –1.24, –0.22) downstream of a dam, and (iii) altered flows are associated with reduced abundance of fluvial specialists (–0.42; 95% CIs, –0.81, –0.02) but not habitat generalists (overall effect size, –0.14; 95% CIs, –0.61, 0.32). Publication bias is evident in several of the meta-analyses; however, multiple experiments from a single study may be contributing to this bias. Fail-safe Ns suggest that many (>100) studies showing positive or no effects of water management on the selected endpoints would be required to qualitatively change the results of the meta-analysis, which in turn suggests that the conclusions are reasonably robust.

Spatial and temporal variation in prey selection of brown trout in a cold Arkansas tailwater

Abstract – We examined prey utilisation patterns of brown trout (Salmo trutta Linnaeus) as related to available benthos in a cold tailwater (Little Red River, Arkansas) having low biodiversity of benthic macroinvertebrates and low availability of fish as prey. Benthic macroinvertebrates were sampled monthly in upstream and downstream sites over a 1‐year period. Stomach content analysis was also performed on 30 brown trout (10 each of three size classes) collected monthly from each site. Benthic and diet diversities were low at both sites, as isopods (Lirceus) accounted for 67% and 51% of the numerical abundance upstream and downstream, respectively, and 80% and 70% of all prey consumed by upstream and downstream brown trout. Physid snails (upstream) and chironomid larvae (downstream) accounted for most of the remaining prey taxa consumed. Piscivory and consumption of terrestrial invertebrates were rare. There was no clear relation between diet diversity and trout size. Densities of benthic macroinvertebrates and prey consumption were significantly greater upstream than downstream. Benthic macroinvertebrate assemblages and prey consumption were seasonally constant at both sites. Generally, prey consumption was consistent with availability of each prey in the benthos; the lack of benthic diversity and overwhelming abundance of one taxon (Lirceus) most likely contributed to food selection patterns. Although isopods are abundant within this tailwater to serve as a forage base, the displacement of native fish fauna because of the thermal regime of hypolimnetic release from Greers Ferry Reservoir probably serves as a major limitation to brown trout growth.

Emigration of Fish from Two South Carolina Cooling Reservoirs

AbstractWe assessed fish escapement from two South Carolina reservoirs, Par Pond and L Lake, from spring 2002 through summer 2003. Escapement was greatest in the spring and early summer, with lake chubsucker Erimyzon sucetta dominating the escapement catch in early spring and several sunfishes Lepomis spp. dominating in late spring and early summer. Most of the escaping centrarchids were bluegill L. macrochirus, warmouth L. gulosus, and redbreast sunfish L. auritus in L Lake and warmouths, bluegills, dollar sunfish L. marginatus, and spotted sunfish L. punctatus in Par Pond. Escapement was enhanced by high reservoir water levels and surface discharge over the spillway. Escapement declined during periods of hypolimnetic release from bottom discharge gates. Location of the Par Pond discharge structure in the littoral zone rather than the pelagic zone as in L Lake contributed to greater escapement of littoral species in Par Pond. Species composition in the escapement catches and reservoirs was not significantly correlated. Relatively low escapement from L Lake and Par Pond compared with that in other reservoirs may have been related to the configuration of the discharge structures, low water levels during 2002, fish habitat preferences, species composition and abundance in the reservoirs, and low rates of discharge.

A Bioenergetic Analysis of Factors Limiting Brown Trout Growth in an Ozark Tailwater River

We examined prey utilization and energy consumption by brown trout, Salmo trutta, in a cold tailwater (Little Red River, Arkansas, USA; LRR) having low biodiversity and low availability of fish as prey. Stomach content analysis and age estimation were performed on thirty brown trout (10 each of three size classes for a total of 710 trout) collected monthly from an upstream and downstream site over a 1-year period. Diet diversity was low at both sites, as 80% and 70% of all prey consumed by upstream and downstream brown trout, respectively, were isopods. Piscivory ( < 0.5% of individuals sam- pled) and consumption of terrestrial invertebrates were rare. There was no relation between diet diversity and trout age, and a very small ontoge- netic shift in brown trout diet. Second, we inves- tigated brown trout growth rates relative to prey consumption and temperature. Temperatures and availability of prey were less than required for maximal trout growth. However, prey availability limited trout growth directly, but sub-optimal temperatures probably buffered the effect of this reduced energy consumption by reducing meta- bolic energy expenditures. Brown trout growth was 54.8-57.0% of the maximum predicted by a bioenergetics model. Instantaneous growth rates for age 1 and adult brown trout were slightly higher for those downstream (0.195) versus those upstream (0.152). Although isopods are abundant within this tailwater to serve as a forage base, the displacement of native fish fauna and subsequent lack of establishment of cold-tolerant forage fish species due to the thermal regime of hypolimnetic release from Greers Ferry Reservoir probably serves as a major barrier to brown trout growth.

The Fundamental Thermal Niche of Adult Landlocked Striped Bass

AbstractResearchers have described the temperatures selected by landlocked striped bass Morone saxatilis in different locales throughout the USA. However, seasonally low concentrations of dissolved oxygen (DO) in many systems prevented striped bass from using the cool waters (&lt;22°C) they may have preferred. In Melton Hill Reservoir, a 92‐km‐long impoundment on the Clinch River in east Tennessee, 15 adult striped bass were tagged with temperature‐sensing radio tags and tracked for an average of 418 d in 1999–2000. Cold, hypolimnetic discharges from an upstream dam and heated discharge from a steam‐generating electric facility near the midpoint of this run‐of‐the‐river reservoir provided a broad range of temperatures in most seasons, and hypoxic habitats were uncommon even during stratification. The mean temperature occupied by striped bass varied seasonally (repeated‐measures analysis of variance, P &lt; 0.0001) and was highest in summer (17.5°C), intermediate in spring and fall (15.4–16.9°C), and lowest in winter (13.0°C). The mean and modal temperatures occupied during the growing season (May–October 1999) were 17.5°C and 19.0°C, respectively; 30% of the observations were between 9°C and 15°C. These data indicate that the fundamental thermal niche of adult landlocked striped bass may be lower than literature estimates. These results also represent the first unbiased field estimates of the influence of season on the thermal ecology of adult striped bass. The thermal characteristics of habitats considered optimal in habitat suitability index models for adult landlocked striped bass (i.e., 18– 24°C) should be revised to include cooler waters.

The organisation of fish assemblages in the regulated Lima basin, Northern Portugal

In order to understand the structure of fish assemblages in the modified Lima basin (Northern Portugal), two distinct datasets concerning the presence and abundance of fish species were subjected to multivariate analysis. On the River Lima two types of flow modification are present within kilometres of one another: (a) a reduced and constant flow due to hypolimnetic release; and (b) an intense and irregular flow. A comparison of their influence on fish assemblages revealed a gradient of assemblage types from tributaries to main river sites. The latter were characterised by a strong dominance of cyprinids, particularly Iberian barbel (Barbus bocagei). The former harboured two kinds of fish assemblages: those closer to the river mouth were dominated by the cyprinids Iberian chub (Squalius carolitertii) and Iberian nase (Chondrostoma polylepis), which were also frequently present in the main river; while in those further upstream the predominant species was the brown trout (Salmo trutta). Although explanatory variables such as distance from source, altitude, substrate coarseness and width were the primary correlates of fish assemblage composition, dam construction and flow regulation also had a significant effect upon assemblage structure, particularly by: i) reducing the importance of migratory species; ii) constraining the presence of trout in the regulated segments; and iii) simplifying the community, especially in the case of the constant and reduced flow regime.

The influence of flow impoundment and river regulation on the distribution of riverine macroinvertebrates at Mammoth Cave National Park, Kentucky, U.S.A.

The effects of impoundment by a low-head dam and hypolimnetic release from a reservoir on benthic macroinvertebrate assemblages were studied in two lowland rivers. The first river (Green River) was initially divided into three zones (impounded, transitional, erosional) according to hydrological characteristics. The entire reach of the second river (Nolin River) was a regime unit. Only the Green River erosional zone was free-flowing with a linear sequence of riffle-run-pool reaches. A detrended correspondence analysis showed that the Green River impounded and transitional zones were taxonomically indistinguishable while the Green River erosional zone and the Nolin River were each distinct. A canonical correspondence analysis revealed that higher surface velocity, higher summer water temperatures and more turbid conditions, and lower water temperatures were contributing parameters to the separation of the Green River erosional zone, Green River transitional/impounded zones, and the Nolin River, respectively, in ordination space. A series of one-way ANOVA’s testing for differences of macroinvertebrates assemblages between the three Green River zones according to five metrics showed that the Green River erosional zone demonstrated significantly higher values and the transitional and impounded zones were ecologically similar.