Growth,Condition and Survival of Three Forage Fish Species Exposed to Two Different Experimental Hydropeaking Regimes in a Regulated River

Hydroelectric dam operation can alter discharge and temperature patterns, impacting fish populations downstream. Previous investigations into the effects of river regulation on fish have focused on a single species within a river, yet different results among studies suggest the potential for species‐specific impacts. Here, we compare the impacts of two different hydropeaking regimes relative to a naturally flowing river on three ecologically important members of the forage fish community: longnose dace (Rhinichthys cataractae), slimy sculpin (Cottus cognatus) and trout‐perch (Percopsis omiscomaycus). Annual growth, estimated from otolith back‐calculations, was higher for each of the species in the regulated river relative to the naturally flowing river but did not differ between hydropeaking regimes. Condition was assessed using weight–length relationships and differed between rivers for each species, and between hydropeaking regimes for longnose dace and slimy sculpin. Survival of longnose dace and slimy sculpin was lower in the regulated river relative to the naturally flowing river, but comparable between rivers for trout‐perch. Annual growth was significantly related to mean summer discharge in the regulated river and to mean summer water temperature in the naturally flowing river for each species, and significantly different slopes among species indicate species‐specific responses to discharge and temperature alterations. This study demonstrates different biological responses among fish species within rivers to regulation in general, as well as to specific hydropeaking regimes. Future studies should focus on multiple species and multiple indicators of fish health to more fully characterize the impacts of river regulation on downstream fish communities. Copyright © 2016 John Wiley & Sons, Ltd.     

ECOSYSTEM ENGINEERS MODULATE EXOTIC INVASIONS IN RIPARIAN PLANT COMMUNITIES BY MODIFYING HYDROGEOMORPHIC CONNECTIVITY

Patterns of native and exotic plant species richness and cover were examined in relation with ecosystem engineer effects of pioneer vegetation within the Mediterranean gravel bed river Tech, South France. The floristic composition was characterized according to two distinct vegetation types corresponding to two habitats with contrasted conditions: (i) open and exposed alluvial bars dominated by herbaceous communities; and (ii) islands and river margins disconnected from annual hydrogeomorphic disturbances and covered by woody vegetation. A significant positive correlation between exotic and native plant species richness and cover was observed for both vegetation types. However, significant differences in native and exotic species richness and cover were found between these two vegetation types. Higher values of total species richness and Shannon diversity were attained within the herbaceous vegetation type than within the woody type. These differences are most likely related to changes in local exposure to hydrogeomorphic disturbances driven by woody engineer plant species and to vegetation succession. A lower exotic species cover within the woody vegetation type than within the herbaceous type suggested an increase of resistance to invasion by exotic species during the biogeomorphic succession. The engineer effects of woody vegetation through landform construction resulted in a decrease of alpha (α) diversity at the patch scale but, in parallel, caused an increase in gamma (γ) diversity at the scale of the studied river segment. Our study corroborates recent investigations that support the theory of biotic acceptance of exotic species by native species at the local scale (generally <10 m²) within heterogeneous and disturbed environments. Furthermore, we suggest that in riparian contexts such as the River Tech exotic species trapp sediment at the same time as native species and thus contribute to the increase in ecosystem resistance during the biogeomorphic succession. Copyright © 2012 John Wiley & Sons, Ltd.     

Sensitivity to aerial exposure: potential of system-wide drawdowns to manage zebra mussels in the Mississippi River

We report the results of five experiments performed during periods of lowered river water level in Pool 26 of the Mississippi River. Four experiments compared survivorship of zebra mussels (Dreissena polymorpha) and unionid mussels (Unionidae). Under mild spring conditions (March), survivorship of zebra mussels was similar to that of unionid mussels, but during summer (July) survivorship of zebra mussels was lower than that of unionid mussels. Survivorship of zebra mussels was greatest when attached to native unionids, compared with detached zebra mussels and zebra mussels attached rocks. A fifth experiment compared survivorship of 10 species of unionid mussels after 24 hours of aerial exposure. In general, survivorship of thick-shelled species, such as Amblema plicata, was greater than survivorship of thin-shelled species, such as Potamilus ohiensis. The experiments conducted suggest that drawdowns during warm summer conditions could have a profound, negative influence on zebra mussel demography and distribution. In contrast, unionid mussel survival was unaffected by aerial exposure of up to 24 hours during a midsummer drawdown. © 1997 John Wiley & Sons, Ltd.     

Effects of spates of different magnitudes on a Phragmites japonica population on a sandbar of a frequently disturbed river

In flow‐regulated rivers, natural flow patterns and related sedimentation processes are disturbed, thus interrupting the magnitude and duration of spates. We hypothesized that this in turn might alter the impact of spates on plant communities inhabiting sandbars along the river channel. In order to test this hypothesis, the effects of spates of different magnitudes and durations on spatial variation and composition of a Phragmites japonica Steud. community on a sandbar during a 2‐year period were investigated. The flow variation of the river was continuously monitored, and the spates were categorized as small, medium or large, based on the magnitude (depth). Growth parameters (shoot length and shoot density), above‐ and belowground biomass of the plant community and the quantity of litter retained within selected quadrats, and ash‐free dry weight (AFDW) of the soil were observed at seven different locations: two each at the extreme upstream, middle and extreme downstream and one location in the downstream reaches of the sandbar. Our results showed that the aboveground biomass declined due to large‐scale spates; however, these spates triggered the formation of secondary shoots if sediments were accumulated as a result of the spates. Conversely, the aboveground biomass did not recover if the substrate was eroded. Accumulated litter beneath shoots was easily washed away due to spates, positively affecting shoot re‐growth. The effects of spates on the P. japonica plant community depended highly on the location of the plants on the sandbar and the magnitude and duration of the spates. The different erosion–sedimentation processes, depending on the local river channel morphology and spate types, significantly affected the growth and morphological characteristics of P. japonica population. Copyright © 2008 John Wiley & Sons, Ltd.     

Physical engineering of an island‐braided river by two riparian tree species: Evidence from aerial images and airborne lidar

Following a review of the European distribution, habitat requirements and life history characteristics of two riparian tree species, Alnus incana (L.) Moench (grey alder, a member of the Betulaceae family) and Populus nigra L. (black poplar, a member of the Salicaceae family), we explore their changing spatial distribution and topographic position within a 7 km, island braided reach of the Tagliamento River, Italy, where Populus nigra dominates the woody vegetation cover. Combining field observations and information extracted from aerial images, airborne lidar data and river flow time series for the period 1986–2017, we investigate (a) the changing spatial distribution of all (P. nigra‐dominated) woody vegetation and of A. incana alone; (b) whether river bed topography can be associated with these changing spatial distributions and (c) we consider whether A. incana displays any particular characteristics in its spatial and topographic distribution that may indicate that it is complementing the physical engineering role of P. nigra. We show that A. incana predominantly grows in lines along channel, island and floodplain edges, bordering wooded areas dominated by P. nigra and that areas supporting A. incana are associated with the topographic development of the river bed. We conclude that A. incana appears to be acting as a complementary physical engineer to P. nigra, suggesting that similar complementary physical engineering of river beds may be achieved by species with different life history traits to influence landform development in other river environments.     

Legacies of flood reduction on a Dryland river

The Bill Williams (Arizona) is a regulated dryland river that is being managed, in part, for biodiversity via flow management. To inform management, we contrasted riparian plant communities between the Bill Williams and an upstream free‐flowing tributary (Santa Maria). Goals of a first study (1996–1997) were to identify environmental controls on herbaceous species richness and compare richness among forest types. Analyses revealed that herbaceous species richness was negatively related to woody stem density, basal area and litter cover and positively related to light levels. Introduced Tamarix spp. was more frequent at the Bill Williams, but all three main forest types (Tamarix, Salix/Populus, Prosopis) had low understory richness, as well as high stem density and low light, on the Bill Williams as compared to the Santa Maria. The few edaphic differences between rivers (higher salinity at Bill Williams) had only weak connections with richness. A second study (2006–2007) focused on floristic richness at larger spatial scales. It revealed that during spring, and for the study cumulatively (spring and fall samplings combined), the riparian zone of the unregulated river had considerably more plant species. Annuals (vs. herbaceous perennials and woody species) showed the largest between‐river difference. Relative richness of exotic (vs. native) species did not differ. We conclude that: (1) The legacy of reduced scouring frequency and extent at the Bill Williams has reduced the open space available for colonization by annuals; and (2) Change in forest biomass structure, more so than change in forest composition, is the major driver of changes in plant species richness along this flow‐altered river. Our study informs dryland river management options by revealing trade‐offs that exist between forest biomass structure and plant species richness. Copyright © 2010 John Wiley & Sons, Ltd.     

Longitudinal patterns in flathead catfish relative abundance and length at age within a large river: effects of an urban gradient

We investigated the spatial variation of flathead catfish (Pylodictis olivaris) relative abundance and growth in the 274 km long Kansas River to determine if population dynamics of catfish are related to urbanization. Electrofishing was conducted at 462 random sites throughout the river in summer, 2005–2006 to collect fish. Relative abundance of age 1 fish (≤200 mm), subadult (>200–400 mm) and adult fish (>400 mm) ranged from 0.34 to 14.67 fish h^?1, mean length at age 1 was 165 (range: 128–195) mm total length (TL) and mean length at age 3 was 376 mm TL (range: 293–419 mm TL). The proportion of land use within 200 m of the river edge was between 0 and 0.54 urban. River reaches with high relative abundance of age 1 flathead catfish had high relative abundance of subadult and adult catfish. River reaches with fast flathead catfish growth to age 1 had fast growth to age 3. High urban land use and riprap in the riparian area were evident in river reaches near the heavily populated Kansas City and Topeka, Kansas, USA. Reaches with increased number of log jams and islands had decreased riparian agriculture. Areas of low urbanization had faster flathead catfish growth (r = 0.67, p = 0.005). Relative abundance of flathead catfish was higher in more agricultural areas (r = ?0.57, p = 0.02). Changes in land use in riverine environments may alter population dynamics of a fish species within a river. Spatial differences in population dynamics need to be considered when evaluating riverine fish populations. Published in 2008 by John Wiley & Sons, Ltd.     

Simulated recruitment of riparian trees and shrubs under natural and regulated flow regimes on the Wisconsin River,USA

Models that link ecological responses to hydrologic changes are important for assessing the effects of flow regulation on aquatic and riparian ecosystems. Based on the Recruitment Box Model, a graphical model used to prescribe environmental flows for cottonwood (Populus spp.) recruitment, we designed a simulation model to represent the influence of river flow dynamics on seedling recruitment of riparian pioneer woody plants. The model simulates the influence of temporal patterns of river stage on dispersal, germination, initial recruitment and over‐winter survival of first‐year seedlings of riparian pioneer shrubs and trees. We used the model to simulate seedling recruitment patterns for five species (Acer saccharinum, Betula nigra, Populus deltoides, Salix nigra and Salix exigua) on the Wisconsin River (Wisconsin, USA) under three flow scenarios: historic (1935–2002), simulated natural (1915–1975) and simulated regulated flows (1915–1975). Simulation results agreed well with field‐observed relative differences among years (1997–2000) in seedling densities for the five focal species. Simulated successful recruitment years were highly synchronous among species, but species differed in their sensitivity to flows at different times during the growing season, consistent with among‐species differences in seed dispersal timing. Comparison of simulated natural and regulated flows for 1915–1975 showed that flow regulation decreased monthly flow variability, increased late summer to winter baseflow and reduced the magnitude of spring peaks. Simulated recruitment and over‐winter survival of tree seedlings of all species was enhanced under the regulated flow scenario, likely due to increased summer baseflow and reductions in peak flood magnitude. Our analyses show the utility of extending the Recruitment Box Model to include multiple species of riparian shrubs and trees, and the effects of post‐colonization flows on their recruitment success. However, some key functional relationships between flow patterns and woody seedling demography (e.g. shear stress thresholds for seedling mortality) have not been adequately quantified and merit further study. Copyright © 2006 John Wiley & Sons, Ltd.     

River impoundment and sunfish growth

Impoundment of rivers by dams is widespread and one of the most devastating anthropogenic impacts to freshwater environments. Linking theoretical and applied research on river impoundment requires an improved capacity for predicting how varying degrees of impoundment affects a range of species. Here, growth of 14 North American sunfish species resilient to river impoundment was compared in rivers versus impoundments. Growth response to river impoundment varied widely, but consistently among taxa: five species (shadow bass, rock bass, flier, redbreast sunfish and green sunfish) showed significantly higher growth in riverine ecosystems, four species (largemouth bass, smallmouth bass, spotted bass and longear sunfish) showed significantly higher growth in impounded ecosystems, and five species (bluegill, black crappie, white crappie, redear sunfish and warmouth) displayed no difference in growth between rivers and impoundments. Furthermore, significant linear models were developed for predicting growth of two species (largemouth bass, R² = 0.75 and warmouth, R² = 0.44) based on a physiographically specific index of reservoir retention time. For another species (white crappie), growth could not be predicted by the retention time index in Central Lowlands rivers (R² = 0.001), but was strongly predicted by this factor in southeastern Coastal Plain rivers (R² = 0.76) showing how impacts of impoundment, and prediction of its consequences, can vary across river landscape types. Further analysis of fish growth in response to river impoundment, regulation and fragmentation could greatly enhance conservation biology, restoration ecology and basic land use decisions in riverine landscapes. Copyright © 2010 John Wiley & Sons, Ltd.     

FISH RECRUITMENT IS INFLUENCED BY RIVER FLOWS AND FLOODPLAIN INUNDATION AT APALACHICOLA RIVER,FLORIDA

High human demand for limited water resources often results in water allocation trade‐offs between human needs and natural flow regimes. Therefore, knowledge of ecosystem function in response to varying streamflow conditions is necessary for informing water allocation decisions. Our objective was to evaluate relationships between river flow and fish recruitment and growth patterns at the Apalachicola River, Florida, a regulated river, during 2003–2010. To test relationships of fish recruitment and growth as responses to river discharge, we used linear regression of (i) empirical catch in fall, (ii) back‐calculated catch, via cohort‐specific catch curves, and (iii) mean total length in fall of age 0 largemouth bass Micropterus salmoides, redear sunfish Lepomis microlophus and spotted sucker Minytrema melanops against spring–summer discharge measures in Apalachicola River. Empirical catch rates in fall for all three species showed positive and significant relationships to river discharge that sustained floodplain inundation during spring–summer. Back‐calculated catch at age 0 for the same species showed positive relationships to discharge measures, but possibly because of low sample sizes (n = 4–6), these linear regressions were not statistically significant. Mean total length for age 0 largemouth bass in fall showed a positive and significant relationship to spring–summer discharge; however, size in fall for age 0 redear sunfish and spotted sucker showed no relation to spring–summer discharge. Our results showed clear linkages among river discharge, floodplain inundation and fish recruitment, and they have implications for water management and allocation in the Apalachicola River basin. Managed flow regimes that reduce the frequency and duration of floodplain inundation during spring–summer will likely reduce stream fish recruitment. Copyright © 2012 John Wiley & Sons, Ltd.     

Population characteristics of five major commercial fish species of the Tono Reservoir in northern Ghana

Five major commercial fish species, comprising Sarotherodon galilaeus, Oreochromis niloticus, Coptodon zillii, Clarias gariepinus and Auchenoglanis occidentalis of the Tono Reservoir fishery, were studied from January, 2015 to December, 2016 using length‐based models to establish the state of the stocks. Routines in the FAO‐ICLARM Stock Assessment Tools II (FISAT II) were used to determine the growth and mortality parameters, exploitation rates (E) and recruitment patterns of the length–frequency data generated from commercial fish landings and experimental catches. The growth coefficients (K) of the five fish species ranged 0.48–4.89 per year; asymptotic lengths (L_∞) ranged from 18.38 to 76.16 cm; and hypothetical ages (t₀) ranged between 0.03 and 0.28 years. All five fish species were exploited above the optimal exploitation rate (E_opt = 0.5), indicating overfishing of the species. There was a year‐round recruitment, with major and minor peaks corresponding with the major and minor spawning seasons. Fishing gear regulation and community‐based co‐management system were some management options recommended to enhance sustainable exploitation and management of the fishery.     

Spatial Variation of Woody Riparian Vegetation across the Riverbank Topographic Gradient in Mediterranean Rivers: Species and Growth Categories

The composition and structure of riparian vegetation are linked to the natural hydrological variability and variation of environmental parameters in several spatial scales. The objective of this study is to determine the relationship between the spatial distribution of the woody riparian vegetation and the variation in the riverbank topographic gradient, verifying whether this variation was significant between species and/or growth categories. Specifically, our research examined the location of the woody species with respect to the thalweg along two reaches of the Jarama River in Central Spain. The positioning variables of each individual and distance and elevation above the thalweg were evaluated for four growth categories using statistical analysis. This study revealed that the positioning of the species along reaches is not random and differs with the species and growth categories. In addition, groups of species were specified in the different growth categories using similar positioning patterns with respect to the thalweg. Examples of similar groups of species in a specifically growth category were as follows: Alnus glutinosa–Salix alba–Salix fragilis for one reach and Alnus glutinosa–Populus nigra–Salix alba–Salix salviifolia for the other one. Topographical preference ranges of the riparian species and groups of the Jarama River were also obtained. The integration of data relative to the distribution of species along the topographic gradient can be very useful in identifying species with a preference for specific locations and can also contribute to the success of the measures adopted to restore these frequently highly degraded environments. Copyright © 2015 John Wiley & Sons, Ltd.     

Abundance of Invasive,Non‐Native Riparian Herbs in Relation to River Regulation

River regulation is associated with vegetation encroachment and invasions of some non‐native species in the semi‐arid west. Shifts in the abundance of native and non‐native woody riparian species are an interplay of regulation, life history traits and an array of flow and physical environmental variables. We sought to compare plant densities and per cent cover of several invasive species over two time periods in a paired river study, contrasting three different degrees of regulation along reaches of the Green and Yampa rivers in Colorado and Utah, USA. We censused patches of non‐native plants and recorded per cent cover in quadrats along 171 river km. The upper Green (10.1 patches ha^?1) had the highest invasive plant patch density followed by the lower Green (4.4 per ha) and the Yampa (3.3 per ha). Invasive species were present in 23%, 19% and 4% of sample quadrats, and an average of 0.28, 0.22 and 0.04 invasive species detected per square metre was recorded along the upper Green, lower Green and Yampa Rivers, respectively. Most species had significantly (p ≤ 0.02) higher percent cover on the upper Green than either or both the lower Green and the Yampa River. Whereas the less regulated river reaches maintain lower densities of invasive species than the most regulated reach, long‐term persistence of this pattern is still in question as some species patches showed notable increases on the Yampa and lower Green Rivers from 2002–2005 to 2010–2011. Although invasion is enhanced by flow regulation, life history traits of some species suggest invasion is likely, regardless of flow regulation. Published 2015. This article is a U.S. Government work and is in the public domain in the USA     

Spawning and recruitment patterns of major fish species in Bontanga Reservoir, Ghana, West Africa

The spawning and recruitment patterns of the major Cichlid fish species, including Hemichromis fasciatus, Oreochromis niloticus, Sarotherodon galilaeus, Tilapia zillii and other species, Auchenoglanis occidentalis, Brycinus nurse, Clarias gariepinus and Marcusenius senegalensis were studied for 24 months in Bontanga Reservoir, Ghana, using length‐based fish stock assessment approaches. The species spawned throughout the year, with two spawning pulses described as major and minor spawning seasons. The major spawning season occurred from March to September for the Cichlids, and from May to September for the other species. The minor spawning season, indicative of extended spawning, occurred from October to March for all the fish stocks. Fish spawning began with the onset of the rains in April/May, peaking by June/July, before the rainfall peak in August for all the fish stocks studied. Recruitment was found to occur throughout the year, with major and minor pulses coinciding with the major and minor spawning seasons. Accordingly, the most appropriate time for a possible closed fishing period appears to be from June to August, 1 month after the start of, and before the end of, the rains. The estimated mean standard length (L_m) for first time spawners of A. occidentalis, B. nurse, C. gariepinus and H. faciatus were 11.7, 12.7, 2.7 and 7.5 cm respectively. The estimated maturity–length ratio of 0.4 and 0.2 for O. niloticus, S. galilaeus and T. zillii were lower than the known 0.7 for normal growth, suggesting the tilapias matured faster, and at a smaller size, in the reservoir. Apparent sexual precocity associated with early maturity, year‐round spawning and recruitment were some important adaptations found to have sustained the reservoir fisheries, even during high fishing pressures. For conservation and sustainable exploitation of the fisheries, instituting a closed fishing season, mesh size regulations, withdrawal rights and a community‐based fishery management system are recommended.     

A Community‐Level,Mesoscale Analysis of Fish Assemblage Structure in Shoreline Habitats of a Large River Using Multivariate Regression Trees

The mesoscale (10⁰–10² m) of river habitats has been identified as the scale that simultaneously offers insights into ecological structure and falls within the practical bounds of river management. Mesoscale habitat (mesohabitat) classifications for relatively large rivers, however, are underdeveloped compared with those produced for smaller streams. Approaches to habitat modelling have traditionally focused on individual species or proceeded on a species‐by‐species basis. This is particularly problematic in larger rivers where the effects of biological interactions are more complex and intense. Community‐level approaches can rapidly model many species simultaneously, thereby integrating the effects of biological interactions while providing information on the relative importance of environmental variables in structuring the community. One such community‐level approach, multivariate regression trees, was applied in order to determine the relative influences of abiotic factors on fish assemblages within shoreline mesohabitats of San Pedro River, Chile, and to define reference communities prior to the planned construction of a hydroelectric power plant. Flow depth, bank materials and the availability of riparian and instream cover, including woody debris, were the main variables driving differences between the assemblages. Species strongly indicative of distinctive mesohabitat types included the endemic Galaxias platei. Among other outcomes, the results provide information on the impact of non‐native salmonids on river‐dwelling Galaxias platei, suggesting a degree of habitat segregation between these taxa based on flow depth. The results support the use of the mesohabitat concept in large, relatively pristine river systems, and they represent a basis for assessing the impact of any future hydroelectric power plant construction and operation. By combing community classifications with simple sets of environmental rules, the multivariate regression trees produced can be used to predict the community structure of any mesohabitat along the reach. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of river discharge on abundance and instantaneous growth of age‐0 carpsuckers in the Oconee River,Georgia, USA

The Oconee River in middle Georgia, U.S.A., has been regulated by the Sinclair Dam since 1953. Since then, the habitat of the lower Oconee River has been altered and the river has become more incised. The altered environmental conditions of the Oconee River may limit the success of various fish populations. Some obligate riverine fishes may be good indicator species for assessing river system integrity because they are intolerant to unfavourable conditions. For example, many sucker species require clean gravel for feeding and reproduction. Further, age‐0 fishes are more vulnerable than adults to flow alterations because of their limited ability to react to such conditions. In this study, we investigated the relationship between abundance and growth of age‐0 carpsuckers to river discharge in the Oconee River. A beach seine was used to collect age‐0 carpsuckers (Carpiodes spp.) from littoral zones of the lower Oconee River from May through July of 1995 to 2001. Regression models were used to assess whether 12 river discharge categories (e.g. peak, low, seasonal flows) influenced age‐0 carpsucker abundance or instantaneous growth. Our analysis indicated that abundance of age‐0 carpsuckers was significantly negatively related to number of days river discharge was >85 m³ s^?1(r² = 0.61, p = 0.04). Estimates of instantaneous growth ranged from 0.10 to 0.90. Instantaneous growth rates were significantly positively related to summer river discharge (r² = 0.95, p <0.01). These results suggest that (1) moderate flows during spawning and rearing are important for producing strong‐year classes of carpsuckers, and (2) river discharge is variable among years, with suitable flows for strong year‐classes of carpsuckers occurring every few years. River management should attempt to regulate river discharge to simulate historic flows typical for the region when possible. Such an approach is best achieved when regional climatic conditions are considered. Published in 2007 by John Wiley & Sons, Ltd.     

Hydrodynamic behaviour of European black poplar (Populus nigra L.) under coppice management along Mediterranean river ecosystems

This study examined the hydrodynamic behaviour of European black poplar (Populus nigra L.) under coppice management in riparian areas with a multidisciplinary approach. An innovative procedure on the basis of the combination of plant allometric relations and hydrodynamic models was applied to assess drag forces and plant hydrodynamic bending as function of the basal diameter and module of elasticity, with a probabilistic approach. Cuttings of European black poplar from 2 close riverine environments of Southern Italy have been planted and subjected to the same coppice management strategy. The 2 different 3‐year‐old shoot poplar ensembles exhibited statistically similar morphological traits but stems with different module of elasticity. Drag forces were simulated with a model on the basis of the vegetative Cauchy number. Plant deformation under the hydrodynamic load was modelled as a base‐mounted cantilever beam. The differences in the observed elasticity were verified to be also significant from a hydrodynamic perspective. Diagrams were drawn to describe plant bending, drag forces, and basal momentum as function of basal diameter, accounting for the uncertainty in the module of elasticity. These results can be exploited for identifying objective hydrodynamic criteria to be adopted for coppice management of riparian woody vegetation in human controlled river ecosystems.     

Advection,growth and nutrient status of phytoplankton populations in the lower River Murray,South Australia

To investigate the link between river flow, nutrient availability and development of algal blooms, growth rates of the major phytoplankton species were examined in situ in the lower River Murray, South Australia over the 1994/1995 summer. Eight sites were selected over a 54 km reach between Lock 1 and Nildottie and growth rates estimated by monitoring mean cell density in time‐aligned parcels of water as they travelled downstream. Discharge at Lock 1 during the period of study (3000–5000 ML day⁻¹) typified summer entitlement flows to South Australia. A large, shallow floodplain lake (lagoon), with an hydraulic connection to the river, supported a large population of cyanobacteria in summer, but inputs to the main channel did not substantially affect the abundance and composition of river phytoplankton. Mean net growth rates of Anabaena circinalis and A. flos‐aquae f. flos‐aquae were 0.132 and 0.176 day⁻¹, respectively, although individual rates varied from positive to negative. In contrast, the mean growth rate of the filamentous diatom Aulacoseira granulata was −0.15 day⁻¹, reflecting a decrease in population size with advection downstream. Mean cell densities of the three species did not exceed 5000 cells mL⁻¹ throughout the study. Growth bioassays conducted in the laboratory indicated that nitrogen was often the nutrient limiting algal growth, although it was not established whether nitrogen was limiting in situ. A conceptual model is presented, linking these findings with those of other work on the lower River Murray, to summarize the physical and chemical environmental factors governing the abundance of cyanobacteria in this reach of the river. Copyright © 2000 John Wiley & Sons, Ltd.     

Effects of river regulation on the structure of a fast-growing brown trout (Salmo trutta L.) population

During the last 70 years, the Norwegian lake Mjøsa and its inflowing rivers have been subjected to serious changes due to hydroelectric power development. Regulation of the main inlet river, Gudbrandsdalslagen, started in 1919. The river power station at the Hunder fall was completed in 1964. This resulted in a reduction of winter water flow below the Hunder dam from approximately 26m³s^?1 to 2m³s^?1, which affected the most important spawning area of the fast-growing population of brown trout, Salmo trutta L. The population was investigated in detail in 1907, 1909, 1961, and 1985, and river growth, smolt age, and growth in Lake Mjøsa are compared. Only wild fish were included in the study. The main pattern throughout this period shows an increased river growth rate before smoltification and reduced smolt age. The average smolt age dropped from 4.7 years in 1909 to 4.1 years in 1985, and at the same time smolt size decreased from 26.8 cm to 25.1 cm. Considering the major changes in abiotic factors in the river spawning section, the changes in age structure and growth of brown trout smolt are comparatively small. In Lake Mjøsa, increased productivity due to input of nutrients has obviously favoured forage fish such as smelt (Osmerus eperlanus (L.)) and vendace (Coregonus albula (L.)). The growth rate of brown trout in the lake has improved from 1909 to 1961 and 1985, followed by a reduced spawning age. However, due to increased human exploitation the average length of ascending fish (approximately 68 cm) and condition factor ( K = 1.14–1.16) have altered little.     

北京妫水河湿地植物多样性调查与评价研究

为探究北方缺水地区湿地植物物种多样性变化,选取北京市延庆区妫水河为研究区,采用物种丰富度(R),Shannon-Wiener指数(H),Simpson指数(D)和Peilou均匀度指数(J)作为植物群落多样性指标,使用方差法和典型对应分析(CCA)对不同河段和区域间湿地植物物种分布与环境因子之间的响应关系进行研究。研究结果表明:研究区现有湿地植物93种,隶属于44科76属,优势科有菊科(Asteraceae)、禾本科(Poaceae)、豆科(Fabaceae)和莎草科(Cyperaceae); 93种植物的Shannon-Wiener指数(H)小于3,Simpson指数(D)和Peilou均匀度指数(J)介于0. 5～0. 75之间,物种数量总体不多,但分布较均匀,处于中等水平;对湿地植物群落形成及生长影响较大的水质因子依次为全氮(TN)、p H和化学需氧量(COD)。