Habitat loss as the main cause of the slow recovery of fish faunas of regulated large rivers in Europe: the transversal floodplain gradient

In large European rivers the chemical water quality has improved markedly in recent decades, yet the recovery of the fish fauna is not proceeding accordingly. Important causes are the loss of habitats in the main river channels and their floodplains, and the diminished hydrological connectivity between them. In this study we investigate how river regulation has affected fish community structure in floodplain waterbodies of the rivers Rhône (France), Danube (Austria), Rhine and Meuse (The Netherlands). A typology of natural and man‐made aquatic habitats was constructed based on geomorphology, inundation frequency and ecological connectivity, along the transversal river–floodplain gradient, i.e. perpendicular to the main stream of the river. Fish species were classified in ecological guilds based on their flow preference, reproduction ecology and diet, and their status on national red lists was used to analyse the present state of the guilds and habitats. Ecological fish guilds appear to be good indicators of ecological integrity and functioning of river–floodplain systems. A transversal successional gradient in fish community structure that bears some resemblance to the gradient found in natural rivers can still be discerned in heavily regulated rivers. It resembles the longitudinal river gradient; even some predictions of the River Continuum Concept are applicable. Overall, richness and diversity of species and ecological guilds decrease with decreasing hydrological connectivity of floodplain waterbodies. Anthropogenic disturbances have affected fish species unevenly: guilds of specialized species that are highly adapted to specifically riverine conditions have declined far more than generalist species. Fish habitats in the main and secondary channels have suffered most from regulation and contain the highest percentage of threatened species. Rheophilic fishes have become rare because their lotic reproductive habitats are severely degraded, fragmented, absent or unreachable. Limnophilic fishes have become rare too, mainly as a result of eutrophication. Eurytopic fishes have become dominant everywhere. Copyright © 2003 John Wiley & Sons, Ltd.     

SHORT‐TERM IMPACTS OF LATERAL HYDROLOGICAL CONNECTIVITY RESTORATION ON AQUATIC MACROINVERTEBRATES

In floodplain ecosystems, the lateral hydrological connectivity between the main river channel and the secondary channels plays a major role in shaping both the habitat conditions and the macroinvertebrate diversity. Among other threats, human activities tend to reduce the lateral connectivity, which increases floodplain terrestrialization and induces a loss of aquatic biodiversity. Consequently, the restoration of lateral connectivity is of growing concern. We studied four secondary channels of the Rhône floodplain that were subjected either to no restoration or to three different restoration measures (river flow increase only, flow increase plus dredging and flow increase plus reconnection to the river). Macroinvertebrate and environmental data were analysed one year before and during a period of five years after restoration. We expected a progressive increase of lateral connectivity according to the type of restoration. Changes in macroinvertebrate assemblages were predicted to be towards more rheophilic communities and proportionally related to the changes in lateral connectivity. In the reconnected channel, lateral connectivity increased and remained high five years after restoration. In the dredged channel, the immediate increase of the lateral connectivity metric induced by sediment removal was followed by a rapid decrease. In the unrestored channel and the channel only influenced by flow increase, the metric remained constant in time. The macroinvertebrate composition and the rarefied EPT richness changes were proportionally related to the changes in lateral connectivity. Alien species richness and densities increased progressively in all channels after restoration. Our results showed that modifications of the lateral connectivity lead to predictable changes in macroinvertebrate diversity. Synergistic interactions between restoration and longer‐term changes (e.g. climatic change, invasion of alien species) encourage long‐term monitoring to assess the durability and trends of restoration measures. Copyright © 2012 John Wiley & Sons, Ltd.     

Benefits to 0+ fishes of connecting man‐made waterbodies to the lower River Trent,England

Floodplain waterbodies are reputed to enhance recruitment of riverine fish populations via provision of spawning and nursery habitat, refuge from floods, and increased availability of planktonic food resources compared with main river channels. Notwithstanding, there have been few parallel studies of fishes and their food resources at both main river and floodplain sites. Thus, this study investigated the 0+ fishes, zooplankton and phytoplankton (chlorophyll a) at four main river and four (man‐made) floodplain sites on the lower River Trent, England, between May 1999 and October 2004 inclusive. All sites shared the same key fish species, and there were no consistent differences in the densities, growth or condition of 0+ fishes from main river and floodplain sites. Similarly, all sites shared the same key zooplankton taxa. However, zooplankton densities, notably of large‐bodied cladocerans, and chlorophyll a concentrations, were significantly higher at floodplain sites than at main river sites. Thus, connection of man‐made waterbodies has the potential to enhance recruitment of riverine fish populations via provision of important spawning and nursery habitat, and superior feeding opportunities for 0+ fishes compared with main river channels. Copyright © 2007 John Wiley & Sons, Ltd.     

Exploring functional flow heterogeneity in regulated flow regime: Fish species turnover along hydraulic gradients in an artificial waterway network

Humans have altered river flows leading to a loss of connection with floodplain habitats. The expansion of agriculture in floodplains has resulted in landscapes dominated by irrigated farmland. A key challenge in water management is to conserve existing ecological communities and habitat heterogeneity, while simultaneously maintaining engineered infrastructure for agriculture. In this study, we focused on an artificial channel network for irrigation with a regulated flow regime and its function as habitat for various fish species. Differences in hydraulic conditions among channels and compositional changes in fish species were examined to clarify functional flow heterogeneity. Analyzed using pairwise Simpson dissimilarity among sampling reaches, species turnover was positively associated with Froude number (flow intensity) differences at intermediate discharges, and with differences in cross-sectional areas (flow magnitude) at low discharges. Drastic changes in inflows should be considered for the effective conservation of flow heterogeneity, even under a regulated flow regime. Improved engineering design to manage the hydraulic environment is one option for maintaining the ecological value of lateral waterbodies in human-dominated landscapes. Our findings provide insights into the importance of functional flow heterogeneity to conserve fish species diversity.     

Reconstruction of the characteristics of a natural alluvial river–floodplain system and hydromorphological changes following human modifications: the Danube River (1812–1991)

Based on detailed historical surveys from 1812, the natural riverine landscape of a 10.25‐km‐long reach of the Danube River in the Austrian Machland region prior to channelization is analysed. Anthropogenically induced changes of fluvial dynamics, hydrological connectivity and aquatic habitat composition are discussed, comparing the situations following channelization (1925) and flow regulation (1991). In 1812 the alluvial river–floodplain system of the Danube River comprised a highly complex channel network, numerous gravel bars and extensive islands, with the main channel and side arms (eupotamon) representing about 97% of the entire water surface at low flow. The floodplain was characterized by relatively flat terrain and numerous natural trenches (former active channels) connected to the main channel. These hydromorphological conditions led to marked expansion/contraction of the water surface area at water level fluctuations below bankfull (‘flow pulse’). The high degree of hydrological connectivity enabled intensive exchange processes and favoured migrations of aquatic organisms between the river and floodplain habitats over a period of approximately 90 days per year. Overall in 1812, 57% of the active zone (active channels and floodplain) was inundated at bankfull water level. Channelization and construction of hydropower plants resulted in a truncated fluvial system. Consequently, eupotamal water bodies decreased by 65%, and gravel/sand bars and vegetated islands decreased by 94% and 97%, respectively, whereas the area of the various backwaters doubled. In 1991 the former ‘flow pulse’ was halved due to artificial levees and embankments, greatly diminishing hydrological connectivity and decoupling large areas of the floodplain from the main channel. Active overflow, formerly playing an important role, is now replaced by backwater flooding and seepage inflow in isolated water bodies. Copyright © 2003 John Wiley & Sons, Ltd.     

Mesoscale river restoration enhances the diversity of floodplain vegetation

Effective river restoration aims for the recovery of ecosystem functions by restoring processes and connectivity to the floodplain. At the straightened lowland river Stör in northern Germany, a sequence of 15 new meanders was created in 2008, with wavelengths up to 70 m. The newly created areas within the meander bends range in size from 215 to 1,115 m² and function as a series of 15 restored floodplain sites, which are subject to succession. After 7 years of restoration measures, we investigated the vegetation dynamics on the (a) restored floodplains and compared them with adjacent floodplain sites that were used as (b) low‐intensity grazed grassland or as (c) abandoned grassland. We analysed the species diversity, functional vegetation parameters, and plant communities of 200 plots within the floodplain area of the three floodplain types and of 246 plots at their river banks. Plant species diversity and composition differed with respect to restoration measure and site management. Restored floodplains revealed a higher coverage in species of wet grasslands and softwood forests and higher species diversity than abandoned grasslands. Grazed grasslands showed the highest species number and coverages of pioneer vegetation. The banks indicated fewer differences in species composition between floodplain types. The construction of restored floodplains revealed greater overall plant diversity due to promoting the development of typical floodplain vegetation. Shallow meanders with increased flooding intensity and the creation of a varying microreliefs are recommended as combined river/floodplain measures in order to foster processes and connectivity between valley components.     

Fish community response to habitat variables in two restored side channels of the lower Platte River,Nebraska

Anthropogenic alterations to large rivers ranging from impoundments to channelization and levees have caused many rivers to no longer access the floodplain in a meaningful capacity. Floodplain habitats are important to many riverine fishes to complete their life‐history strategies. The fish community and species of fish that inhabit floodplain habitats are often dictated by the type of habitat and the conditions within that habitat (e.g., temperature, water velocity, depth, and discharge). As mitigation and restoration projects are undertaken, it is imperative that managers understand how various habitat components will affect the fish community in floodplain habitats. We collected fish and habitat data from two restored side channels with different structural designs on the lower Platte River, Nebraska, to determine how habitat variables predicted species diversity and individual species presence. We found a decrease in discharge in the main‐stem river resulted in increased diversity in one of the side channels, with the greatest diversity values occurring during summer. No habitat variables performed well for predicting fish species diversity for an adjacent side channel with more uniform depth and velocity and no groundwater inputs. However, several native riverine fish species in this side channel were shown to be associated with high temperature, dissolved oxygen, main‐stem discharge, and discharge variability. These results highlight the importance of considering the physical design of restored floodplain habitats when attempting to enhance fish communities.     

Patchiness in a Large Floodplain River: Associations Among Hydrology,Nutrients, and Fish Communities

Large floodplain rivers have internal structures shaped by directions and rates of water movement. In a previous study, we showed that spatial variation in local current velocities and degrees of hydrological exchange creates a patch‐work mosaic of nitrogen and phosphorus concentrations and ratios in the Upper Mississippi River. Here, we used long‐term fish and limnological data sets to test the hypothesis that fish communities differ between the previously identified patches defined by high or low nitrogen to phosphorus ratios (TN:TP) and to determine the extent to which select limnological covariates might explain those differences. Species considered as habitat generalists were common in both patch types but were at least 2 times as abundant in low TN:TP patches. Dominance by these species resulted in lower diversity in low TN:TP patches, whereas an increased relative abundance of a number of rheophilic (flow‐dependent) species resulted in higher diversity and a more even species distribution in high TN:TP patches. Of the limnological variables considered, the strongest predictor of fish species assemblage and diversity was water flow velocity, indicating that spatial patterns in water‐mediated connectivity may act as the main driver of both local nutrient concentrations and fish community composition in these reaches. The coupling among hydrology, biogeochemistry, and biodiversity in these river reaches suggests that landscape‐scale restoration projects that manipulate hydrogeomorphic patterns may also modify the spatial mosaic of nutrients and fish communities. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.     

Impacts of water resource development on hydrological connectivity of different floodplain habitats in a highly variable system

Floodplains and their associated wetlands are important features of semiarid and arid landscapes, providing habitat and refugia for native species as well as contributing to human needs for freshwater. Globally, floodplain habitats are some of the most modified ecological communities because of water resource development and land‐use changes. However, the hydrological changes that have occurred in highly variable semiarid and arid systems are rarely quantified in a way that helps us understand the consequences for different floodplain habitat types. This study investigated changes in floodplain‐river connectivity that have occurred because of water resource development on four floodplain habitat types in the Lachlan River Catchment, Australia: (a) temporary floodplain lakes, (b) intermittent river red gum (Eucalyptus camaldulensis) swamps, (c) intermittent black box (Eucalyptus largiflorens) swamps, and (d) terminal wetlands (wetlands along distributary creeks). Changes to floodplain‐river connectivity characteristics were calculated using their commence to fill thresholds and flow scenarios derived from a river hydrology model, enabling comparison of long‐term data sets (120 years) encompassing a range of climate conditions. Connection regime metrics have changed significantly in all floodplain habitats except intermittent black box swamps. Temporary floodplain lakes have experienced the greatest reduction in number of connection events (60% reduction), followed by intermittent river red gum swamps (55% reduction). Intermittent black box swamps and terminal wetlands have experienced the least change in number of connection events (35% reduction). The nature of the change in connection suggests a change in vegetation communities will occur in response to long‐term hydrological change.     

THE INFLUENCE OF HYDROLOGICAL CONNECTIVITY ON FOOD WEB STRUCTURE IN FLOODPLAIN LAKES

Hydrological connectivity is an important driver of ecosystem structure in floodplain rivers; however, little is known of how hydrological connectivity affects the structure and functioning of food webs in these systems. This study examines aquatic food web structure in 10 floodplain lakes on a dryland river floodplain in eastern Australia across a connectivity gradient. Results for fishes suggest that benthic carbon sources are more important in high connectivity billabongs than in low connectivity billabongs and that pelagic sources are more important in low connectivity billabongs than in high connectivity billabongs. Fishes in less connected billabongs were also found to feed at higher trophic levels than in more connected billabongs. We hypothesize that in high connectivity billabongs, where suitable benthic primary sources are abundant, common fish species such as carp and bony bream feed as detritivores or herbivores; while in low connectivity billabongs, where benthic sources are less abundant, the same species feed as planktivores, insectivores or piscivores. This dietary difference may also be promoted by greater predation efficiency in less structurally complex low connectivity billabongs. The feeding behaviour of these fish species subsequently influences the trophic positions of fishes higher in the food chain and ultimately the total food chain lengths that high and low connectivity billabongs support. The results of this study highlight the importance of hydrological connectivity to the structure of food webs in these systems and the potential for them to be affected by water resource development. Copyright © 2011 John Wiley & Sons, Ltd.     

Hydrologic connectivity and land cover affect floodplain lake water quality,fish abundance,and fish diversity in floodplain lakes of the Wabash-White River basin

Floodplain lakes are important aquatic resources for supporting ecosystem services, such as organismal habitat, biodiversity, and the retention of nutrients and sediment. Due to geomorphic alteration of river channels and land-cover change, degradation to floodplain lakes in the Ohio River basin is occurring at a rate that will escalate as climate change causes increased flood intensity and the seasonal redistribution of rainfall. A better understanding of the local drivers that affect oxbow lakes is needed for targeted floodplain restoration efforts designed to slow degradation. We examined the effects of land cover, topography, and hydrologic connectivity on water quality and fish diversity and abundance in nine floodplain lakes with potentially high remnant ecological function in the Wabash-White watershed (Indiana, Ohio, and Illinois). Data collection included water-quality parameters; stable water isotopes; total phosphorus, total nitrogen, and chlorophyll-a; and fish community diversity and abundance. Results indicate that hay/pasture land cover and decreased topographic relief in the local oxbow watersheds, along with reduced river hydrologic connectivity, were related to an increase in total phosphorus, total nitrogen, and chlorophyll-a. Greater biodiversity and abundance in fish assemblages were evident in oxbow lakes that were more disconnected from the main channel. The results of this study suggest that hydrologic connectivity of oxbow lakes with the contributing drainage area and the main channel influence nutrients and fish communities. Knowing the influencing factors can help ecosystem managers better protect these valuable floodplain lake ecosystems and prioritize restoration efforts amidst increasing stressors due to climate and land-use changes.     

GEOMORPHOLOGY AND FLOODING SHAPE FISH DISTRIBUTION IN A LARGE‐SCALE TEMPERATE FLOODPLAIN

Natural river‐floodplain systems are characterized by their dynamic hydrology and diverse geomorphology resulting in a wide range of habitats that support high fish diversity and production. Various factors (e.g. hydrological dynamics, water quality, and biotic processes) have been proposed to explain fish distribution in large river floodplains, but it is still widely acknowledged that the mechanisms involved may vary in diverse floodplain systems and that they are not fully understood. To determine how flooding dynamics and floodplain geomorphology influence fish species distributions across the Volga‐Akhtuba floodplain, Russian Federation, we examined the distributions of eight species with respect to variables reflecting floodplain hydrology and geomorphology. On the basis of fish catches in 40 floodplain water bodies at the end of summer in 2006–2008, we found that frequency of occurrence of most fish species remained stable along the time. The distribution of fish species was strongly influenced by the size and shape of water bodies as well as flood extent. Therefore, the long‐term flood variability that drives the geomorphic heterogeneity of the floodplain creates suitable habitats across ranges of fish flow guilds (rheophilic, eurytopic, and limnophilic), resulting in high diversity of the floodplain ichthyofauna. We conclude that this diverse habitat availability is a highly significant factor influencing fish distribution in the Volga‐Akhtuba floodplain. Copyright © 2012 John Wiley & Sons, Ltd.     

Temporal variations in organic carbon utilization by consumers in a lowland river

Understanding temporal trophic interactions in riverine food webs is essential for predicting river ecosystem function and improving management of these ecosystems. Temporal changes in energy flows through riverine food webs are readily assumed but are rarely tested. Temporal variability in food webs from two reaches of a lowland river (Ovens River, south‐eastern Australia) with differing levels of floodplain connectivity were examined over 12 months. We investigated how seasonal changes, flow variability and floodplain connectivity influence (i) stable isotope signatures of basal organic carbon sources (terrestrial sources: trees and grasses; aquatic sources: seston, biofilm and filamentous algae) and consumers (macroinvertebrates and fish) and (ii) the relative proportions of organic carbon sources contributing to consumer biomass using mixing models. We hypothesized (i) that during high flows, increased floodplain connectivity would increase the lateral exchange of terrestrial carbon subsidies to main channel consumers and (ii) that during low flow periods, main channel consumers would derive the majority of their carbon from aquatic benthic sources. Results indicated that isotope signatures for basal sources and for most of the consumers varied temporally and spatially. Mixing models indicated that increased floodplain connectivity did not increase terrestrial subsidies to consumers during high flows. Seston was the primary source during high flows whilst terrestrial vegetation increased in importance during low flows. Filamentous algae was also important during low flows for some consumers. These findings indicate that it is essential to include temporal variability in order to understand energy flows in lowland rivers, thus allowing for the dynamic nature of these ecosystems. Copyright © 2010 John Wiley & Sons, Ltd.     

Dynamics of river fish populations in response to hydrological conditions: a simulation study

Increasing multi‐sectoral demands on water resources have led to water abstraction and transfer activities, and the construction of dams and embankments that have significantly altered the flood regimes of rivers throughout the world resulting in the loss of fish production and biodiversity. The current emphasis on sustainable development and biodiversity conservation is leading efforts to mitigate these impacts by means of interventions such as the release of artificial floods downstream of dams and the manipulation of water levels within impounded floodplains. Whilst much work has been done to determine the hydrological requirements for the maintenance of salmonid populations, few equivalent studies are available from which to develop criteria for the management of hydrological regimes for fishes and fisheries in large floodplain–river systems such as the Mekong. The population dynamics of fish in such rivers are believed to respond to hydrological conditions in a density‐dependent manner. An age‐structured population dynamics model incorporating sub‐models describing density‐dependent growth, mortality and recruitment was used to explore how hydrological conditions within a theoretical floodplain–river system affect the dynamics of a common floodplain–river fish species. Graphical summaries of the response of exploitable biomass to a range of different drawdown rates, dry and flood season areas and volumes, and flood season durations are presented under five different model assumptions concerning density‐dependent processes. Optimal flooding patterns are also described for the model species and theoretical river system. The patterns of predictions that emerge from the simulations provide guidelines for managing or manipulating hydrological conditions in river systems for both fixed and variable volume hydrological scenarios. As a general rule of thumb, exploitable biomass is maximized by minimizing the rate of drawdown and maximizing the flood duration and flood and dry season areas or volumes. However, experiences from dam and other hydraulic engineering projects suggest that these predictions should be treated with caution until we better understand the influence of hydrology on spawning behaviour, system primary production, and critical habitat availability. Copyright © 2004 John Wiley & Sons, Ltd.     

DISCONNECTING THE FLOODPLAIN: EARTHWORKS AND THEIR ECOLOGICAL EFFECT ON A DRYLAND FLOODPLAIN IN THE MURRAY–DARLING BASIN,AUSTRALIA

Globally, dams and water extractions are well‐recognised disruptors of flow regimes in floodplain wetlands, but little is known of the hydrological and ecological impacts of floodplain earthworks constructed for irrigation, flood mitigation and erosion control. We mapped the distribution of earthworks with high‐resolution SPOT (Système Probatoire d'Observation de la Terre) imagery in an internationally recognised Ramsar wetland, the Macquarie Marshes of the Murray–Darling Basin, Australia. There were 339 km levees, 1648 km channels, 54 off‐river storages and 664 tanks (0.5–5 m high), detected within the 4793 km² floodplain study area. Earthworks reduced localised flooding compared with undeveloped sites. The most pronounced disconnection of the original floodplain (73.0%) occurred where earthworks were most concentrated compared with areas with few earthworks (53.2%). We investigated relationships between hydrological connectivity and mortality of the perennial flood‐dependent river red gum Eucalyptus camaldulensis at 55 floodplain sites (225 × 150 m). Over half of the river red gums were dead at 21.8% of the sites. Earthworks blocked surface flows to flood‐dependent vegetation and drowned vegetation in artificially inundated off‐river storages. Mortality was due to impacts of earthworks and potentially exacerbated by effects of river regulation, water extraction and climate. River red gums were healthiest in narrow river corridors where earthworks confined flows and flows could recede freely. Rehabilitation of flood‐dependent ecosystems should focus on reinstating lateral connectivity and protecting environmental flows. Copyright © 2011 John Wiley & Sons, Ltd.     

HYDROLOGICAL CONNECTIVITY OF ABANDONED CHANNEL WATER BODIES ON A COASTAL PLAIN RIVER

Oxbow lakes, sloughs and other floodplain depressions associated with former channel positions are critical elements of floodplain hydrology, geomorphology and ecology. They comprise key elements of wetland and aquatic habitats and have important influence on the storage and routing of floodwaters. The hydrological connectivity between active river channels and floodplain depressions varies considerably in a qualitative sense, even within a single fluvial system. Several oxbows, sloughs and paleochannels were examined on the lower Sabine River, Texas/Louisiana, during a period of high but sub‐bankfull flow as well as at lower flows. Six different types of surface water connectivity with the main, active channel were identified: (i) flow through—a portion of the river flow regularly passes through the feature and returns to the main channel; (ii) flood channel—there is no hydraulic connection at normal flows, but at high flows the channels convey discharge, at least part of which returns to the main channel; (iii) fill and spill—the features fill to a threshold level at high flows and then overflow (mainly via ephemeral channels) into flood basins; (iv) fill and drain—the features fill at high river discharges but do not (except in large floods) overflow because as river discharge declines, water drains back to the river; (v) tributary occupied—tributaries draining to the abandoned channel continue to occupy it, flowing through it to the active channel; and (vi) disconnected—no flow is exchanged except during large floods. The age or stage of infilling and the relative elevation of abandoned channels are important first‐order controls of hydrological connectivity, but the lateral distance from the active channel is poorly related. Other critical controls are whether the cutoff section receives tributary input and whether a tie channel forms. The alluvial valley geomorphic context—specifically the presence of a meander belt ridge and flood basins—is also critical. Copyright © 2011 John Wiley & Sons, Ltd.     

HYDROLOGIC CONNECTIVITY OF FLOODPLAINS,NORTHERN MISSOURI—IMPLICATIONS FOR MANAGEMENT AND RESTORATION OF FLOODPLAIN FOREST COMMUNITIES IN DISTURBED LANDSCAPES

Hydrologic connectivity between the channel and floodplain is thought to be a dominant factor determining floodplain processes and characteristics of floodplain forests. We explored the role of hydrologic connectivity in explaining floodplain forest community composition along streams in northern Missouri, USA. Hydrologic analyses at 20 streamgages (207–5827 km² area) document that magnitudes of 2‐year return floods increase systematically with increasing drainage area whereas the average annual number and durations of floodplain‐connecting events decrease. Flow durations above the active‐channel shelf vary little with increasing drainage area, indicating that the active‐channel shelf is in quasi‐equilibrium with prevailing conditions. The downstream decrease in connectivity is associated with downstream increase in channel incision. These relations at streamflow gaging stations are consistent with regional channel disturbance patterns: channel incision increases downstream, whereas upstream reaches have either not incised or adjusted to incision by forming new equilibrium floodplains. These results provide a framework to explain landscape‐scale variations in composition of floodplain forest communities in northern Missouri. Faust ( 2006 ) had tentatively explained increases of flood‐dependent tree species, and decreases of species diversity, with a downstream increase in flood magnitude and duration. Because frequency and duration of floodplain‐connecting events do not increase downstream, we hypothesize instead that increases in relative abundance of flood‐dependent trees at larger drainage area result from increasing size of disturbance patches. Bank‐overtopping floods at larger drainage area create large, open, depositional landforms that promoted the regeneration of shade‐intolerant species. Higher tree species diversity in floodplains with small drainage areas is associated with non‐incised floodplains that are frequently connected to their channels and therefore subject to greater effective hydrologic variability compared with downstream floodplains. Understanding the landscape‐scale geomorphic and hydrologic controls on floodplain connectivity provides a basis for more effective management and restoration of floodplain forest communities. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

River regulation influences the composition and distribution of inland frog communities

Frogs are widespread through inland rivers and floodplains and are an important component of floodplain food chains. Despite this, studies of frog communities in inland river systems are limited and the impacts of river regulation on frog communities have received very little attention. Surveys for frogs, tadpoles and egg masses along with assessment of vegetation, hydrology and water chemistry were conducted along 10 km reaches of three creek systems in the Lachlan River catchment, a major regulated river in Australia's Murray–Darling Basin. A total of 23 sample sites were surveyed at locations above and below in‐stream weirs as well as adjacent floodplain depressions. The hydrological regimes of sample sites were classified according to the length of time that they were known to hold water (water permanence). The sample sites fell into two distinct categories, 14 were classified as permanent and occurred upstream of weirs while nine were classified as temporary and were located downstream of weirs and in depressions adjacent to the weir pool. Permanent sample sites had a significantly higher percentage of dead standing timber and were deeper with less aquatic vegetation cover than temporary sample sites. Seven frog species were identified; there were no significant differences in species richness between permanent and the temporary waterbodies but the composition of frog assemblages differed significantly between them. This suggests that alteration to the hydrology of inland creek systems can lead to changes in the distribution frog species, with some becoming more common due to increases in the availably of permanent waterbodies while others decline due to reductions in the availability of seasonally flooded waterbodies. Copyright © 2010 John Wiley & Sons, Ltd.     

HYDROLOGICAL GRADIENT AND SPECIES TRAITS EXPLAIN GASTROPOD DIVERSITY IN FLOODPLAIN GRASSLANDS

Floodplains are characterized by high spatial and temporal heterogeneity. Despite low active mobility, Central European floodplain gastropod communities show a high species diversity. They are supposed to have developed a large range of resistance and/or resilience strategies to survive in the highly variable and frequently disturbed floodplain habitats. Relating gastropod diversity and species traits to hydrological conditions, we tested how different groundwater and flood regimes affect gastropod diversity and identified the main species traits favouring their survival in highly dynamic floodplain grasslands. Species richness, species diversity and functional diversity peaked at intermediate flood disturbance and moisture levels. Harsher environmental conditions in either dryer or frequently flooded habitats restricted the gastropod communities to a few specialized species. Morphological and life‐history traits showed significant variations along the hydrological gradient. Shell character and mode of reproduction proved to be important functional determinants for gastropod community composition. Species with strongly calcified shells, which limit the risk of injuries in case of dislodgment, were more often found in flood prone sites. Uniparental reproduction dominated in the driest as well as in highly flood‐disturbed habitats, providing reproductive assurance where harsh environmental conditions may reduce the number of potential mates. Intermediate disturbance and moisture levels favoured local gastropod diversity whereas dryer or highly flood‐disturbed habitats sheltered specialized species. Therefore, the maintenance of areas with different disturbance and moisture levels is of major importance in favouring taxonomical and functional mollusc diversity across the whole floodplain. Copyright © 2011 John Wiley & Sons, Ltd.     

Flooding and Surface Connectivity of Taxodium‐Nyssa Stands in a Southern Floodplain Forest Ecosystem

An understanding of the factors controlling the permanent and episodic links between the main stem of a river and the ecosystems of its alluvial floodplain is necessary for evaluating the influence of modern river processes on floodplain ecology and habitat diversity and for the successful implementation of flow regimes that meet human needs for water in a manner that sustains the ecological integrity of affected systems. In this study, we examined relationships between river hydrology and lateral hydrological connectivity, which is crucial to directing fluxes of water, material, and organisms into and across a floodplain. We did so by translating measures of river discharge for the Congaree River into high resolution maps of flood conditions for the floodplain at Congaree National Park using a 2D flood inundation model. Utilizing a graph network approach, we then analyzed the connectivity of a key wetland ecosystem, Taxodium‐Nyssa forested swamps, to the main stem river and to each other under different flows. Our results underscore that floodplain connectivity is initiated at sub‐bankfull discharges and does not depend on levee overtopping, while also clarifying that various sources of connectivity are triggered at different flow levels in specific reaches. Further, our findings demonstrate the sensitive and non‐linear response of floodplain connectivity to river flows and provide useful information to facilitate the management of flood processes in the Congaree River watershed. Copyright © 2014 John Wiley & Sons, Ltd.