Regulated Lowland River Research Articles

Diel patterns of habitat use and movement by juvenile and subadult trout cod (Maccullochella macquariensis) in a regulated lowland river

Improved knowledge of habitat use and movements is needed to support recovery of threatened riverine fish species. The trout cod (Maccullochella macquariensis) is a nationally threatened fish species endemic to rivers of the southern Murray–Darling Basin (MDB), Australia. This study investigated diurnal and nocturnal habitat use and movements of trout cod in the Goulburn River, using radio-telemetry. Trout cod typically occupied small reaches (<300 m) of stream during the late spring–summer study period. Larger-scale movements (mostly home-range shifts) and movements onto inundated riparian habitats also occurred during a period of high discharge. Trout cod occupied deeper water further from the bank during the day and shallower water closer to the bank during the night and used areas with slower velocities more than was their availability, particularly during the earlier hours of the day. They also made extensive use of in-stream cover (i.e. wood). Our information on diel habitat use provides a platform to guide informed management (e.g. availability of preferred habitats under different river-discharge scenarios) to improve trout cod population sustainability.

Flow event size influences carbon, nutrient and zooplankton dynamics in a highly regulated lowland river

AbstractRiver regulation and water extraction has significantly altered flow regimes and reduced flood events in many inland river systems. Environmental flows have been adopted in many systems to mitigate the ecological impacts of river regulation, however a lack of knowledge regarding the interrelationship between flow regimes, carbon transport and instream productivity make prioritising water management difficult. To address this knowledge gap, we conducted a study on the Namoi River in the Murray-Darling Basin, Australia, monitoring changes in dissolved organic carbon (DOC), nutrient dynamics and planktonic food web structure during a period of variable flows. Nutrient and DOC concentrations were positively correlated with river discharge and zooplankton concentrations were highest post flow events. Planktonic chlorophyll-a, increased DOC concentration and higher discharge were the most influential drivers of change in zooplankton communities. Further, our results indicated that flow events increased production through both heterotrophic and autotrophic pathways, significantly boosting zooplankton concentration compared to base flow conditions across all measured flow events. We suggest even small in-channel flow events can be important for increasing basal and zooplankton production in rivers, and therefore should be protected or promoted by environmental flow management, particularly during drought conditions.

Towards ecological flows: status of the benthic macroinvertebrate community during summer low-flow periods in a regulated lowland river

Climate change along with the increasing exploitation of water resources exacerbates low-flow periods, causing detrimental effects on riverine communities. The main mitigation measure currently adopted to counteract hydrological alterations induced by off-stream diversion is the release of minimum flows (MFs), even if within the European Union Water Framework Directive an upgrade towards ecological flows is urgently required to achieve good ecological status (GES). In this study, we investigated the temporal evolution of the benthic macroinvertebrate community in an Italian-regulated lowland river (Ticino River) to clarify the ecological effects of summer low flows, and we evaluated the current MFs in the perspective of meeting GES standard. Biomonitoring was carried out for four consecutive years (2019-2022), in a river site immediately below a large off-stream diversion. The four study years were characterized by different streamflow patterns, thus allowing us to compare the temporal trajectories of the community under different flow conditions. Moreover, the interruption of the low-flow periods due to overflow spilled by the upstream dam gave us the opportunity to assess the effects of experimental flow peaks. Contrary to the expectation, the macroinvertebrate assemblage kept almost unvaried across the years, showing great resistance and resilience to hydrological changes. Even in extraordinarily dry 2022, the community composition varied only slightly, with a reduction of mayflies and an increase of mollusks. However, a deterioration of the ecological status below GES standard was recorded that summer, indicating the need for an upgrading of the current MFs. This upgrade would include experimental flow peaks in critical periods, which act as intermediate disturbances, enhancing community richness, diversity, and overall quality, as well as compliance with a threshold of an index specifically developed for the hydrological pressure.

Hydrological management strategies for the control of algal blooms in regulated lowland rivers

AbstractReservoirs of lowland floodplain rivers with eutrophic backgrounds cause variations in the hydrological and hydraulic conditions of estuaries and low‐dam reservoir areas, which can promote planktonic algae to proliferate and algal bloom outbreaks. Understanding the ecological effects of variations in hydrological and hydraulic processes in lowland rivers is important for algal bloom control. In this study, the middle and lower reaches of the Han River, China, a typical regulated lowland river with a eutrophic background, are selected. Based on the effect of hydrological and hydraulic variability on algal blooms, a hydrological management strategy for river algal bloom control is proposed. The results showed that (a) differences in river morphology and background nutrient levels cause significant differences in the critical threshold flow velocities for algal bloom outbreaks between natural river and low‐dam reservoir sections; there is no uniform threshold flow velocity for algal bloom control. (b) There are significant differences in the river hydrological/hydraulic conditions between years with and without algal blooms. The average river flow, water level and velocity in years with algal blooms are significantly lower than those in years without algal blooms. (c) For different river sections where algal blooms occur and to meet the threshold flow velocities, the joint operation of cascade reservoirs and diversion projects is an effective method to prevent and control algal blooms in regulated lowland rivers. This study is expected to deepen our understanding of the ecological significance of special hydrological processes and guide algal bloom management in regulated lowland rivers.

Environmental influences on migration patterns and pathways of a threatened potamodromous fish in a regulated lowland river network

AbstractIn riverine ecosystems, movements of biota among different parts of the river network are fundamental to the functioning and sustainability of populations, especially for fish. Globally, human activities (e.g., dam construction and altered flow regimes) have impaired riverine fish movements through reduced river connectivity, leading to well‐documented population declines. This study examined the timing and extent of movement of a nationally threatened potamodromous fish, silver perch (Bidyanus bidyanus), among different parts of the mid‐Murray River network in south‐eastern Australia using acoustic telemetry to identify the relative importance of different environmental variables (including river discharge) influencing timing and extent of movements. A total of 203 silver perch (fork length range 120–415 mm) was tagged in the Murray River. Silver perch occupied extended reaches of river (hundreds of kilometres) and multiple habitats throughout the river network, ranging from a large mainstem river to inflowing tributaries, anabranches and floodplain habitats. Movements were associated with aspects of the flow regime, such as small to moderate mainstem discharge rises and elevated tributary discharge. These results emphasise the importance of managing silver perch populations within an integrated riverscape context rather than solely focusing on reach/river scales. The inter‐relationship between silver perch movement and river discharge highlights the susceptibility of the species to river regulation, which has previously been implicated in its decline. The information on migratory behaviours in this study provides empirical evidence to support refinement and implementation of targeted restoration strategies, such as environmental flows, to assist the management and conservation of an imperilled species.

Hydrology and water temperature influence recruitment dynamics of the threatened silver perch Bidyanus bidyanus in a regulated lowland river

Understanding the influence of extrinsic factors such as hydrology and hydraulics on recruitment provides essential insight to inform management of fish populations. The critically endangered silver perch Bidyanus bidyanus is a long-lived, potamodromous pelagophil endemic to Australia’s Murray–Darling Basin. Declines of this species are often attributed to river regulation, although quantitative studies linking hydrology and hydraulics to key aspects of its life cycle are sparse. We used a multidecadal age-structured dataset collected from a locally abundant population of silver perch to quantify the relative importance of different abiotic drivers on year-class strength (recruitment). Silver perch recruited across highly variable hydrological conditions. The strongest year classes were associated with a combination of low to average river discharge (i.e. within channel) and high water temperatures over the peak spawning period, followed in the next year by extended high flows and widespread flooding that promoted survival of age-1+ juvenile fish. We suggest that conditions affecting the growth and dispersal of juvenile fish, in addition to the spawning period, are critical in governing recruitment dynamics. This highlights the need for multiyear flow plans for freshwater fish populations.

Behaviour, escapement and mortality of female European silver eels within a regulated lowland river draining into the Baltic Sea

AbstractMigration patterns of 146 female silver eels on their way through fresh and brackish water stretches of the Warnow River into the Baltic Sea were studied using acoustic telemetry. The majority of eels (77%) regained their expected downstream migration behaviour after a median rehabilitation time of 2 days. Positive rheotactic behaviour was observed for 23% of tagged eels after release, probably influenced by experimental procedures. The observed silver eel escapement rate was 63%. Two migratory groups were classified by significantly different migratory speed, time of residence in brackish waters and biometric silvering characteristics. Although the classification of silvering stages is based on these measures, the proportions of these stages did not reflect the observed differences in migratory behaviour. In the context of a permanent silver eel descent in the freshwater part of the Warnow River, a permanent escapement was likewise occurring into the Baltic Sea.

Quantifying habitat benefits of channel reconfigurations on a highly regulated river system, Lower Missouri River, USA

We present a quantitative analysis of habitat availability in a highly regulated lowland river, comparing a restored reach with two reference reaches: an un-restored, channelized reach, and a least-altered reach. We evaluate the effects of channel modifications in terms of distributions of depth and velocity as well as distributions and availability of habitats thought to be supportive of an endangered fish, the pallid sturgeon (Scaphirhynchus albus). It has been hypothesized that hydraulic conditions that support food production and foraging may limit growth and survival of juvenile pallid sturgeon. To evaluate conditions that support these habitats, we constructed two-dimensional hydrodynamic models for the three study reaches, two located in the Lower Missouri River (channelized and restored reaches) and one in the Yellowstone River (least-altered reach). Comparability among the reaches was improved by scaling by bankfull discharge and bankfull channel area. The analysis shows that construction of side-channel chutes and increased floodplain connectivity increase the availability of foraging habitat, resulting in a system that is more similar to the reference reach on the Yellowstone River. The availability of food-producing habitat is low in all reaches at flows less than bankfull, but the two reaches in the Lower Missouri River – channelized and restored – display a threshold-like response as flows overtop channel banks, reflecting the persistent effects of channelization on hydraulics in the main channel. These high lateral gradients result in punctuated ecological events corresponding to flows in excess of bankfull discharge. This threshold effect in the restored reach remains distinct from that of the least-altered reference reach, where hydraulic changes are less abrupt and overbank flows more gradually inundate the adjacent floodplain. The habitat curves observed in the reference reach on the Yellowstone River may not be attainable within the channelized system on the Missouri River, but the documented hydraulic patterns can be used to inform ongoing channel modifications. Although scaling to bankfull dimensions and discharges provides a basis for comparing the three reaches, implementation of the reference reach concept was complicated by differences in flow-frequency distributions among sites. In particular, habitat availability in the least-altered Yellowstone River reach is affected by increased frequency of low-flow events (less than 0.5 times bankfull flow) and moderately high-flow events (greater than 1.5 times bankfull flow) compared to downstream reaches on the Lower Missouri River.

Ecohydrological and morphological relationships of a regulated lowland river; based on field studies and hydrological modeling

The planning of the correct channel reconstruction is a priority in the rehabilitation of regulated surface streams. The present study examines the most important morphological and ecohydrological factors of a highly modified watercourse. A comparative analysis was performed among modelled and measured morphological, river geometrical, hydraulic, and vegetation status indicators. Reduction of data redundancy and examination of the relationships was done by factor analysis. This was found during the data reduction to six or seven attributes (component) and it was sufficient for imaging the original information content. There are mathematically derived hydrological background variables that are extremely significant in characterizing the watercourses, for example; the Reynolds number, Froude number, ratio of slope Froude-number, speed-to-depth ratio, depth-to-width ratio, etc. The partially regulated and artificial river sections were numerically separated from each other in the analysis. The functional aquatic plant habitats and Manning’s n Roughness have got a higher factor weight in the less regulated river section, than in artificial stage. The weakest factor weights were those status indicators which were most changed by the regulations. The results provide support for defining and monitoring the required environmental geometric riverbed interventions.

Early recolonization of a dredged lowland river by dragonflies (Insecta: Odonata)

The influence of dredging on the dragonfly assemblages of the small regulated lowland River Krąpiel (north-western Poland) was analyzed a short time after the dredging. Dragonfly assemblages were destroyed, but they began to recover rapidly. Many biocoenotic indices reached high values at just six months after the dredging. The recolonization first occurred as a result of larval drift, and then, via dispersion of adult dragonflies. This process took place in conditions different from the prevailing conditions in the period before dredging, in terms of microhabitat availability and physico-chemical conditions. Compared to the previous assemblage, the emerging assemblage was more typical of assemblages found in small, natural running waters. Therefore, dredging (carried out for economic reasons) could be regarded as a process that unintentionally had a positive influence on odonate assemblages. Currently, when most small watercourses are regulated, dredging that is properly planned and controlled has proven to protect the natural fauna. It is worthwhile to apply lessons learned from examples of active fauna protection to what is currently known as “the rotational model” for dredging.

Floods reduce the prevalence of exotic plant species within the riparian zone: evidence from natural floods

AbstractQuestionsDoes flooding reduce the prevalence of exotic plant species and promote native riparian plant species?LocationField study, two regulated lowland rivers, SE Australia.MethodsWe used a large flood in spring to test predictions of vegetation response to flooding. We predicted that the species richness of exotic taxa (mostly terrestrial annual/biennial grasses and forbs in the study systems) would be reduced by flooding, while the richness of native riparian species adapted to fluvial disturbance would increase. We surveyed the riparian vegetation at three sites on each of two regulated rivers in the summers before and after a large flood in early spring that inundated all of the sites on one river, and one site on the other river; the remaining sites were not flooded. Overall, the study was a natural experiment with a before–after control–impact design.ResultsAs predicted, flooding dramatically reduced the richness of exotic taxa, particularly of terrestrial grasses. However, some invasive flood‐tolerant exotic taxa, such as blackberries and willows, were promoted by the floods. Flooding reduced the cover of most native taxa, but did not affect overall native taxon richness. In contrast, the richness and cover of native summer annuals increased following the flood.ConclusionsOur study provides evidence to support the use of managed floods to reduce the prevalence of terrestrial exotic taxa within the riparian zones of regulated rivers. The frequency and intensity of flow pulses necessary to keep terrestrial exotics at low levels requires further research.

Human-mediated contact zone between endemic and invasive Barbus species (Osteichthyes: Cyprinidae) in a regulated lowland river: genetic inferences and conservation implications

Genetic surveys of the endemic Padanian barbel Barbus plebejus (Osteichthyes: Cyprinidae) were completed on the lowland Adda River, a heavily regulated Italian Alpine river in which the European Barbus barbus is invasive. Both Barbus species are of the large fluvio-lacustrine ecophenotype and thus they share habitats in the river. Mitochondrial (N = 35) and nuclear (N = 256) DNA sequences were completed with the aim of estimating the extent and genetic consequences of the B. barbus invasion. The phylogenetic analyses, based on cyt b mtDNA, S7 ribosomal protein and growth hormone (GH) nuclear genes, revealed 59 haplotypes as B. plebejus and 37 as B. barbus. Genetic diversity values (H and π) and the mismatch distribution analyses indicated B. plebejus was still experiencing demographic radiation from a modest number of founders. The B. barbus cyt b haplotypes suggested their original source was the River Danube. Through analysing the nuclear paralog sequences, evidence was provided of consistent genetic admixture between B. plebejus and B. barbus, being recorded in over 40% of the sampled fishes, indicating that their ecological and biological interactions are disrupting the genetic integrity of B. plebejus. Bayesian analysis and factorial correspondence analysis then revealed that the habitat fragmentation of the Adda River is not limiting population connectivity. It was apparent there has been recent and rapid upstream dispersion of B. barbus, with individuals overcoming barriers to their migration through using fish passes and/or through translocation between impounded sections by anglers. The potential conservation implications of this for the endemic Barbus species are discussed.

Floodplain connectivity facilitates significant export of zooplankton to the main River Murray channel during a flood event

This study assessed the role that floods play in providing lateral connectivity between riverine habitats and floodplains, stimulating productivity and contributing zooplankton from the floodplain to the river channel. The study took place on the Chowilla Floodplain of the River Murray, Australia, and the adjacent River Murray Channel throughout the 2010–2011 floods. We found that a considerable transfer of zooplankton from the floodplain into the river channel occurred. Average zooplankton abundance was higher on the floodplain than the main river channel and increased the zooplankton abundance in the river channel downstream. At the peak of the flood, flows reached ~93 000 megalitres per day (ML d−1), inundating ~67 km2 of floodplain. At the time of this study, up to 6.3 ± 1.6 (SD) tonnes per day of zooplankton (dryweight) was being exported from the Chowilla floodplain. Differences in species assemblages were also observed within the River Murray, which seemed to be caused by the influence of the Chowilla Floodplain. This study demonstrated that floodplains provide significant zooplankton biomass, which constitutes a resource input into the riverine food web. These results provide some evidence for the Flood Pulse Concept, which highlights the importance of lateral hydrological connectivity between riverine habitats and floodplains in stimulating productivity and providing a linkage between habitats for biota. Management of regulated lowland rivers should consider not only the provision of water to the floodplain, but also the return of the floodplain waters to the river to sustain riverine food webs .

Flow-induced alterations to fish assemblages, habitat and fish–habitat associations in a regulated lowland river

Understanding the cause–effect response of aquatic biota to hydrological variability is fundamental to the restoration of regulated rivers. Spatio-temporal variation in fish assemblage structure, microhabitat cover and fish–habitat associations were investigated in the main channel of the regulated lower River Murray, Australia, during a prolonged period of low within-channel flows and following a high flow event and flood. Several small-bodied species (e.g. carp gudgeon, Hypseleotris spp.), were abundant and significantly associated with submerged macrophytes during low flows, but were absent or significantly less abundant following flooding, and the loss of these microhabitats. Large-bodied riverine species that spawn in response to increases in flow (e.g. golden perch, Macquaria ambigua ambigua) or spawn and recruit in inundated floodplain habitats (e.g. common carp, Cyprinus carpio), exhibited flexible microhabitat use and were significantly more abundant following flooding. In the lower River Murray, high flow events appear integral in structuring fish assemblages, indirectly influencing the abundance of small-bodied fish by re-structuring macrophyte cover and directly influencing the abundance of large-bodied species by facilitating critical life history processes (e.g. recruitment). These results highlight species-specific differences in cause–effect responses to flow variability and have implications for managing flow in regulated rivers.

Downstream migration dynamics of female and male silver eels (Anguilla anguilla L.) in the regulated German lowland Warnow River

AbstractIn the light of the European wide efforts to increase the spawning biomass of the European eel, a reliable measurement of the escapement of mature silver eel is necessary to prove the effectiveness of the conservation management measures. The seaward migration of mature eel is commonly viewed as a seasonal phenomenon with concentrated migration peaks occurring in spring and autumn. To verify the assumed seasonal silver eel migration events for regulated lowland rivers, a stow‐net system was installed in the Warnow River located in north‐eastern Germany. Between 2008 and 2011, the stow‐net system was operated from March to December each year. The eel harvest was documented on a weekly base including the documentation of weight and length, the silvering stage and the tissue sampling for the molecular identification of the eel species. During the 4 year monitoring period, a continuous downstream migration of female and male silver eels was observed. Additionally, single migration peaks were recorded in each year occurring between April and December. Moreover, female and male silver eels showed varying downstream migration dynamics. Based on a Chi‐squared Automatic Interaction Detection (CHAID) tree analysis, it was shown that during periods of a daily minimum air temperature over 10.4 °C, increased discharge levels and increased wind speeds, higher weekly migration rates of silver eels were likely. Furthermore, the results indicated that both sexes differed in their responses to migration triggering environmental factors. The presented results might be helpful to design more efficient eel conservation management strategies in regulated lowland rivers.

Escapement, route choice, barrier passage and entrainment of seaward migrating European eel, Anguilla anguilla, within a highly regulated lowland river

Fluvial disconnectivity can have important impacts on fish populations, including hindering movement between habitats required for different ontogenic stages. Recruitment of the European eel (Anguilla anguilla) has reduced by over 90% since the early 1980's, in part due to the effect of riverine barriers on its catadromous migration. There is a legislative requirement to restore free passage, increase habitat availability, and limit anthropogenic losses at intakes to aid eel recovery and good ecological status; necessitating an improved understanding of underlying processes. Escapement, route choice, delay at structures, and entrainment at water abstraction points of downstream migrating silver eels were examined using acoustic and Passive Integrated Transponder (PIT) telemetry in the heavily regulated lower river Stour, UK. Downstream migrating adult eel (n=69) were trapped approximately 10km upstream of the tidal limit, surgically implanted with an acoustic transducer and PIT transponder, and released between October and December in 2009 and 2010. Movements of tagged individuals were monitored by a linear array of 19 fixed acoustic receivers extending from the release site, through the last 9.2km of the freshwater catchment. Three groups of water control structures, two water abstraction intakes and several possible routes of migration are present in the reach. Seventy six and 65% of tagged eels escaped from the study reach in 2009 and 2010, respectively. Entrainment at a single intake was the principal cause of loss and positively related to rapid increases in abstraction whilst eels were in the vicinity of the intake. Route choice into the estuary was dependent on discharge over a large intertidal weir; opening regimes of a tidal gate at the termination of the alternative channel; and abstraction rate at a nearby water intake. Long delays (up to 68.5 days) and recurrent behaviour were associated with several structures in the study reach; high variability between individuals reflected the management of spill at weirs. Potential scenarios for minimising entrainment and delay through integrated management of water level control structures and abstraction rates are discussed.

Key environmental variables affecting the ichthyofaunal composition of groyne fields in the middle Elbe River, Germany

During its development into an important waterway the middle Elbe River underwent many anthropogenic influences, all of which strongly impacted the fish fauna assemblage. Both the river morphology and hydrology were affected by groyne field construction, with consequential changes of fish habitats along river banks. Variables in the ichthyofaunal composition of middle Elbe River groyne field areas and types were analyzed for their environmental influence based on data using a random point-abundance-sampling-strategy with an electrofishing device. Altogether 22 fish species and hybrids of cyprinids from eight families, mostly cyprinids and percids, were found. Habitat guilds were dominated by eurytopic fish species (40%), followed by the oligorheophilic guild (24%). The reproductive guilds were represented mainly by phyto-lithophils (36%) and phytophils (28%). The feeding guilds were dominated by benthivorous species (32%), followed by omnivorous and piscivorous species (each 24%). Most frequent were roach ( Rutilus rutilus , 32.6%), perch ( Perca fluviatilis , 29.7%), and ide ( Leuciscus idus , 25.6%). The canonical correspondence analysis indicated that substrate, slope, and vegetation cover were the most important factors affecting the entire fish assemblage, including all age groups. The types of groyne fields were found to be mostly of minor importance with regard to differences in the ichthyofaunal structure. Fish assemblage at groyne heads and surrounding areas were compared to those in the siltation and static flow areas, whereby rheophilic fish species occurred most often at the groyne head and surrounding areas. The greatest temporal changes within the young-of-the-year (YOY) fish fauna were during the summer months June to August, where habitat shifts among the groyne field areas were a result of changes in YOY habitat requirements throughout the year. Hence, this study indicates that groyne fields are used as alternative habitats by a high number of different fish species and life stages and could therefore contribute to sustain a species-reach fish fauna in regulated lowland rivers.

How do hydromorphological constraints and regulated flows govern macroinvertebrate communities along an entire lowland river?

ABSTRACTMacroinvertebrates' response to hydromorphological alterations and regulated flows along lowland rivers is still poorly known despite ecohydrology's fundamental role in river science. Along the Oglio River (Northern Italy), several water abstractions and dams break it into segments with varying hydraulic and morphological properties. Three types of a priori different environments were identified (dammed, downstream and free flowing sections), and macroinvertebrate communities were sampled from each zone. This study aimed (i) to investigate patterns of macroinvertebrate communities along a regulated lowland river by testing the a priori zones; (ii) to find macroinvertebrate taxa that served as indicators of the various hydrological conditions and (iii) to verify hydromorphological control over ecological macroinvertebrate traits resulting in different trait values in each identified zone. Macroinvertebrate community was characterized in a total of 63 stations by means of two quantitative approaches, each exploring a surface of 0.5 m2. The lowest richness values were found in dammed sites that tended toward lentic conditions. Ecnomidae (dammed zones), Limoniidae (downstream zones) and Heptageniidae (free flowing section) were identified as the best indicators of varying hydrological conditions. As suggested by the results of 4th Corner Method, environmental constraints define communities with different ecological traits. These results highlight hydromorphological control over macroinvertebrate community structure and reflect how regulated flows affect the Oglio River in terms of biodiversity, indicator taxa and ecological traits. The authors wish to stress the importance of considering the ecological effects of dams and impoundments on river systems in upstream areas as well as downstream. Copyright © 2012 John Wiley & Sons, Ltd.

Environmental Water Allocations in regulated lowland rivers may encourage offstream movements and spawning by common carp, Cyprinus carpio: implications for wetland rehabilitation

Environmental Water Allocations (EWAs) are used to enhance native flora and fauna in regulated rivers, but may also benefit alien invasive species like common carp (Cyprinus carpio). We examined the invasion and spawning risk posed by adult common carp during an EWA delivered from the River Murray to a flow-through wetland in South Australia from June to December 2008. Offstream movements of fish and turtles were monitored continuously via the inlet and outlet creeks. Long-necked turtles (Chelodina longicollis, n = 129) dominated at the inlet where few fish were collected (n = 24), whereas much larger numbers of common carp in prime spawning condition (n = 4709), alien goldfish (Carassius auratus, n = 1201) and native bony herring (Nematalosa erebi, n = 93) were attracted to the outlet and displayed distinct movements. Adult common carp movements began in August, in response to increasing water temperatures, peaked in mid-September before spawning, then declined and were close to zero by December. The timing of EWA deliveries potentially could be manipulated to reduce adult carp invasion and spawning potential while providing some advantage to native fish, but the benefits may be short-lived without additional carp management interventions such as wetland drying.

Dissolved organic carbon mobilisation in relation to variable discharges and environmental flows in a highly regulated lowland river

The relationships between discharge and dissolved organic carbon (DOC) have been extensively studied in rainfall runoff-driven stream systems. Less is known about discharge and DOC relationships in river systems dependent on floodplain inundation. We examined DOC dynamics and mobilisation over low discharge periods and several larger discharge events in the highly regulated lowland Namoi River, Australia. Stable isotope signatures (δ13C) of various water-column fractions (e.g. 200 µm, fine particulate organic matter) were used to determine the sources of DOC. DOC concentrations over low discharge periods were fairly similar among sites and ranged between 5 and 10 mg L–1. Concentrations during a high-discharge event increased substantially with a mean of 20.4 mg L–1 and a maximum of 44 mg L–1. Significant positive linear relationships were found between DOC concentrations and discharge (P < 0.001, r2 = 0.45). The 13C composition of DOC sampled across the three sites (e.g. –26.2‰) suggests a mixture of terrestrial and aquatic sources, with little downstream variation; however, we would envisage that during periods of high discharge allochthonous sources would dominate. Environmental flows (that are ecologically beneficial, potentially reversing changes brought about by flow regulation) have been allocated to the river, with the intention to increase the amount of DOC delivered to the river. The relationship between DOC and discharge was used to estimate DOC loads to the river under different modelled flow-management scenarios, including without environmental flow, with environmental flow, and simulated natural (low development) flow. On the basis of the modelling results, environmental flows should increase the amount of allochthonous DOC transported within the river in years with moderate and large flow events. Years with low flows did not deliver large loads of allochthonous DOC. The present results showed the potential variability in DOC delivery in relation to floodplain inundation in a lowland river that may not otherwise be detected in rainfall/runoff-driven headwater streams.