Large River Systems Research Articles

Fine‐scale environmental heterogeneity shapes fluvial fish communities as revealed by eDNA metabarcoding

AbstractConservation of freshwater biodiversity requires being able to track the presence and abundance of entire fish communities. However, studying fish community composition within rivers remains a technical challenge because of high spatial and temporal physico‐chemical variability, anthropic activities and connections with other river catchments, which may all contribute to important variations in local ecology and communities. Here, we used environmental DNA metabarcoding to document spatial variation in fish communities at a small geographic scale in a large river system. The study was conducted in the Contrecoeur sector (5.5 km long and approximately 1–1.5 km wide) of the St. Lawrence River (Québec, Canada), where two water masses with different physico‐chemical properties, known as "brown waters" and "green waters," flow in parallel with limited admixing. Water samples were collected during two consecutive days at 53 stations located in both water masses. Using universal PCR MiFish 12S primers, Illumina MiSeq sequencing, and the Barque (www.github.com/enormandeau/barque) eDNA analysis software developed by our group, a total of 67 fish species were detected. PERMANOVA and redundancy analyses (RDA) performed on relative read abundance revealed that each water mass comprised distinct communities that depended on turbidity, depth, and to a lesser extent on the upstream versus downstream position along the study area. eDNA metabarcoding results were compared with those of traditional surveys conducted previously in the sector and up to 40 km upstream of it. As previously reported, higher species diversity was detected by eDNA and with substantially lower sampling effort. Our results represent one of the few studies documenting the potential of eDNA metabarcoding to investigate small‐scale variation in 2D spatial patterns of distribution of whole fish communities associated with habitat characteristics within a lotic system.

Local rainfall or river overflow? Re-evaluating the cause of the Great 2011 Thailand flood

The Great 2011 Thailand flood is one of the most catastrophic flood disasters recorded worldwide in modern history, which covered about 100,000 km2 of the country and resulted in 813 casualties and incurring US$ 46 billion in economic losses. In this paper, we re-evaluate the flood event to identify the root source of the catastrophe. By analyzing hydrological data from gauge stations along the Chao Phraya River (CPR) and remote sensing data, we decoupled the volumetric contributions of river overflow and local rainfall to the lowland reach of the basin and found that the latter contributed most to the flooding. More specifically, out of the total of 77.6 km3 of floodwaters estimated, 73.7% was from precipitation, while only 26.3% was from river discharges. This finding differs from the official reports or previous studies, which attributed the main cause of the river overbank flow. Therefore, it is also inferred that the upstream dam operations would have been only marginally helpful in mitigating the flood since rainfall made up most of the floodwaters in the downstream reach of the river. Our finding offers a new perspective that the local rainfall could be a significant source of the floodwater, rather than river overflow in the lower reaches of a large monsoonal river system in Southeast Asia. Thus, this paper contributes to the understanding of complex flood processes in large river basins and provides fresh insights for efficient flood control and stormwater management.

Quartz electron spin resonance signal intensity of Al and Ti–Li centers as a provenance indicator: An example from the Yangtze River Basin

The quartz electron spin resonance (ESR) signal intensity (SI) of the Al and Ti–Li centers in sediment samples collected from the mainstream and major tributaries of the Yangtze River Basin were measured to determine whether they can be used as fluvial sediment provenance indicators. The results show significant spatial variation in the ESR-SI between the bulk, coarse (200–355 μm), and fine (75–154 μm) grain-size fractions. For all three size fractions, the ESR-SIs of sediment from the mid-lower-reaches of the basin were higher than those of the upper reaches, demonstrating that the ESR-SI can be used to distinguish between quartz contributed by the upper and mid-lower reaches of the basin. Furthermore, the quartz ESR-SI of the major tributaries exhibited distinct spatial variation, according to the distribution of the type of bedrock. Thus, the quartz ESR-SI can be used to trace the provenance of fluvial sediments, even in large river systems. Additionally, the upper Yangtze River Basin contributed a large amount of material to the lower-mid mainstream, and even the fine fraction of sediment in the lower Yangtze River originated mainly from the upper Yangtze River Basin.

Sediment and Submarine Groundwater Discharge Mediated Arsenic Flux into the Bay of Bengal, India: an Appraisal

The Ganga-Brahmaputra delta is among the largest river systems in terms of sediment discharge and the amount of arsenic present in the groundwater. Any slim possibility of arsenic intrusion in the coastal estuaries on account of active submarine groundwater discharge (SGD) can prove fatal to the biotic population at large. In the present study, a working hypothesis for the probable intrusion of arsenic through SGD has been postulated by re-analyzing the data of river discharge, sediment load, and arsenic concentration in the Bengal delta. Our study puts forward a strong argument for the coastal vulnerabilities of Bay of Bengal as the arsenic flux has reached at 1275 × 103 kg/year and 865.8 × 103 kg/year in the suspended and the dissolved form respectively. The enriched arsenic adsorbed on the surface of the Cenozoic sediments (dissolved and suspended) is contributing to the majority of arsenic flux to the Bay of Bengal. Furthermore, the flux resulting due to SGD contributes roughly 1% of the total flux, which if unregulated can prove catastrophic to the primary autotrophs. More so, the existence of flood-like conditions, as well as the region susceptibility to the fast-changing climatic trends, significantly increases the risk for the region in the near future. Therefore, ignoring the contributions of SGD in aiding the arsenic carcinogenicity in the coastal region of Bengal cannot continue for long and the time has come when a strong framework for SGD monitoring should be put in place, especially for regions falling under the purview of arsenic hazard zones.

COMPOSITION AND STRUCTURE OF THE NEPSKY AND TIRSKY FORMATION OF THE PRILENSK-NEPSKY STRUCTURAL-FACIES ZONE OF THE NEPA-BOTUOBA ANTECLISE BY THE RESULTS OF CORE MATERIAL STUDIES

Background. Clarification of the structure of sedimentary formations located on the Siberian Platform by means of lithological and petrographic core studies has a theoretical and practical significance for the creation of geological models for oil and gas prospecting and exploration works. Aim. To characterize and compare the composition of the upper Vendian of the central and southern parts of the Prilensk-Nepsky structural-facies zone. Materials and methods . Core samples for analysis were obtained from boreholes in the Preobrazhenskaya, Verkhnechenskaya, Danilovskaya, Yaraktinskaya and other areas. The article is based on the results of a layered lithological description of the cores from 15 wells with a total core output of 560 m in the interval of the Nepsky and Tirsky formations; the petrographic analysis of 540 thin sections; the determination of grain size distribution of rocks by laser light scattering in 220 samples; and X-ray diffraction analysis of 540 samples. Results . The Nepsky formation is represented by the following lithotypes: conglomerates, gravelites, sandstones, aleurolites and argillites. The section of the southern part of the structural-facies zone, compared to its central part, is characterised by increased claying. This is associated with the location of this zone in transitional coastal-marine environments, where large river systems were discharged carrying sandy-clay material into the basin. The source of the terrigenous rocks was felsic igneous rocks. The section of the Tirsky formation of the central part is represented (from bottom to top) by a transition from argillites and aleurolites to dolomites. In the section of the southern part, basal sandstones lie at the base, alternating upstream with clay-carbonate rocks. Conclusion . The material composition of the studied sediments reflects the specifics of sedimentation during the Nepsky and Tirsky time periods. For the first time, the Tirsky formation of the southern part of the structural-facies zone was divided into four lithological members. The formation of basal sandy rocks at the bottom of the sub-formations is associated with the stages of tectonic activity and positive tectonic movements that initiated relatively rapid weathering and transportation of terrigenous material from adjacent heights to the sedimentation basin.

Current research status of large river systems: a cross-continental comparison.

Rivers play an irreplaceable role in nature and human society but are the most vulnerable ecosystem in the world to multiple environmental stressors. However, the global-scale research status and the distribution patterns of major stressors in large rivers remain unclear. This study analysed research publications (12,807 documents from 1900 to 2019) related to six large rivers with continental representativeness to tackle these knowledge gaps. The results showed that the total outputs have grown rapidly over the study period, particularly since the 1990s. Consistent with the varied environmental characteristics and problems among the rivers, the research outputs and focuses demonstrated clear differences, which could further be attributed to geographical location, journal preferences and the economic strength of the country in which the river is located. Overall, climate change was the most frequently and widely considered environmental stressor in large rivers. Regardless of climate change, species diversity and hydropower development were widely addressed in the Amazon, Congo and Mekong river basins. Water pollution was the main stressor studied in the Rhine River and Mississippi River, while agricultural irrigation and drought were the most frequently addressed research subjects in the Murray-Darling River. This study provides a comprehensive understanding of the research status and stressor distribution in large global rivers, highlighting the relationship between river research and geographical regions, pointing out future research directions and providing management guidance for large rivers.

Sediment Transit Time and Floodplain Storage Dynamics in Alluvial Rivers Revealed by Meteoric10Be

AbstractQuantifying the time scales of sediment transport and storage through river systems is fundamental for understanding weathering processes, biogeochemical cycling, and improving watershed management, but measuring sediment transit time is challenging. Here we provide the first systematic test of measuring cosmogenic meteoric Beryllium‐10 (10Bem) in the sediment load of a large alluvial river to quantify sediment transit times. We take advantage of a natural experiment in the Rio Bermejo, a lowland alluvial river traversing the east Andean foreland basin in northern Argentina. This river has no tributaries along its trunk channel for nearly 1,300 km downstream from the mountain front. We sampled suspended sediment depth profiles along the channel and measured the concentrations of10Bemin the chemically extracted grain coatings. We calculated depth‐integrated10Bemconcentrations using sediment flux data and found that10Bemconcentrations increase 230% from upstream to downstream, indicating a mean total sediment transit time of 8.4 ± 2.2 kyr. Bulk sediment budget‐based estimates of channel belt and fan storage times suggest that the10Bemtracer records mixing of old and young sediment reservoirs. On a reach scale,10Bemtransit times are shorter where the channel is braided and superelevated above the floodplain, and longer where the channel is incised and meandering, suggesting that transit time is controlled by channel morphodynamics. This is the first systematic application of10Bemas a sediment transit time tracer and highlights the method's potential for inferring sediment routing and storage dynamics in large river systems.

Severe western Canadian wildfire affects water quality even at large basin scales

Wildfires can have severe and lasting impacts on the water quality of aquatic ecosystems. However, our understanding of these impacts is founded primarily from studies of small watersheds with well-connected runoff regimes. Despite the predominance of large, low-relief rivers across the fire-prone Boreal forest, it is unclear to what extent and duration wildfire-related material (e.g., ash) can be observed within these systems that typically buffer upstream disturbance signals. Following the devastating 2016 Fort McMurray wildfire in western Canada, we initiated a multi-faceted water quality monitoring program that suggested brief (hours to days) wildfire signatures could be detected in several large river systems, particularly following rainfall events greater than 10 mm. Continuous monitoring of flow and water quality showed distinct, precipitation-associated signatures of ash transport in rivers draining expansive (800–100,000 km2) and partially-burned (<1–22 percent burned) watersheds, which were not evident in nearby unburned regions. Yields of suspended sediment, nutrients (nitrogen, phosphorus) and metals (lead, others) from impacted rivers were 1.2–10 times greater than from those draining unburned regions. Post-fire suspended sediment concentrations in impacted rivers were often larger than pre-fire 95% prediction intervals based on several years of water sampling. These multiple lines of evidence indicate that low-relief landscapes can mobilize wildfire-related material to rivers similarly, though less-intensively and over shorter durations, than headwater regions. We propose that uneven mixing of heavily-impacted tributaries with high-order rivers may partially explain detection of wildfire signals in these large systems that may impact downstream water users.

Million-year lag times in a post-orogenic sediment conveyor.

Understanding how sediment transport and storage will delay, attenuate, and even erase the erosional signal of tectonic and climatic forcings has bearing on our ability to read and interpret the geologic record effectively. Here, we estimate sediment transit times in Australia's largest river system, the Murray-Darling basin, by measuring downstream changes in cosmogenic 26Al/10Be/14C ratios in modern river sediment. Results show that the sediments have experienced multiple episodes of burial and reexposure, with cumulative lag times exceeding 1 Ma in the downstream reaches of the Murray and Darling rivers. Combined with low sediment supply rates and old sediment blanketing the landscape, we posit that sediment recycling in the Murray-Darling is an important and ongoing process that will substantially delay and alter signals of external environmental forcing transmitted from the sediment's hinterland.

Physio-geographical characteristics of the marine regions and their catchment areas of the Mediterranean Sea and Black Sea marine system

ABSTRACT The Mediterranean Sea and the Black Sea comprise a semi-enclosed intercontinental Marine System (MBMS), bordered by the Eurasian and African continents. Its total marine surface is circa 3 × 106 km2, while its drainage basin exceeds 7 × 106 km2; the latter includes the watersheds of 39 large (>104 km2) river systems, which correspond to about 83% of its total area. The MBMS includes four basins, that is, Western, Central, Eastern Mediterranean (WMED, CMED, EMED) and Black Sea (BLS), which are subsequently divided into 11 marine regions. Thus, the WMED includes the Alboran (ALB), the West MED (WEST) and the Tyrrhenian (TYR); the CMED consists of the Adriatic (ADR), the Ionian (ION) and the Centric MED (CEN); the EMED includes the Levantine (LEV), the Aegean (AEG) and the Marmara (MAR) and the BLS except of the Black Sea (BLA) includes the Azov Sea (AZOV). The present work aims to: (i) describe geographically the MBMS marine regions and their boundaries according to International Hydrographic Office and the common practice for environmental studies; (ii) provide and compare key-morphometric characteristics of all marine regions and their catchment areas and (iii) examine the possibility of a better separation of the existing marine regions.

Perfluoroalkyl substances in the Yangtze River: Changing exposure and its implications after operation of the Three Gorges Dam

Potential ecological risk from perfluoroalkyl substances (PFASs) under changing exposure in large river systems has become a new challenge recently. Based on systematic monitoring of PFASs at 43 hydrologic stations along a 4300 km continuum of the Yangtze River, we investigated the ecological risk of PFASs under changing exposure resulting from operation of the Three Gorges Dam (TGD). Importantly, perfluorooctanoic acid (PFOA) was found extensively exposed in most mainstream water samples, accounting for about 90% of the total content of PFASs in both spring and autumn, while short-chain PFASs contributed more than PFOA in sediment. The significant inversion of long-chain PFASs occurrence from sediment to water reflected a profound change in exposure due to loss of finer sediments resulting from long-distance and long-term scour of the riverbed downstream of the TGD. The coarsening of bed materials would weaken sorption of long-chain PFASs in sediments and enhance their exposure in water, resulting in substantial increase of ecological risk to representative aquatic organisms. In the long term, particular attention should be paid to reduction of PFOA discharge to downstream of the TGD from typical industries. This also highlights the significance of huge dams to alternative exposures of persistent organic pollutants and the necessity of new strategy for ecological risk management of large river systems.

Spatial and temporal variations in mire surface water chemistry as a function of geology, atmospheric circulation and zonal features in the south-eastern part of Western Siberia

This study analysed the chemistry of water samples collected from mires in the south-eastern part of Western Siberia. The aim was to determine spatial and temporal patterns in the hydrochemistry of mires in relation to the environmental conditions and geological history of the region. The results show that concentrations of Fetotal, Са2+, Mg2+, Na+, and HCO3– ions are higher than those obtained in other regions of Russia and Europe, which is consistent with the geological conditions of Western Siberia. High Fetotal concentrations are associated with the distribution of sedimentary ore in the region's largest iron ore basin, the Bakchar deposit, which is represented by strata of various Cretaceous sedimentsary rock at depths of 150–200 m. The Fe-rich rocks contribute to the high Fe concentrations in deep groundwater that feeds into the fens. The increase in Са2+, Mg2+, Na+, Cl−, and HCO3– in the water resulted from a xerothermic period that occurred in the southern part of Western Siberia (4500–2500 years ago) that increased the carbonation and salinity of the rocks. A feature characteristic of the hydrochemical regime in raised bogs is the transformation of the chemical composition during the year, with a change in dominant anions from HCO3– to Cl− or SO42−. The water in transitional mires and rich fens were characterised by the predominance of HCO3– and Са2+. Studies have shown that the size and stage of development play leading roles in the formation of the water chemistry in the mires. Small bogs and transitional mires were characterised by a rich mineral nutrition; while, the chemical composition of fens was mainly determined by their geomorphologic position and proximity to large river systems. The results of this study provide a better understanding of the functions of peatland ecosystems in Western Siberia and improve water sampling strategies for future research.

Microplastics in Lake Mead National Recreation Area, USA: Occurrence and biological uptake.

Microplastics are an environmental contaminant of growing concern, but there is a lack of information about microplastic distribution, persistence, availability, and biological uptake in freshwater systems. This is especially true for large river systems like the Colorado River that spans multiple states through mostly rural and agricultural land use. This study characterized the quantity and morphology of microplastics in different environmental compartments in two large reservoirs along the Colorado River: Lakes Mead and Mohave, within Lake Mead National Recreation Area. To assess microplastic occurrence, surface water and surficial sediment were sampled at a total of nine locations. Sampling locations targeted different sub-basins with varying levels of anthropogenic impact. Las Vegas Wash, a tributary which delivers treated wastewater to Lake Mead, was also sampled. A sediment core (33 cm long, representing approximately 19 years) was extracted from Las Vegas Bay to assess changes in microplastic deposition over time. Striped bass (Morone saxatilis), common carp (Cyprinus carpio), quagga mussels (Dreissena bugensis), and Asian clams (Corbicula fluminea) were sampled at a subset of locations to assess biological uptake of microplastics. Microplastic concentrations were 0.44–9.7 particles/cubic meter at the water surface and 87.5–1,010 particles/kilogram dry weight (kg dw) at the sediment surface. Sediment core concentrations were 220–2,040 particles/kg dw, with no clear increasing or decreasing trend over time. Shellfish microplastic concentrations ranged from 2.7–105 particles/organism, and fish concentrations ranged from 0–19 particles/organism. Fibers were the most abundant particle type found in all sample types. Although sample numbers are small, microplastic concentrations appear to be higher in areas of greater anthropogenic impact. Results from this study improve our understanding of the occurrence and biological uptake of microplastics in Lake Mead National Recreation Area, and help fill existing knowledge gaps on microplastics in freshwater environments in the southwestern U.S.

Climate-induced trends in global riverine water discharge and suspended sediment dynamics in the 21st century

Abstract Anthropogenic climate change, particularly through increased greenhouse gas (GHG) emissions, is projected to impact 21st century precipitation distribution, altering fluvial processes such as riverine water discharge and sediment dynamics, worldwide. Changes in fluvial water and sediment discharges can have profound impacts on the functioning and connectivity of earth's natural systems. In this paper, we study the natural sensitivity of water discharge and suspended sediment fluxes in large global river systems to predicted climate change in the 21st century. A global-scale hydro-geomorphic model (WBMsed) was forced with precipitation and temperature projections generated by five General Circulation Models (GCMs), each driven by four Representative Concentration Pathways (RCPs). Anthropogenic drivers were excluded from the simulations in order to isolate the signal of 21st century climate change. The results, based on an ensemble of model outputs, revealed that global river discharge and sediment dynamics are highly sensitive to anthropogenic climate change in the 21st century. Increasing global warming will lead to more extreme changes and greater rates of changes (increasing or decreasing) in both variables. Despite substantial regional heterogeneity, a global net increase is projected for both natural river discharge and sediment flux toward the end of the 21st century under all climate change scenarios. These increases are larger with increasing levels of atmospheric warming. At the end of this century, projected climate changes under RCP 2.6, 4.5, 6.0 and 8.5 scenarios, will lead to 2%, 6%, 7.5% and 11% increases respectively in mean global river discharge relative to the 1950–2005 period, while mean global suspended sediment flux will show 11%, 15%, 14% and 16.4% increases under pristine conditions. In addition to magnitudes, inter-annual variability also increases with increasing warming. Changes in sediment flux closely follow the patterns predicted for discharge, and are mostly driven by climate warming-induced spatial and temporal variation in precipitation. However, the relationship between precipitation, discharge and sediment flux was found to be non-linear both in space and time, demonstrating the utility of explicit modeling of both hydrology and geomorphology.

Acoustic Sediment Estimation Toolbox (ASET): A software package for calibrating and processing TRDI ADCP data to compute suspended-sediment transport in sandy rivers

Quantifying suspended-sediment transport is critical for a variety of disciplines related to the management of water resources. However, the number of gauging stations and monitoring networks in most rivers around the world is insufficient to improve understanding of river dynamics and support water resource management decisions. This is mainly due to the high operational costs and intensive labor involved in traditional sediment measurement techniques, especially in sand bed rivers where coarse material varies spatially in the river cross section. Recently, the acoustic surrogate method has received attention as a potentially accurate surrogate technology for estimating suspended-sediment concentrations. In addition, the acoustic surrogate method, through use of acoustic Doppler current profilers (ADCPs), has the advantage of being able to simultaneously measure the flow velocity field and cross-sectional area when moving-boat measurements are performed. In spite of the important advances made in the implementation of this technique, there are no widely-available, free tools for processing the ADCP acoustic signal cross section measurements which include options to extrapolate velocity and sediment in unmeasured ADCP zones and develop calibrations with physical samples. This paper presents a new software called Acoustic Sediment Estimation Toolbox (ASET), which enables the user to develop a calibration between the acoustic signal collected with a down-looking Teledyne RD Instruments ADCP and sediment concentrations determined using traditional sediment sampling techniques. Moreover, ASET software uses dynamic ADCP measurements to estimate the total suspended-sediment transport through a river cross section. The theoretical framework and data processing routines applied by each module in ASET are presented. Finally, a comparison is made between the results obtained by ASET and by traditional methodologies for computing suspended-sediment transport in a large river system (Paraná River, Argentina).

Hydro-climatology study of the Ogooué River basin using hydrological modeling and satellite altimetry

Hydrological models are important tools for the simulation of water storage and hydrological fluxes in large basins and complex river systems. The hydrological models can compensate the lack of observed data in ungauged basins. In this study, the hydrological model of large basins MGB (for Model of Large Basins in Portuguese) is used to evaluate the hydrological processes of the Ogooué River Basin (ORB), which has been mostly unmonitored for about three decades. Simulations were carried out over an 18-year period from 1998 to 2015 using TRMM 3B42 daily rainfall data from the Tropical Rainfall Measurement Mission (TRMM) as forcing and in situ and altimetry-based river discharges from Envisat, Saral Altika and Jason-2 for calibration and validation. The results of the model were in good agreement with the flows measured at stations upstream and downstream of the Ogooué basin (Nash-Sutcliffe Efficiency (NSE) > 0.56 for all calibration gauges). The MGB model efficiently describes the seasonal and interannual variations of the flow in the Ogooué River and its major tributaries which were found to be highly correlated to the rainfall (r ranging from 0.72 to 0.90 and 0.56 to 0.87 at seasonal and interannual time-scales respectively). Interannual variations of precipitation and river discharge of the ORB are linked to the El Niño Southern Oscillation (ENSO) in the tropical eastern Pacific Ocean and southeastern tropical Atlantic Niño. Also, the Ogooué River discharge was found to be strongly correlated with Sea Surface Temperature (SST) at annual and semi-annual time-scales.

Assessment of impacts of utilization on water resources in the basin of trans-boundary Red river system

The Red river system is the large trans-boundary river system, there has been no united system of hydrology stations as well as integrated plan for the water use and management in the whole basin. The trend of water resources change in the Red river system basin has been assessed on the basic of statistic analyses of data observed during the studies, especially in the time when the exploitation of water resources has been intensified for the multisectoral development. This paper shows some of the results from considerations of the water use in the highlands that is influential in water resources in the Red river system basin and the planned reservoirs which are built in the basin of Red river system. The results include the assessment of the state and trend of water resources in the Red river system basin, the trend of water level lowering in the lowlands and its impacts.

Lagrangian tracking measurements revealed the temporal dynamics of nitrogen and phosphorus spiralling in a large Japanese river

The in-stream processing of nutrients plays an important role in the fluvial nutrient transport from lands to the ocean, but few empirical studies have addressed the temporal dynamics of net nutrient removal rates in large river systems. The present study applied the recently developed Lagrangian approach to the sixth-order Fuji River to identify the seasonal dynamics of in-stream uptake rates for inorganic nitrogen and phosphorus by tracking a specific water parcel along the river course. The Lagrangian measurements revealed temporal variability of nutrient uptake metrics, but the overall pattern showed a net positive uptake of ammonium (NH4) and a net release of nitrite (NO2) and nitrate (NO3) from the riverbed from May through December. The net uptake rates of phosphate (PO4) showed both positive and negative values depending on the sampling periods. Environmental correlates of the net areal uptake rates (Unet) suggested that biological transformation processes such as assimilation for organic matter synthesis and utilization for energy in oxidation–reduction reactions, as well as abiotic sorption processes, influenced the nutrient removal in the Fuji River. The present study highlights that physical factors (temperature and light), reaction substrates and epilithic organisms may responsible for nutrient dynamics in large river ecosystems.

Modelling patterns of coexistence of three congeneric headwater fishes

Abstract Mechanisms driving patterns of occurrence and co‐occurrence among North American freshwater fishes are poorly understood. In particular, the influence of biotic interactions on coexistence among stream reaches and their effects on regional species distribution patterns is not well understood for congeneric headwater fishes. Occupancy models provide a useful framework for examining patterns of co‐occurrence while also accounting for imperfect detection. Occupancy models may be extended to test for evidence that a dominant species influences the occurrence of a subordinate species and thus evaluate support for the hypothesis that species interactions drive patterns of coexistence. We examined patterns of occurrence and co‐occurrence at the stream‐reach scale among three species of darters (Percidae: Etheostomatinae) that occupy headwater streams within a Gulf Coastal Plain drainage in the south‐eastern U.S.A. We assessed species occurrences at 97 sites in first‐ to third‐order streams on one occasion each and used data from four sub‐reaches sampled with equal effort at each site to estimate species‐specific detection probabilities. Following sampling, a suite of habitat variables was collected at three equidistant points along each of the three transects established within a sub‐reach. Coarse (stream‐segment, catchment, network) scale variables were also incorporated using geospatial data. Single‐species and two‐species occupancy models were used to examine patterns of occupancy and coexistence. The occupancy of each species was influenced by distinct habitat variables. Goldstripe darters (Etheostoma parvipinne) were constrained by a stream size gradient, groundwater input appeared to influence the occurrence of Yazoo darters (Etheostoma raneyi), and local habitat heterogeneity (e.g. variation in depth and current velocity) appeared to influence the occupancy of redspot darters (Etheostoma artesiae). We found no evidence that the presence of one species influenced the occurrence of another within a stream‐reach based on two‐species occupancy models. Rather, species co‐occurrences were best explained as independent occurrences within a stream‐reach according to species‐specific habitat associations. Occupancy modelling may provide a suitable framework for evaluating the influence of biotic interactions among congeneric stream fishes along species‐specific habitat gradients at the stream reach scale. Our study offers insight into how habitat variation can influence coexistence of potential competitors across a large river system.

Rapid change in Yangtze fisheries and its implications for global freshwater ecosystem management

AbstractFreshwater capture fisheries are globally essential for food security and aquatic biodiversity conservation. The Yangtze River Basin is the third longest and one of the most human‐influenced drainage basins worldwide. Since the founding of P. R. China in 1949, this large river system has suffered increasing human perturbation and its sustainable development is now severely challenged. Meta‐analysis showed that Yangtze River fisheries have experienced an extraordinary process of utilization–over‐exploitation–protection during the past 70 years, to an extent that other globally important rivers may never have encountered. Its fisheries appear to have collapsed over the past four decades, with yield decreasing to only 25% of an historical peak of 400,000 metric tonnes in the late 1950s. Endemic, migratory and rare fishes have been highly threatened with obvious changes in fish community structure and aquatic biodiversity. Anthropogenic activities, including impoundment of water in dams, discharge of pollutants and riverine modification for vessel navigation, have caused large decreases in fisheries yields. Projections from surplus production modelling showed potential for improvement under fishing ban scenarios, but without any prospect for full recovery to historical stock status. This study revealed that the change in fisheries resources was dominated by the social–ecological watershed system, and an integrated approach to river basin management is warranted. Better management of freshwater ecosystems to integrate food security with biodiversity conservation is urgently needed throughout the world, and the changes evident in the Yangtze River fish populations can serve as an informative global reference.