Large Floodplain Research Articles

Seasonal and spatial variability of CO2emission from a large floodplain lake in the lower Amazon

[1] The inundation status of the Amazon floodplain affects biogenic gas production and evasion. We analyzed spatial variability of dissolved CO2 concentration and gas evasion in a large floodplain lake in the lower reach of the Amazon River in four hydrological phases. We calculated surficial CO2 concentrations from measurements of pH, dissolved inorganic carbon, temperature, and conductivity and used meteorological data to calculate gas transfer coefficients to estimate CO2 evasion. Gas transfer coefficients that take into account both wind and heating and cooling at the lake's surface are on the order of 10 cm hr−1, approximately four times higher than values previously used in regional estimates of gas evasion from lakes on the Amazon floodplain. Supersaturation of CO2 occurred throughout the lake and was higher in the littoral zone and in regions receiving Amazon River inflows. CO2 concentration was reduced in regions with phytoplankton blooms. The range of CO2 concentrations was least at low water, 47 μM to 233 μM, and largest at high water, 1 μM to 656 μM; the average annual value was 125 μM. We estimate mean (±standard deviation) fluxes from open-water in L. Curuai to the atmosphere of 44 ± 15, 348 ± 13, 371 ± 23, and 364 ± 20 mmol CO2 m−2 d−1 during receding, low, rising, and high water, respectively. The error associated with these values reflects, for each hydrological phase, the spatial variation in CO2 concentration in L. Curuai, a likely range in atmospheric CO2 levels and temporal variations in gas transfer coefficient within 10-day periods.

Fish mediate high food web connectivity in the lower reaches of a tropical floodplain river

High levels of hydrological connectivity during seasonal flooding provide significant opportunities for movements of fish between rivers and their floodplains, estuaries and the sea, possibly mediating food web subsidies among habitats. To determine the degree of utilisation of food sources from different habitats in a tropical river with a short floodplain inundation duration (~2 months), stable isotope ratios in fishes and their available food were measured from three habitats (inundated floodplain, dry season freshwater, coastal marine) in the lower reaches of the Mitchell River, Queensland (Australia). Floodplain food sources constituted the majority of the diet of large-bodied fishes (barramundi Lates calcarifer, catfish Neoarius graeffei) captured on the floodplain in the wet season and for gonadal tissues of a common herbivorous fish (gizzard shad Nematalosa come), the latter suggesting that critical reproductive phases are fuelled by floodplain production. Floodplain food sources also subsidised barramundi from the recreational fishery in adjacent coastal and estuarine areas, and the broader fish community from a freshwater lagoon. These findings highlight the importance of the floodplain in supporting the production of large fishes in spite of the episodic nature and relatively short duration of inundation compared to large river floodplains of humid tropical regions. They also illustrate the high degree of food web connectivity mediated by mobile fish in this system in the absence of human modification, and point to the potential consequences of water resource development that may reduce or eliminate hydrological connectivity between the river and its floodplain.

Erratum to: Limnogeology in Brazil’s ‘‘forgotten wilderness’’: a synthesis from the large floodplain lakes of the Pantanal

Erratum to: Limnogeology in Brazil’s ‘‘forgotten wilderness’’: a synthesis from the large floodplain lakes of the Pantanal

THE RIVER MACHINE: A TEMPLATE FOR FISH MOVEMENT AND HABITAT, FLUVIAL GEOMORPHOLOGY, FLUID DYNAMICS AND BIOGEOCHEMICAL CYCLING

ABSTRACTThe dynamics of many large floodplain rivers are dominated by the flood pulse. The high kinetic energy of the erosive flows associated with the flood pulse forms and reforms the river channel. In general, the flood pulse supports the immense abundance and diversity of river life by transporting nutrients and organic matter into backwaters in spring, supporting primary and secondary production during the summer and redistributing these products to channels as water levels recede. Both North American and South American fluvial‐dependent large river fishes exhibit complex, system‐level longitudinal and/or lateral movements across life stages that allow them to exploit flood pulse‐driven spatial heterogeneity and seasonal connectivity to feed, reproduce and avoid harsh conditions. We argue that two hydraulic variables, the magnitudes of velocity and the spatial velocity gradient, are necessary and sufficient to both understand fish ‘hydro‐navigation’ as well as explain patterns in biogeochemistry and fluvial geomorphology and thereby create a new conceptual framework for large floodplain rivers integrating fluid dynamics, channel morphology, biogeochemical cycling and important elements of fish ecology. We illustrate the framework using summary data from the São Francisco River, Brazil that contains sub‐basins possessing different levels of impact and also from the lower Paraná River (Argentina) where natural processes can still be studied. We believe the framework is an important element of large river restoration because it directly links the unique physical and chemical processes of large floodplain rivers to life requirements important to fishes and other biota. Copyright © 2011 John Wiley & Sons, Ltd.

Accelerated overbank accumulation after nineteenth century river regulation works: A case study on the Maros River, Hungary

In the nineteenth century, the meandering rivers of the Carpathian Basin were extensively regulated, and their large (up to 100 km wide) floodplains were confined by artificial levees. On the narrow (0.5–4 km) artificial floodplains confined by these levees, overbank sedimentation has become the dominant geomorphological process, resulting in rising floodplain levels during the last century. The Maros River is an extreme example of this process, as it carries a significant amount of suspended sediment (4.6 × 10 6 m 3/y) and because its lowland reach was drastically shortened by cutoffs, resulting in bed scour and an increased sediment load. Simultaneously, a flood embankment system was constructed, reducing the floodplain width by 70–80%. The resulting accelerated overbank sediment accumulation increases the flood hazard along the river. The aim of this study is to determine the spatial and temporal pattern of overbank sediment accumulation on the artificial floodplain of the lower Maros River over the last 150 years and to identify its influencing factors. The lateral slope of the protected (natural) floodplain was projected towards the present-day channel, allowing the volume (m 3) and rate (cm/y) of overbank sedimentation to be calculated using digital terrain modelling (DTM). Changes in the rate of sedimentation were determined by pollen analysis. The studied reach was divided into five sections based on the rate of aggradation: (i) the distal surface of the alluvial fan, where no sedimentation was measured, only incision; (ii) the original front of the alluvial fan (1.0 ± 0.4 cm/y); (iii) the recently formed secondary alluvial fan (1.4 ± 0.3 cm/y); (iv) the low-lying floodplain (0.4 ± 0.2 cm/y); and (v) the outlet unit (2.0 ± 0.5 cm/y). The aggradation rate was slower on elevated forms, such as the pre-regulation natural levee (0.2 cm/y), and greater on the low-lying floodplain (0.6 cm/y). The greatest accumulation rate (1.3–2.4 cm/y) was measured in cutoffs. The sedimentation rate was high during the regulation period (1.9–2.4 cm/y) but later decreased to 0.5–0.9 cm/y. The rate of aggradation was determined by the width, height and slope conditions of the floodplain, the impoundment of the main river and the land use in the area.

Landsat mapping of annual inundation (1979–2006) of the Macquarie Marshes in semi-arid Australia

Measuring inundation over long timeframes is essential for understanding the responses of large floodplain wetlands on regulated rivers, such as the internationally Ramsar listed Macquarie Marshes (2000 km2) in central-eastern Australia. We used near-spring Landsat images (Multispectral Scanner (MSS) and Thematic Mapper (TM) imagery) over 28 years (1979–2006) and classified for inundation, integrating water and vegetation response using Iterative Self-Organizing Data Analysis (ISODATA) clustering. A spatially explicit inundation index showed that zones inundated with high frequency were mostly in the northern region. Change detection of inundation indices over three consecutive water management periods (period 1 (1979–1987), period 2 (1988–1996) and period 3 (1997–2006)) showed that zones inundated with high frequency across the Macquarie Marshes contracted, equating to the loss of three or more spring floods from each 9-year period, despite no corresponding change in annual catchment or local rainfall. Landsat represents the only effective available long-term information for analysing long-term changes in inundation patterns of floodplain wetlands.

Acute Physicochemical Effects in a Large River-Floodplain System Associated with the Passage of Hurricane Gustav

On 1 September 2008, Hurricane Gustav passed over the Atchafalaya River Basin (ARB) in south-central Louisiana. Anticipating physicochemical shifts due to concentrated precipitation and wind stress generated by this strong category 2 storm, we deployed a continuous recording multiparameter water quality sonde in a southern ARB bayou 3 days prior to storm arrival to document conditions before, during, and after hurricane landfall. Quarter-hourly physicochemical measurements taken over a 2-week period indicated that dissolved oxygen (DO), pH, and specific conductance all reached annual lows immediately following storm passage. The most pronounced post-storm fluctuation involved DO. Daily mean DO concentration dropped to hypoxic level (DO ≤ 2 mg/L) within 3 days of landfall, followed by near anoxic conditions within 5 days that resulted in extensive system-wide fish kills. Within 6 weeks, however, DO returned to, and pH was near pre-storm levels. To evaluate the impact of Hurricane Gustav on ARB physicochemistry, we contrasted data on DO, pH, temperature, and specific conductance collected from 16 lower ARB sampling sites over a 54-day interval prior to storm landfall with data collected during a 45-day post-storm period. Results indicated that water quality was highly dissimilar (P < 0.0001) between the two periods.

Fish recruitment in a large, temperate floodplain: the importance of annual flooding, temperature and habitat complexity

1. Large river floodplains are considered key nursery habitats for many species of riverine fish. The lower Volga River floodplains (Russian Federation) are still relatively undisturbed, serving as a suitable model for studying the influence of flooding and temperature on fish recruitment in floodplain rivers. 2. We examined the interannual variability in recruitment success of young-of-the-year (YOY) fish in the lower Volga floodplain in relation to flood pulse characteristics and rising water temperatures in the spring. We sampled four areas with different flooding regimes, in three consecutive years (2006–2008). 3. Extensive areas with a long duration of flooding accommodated high densities of young fish. This suggests that extended inundation improves the recruitment success of river fish. In areas with extensive flooding, the biomass of YOY of most fish species was about three times higher in 2006 and 2007 than in 2008. We hypothesise that low spring temperatures in 2008 may have caused this reduced recruitment and that a flood synchronised with rising temperature enhances recruitment success. 4. Extensive flooding was particularly favourable for species characterised by large body size, delayed maturation, high fecundity and low parental investment, such as pike Esox lucius, roach Rutilus rutilus and ide Leuciscus idus. Gibel carp Carassius gibelio, a species tolerant of high temperature and hypoxia, did particularly well in small waterbodies in the driest parts of the floodplain. 5. Structural characteristics of floodplain waterbodies explained much of YOY fish density. These species–environment associations varied from year to year, but some species such as common bream Abramis brama, roach and gibel carp showed consistent relationships with structural habitat characteristics in all years, despite large interannual fluctuations in flood pulse and spring temperature

Spatial patterns of aquatic habitat richness in the Upper Mississippi River floodplain, USA

Interactions among hydrology and geomorphology create shifting mosaics of aquatic habitat patches in large river floodplains (e.g., main and side channels, floodplain lakes, and shallow backwater areas) and the connectivity among these habitat patches underpins high levels of biotic diversity and productivity. However, the diversity and connectivity among the habitats of most floodplain rivers have been negatively impacted by hydrologic and structural modifications that support commercial navigation and control flooding. We therefore tested the hypothesis that the rate of increase in patch richness (# of types) with increasing scale reflects anthropogenic modifications to habitat diversity and connectivity in a large floodplain river, the Upper Mississippi River (UMR). To do this, we calculated the number of aquatic habitat patch types within neighborhoods surrounding each of the ≈19 million 5-m aquatic pixels of the UMR for multiple neighborhood sizes (1–100 ha). For all of the 87 river-reach focal areas we examined, changes in habitat richness ( R) with increasing neighborhood length ( L, # pixels) were characterized by a fractal-like power function R = L z ( R 2 > 0.92 ( P < 0.05)). The scaling exponent ( z) measures the rate of increase in habitat richness with neighborhood size and is related to a fractal dimension. Variation in z reflected fundamental changes to spatial patterns of aquatic habitat richness in this river system. With only a few exceptions, z exceeded the river-wide average of 0.18 in focal areas where side channels, contiguous floodplain lakes, and contiguous shallow-water areas exceeded 5%, 5%, and 10% of the floodplain respectively. In contrast, z was always less than 0.18 for focal areas where impounded water exceeded 40% of floodplain area. Our results suggest that rehabilitation efforts that target areas with <5% of the floodplain in side channels, <5% in floodplain lakes, and/or <10% in shallow-water areas could improve habitat diversity across multiple scales in the UMR.

Limnogeology in Brazil’s “forgotten wilderness”: a synthesis from the large floodplain lakes of the Pantanal

Sediment records from floodplain lakes have a large and commonly untapped potential for inferring wetland response to global change. The Brazilian Pantanal is a vast, seasonally inundated savanna floodplain system controlled by the flood pulse of the Upper Paraguay River. Little is known, however, about how floodplain lakes within the Pantanal act as sedimentary basins, or what influence hydroclimatic variables exert on limnogeological processes. This knowledge gap was addressed through an actualistic analysis of three large, shallow ( Si4+ > Ca2+), mildly alkaline, freshwater systems, the chemistries and morphometrics of which evolve with seasonal flooding. Lake sills are bathymetric shoals marked by siliciclastic fans and marsh vegetation. Flows at the sills likely undergo seasonal reversals with the changing stage of the Upper Paraguay River. Deposition in deeper waters, typically encountered in proximity to margin-coincident topography, is dominated by reduced silty-clays with abundant siliceous microfossils and organic matter. Stable isotopes of carbon and nitrogen, plus hydrogen index measured on bulk organic matter, suggest that contributions from algae (including cyanobacteria) and other C3-vegetation dominate in these environments. The presence of lotic sponge spicules, together with patterns of terrigenous sand deposition and geochemical indicators of productivity, points to the importance of the flood pulse for sediment and nutrient delivery to the lakes. Flood-pulse plumes, waves and bioturbation likewise affect the continuity of sedimentation. Short-lived radioisotopes indicate rates of 0.11–0.24 cm year−1 at sites of uninterrupted deposition. A conceptual facies model, developed from insights gained from modern seasonal processes, can be used to predict limnogeological change when the lakes become isolated on the floodplain or during intervals associated with a strengthened flood pulse. Amplification of the seasonal cycle over longer time scales suggests carbonate, sandy lowstand fan and terrestrial organic matter deposition during arid periods, whereas deposition of lotic sponges, mixed aquatic organic matter, and highstand deltas characterizes wet intervals. The results hold substantial value for interpreting paleolimnological records from floodplain lakes linked to large tropical rivers with annual flooding cycles.

Riffle‐pool morphometry and stage‐dependant morphodynamics of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany)

ABSTRACTRiffle‐pool sequences are a common feature of gravel‐bed rivers. However, mechanisms of their generation and maintenance are still not fully understood. In this study a monitoring approach is employed that focuses on analysing cross‐sectional and longitudinal channel geometry of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany) with a high temporal and spatial resolution, in order to conclude from stage‐dependant morphometric changes to riffle and pool maintaining processes.In accordance with previous authors, pool cross‐sections of the Mulde River are narrow and riffle cross‐sections are wide suggesting that they should rather be addressed as two general types of channel cross‐sections than solely as bedforms. At high flows, riffles and pools in the study reaches changed in length and height but not in position. Pools were scoured and riffles aggraded, a development which was reversed during receding flows below the threshold of 0·4Qbf (40% bankfull discharge). An index for the longitudinal amplitude of riffle‐pool sequences, the bed undulation intensity or bedform amplitude, is introduced and proved to be highly significant as a form parameter, its first derivative as a process parameter. The process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation. It is indicated by increasing longitudinal bed amplitudes.According to the observed dynamics of bed amplitudes, maintenance of riffle‐pool sequences lags behind discharge peaks. Maximum bed amplitudes may be reached with a delay of several days after peak discharges. Increasing bed undulation intensity is interpreted to indicate bed mobility. Post‐flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools. This type of transport behaviour is referred to as disconnected mobility.The comparison of two river reaches, one with undisturbed sediment supply, the other with sediment deficit, suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions. It is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Linking Flow Regime, Floodplain Lake Connectivity and Fish Catch in a Large River-Floodplain System, the Volga–Akhtuba Floodplain (Russian Federation)

River-floodplain systems are amongst the most productive—but often severely impacted—aquatic systems worldwide. We explored the ecological response of fish to flow regime in a large river-floodplain system by studying the relationships between (1) discharge and inundated floodplain area, with a focus on spatial and temporal patterns in floodplain lake connectivity, and (2) flood volume and fisheries catch. Our results demonstrate a non-linear relationship between discharge and floodplain inundation with considerable hysteresis due to differences in inundation and drainage rate. Inundation extent was mostly determined by flood volume, not peak discharge. We found that the more isolated lakes (that is, lakes with a shorter connection duration to the river) are located at higher local elevation and at larger hydrological distance from the main rivers: geographical distance to the river appears a poor predictor of lake isolation. Although year-to-year fish catches in the floodplain were significantly larger with larger flood volumes in the floodplain, they were not in the main river, suggesting that mechanisms that increase catch, such as increased floodplain access or increased somatic growth, are stimulated by flooding in the floodplain, but not in the river. Fish species that profit from flooding belong to different feeding guilds, suggesting that all trophic levels may benefit from flooding. We found indications that the ecological functioning of floodplains is not limited to its temporary availability as habitat. Refugia can be present within the floodplain itself, which should be considered in the management of large rivers and their floodplain.

Analysing riffle-pool dynamics of a large floodplain river with a system-oriented approach

Analysing riffle-pool dynamics of a large floodplain river with a system-oriented approach

SPATIAL RELATIONS OF TOPOGRAPHY, LITHOLOGY AND WATER QUALITY IN A LARGE RIVER FLOODPLAIN

ABSTRACTFloodplains exert important controls on water and nutrient processing, yet spatial heterogeneities in floodplain characteristics result in variable effectiveness. In this study, we evaluated the spatial relations among topographic, lithologic and water quality features within the Cedar River floodplain located in southeastern Iowa. Floodplain topography and lithology were dominated by a series of sandy ridges and fine‐textured swales typical of a natural meandering river floodplain complex. Groundwater sampling results from 10 monitoring wells placed in representative ridge and swale environments indicated that water quality varied systematically. Beneath sand‐dominated ridges, water was aerobic and had low specific conductance, and higher concentration of NO3–N and lower concentrations of PO4–P and dissolved organic carbon (DOC). Groundwater beneath swales was anaerobic and typified by high specific conductance, and higher concentrations of NH4–N, PO4–P and DOC. We extrapolated the results from point measurements to the entire floodplain area using surface geophysics and light detection and ranging using co‐kriging to map the distribution of groundwater geochemical environments at the study site. Results are seen to provide an approach to better predict shallow groundwater quality in large river floodplains and improve our ability to manage ecosystem services in these strategic locations. Copyright © 2011 John Wiley &amp; Sons, Ltd.

CAN WE IMPROVE MANAGEMENT PRACTICE OF FLOODPLAIN LAKES USING CLADOCERAN ZOOPLANKTON?

ABSTRACTNatural resource managers across Australia intend to promote healthy floodplain lake ecosystems with rich diversity and composition of biota because such ecosystems provide economically valuable services to society. However, management practice of these floodplain lake ecosystems is impeded by confounding effects of anthropogenic impacts and natural climate variability in recent decades. Yet, there are a few potential biological markers available that profoundly respond to ecological effects of climate change and human disturbances. Cladoceran zooplankton plays an intermediary role in food web dynamics. They show distinct responses to changes in temperature and environmental perturbations, such as acidification, nutrient loading and salinization. The effects of temperature and land‐use changes on food web dynamics and water quality, in particular, are major concerns for shallow lowland large river floodplain lakes management in Australia. Information on zooplankton assemblages and diversity can help increase our understanding of ecological processes in a wide range of environmental exposures. The study of cladoceran fossils and their ephippia preserved in floodplain lake sediment has substantially furthered our understanding of species–environment relationships at different temporal and spatial scales and allowed us to develop powerful inference models for degraded floodplain lake ecosystems. This consequently defines a benchmark of a shift from a naturally intact ecosystem to an ecologically poor regime. In this paper, I have made an attempt to persuade wetland managers through application of contemporary and palaeocladoceran communities to improve management practice of floodplain lake ecosystems in Australia by providing a range of examples. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Longitudinal spatial variation in ecological conditions in an in‐channel floodplain river system during flow pulses

AbstractA field survey was conducted during flow pulses to investigate the longitudinal spatial patterns in water quality, dissolved inorganic and organic matter, phytoplankton, planktonic bacteria, zooplankton, gross primary productivity (GPP) of phytoplankton and planktonic respiration (PR) in channels of the large floodplain system (∼124 km in length) of the Macquarie Marshes, south‐eastern Australia. Four river reaches (areas) with distinct hydrogeomorphological characteristics within the distributary zone of the lower Macquarie River were chosen for analysis of abiotic and biotic variables in their in‐stream environments. The results showed marked longitudinal spatial variation in the values within and among the measured environmental variables including such functional aspects as primary productivity and PR. The variables that tended to have increasing values in a downstream direction were conductivity, total nitrogen (TN), total phosphorus (TP), dissolved reactive phosphorus (DRP), dissolved silica, dissolved organic carbon (DOC), dissolved organic nitrogen (DON), dissolved organic phosphorus (DOP), ratio of DOC/DON and counts of planktonic bacteria. Conversely, the values that tended to decrease downstream were the ratios of TN/TP, DIN/DRP, DOC/DOP, DON/DOP and GPP/PR. Variables that had a localized peak(s) were dissolved oxygen, turbidity, dissolved inorganic nitrogen, GPP, PR and counts of cyanobacteria, diatoms, green algae, cryptomonads, protozoans, rotifers, copepods and cladocerans. Overall, two distinct ecological zones were identified within the broader distributary functional process zone (FPZ): these being the upstream zone with relatively high levels of DO, turbidity, diatoms and GPP/PR ratio, and the downstream zone with relatively high levels of nutrients, dissolved organic matter, cyanobacteria, planktonic bacteria, protozoans and cladocerans. The results of this study describe the spatial connectivity of ecological processes related to hydrogeomorphological factors within a FPZ of a riverine ecosystem, and support the predictions of the riverine ecosystem synthesis framework that ecological patterns and processes can be discontinuous on a longitudinal spatial scale. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Seasonal zooplankton dynamics in main channel and backwater habitats of the Upper Mississippi River

This study used stratified random sampling to examine the spatial and temporal distribution of zooplankton communities in a large floodplain river (Mississippi River, USA). Potential mechanisms controlling zooplankton abundance and community structure were considered. Main channel and backwater habitats included in this study differed between a turbid upper pool reach where aquatic macrophytes were sparse and a lower pool reach which was considerably less turbid and had extensive aquatic macrophyte coverage. Samples were collected monthly during the summer over a 2-year period and multivariate analysis was used to examine the spatial and temporal distribution of zooplankton. Significant differences were found in zooplankton density and community composition among habitats and reaches within the pool. Rotifers were the dominant taxa and seasonality was pronounced, with peak densities often occurring in late-spring. Community structure varied by habitat and reach, which suggests that water quality, physical habitat characteristics, presence of aquatic macrophytes, and zooplankton sources can all influence the zooplankton communities of the Upper Mississippi River. Characterization of the zooplankton communities provides a basis for understanding changes in the river ecosystem and examination of zooplankton communities among habitats provides insight into the mechanisms affecting zooplankton dynamics.

Reconstructing Sakhalin Taimen Parahucho perryi Historical Distribution and Identifying Causes for Local Extinctions

AbstractThe Sakhalin taimen Parahucho perryi is an endangered salmonid with a natural range limited to the Russian Far East and Japan. We constructed a classification tree to determine the environmental factors shaping the historical global distribution of this species and then predicted its potential geographic range. The distribution was most strongly influenced by a spatial autocorrelation term, indicating that it is highly contiguous. Large drainage basins with low topographic relief and large floodplains had a higher probability of taimen occurrence. The boundary of the global distribution was delineated by mean monthly precipitation within the range of 54–96 mm. The presence of Sakhalin taimen was predicted in many drainage basins where it has never been recorded. We also modeled the status of 48 taimen populations in Japan, where it was possible to classify them into three categories: currently stable (7), endangered (5), and extinct (36). The most significant factor differentiating the 12 extant populations from the 36 extinct populations was mean annual air temperature, the extant populations being distributed exclusively in areas where the air temperature is below 5.2°C and agricultural development is minimal. The extant populations were found in drainages with significantly lower elevations and a smaller percentage of farmland compared with drainages where populations have been extirpated. The presence of lagoons was a common characteristic of the drainages with the 7 stable populations, suggesting that lagoons represent critical refugia for the species. The implications of this study for taimen conservation are discussed.

The contribution of long-term isolated water bodies to floodplain fish diversity

Summary 1. Floodplains and their water bodies are typical ecotopes of large lowland rivers. The lowland Oder River, Germany, provided a rare opportunity to study fish assemblages of comparable floodplain water bodies differing by >50 years of isolation history. We hypothesised that true floodplain specialists peak in rarely connected water bodies, while frequently flooded waters support tolerant generalists. 2. Three macrohabitats, main channel (MC), temporarily connected and isolated floodplain water bodies, were sampled by electro-fishing and their fish assemblage characters recorded. 3. Long-term isolation of floodplain water bodies had a significant effect on the fish assemblage by promoting species preferring still water. Limnophilic and floodplain specialist species significantly increased with isolation. 4. Fish densities, species richness and diversity clearly differed between MC sites and floodplain water bodies. Shannon’s species diversity index peaked in both the MC and isolated waters and was lowest in the temporarily flooded waters where eurytopic fish dominated. 5. The significant gain in abundance and numbers of limnophilic species in the isolated compared to the temporarily flooded water bodies underpinned the significant contribution of long-term isolated waters to the gamma-diversity of large floodplain rivers, which should be considered in floodplain rehabilitation.

Paleoecological responses to avulsion and floodplain evolution in a semiarid Australian freshwater wetland

Paleoecological responses to coupled geomorphic and hydrological changes have rarely been studied using a multiproxy approach in freshwater floodplain wetlands of semiarid Australia. The Macquarie Marshes are a large, multi-channelled and morphodynamic floodplain wetland system in the lowland interior of the Murray-Darling Basin, southeastern Australia. Evidence from historical maps, aerial photographs, geomorphology and sedimentology indicate that avulsion in the southern Macquarie Marshes caused the formation of a major new channel (Monkeygar Creek) and the abandonment of a reach of the Macquarie River in the last 200 years. Paleobiological samples from near-uniform sediment cores were studied to determine the environmental changes related to this avulsion, as well as associated floodplain evolution and hydrological changes in the adjoining flood-reliant wetlands. Fifteen macrophyte taxa, seven charophyte taxa, 28 diatom taxa and 18 aquatic invertebrate taxa were identified in sediments from the terminal reach of the Macquarie River and the active lower reach of Monkeygar Creek. Paleoecological reconstruction indicates that the slowly accreting marsh (ca 0.032 to 0.037 cm a−1) around the Macquarie River (Terminus Marsh) was more frequently and deeply inundated in the past (presence of limnetic zooplankton before ca 1626 cal. years BP) than during the penultimate pre-avulsion phase (absence of limnetic zooplankton and more diverse terrestrial and aquatic vegetation from ca 540 cal. years BP). In contrast, the floodplain near lower Monkeygar Creek was only periodically inundated prior to avulsion (high diversity of charophytes and low diversity of diatoms and invertebrates before ca 149 years BP), before experiencing disturbance, rapid accretion and ecological succession during the avulsion (paucity of wetland biota in distinct flood facies ca 149 years BP). This part of the floodplain became a substantial wetland, dominated by algae (charophytes and diatoms) and submerged macrophytes, due to a more consistent flood regime following the avulsion. The contemporary marsh on lower Monkeygar Creek has evolved in the last ca 54 years into a shallower, less frequently flooded wetland (new dominance of emergent macrophytes, charophytes and littoral cladocerans), probably due to changes in inundation caused by rapid local floodplain accretion (ca 0.409 to 0.556 cm a−1) and possibly river regulation. This multiproxy approach provides some of the first evidence of significant changes in wetland communities and ecological successions that are directly related to the geomorphic processes of avulsion and floodplain evolution in a semiarid wetland system such as the Macquarie Marshes.