Frogs during the flood: Differential behaviours of two amphibian species in a dryland floodplain wetland

Flow regimes structure biotic processes and communities of floodplain wetlands. Most current understanding of the influence of the flood pulse on vertebrates is based on knowledge of relationships between flow regimes and fish and bird populations. Its influence on amphibian populations is poorly known, and possibly difficult to disentangle due to the strong influence of weather on amphibian movement and breeding. We predicted that the behaviour of two functionally different amphibian species would vary due to different physiological constraints and life histories driving habitat use and activity periods. We radio‐tracked 52 frogs, 27 barking marsh frogs (Limnodynastes fletcheri, Limnodynastidae) and 25 green tree frogs (Litoria caerulea, Hylidae), to investigate the influence of weather and flooding on habitat use, size of activity area and nightly movement. Activity area sizes and nightly distances of the species were highly variable. Limnodynastes fletcheri had a greater area of activity during floods (mean area: 5237 ± 2556 (SE) m² cf. 639 ± 247 m² post flood) while the tree frog had a greater area after flooding (207 ± 68 m² cf. 1946 ± 530 m²). Limnodynastes fletcheri movement was little influenced by weather or inundation, with distances slightly restricted by increasing wind speed. However, use of floodplain habitat and coincidental breeding behaviour indicated responsiveness by L. fletcheri to flooding. Litoria caerulea moved further with increasing rainfall and nightly temperatures, seldom used flooded areas and there was no evidence of breeding. Although the flood pulse is the dominant structuring force on the floodplain, its influence varies on amphibian species. Consequently for conservation, L. fletcheri populations may be negatively affected by river regulatory activities, but would respond to environmental flow management, whereas L. caerulea appears unlikely to be affected by river regulation, though the confounding effects of rain and preferred habitat inundation needs exploring.     

Establishing the condition of lowland floodplain rivers: a palaeo-ecological approach

1. Current approaches for determining river ‘health’ rely on the establishment of a ‘reference’ condition. This is often difficult to achieve for lowland floodplain river systems because of the lack of pristine or pre-impacted sites and process models capable of predicting the effect of natural and human disturbances. 2. Using examples from the River Murray, Australia, this paper highlights the benefits of using palaeo-ecological studies in deciding on benchmark or pre-European settlement conditions for lowland floodplain river systems. 3. Physical, chemical and biological analyses of sediment obtained from shallow cores (< 2 m) extracted from a variety of floodplain sites on the River Murray and its tributaries reveal the extent and timing of catchment disturbances on this lowland river. Rates of sedimentation have increased by an order of magnitude since settlement; sediments now being deposited are finer, having implications for the nutrient status of the floodplain and the accumulation of heavy metals associated with these sediments. There has also been a sustained drop in the macrophyte cover of billabongs in the area, as inferred by chydorid cladoceran assemblages. The pre-European settlement diatom assemblages do not resemble those presently accumulating in billabongs in the region. 4. Whilst initially developed for lentic studies, palaeo-ecological studies are relevant to the establishment of benchmark conditions for lowland floodplain rivers. They are relatively straightforward, incorporate both historical and contemporary ecological information and can be applied to any lowland system.     

Development and ecological importance of phytoplankton in a large lowland river (River Meuse,Belgium)

The ecological importance of the River Meuse phytoplankton with regard to carbon and nutrient transport has been examined in two reaches of the Belgian course of the river.Field measurements of total particulate organic carbon (POC), particulate organic nitrogen (PON) and particulate phosphorus (PP) show that the large autochtonous production of organic matter strongly affects the carbon and nutrient budget of the aquatic system. During the growing season, phytoplankton accounts for nearly 60% of the POC and dominates the PON. Calculations of the carbon and oxygen budget in the upper reach of the Belgian Meuse demonstrates that the ecosystem is autotrophic, i.e. that autochtonous FPOM (fine particulate organic matter) production is the major carbon input. This suggests that in large lowland rivers, primary production (P) may exceed community respiration (R), i.e. P:R>1, whereas they are assumed to be heterotrophic (P:R<1) in the River Continuum concept.The question of maintenance of phytoplankton in turbid mixed water columns is also addressed, and the case of the River Meuse is treated on the basis of studies of photosynthesis and respiration (ETS measurements). The results suggest that the potamoplankton may show some low-light acclimation, through an increase of chlorophyll a relative to biomass, when it comes to deep downstream reaches, and that algal respiration rate may be reduced. A simulation of the longitudinal development of the algal biomass shows the different phases of algal growth and decline along the river and brings support to the importation hypothesis for explaining maintenance of potamoplankton in the downstream reaches.     

Connectivity and complexity of floodplain habitats govern zooplankton dynamics in a large temperate river system

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The effect of flooding and flood timing on leaf litter breakdown rates and nutrient dynamics in a river red gum (Eucalyptus camaldulensis) forest

Comparisons of litter standing-stocks in low-lying and higher areas of the floodplain and the effects of controlled flooding events on leaf litter decomposition and leaf litter nutrients were examined during autumn and winter in a southeastern Australian river red gum (Eucalyptus camaldulensis) floodplain forest. The mean mass of total litter and some litter components was significantly greater in autumn than in winter but there were few differences in litter mass between low-lying flood runners and higher sites (1.5 m) on the floodplain, regardless of season. Leaf decomposition was more rapid in flooded areas than in non-flooded areas and was significantly faster in autumn than in winter. In flooded leaves, concentrations of phosphorus and nitrogen dropped rapidly during the first 3 days of each experiment, increased to near original after 7–10 weeks and then decreased again. After 112 days of decomposition the C:N:P ratios of leaf litter increased, but this effect was most marked for flooded leaves. Simple models of leaf litter dynamics indicated that leaf litter standing-stocks in low-lying flood runners would be reduced by flooding, particularly during autumn. In contrast, models predicted a net gain in standing-stocks of leaf litter to be higher on the floodplain, particularly in autumn. Alteration to the seasonal timing of floods by river regulation has probably decreased litter standing-stocks and nutrients available in low-lying areas of the floodplain to support the production of macrophytes and biofilms during winter and spring floods.     

Thermal patterns in the surface waters of a glacial river corridor (Val Roseg, Switzerland)

SUMMARY 1. We examined the thermal patterns of the surface waters in the catchment of the Roseg River, which is fed by the meltwaters of two valley glaciers. One of the glaciers has a lake at its terminus. The river corridor comprised a proglacial stream reach below one glacier, the glacier lake outlet stream, a 2.5‐km long complex floodplain and a constrained reach extending to the end of the catchment. 2. Temperatures were continuously measured with temperature loggers at 27 sites between 1997 and 1998. Moreover, from 1997 to 1999, spot measurements were taken at 33–165 floodplain sites (depending on water level) at monthly intervals. 3. The temperature regime of glacial streams, including the glacier lake outlet, was characterised by rapidly increasing temperatures in April and May, a moderate decline from June to September (period of glacial melt) and a subsequent fast decline in autumn. During summer, the lake increased temperatures in the outlet stream by 2–4 °C, compared with the adjacent proglacial stream reach. 4. In the main channel (thalweg) of the Roseg River, annual degree‐days (DD) ranged from 176 DD in the upper proglacial reach to 1227 DD at the end of the catchment. 5. Thermal variation among different channels within the floodplain was higher than the variation along the entire main channel. Floodplain channels lacking surface connection to the main channel accumulated up to 1661 annual DDs. 6. Thermal heterogeneity within the floodplain was linked to the glacial flow pulse. With the onset of ice melt, temperatures in the main channel and in channels surface‐connected to the main channel began to decline, whereas in surface‐disconnected channels temperatures continued to increase; as a consequence, thermal heterogeneity at the floodplain scale rose slightly until September. 7. High thermal heterogeneity was not anticipated in the harsh environment of a largely glacierised alpine catchment. The relatively wide range of thermal environments may contribute to the highly diverse zoobenthic community.     

Structural changes and dynamics of the phytoplankton along a highly polluted lowland river of Argentina

The phytoplankton of the Matanza-Riachuelo river (Buenos Aires, Argentina) was studied for a period of 15 months together with different physical and chemical variables, along an increasingly eutrophic in character downstream. 281 taxa were recorded within a seasonal succession, among these 49 percent were euglenophyta, 25 percent bacillariophyta, 15 percent chlorophyta and 10 percent cyanophyta. Multivariate procedures included cluster analyses and ordination by PCA of both species and samples, and stepwise multiple regression analyses were employed to assess the impact of the pollution on the phytoplankton. The effect of the alteration was different in each studied site. With one exception, the changes in the community structure described herein involved modifications of the basic species composition. According to the species present, it was deduced that the main pollutant was the high concentration of organic matter. Results of the Matanza-Riachuelo, Reconquista, and Luján Rivers studies are further discussed.     

Diel distribution patterns of fishes in a temperate large lowland river

During the day, juveniles and typical inshore species in the lower Oder River, Germany, inhabited the littoral (0·05–1·50 m deep) while potamal species inhabited the mid‐channel (40–100 m from the banks). During the night, two behavioural groups were distinguished: 1) potamal species and large individuals of other species, moved from offshore to the littoral, and 2) nocturnal foraging species, became active and increased in inshore and offshore habitats. Inshore, the most significant differences at night was the increasing density of silver bream Blicca bjoerkna increasing both total biomass and total length of fishes caught. Offshore, the increasing density of whitefin gudgeon Gobio albipinnatus at night was most significant. Additional night electrofishing improved the assessment of the abundance, age and size structure of typical potamal fish species. It should be incorporated in large river fish monitoring programmes.     

Priming the productivity pump: flood pulse driven trends in suspended algal biomass distribution across a restored floodplain

1. Chlorophyll a (Chl a) distribution across a 0.36 km² restored floodplain (Cosumnes River, California) was analysed throughout the winter and spring flood season from January to June 2005. In addition, high temporal‐resolution Chl a measurements were made in situ with field fluorometers in the floodplain and adjacent channel. 2. The primary objectives were to characterise suspended algal biomass distribution across the floodplain at various degrees of connection with the channel and to correlate Chl a concentration and distribution with physical and chemical gradients across the floodplain. 3. Our analysis indicates that periodic connection and disconnection of the floodplain with the channel is vital to the functioning of the floodplain as a source of concentrated suspended algal biomass for downstream aquatic ecosystems. 4. Peak Chl a levels on the floodplain occurred during disconnection, reaching levels as high as 25 μg L^?1. Chl a distribution across the floodplain was controlled by residence time and local physical/biological conditions, the latter of which were primarily a function of water depth. 5. During connection, the primary pond on the floodplain exhibited low Chl a (mean = 3.4 μg L^?1) and the shallow littoral zones had elevated concentrations (mean = 4.6 μg L^?1); during disconnection, shallow zone Chl a increased (mean = 12.4 μg L^?1), but the pond experienced the greatest algal growth (mean = 14.7 μg L^?1). 6. Storm‐induced floodwaters entering the floodplain not only displaced antecedent floodplain waters, but also redistributed floodplain resources, creating complex mixing dynamics between parcels of water with distinct chemistries. Incomplete replacement of antecedent floodplain waters led to localised hypoxia in non‐flushed areas. 7. The degree of complexity revealed in this analysis makes clear the need for high‐resolution spatial and temporal studies such as this to begin to understand the functioning of dynamic and heterogeneous floodplain ecosystems.     

Seston sedimentation in a lowland river (River Spree,Germany): their spatial and temporal variations and controlling factors

Sedimentation of organic particles plays a decisive role in streams in relation to pelagic loss as well as retention of nutrients and other substances. The plate sediment trap allows for the direct measurement of these net fluxes. Biweekly measurements were undertaken in the eutrophic lowland River Spree (MQ 14 m³ s^–1) 10 km upstream of Berlin in 1999 and 2000. Trapping rates between 0.5 and 25 g DW m^–2 day^–1 were found near the bank. The variance of seston sedimentation is controlled by the seston concentration, the settling velocity of the particles and the flow velocity. The sinking velocity exhibits significant seasonal fluctuations with highest values in summer. It is shown that the critical flow velocity for sedimentation is another important parameter. This controls the distribution of sedimentation over the width of the river and thus the effective average sedimentation rate for the entire river segment. This average rate ranged between 0.9 and 6.6 g DW m^–2 day^–1.     

Microbial respiration within a floodplain aquifer of a large gravel-bed river

1. Aerobic respiration, productivity and the carbon turnover rate of microbial biofilms were determined at hyporheic and phreatic sites in the Kalispell Valley alluvial aquifer along a transect extending 3.9 km laterally from the main channel of the Flathead River, a sixth order river in Montana (U.S.A.). The effect of experimentally increasing bioavailable organic carbon (acetate) on the respiration rate of biofilms in this carbon‐poor [dissolved organic carbon (DOC) < 2 mg L^?1] aquifer was also measured. 2. Chambers containing natural substratum were placed in‐situ and allowed to colonise for 20 weeks. After 4, 12 and 20 weeks, they were taken to the laboratory where oxygen flux was measured in a computer‐controlled, flow‐through respirometry system. 3. Respiration ranged from 0.01 to 0.33 mg O₂ dm^?3 h^?1 across sites, with means ranging from 0.10 to 0.17 mg O₂ dm^?3 h^?1. Productivity estimates ranged from 0.18 to 0.32 mg C dm^?3 day^?1 (mean 0.25, SE 0.03). The total organic carbon (TOC) of the microbial biofilms ranged from 18.2 to 29.7 mg C dm^?3. Turnover rate ranged from 3.2 to 5.6 year^?1 with a mean of 4.2 year^?1. 4. At the hyporheic site very close to the river, respiration did not significantly increase when samples were supplemented with labile carbon. Respiration increased with increasing DOC addition at hyporheic sites more distant from the river, suggesting a carbon‐limitation gradient within the hyporheic zone. Microbes at the phreatic site did not respond to increasing DOC addition, suggesting that the phreatic biofilm is adapted to low carbon availability. 5. Comparing the volume of the alluvial aquifer (about 0.7 km³) to that of the river benthic sediments (to 0.25 m depth, which amounts to about 1.6 × 10^?4 km³) within the Flathead Valley, leads to the conclusion that interstitial microbial productivity is orders of magnitude greater than benthic productivity. Alluvial aquifers are often voluminous and microbial production is an enormous component of ecosystem production in rivers such as the Flathead.     

The effect of drought on benthic invertebrate communities in a lowland river

Benthic macroinvertebrates were sampled at regular intervals from rural and urban sections of the River Roding, Essex, England, in 1975 and 1976. During the latter year a severe drought led to a marked decline in flows and to desiccation of parts of the river bed. In general, drought conditions resulted in an increase in invertebrate populations and possible reasons for this are presented. A considerable number of individuals of certain groups such as cased caddisfly larvae and prosobranch molluscs were, however, eliminated from the river at this time, mainly as a result of stranding and chemical changes in the environment. The effect of reduced flows on river faunas is briefly discussed.     

Annual movement and migration of adult pikeperch in a lowland river

The downstream migration of 46–66 cm radio-tagged adult pikeperch Stizostedion lucioperca , in the River Gudenaa, Denmark, from late September to late December, coincided with a reduction in the frequent occurrence of prey fish in the tagging area, and hence was interpreted as a feeding migration. The upstream migration from March to mid-April in spring took place prior to spawning which occurred from late April to June.     

Extreme weather events threaten biodiversity and functions of river ecosystems: evidence from a meta-analysis

Both gradual and extreme weather changes trigger complex ecological responses in river ecosystems. It is still unclear to what extent trend or event effects alter biodiversity and functioning in river ecosystems, adding considerable uncertainty to predictions of their future dynamics. Using a comprehensive database of 71 published studies, we show that event – but not trend – effects associated with extreme changes in water flow and temperature substantially reduce species richness. Furthermore, event effects – particularly those affecting hydrological dynamics – on biodiversity and primary productivity were twice as high as impacts due to gradual changes. The synthesis of the available evidence reveals that event effects induce regime shifts in river ecosystems, particularly affecting organisms such as invertebrates. Among extreme weather events, dryness associated with flow interruption caused the largest effects on biota and ecosystem functions in rivers. Effects on ecosystem functions (primary production, organic matter decomposition and respiration) were asymmetric, with only primary production exhibiting a negative response to extreme weather events. Our meta-analysis highlights the disproportionate impact of event effects on river biodiversity and ecosystem functions, with implications for the long-term conservation and management of river ecosystems. However, few studies were available from tropical areas, and our conclusions therefore remain largely limited to temperate river systems. Further efforts need to be directed to assemble evidence of extreme events on river biodiversity and functioning.     

Diel drift of Chironomidae in a large lowland river (Central Poland)

The diel drift patterns of Chironomidae larvae were investigated in a seventh order section of the Warta River (Central Poland) over two diel cycles during May 1989. Three nets (mesh size 400 m) were installed in a cross section of the Warta River.The estimated drift density was low, but was comparable to that calculated for other large rivers. Spatio-temporal fluctuations in abundance and composition of macroinvertebrate drift, including Chironomidae, were observed with the highest density of drifting macrobenthos recorded near the depositional bank of this river. The ratio benthosdrift indicated differing propensities for of the older instars of a given chironomid taxon to drift. Orthocladiinae larvae were the most abundant subfamily of Chironomidae in drift but not in benthos, reaching up to 73% of the total drifting chironomid larvae. More taxa but fewer individuals (about 20% of the chironomid larvae collected) belonged to the tribe Chironomini, the dominant group in benthos.A major part of chironomid drift collection may represent behavioural drift because the net mesh size used in the Warta River was insufficient to catch the earliest instars (distributional drift). Both at the family and subfamily level chironomid larvae exhibited a distinct nocturnal drift periodicity. Nocturnal periodicity was documented for the dominant species, but due to the low density of many chironomid species, it was impossible to determine their diel drift pattern. Some Chironomidae appeared to be aperiodic.     

Habitat characteristics and the distribution of Odonata in a lowland river catchment in eastern England

The influence of habitat characteristics on the distribution of larval and adult Odonata communities in a lowland river in eastern England was studied. There was a longitudinal distribution of Odonata, with larval assemblages being influenced directly by marginal flow velocity, biochemical oxygen demand (BOD) and phosphate concentrations and indirectly by shade and cover of floating vegetation. Adult populations responded directly to shade, reed cover, amenity-managed land use and bank height, and indirectly to BOD and ammonia concentrations. Distribution patterns were strongly associated with both natural changes along the river system and management impacts. River management practices locally disrupted the natural displacement of species along the river, but whilst some forms of human interference on river systems were beneficial to species richness, the effects on stenotopic species were disadvantageous. To conserve Odonata management emphasis should be on the maintenance of suitable conditions for river specialists.     

The importance of rare events in thin film deposition: a molecular dynamics study of tetrahedral amorphous carbon

The transition of diamond-like materials from an sp³ to sp² rich state is of particular interest because of the desirable properties of tetrahedral amorphous carbon (ta-C). Previous works indicate that infrequent processes may dominate this transition, but simulation of these processes presents significant difficulties, since the infrequent processes are activated on the millisecond scale. In this molecular dynamics study the environment dependent interaction potential is used to simulate the thin film deposition of ta-C. Infrequent processes occurring between energetic impacts were activated on the picosecond scale using elevated temperatures. The simulations reveal an abrupt transition in which the ta-C films transform into graphite-like sheets. A similar transformation, albeit at much higher temperatures, is also observed when the films are heated without energetic impacts. These results are found to be in good agreement with published experimental data.     

Juvenile growth of a macrohabitat generalist tied to hydrological character of large‐river system

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