Interactions between period of exposure,grazing and crop growth rate of Cynodon dactylon (L.) Pers. in seasonally flooded areas of the Pongolo River floodplain

Cynodon dactylon (L.) Pers. forms extensive meadows on seasonally inundated areas of the Pongolo river floodplain. As floodwaters receded during winter, the shoots of C. dactylon which had not decomposed during submergence sprouted. Crop growth rate was high (up to 23 kg/ha/d dry mass) and the meadows were heavily grazed by domestic stock which removed up to 15 kg/ha/d. The high crop growth rate was not maintained because as exposure period increased, soils became dry and water stress conditions developed. This caused the C. dactylon to become less palatable and brought about a shift in grazing which allowed a build-up of the grazeable and ungrazeable fraction standing crop. Maximum standing crop occurred at the onset of the next flood. Grazing is shown to have a net negative effect on production, although at different times during exposure defoliation had either a depressing or stimulating effect on growth. It is postulated that the duration of the inundation period is more significant than the duration of the exposure period in maintaining the C. dactylon meadows.     

Just add water: sources of chironomid drift in a large river floodplain

Although seasonal floodplains represent one of the most dynamic and productive of aquatic ecosystems, the sources of this productivity are poorly understood. We examined composition and sources of chironomid drift in the Yolo Bypass, the primary floodplain of the Sacramento River. We found that invertebrate drift during winter floodplain inundation is dominated by a single species, the newly identified chironomid Hydrobaenus saetheri (Diptera: Chironomidae). In order to determine sources of chironomids in the Yolo Bypass, invertebrates were sampled from several potential sources prior to and during initial floodplain inundation. Rehydration of dried floodplain sediments from several locations showed that H. saetheri dominated insect emergence from this colonization pathway. By contrast, H. saetheri was not a substantial component of inundated floodplain ponds or of tributary inputs to the floodplain. We conclude that the initial pulse of invertebrate abundance in Yolo Bypass floodwaters is dominated by chironomid emergence from sediments in multiple regions of the floodplain. Handling editor: S. Declerck     

不同水分条件下毛果苔草枯落物分解及营养动态

于2009年5月至2010年5月采用分解袋法,研究了三江平原典型湿地植物毛果苔草枯落物分解对水分条件变化的响应,探讨了典型碟形洼地不同水位下枯落物分解1a时间内的分解速率与N、P等营养元素动态。分解1a内,无积水环境下枯落物失重率为34.99%,季节性积水环境下为27.28%,常年积水环境下随水位增加枯落物失重率分别为26.99%与30.67%,表明积水条件抑制了枯落物的分解。枯落物的分解随环境变化表现出阶段性特征,分解0—122 d内随水位增加枯落物失重率分别为16.09%、24.25%、23.53%与26.60%,即生长季内积水条件促进了枯落物有机质的分解及重量损失。而随实验进行,分解122—360 d内随水位增加毛果苔草枯落物的失重率分别为18.90%、3.02%、3.46%、4.03%,即在非生长季土壤冻融期积水条件抑制了枯落物分解(P<0.05)。水分条件对毛果苔草枯落物N元素的影响表现为积水条件促进生长季内枯落物的N固定,水位最高处毛果苔草N浓度显著高于无积水环境(P<0.05)。但进入冻融期后积水环境下枯落物N浓度与含量降低;其中季节性积水限制了枯落物的N积累能力,至分解360d时与初始值相比表现出明显的N释放(P=0.01)。毛果苔草枯落物分解61d时P出现富集,其中积水条件下P的富集作用增强,但与水位不相关。分解1a后毛果苔草枯落物表现为P的净释放,不同水分条件下枯落物P元素损失没有明显差异(P>0.05)。     

Differences in processing dynamics of fresh and dried leaf litter in a stream ecosystem

SUMMARY. 1. Although the bulk of litter input to stream ecosystems is in the form of fresh leaves, current understanding of organic matter processing is largely founded on experimental studies made with pre-dried leaves. This paradox points to the critical need for evaluating to what extent those experiments with dried leaves reflect natural litter decomposition.
2. The mass loss rates, patterns of mass loss, and chemical changes during processing of fresh leaf litter were compared with air-dried leaf litter in a stream ecosystem.
3. Although overall mass loss rates were similar between treatments ( k = 0.0213 day⁻¹ and 0.0206 day⁻¹), fresh leaves lost mass at a constant rate, whereas the decay of dried leaves proceeded in two distinct phases. Soluble organic carbon, phosphorus, and potassium were rapidly leached from dried litter, but were largely retained in fresh material for more than a week. Kinetics of concentrations of cellulose and changes in amounts of lignin remaining per leaf pack revealed further differences in decomposition dynamics between treatments, apparently related, either directly or indirectly, to differences in leaching behaviour.
4. Dynamics of nitrogen and protein contents were similar between treatments, indicating that microbial colonization was not greatly delayed on fresh leaves.
5. It is concluded that the retention of labile carbon and nutrients in fresh leaf litter facilitates their utilization by leaf-associated micro-organisms and invertebrates, resulting in an increased importance of biotic processes relative to physical processes such as leaching.
6. At the ecosystem level, retention of carbon and nutrients in streams would be increased, allowing greater overall productivity. Conversely, the availability of labile organic carbon would be reduced in compartments such as the epilithon, fine sediments, and the water column.     

Flood mapping with remote sensing and hydrochemistry: A new method to distinguish the origin of flood water during floods

River flooding is important for the ecological functioning of river floodplains. It is implicitly assumed that in many river floodplains during floods, river water is spreading all over the floodplain. We hypothesize that during flood events a spatial distribution of water types exists, which is correlated to different water sources (river water, atmospheric water and groundwater) and to the spatial distribution of vegetation types. The objective of this paper is to assess a new methodology to determine the extent of flooding and the spatial distribution of different water sources during the flood, using GPS, multispectral remote sensing and hydrochemical analyses. This methodology is applied to the Biebrza River Lower Basin, which has little human impact. Remote sensing resulted in a map distinguishing inundated areas from dry areas, which showed 85% agreement with GPS field measurements. Principal Component Analyses and Cluster Analyses on the measured water chemistry identified different water sources during the flood (river water, groundwater, rainwater) and showed the effects of human impact on water quality. River flood water dominated the entire inundation zone in the northern Lower Basin, which is narrower and steeper than the southern Lower Basin where groundwater and rainwater were significant contributors to the major part of the inundated area. Vegetation in the river flood zone is distinctly different from the rest of the floodplain. Due to mixing of ground- and rainwater, correlation analyses between vegetation and water type were not possible outside the river flood zone. The new methodology is effective in distinguishing inundated areas from dry regions and in separating river flood water from other water sources during a flood.     

Is tolerance to summer flooding correlated with distribution patterns in river floodplains? A comparative study of 20 terrestrial grassland species

It is generally assumed that floods during the growing season have a strong impact on the distribution of grassland plant species in river floodplains but this proposition has never been tested. We examined the survival and growth responses of twenty species, originating from mid- and high-level floodplain grasslands along the River Rhine in the Netherlands, to total submergence for a maximum of two months in an outdoor flooding experiment. Plant survival and biomass reduction with flooding duration was determined as well as biomass recovery after de-submergence.
Our results indicate that species survival is the most prominent factor correlated with species distribution in floodplain areas. Relatively flood tolerant species occurred mainly at low elevations along the floodplain while more flood sensitive species were restricted to high parts of the floodplain gradient. Biomass reduction rates during submergence were only marginally significantly correlated with species lower distribution boundaries along the flooding gradient. Biomass recovery rate was significantly correlated with species distribution patterns in the field only after 2 weeks of complete submergence, but not after 4 and 8 weeks. Our results suggest that the more flood tolerant species can have various ways to survive and recover from flooding, ranging from low rates of biomass loss and low recovery to relatively high rates of biomass loss and quick recovery.
Our results are consistent with the notion that disturbance by floods during the growing season is an important determinant of species lower distribution boundaries in river floodplains. They also suggest that high survival under flooding may be achieved by different physiological mechanisms. Such mechanisms are discussed in this paper.     

Hydrologic control of litter decomposition in seasonally flooded prairie marshes

The effect of seasonal inundation on the decomposition of emergent macrophyte litter (Scolochloa festucacea) was examined under experimental flooding regimes in a northern prairie marsh. Stem and leaf litter was subjected to six aboveground inundation treatments (ranging from never flooded to flooded April through October) and two belowground treatments (nonflooded and flooded April to August). Flooding increased the rate of mass loss from litter aboveground but retarded decay belowground. Aboveground, N concentration decreased and subsequently increased earlier in the longer flooded treatments, indicating that flooding decreased the time that litter remained in the leaching and immobilization phases of decay. Belowground, both flooded and nonflooded litter showed an initial rapid loss of N, but concentration and percent of original N remaining were greater in the nonflooded marsh throughout the first year. This suggested that more N was immobilized on litter under the nonflooded, more oxidizing soil conditions. Both N concentration and percent N remaining of belowground litter were greater in the flooded than the nonflooded marsh the second year, suggesting that N immobilization was enhanced after water-level drawdown. These results suggest different mechanisms by which flooding affects decomposition in different wetland environments. On the soil surface where oxygen is readily available, flooding accelerates decomposition by increasing moisture. Belowground, flooding creates anoxic conditions that slow decay. The typical hydrologic pattern in seasonally flooded prairie marshes of spring flooding followed by water-level drawdown in summer may maximize system decomposition rates by allowing rapid decomposition aboveground in standing water and by annually alleviating soil anoxia.     

Immediate and longer-term effects of managed flooding on floodplain invertebrate assemblages in south-eastern Australia: generation and maintenance of a mosaic landscape

1. We compared assemblages of ground‐active, terrestrial beetles and spiders from different areas of river red gum Eucalyptus camaldulensis floodplain forest in subhumid, south‐eastern Australia before and for 2 years following a managed flood to determine whether the Flood Pulse Concept is an appropriate ecological model for this regulated, lowland river‐floodplain system. 2. Immediately following flooding, the abundance, species richness and biomass of beetles were greatest at sites that had been inundated for the longest period (approximately 4 months). The abundance, species richness and biomass of spiders were not reduced at sites that were flooded for 4 months compared with unflooded or briefly flooded areas. Sites recently flooded for several months had high densities of predatory, hygrophilic beetles (Carabidae) and spiders (Lycosidae). 3. Over the 2 years following the flood, beetles generally were more abundant at sites that had been inundated for longer. At all sampling times, the species richness of beetles at sites increased with the length of time sites were inundated, even before the flood. Neither the abundance nor species richness of spiders was related to duration of flooding. 4. The structure of beetle and spider assemblages at sites that were flooded for different lengths of time did not appear to converge monotonically over the 2 years after the flood. 5. Managed flooding promotes diversity of beetles and spiders both by providing conditions that create a ‘pulse’ in populations of hygrophilic specialists in the short term, and by creating subtle, persistent changes in forest‐floor conditions. Despite its monotypic canopy, river red gum floodplain forest is a habitat mosaic generated by differing inundation histories.     

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.     

An assessment of the importance of water stress in seasonally floodedC. dactylon on the Pongolo River floodplain in the formulation of guidelines for controlled flooding

Daily variation in water potential was measured inCynodon dactylon in seasonally flooded areas of the Pongolo river floodplain. Soil moisture declined during the dry winter season when the plants were exposed, causing them to experience increasing water stress. Fog and mist appeared to relieve stress but it is concluded that water stress is sufficient to reduce production during exposure. The results are discussed in relation to the timing of flood releases from the upstream Pongolapoort dam and it is concluded that the period of exposure is of less significance than the duration and frequency of inundation in the formulation of guidelines for flood releases from the dam.     

The rise and fall of the Lower Mississippi: effects of hydrologic connection on floodplain backwaters

In the last 150 years, the Lower Mississippi River (LMR) floodplain was extensively reduced in area with conversion of once regularly inundated floodplain wetlands to dry land. Yet, between the present levees, there remain substantial remnants of the historical floodplain, including various types of permanent backwater habitats. We hypothesized that degree and timing of hydrologic connection with the river main channel drives variation in physical and chemical properties, and phytoplankton community dynamics, in backwaters. Over 23 months, measurements were made in several sites varying in connection with the main channel. At high stages, the channel and backwaters converged in having elevated turbidity, suspended solids and dissolved nutrient concentrations, reduced algal biomass and production, a seston C:N ratio indicating predominantly allochthonous organic matter sources, and a diatom-dominated community. When connection was reduced, backwaters declined in suspended sediments and nutrients, surged in production and biomass, had a seston C:N ratio reflecting autochthonous production, and switched to a community dominated by cryptomonad algae. With prolonged isolation, biomass-specific production became nitrogen limited and declined. By a conceptual model, we illustrate the seasonally variable role of permanent floodplain backwaters as major sources of algal organic matter, and potentially for nutrient sequestration, within the Lower Mississippi system.     

Macroinvertebrate distribution and production on the floodplains of two lowland headwater streams

SUMMARY.

• 1 Spatial and temporal distribution, abundance and production of the floodplain macroinvertebrate communities of two low-gradient headwater streams in Virginia, U.S.A., were studied over 1 year.
• 2 Spatial and temporal distribution patterns of invertebrates were affected by inundation patterns. Numbers, biomass and production were higher in low than high floodplain areas and on a floodplain continuously inundated over 9 months compared with a floodplain completely flooded only occasionally during storms.
• 3 The predominant species in terms of numbers and production on both floodplains was the harpacticoid copepod Attheyella illinoisensis Forbes. Other species with relatively high production were the sphaeriid clam Pisidium sp., the leptophlebiid mayfly Leptophlebia sp., the isopod Caecidotea racovitzai (Williams), and the chironomids Paratendipes sp. and Polypedilum spp.
• 4 Annual invertebrate production (dry weight) on the floodplain continuously inundated over 9 months was 6.1 g m^?2; production on the periodically inundated floodplain was 1.7 g m^?2. Collector-gatherers accounted for 58–61%, and predators 19–25%, of the production.
• 5 Annual invertebrate production on the floodplains, calculated on the basis of a linear metre of channel length, was 84–490 gm^?1, or about one to two orders of magnitude greater than probable production in the channels, suggesting the significance of floodplain invertebrates to stream system trophic dynamics.

     

Colonising Strategies and Diapause of Planktonic Rotifers (Monogononta,Rotifera) during Aquatic and Terrestrial Phases in a Floodplain (Lower Oder Valley,Germany)

The floodplains of the River Oder are inundated in winter and spring and are desiccated during summer. The plankton community developing towards the end of winter is dominated by monogonont rotifers. Some species maintain populations on the flooded areas as well as in the remaining permanent waters after flooding. Other species are found during the inundation period only. These species produce drought‐resistant diapause stages, beginning at low population densities between 5 to 36 females L^–1. Experiments showed that 9 out of 10 species, which were observed to produce resting eggs during the aquatic phase, were able to survive the terrestrial phase through dormancy in the desiccated floodplain soils. Other species colonise the temporarily inundated areas from the permanent waters again during each flooding period.     

Invertebrate seedbanks: rehydration of soil from an unregulated river floodplain in the south-eastern U.S.

1. We investigated the responsiveness of aquatic invertebrates to rehydration of floodplain soil in a south‐eastern U.S. river floodplain. Non‐inundated soil divots containing invertebrate seedbanks were collected from three floodplain elevations with different inundation histories (inundated for 2, 38, and 78% of a year), and subjected to rehydration (experimental inundation) in aquaria for 10 weeks. 2. Before rehydration, samples were collected to assess the initial density and composition of invertebrates in non‐inundated soil at each elevation. After rehydration, benthic samples were collected biweekly and emergence of aquatic insects was collected weekly from aquaria. 3. The surviving invertebrates were classified as aquatic, semi‐aquatic and terrestrial. Some aquatic invertebrates survived the non‐inundated period in an active state (e.g. Chironomidae and Ceratopogonidae), while others appeared to be dormant (e.g. Heptageniidae, microcrustaceans and Chironomidae). The response for several invertebrates (e.g. Chironominae, Oligochaeta, Collembola and terrestrial invertebrates) depended on the length of rehydration and inundation history. 4. The capacity of aquatic invertebrates to survive and recover was greatest at the most frequently inundated site. Despite variation in floodplain environments, both active and dormant invertebrates persisting in non‐inundated floodplain soil contributed to the floodplain assemblage during flooding. Thus, maintaining the connection between river and floodplain appears to be important in sustaining the invertebrate seedbank and its capacity to recover during inundation. River channelisation and regulation that severs this connection may result in a loss of diversity and abundance.     

Provisioning of bioavailable carbon between the wet and dry phases in a semi-arid floodplain

Ecosystem functioning on arid and semi-arid floodplains may be described by two alternate traditional paradigms. The pulse-reserve model suggests that rainfall is the main driver of plant growth and subsequent carbon and energy reserve formation in the soil of arid and semi-arid regions. The flood pulse concept suggests that periodic flooding facilitates the two-way transfer of materials between a river and its adjacent floodplain, but focuses mainly on the period when the floodplain is inundated. We compared the effects of both rainfall and flooding on soil moisture and carbon in a semi-arid floodplain to determine the relative importance of each for soil moisture recharge and the generation of a bioavailable organic carbon reserve that can potentially be utilised during the dry phase. Flooding, not rainfall, made a substantial contribution to moisture in the soil profile. Furthermore, the growth of aquatic macrophytes during the wet phase produced at least an order of magnitude more organic material than rainfall-induced pulse-reserve responses during the dry phase, and remained as recognizable soil carbon for years following flood recession. These observations have led us to extend existing paradigms to encompass the reciprocal provisioning of carbon between the wet and dry phases on the floodplain, whereby, in addition to carbon fixed during the dry phase being important for driving biogeochemical transformations upon return of the next wet phase, aquatic macrophyte carbon fixed during the wet phase is recognized as an important source of energy for the dry phase. Reciprocal provisioning presents a conceptual framework on which to formulate questions about the resistance and ecosystem resilience of arid and semi-arid floodplains in the face of threats like climate change and alterations to flood regimes.     

Historical record of an alligator gar,Atractosteus spatula (Lacépède, 1803), captured on the floodplain of the Middle Mississippi River at Columbia,Illinois, USA

Historical photographs, newspapers, and interviews can provide useful information on the abundance, distribution, and habitat use of rare fish species and can be useful in the development of conservation and management plans for target species. We report on the historical occurrence of Atractosteus spatula (alligator gar) on the Middle Mississippi River (MMR) floodplain near Columbia, Illinois (Monroe County), approximately 280.5 river kilometers above the Ohio River (River Mile 168) during the spring of 1937. Based on measurements taken from a photograph of a captured specimen and Mr. Paul Lopinot's recollection of the specimen's weight, the alligator gar was approximately 2.0 m total length and weighed 60 kg. An additional 25–30 large alligator gar were observed on the floodplain approximately 3.6 km from the river. The loss of seasonally inundated floodplain habitat due to construction of an extensive agricultural levee system on the MMR may be a contributing factor to the decline of the alligator gar in the northern portion of its range.     

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.     

Impacts of inundation and drought on eukaryote biodiversity in semi‐arid floodplain soils

Floodplain ecosystems are characterized by alternating wet and dry phases and periodic inundation defines their ecological character. Climate change, river regulation and the construction of levees have substantially altered natural flooding and drying regimes worldwide with uncertain effects on key biotic groups. In southern Australia, we hypothesized that soil eukaryotic communities in climate change affected areas of a semi‐arid floodplain would transition towards comprising mainly dry‐soil specialist species with increasing drought severity. Here, we used 18S rRNA amplicon pyrosequencing to measure the eukaryote community composition in soils that had been depleted of water to varying degrees to confirm that reproducible transitional changes occur in eukaryotic biodiversity on this floodplain. Interflood community structures (3 years post‐flood) were dominated by persistent rather than either aquatic or dry‐specialist organisms. Only 2% of taxa were unique to dry locations by 8 years post‐flood, and 10% were restricted to wet locations (inundated a year to 2 weeks post‐flood). Almost half (48%) of the total soil biota were detected in both these environments. The discovery of a large suite of organisms able to survive nearly a decade of drought, and up to a year submerged supports the concept of inherent resilience of Australian semi‐arid floodplain soil communities under increasing pressure from climatic induced changes in water availability.     

Evaluation of the faster initial decomposition of tropical floating leaves ofNymphaea elegans Hook

Contributions of abiotic and biotic processes to the decomposition of floating leaves ofNymphaea elegans were separately evaluated by comparing the rate obtained from anin situ experiment of submerging dry leaf material in a lake, and that from a laboratory experiment of submerging dry leaf material in lake water with a bio-fixing reagent. It took 8 days to decompose 79.4% of the initial dry weight of the floating leaf ofN. elegans in a tropical lake. Of the dry weight loss, 32.9% and 67.1% were atributed to abiotic and biotic decomposition, respectively. The relationship between decomposition rate and the mesh size of the leaf litter bags was examined by the application of a mathematical model. A reasonable value of decomposition loss at an early stage could be obtained using a bag with a mesh opening of 9.9 mm². The decomposition rate of floating leaves is faster than that of other aquatic plants. Rapid decomposition ofN. elegans leaves may be attributed to the fact that the plant has a low carbon to nitrogen ratio.     

Variability of fish diets between dry and flood periods in an arid zone floodplain river

The fish assemblages of an arid zone floodplain river, Cooper Creek, Queensland, Australia, were sampled during two dry periods in isolated waterholes and on the inundated floodplain during the early and late phase of a major flood event. Diets were described for nine native species and compared within and between dry and flood periods. In the dry season, when fishes were restricted to waterholes, diets were characteristically simple with narrow diet breadths. Movement onto the floodplain during flooding clearly increased feeding opportunities, with greater diet breadths evident in all species. Despite obvious potential for terrestrial inputs, diets tended to be dominated by aquatic resources in both the waterholes and on the floodplain. Stomach fullness, however, varied little between dry season waterhole and floodplain samples. Fishes appeared to feed on potentially lower value resources such as detritus and calanoid copepods during the dry season, when waterholes were isolated and food resources were limited. They were then able to capitalize on the 'boom' of aquatic production and more diverse food resources associated with episodic flood events.