Feeding Ecology and Energetic Relationships with Habitat of Blue Catfish, Ictalurus furcatus, and Flathead Catfish, Pylodictis olivaris, in the Lower Mississippi River, U.S.A.

We examined feeding of blue catfish, Ictalurus furcatus, and flathead catfish, Pylodictis olivaris, collected from floodplain lake, secondary (side) river channel, and main river channel habitats in the lower Mississippi River (LMR), U.S.A. We described the feeding ecology of two large river catfish species within the context of whether off-channel habitats in the LMR (i.e., floodplain lakes and secondary channels) potentially provided energetic benefits to these fishes as purported in contemporary theory on the ecology of large rivers. We used diet composition and associated caloric densities of prey consumed as indicators of energetic benefit to catfishes. Differences in diet among habitats were strong for blue catfish, but weak for flathead catfish; consumed foods generally differed among habitats in caloric (energy) content. Caloric densities of consumed foods were generally greatest in floodplain lakes, least in the main river channel, and intermediate in secondary river channels. Strong between-year variation in diet was observed, but only for blue catfish. Blue catfish fed disproportionately on lower-energy zebra mussels in the main river channel during 1997, and higher-energy chironomids and oligochaetes in floodplain lakes during 1998. Results suggested that although off-channel habitats potentially provided greater energetic return to catfishes in terms of foods consumed, patterns of feeding and subsequent energy intake may vary annually. Energetic benefits associated with off-channel habitats as purported under contemporary theory (e.g., the ‘flood-pulse concept’) may not be accrued by catfishes every year in the LMR.     

Macrophytic primary production and nutrient concentrations in a deltaic floodplain marsh of the Lower Paraná River

Biomass changes across an annual cycle were followed at two sampling sites in the floodplain marsh of the Lower Paraná River: close to the river-shore and 800 m inside the floodplain marsh, both dominated by Scirpus californicus and Cyperus giganteus. Tidal influence determines a daily exchange of water between the river and the floodplain marsh.Estimated net primary production was higher in the river (2820 against 1770 g m^–2). Contents of nitrogen and phosphorus in plant tissue decreased from the river to the floodplain (0.62 to 0.45% N and 0.18 to 0.14% P). In spite of the important water exchange between the river and the floodplain, a decrease in nitrate, oxygen and suspended matter, and an increase in soluble reactive phosphorus in the water were observed from the river towards the floodplain marsh.A primary production gradient exists from the river to the inner floodplain marsh, where production is nitrogen-limited, sustained mainly on nutrients supplied by the river. Floodplain marshes are nitrate sinks, probably through denitrification losses and macrophyte uptake.     

Characterisation of the potamal Danube River and the Delta: connectivity determines indicative macrophyte assemblages

The complexity of water bodies in the eu-potamal river corridor and the main delta channels of the Romanian Danube is exemplified by the macrophyte vegetation. Two hypotheses provided the background for our study: (a) is the macrophyte vegetation of large, permanently connected branches significantly separated from that of the main river channel; (b) is the macrophyte composition of the Danube main stem significantly altered when the river divides into the three large navigable Delta channels. Water bodies considered were two contiguous sections of the main river channel, two large branches remaining from the historical floodplain, and the three main Delta channels. We quantified macrophyte diversity and floristic variation. Our data set was prepared from the MIDCC-project data base, in which macrophyte occurrence, abundance and habitat parameters are stored for contiguous survey units of the whole Danube river corridor. Field survey method followed that of Kohler and the European Standard EN14184. Results confirmed our first hypothesis: permanently connected side branches still support significantly different macrophyte assemblages, making them important indicators of floodplain connectivity. The diversion of the Danube into its three large navigable delta channels significantly alters the macrophyte vegetation from the c. 300 km of main stem up-river, substantially supporting our second hypothesis. Our results largely enhance the knowledge on aquatic plant biodiversity in the eu-potamal Danube, forming a solid base for long-term studies. We also discuss the relevance of our results regarding the ecological, as well as the conservational, quality of rivers and their floodplains.     

Spatial and Temporal Trade-Offs by Bluegills in Floodplain River Ecosystems

Ecological trade-offs by organisms to minimize mortality and maximize growth is a foundational theme in ecology. Yet, these trade-offs are rarely examined within spatially complex, temporally variable ecosystems, such as floodplain rivers. Here, we evaluate ecological trade-offs across space and time for the bluegill (Lepomis macrochirus) in two unregulated river ecosystems in southeastern USA. Life-history differences among spatially segregated main channel and floodplain lake populations were used to assess effects of habitat type on bluegill fitness. Growth, condition, and gonadal somatic index were all significantly enhanced in floodplain lakes relative to the main channel. Furthermore, stomach fullness was significantly higher, and predator densities significantly lower in floodplain lakes thereby providing an ecological explanation for the life-history plasticity observed across the riverscape. However, historical observations suggested that although floodplain lakes are highly productive for bluegills, they are also prone to complete desiccation by drought approximately every 5 years, revealing the ultimate value of channel habitat, which does not dry, as desiccation refugia. Bluegills are faced with a balancing act associated with variation in foraging opportunities, and risks to predation and desiccation, that change in both the temporal and the spatial dimensions of floodplain rivers. The differential responses to these opportunities and risks help to explain why both habitats remain actively populated by bluegills, as well as many other organisms, in these and many other natural rivers.     

Distribution of the fish rotan (Perccottus glenii Dybowski, 1877) in the Irtysh River basin and analysis of possible consequences for environment and people

Since 1916, rotan (Perccottus glenii Dybowski, 1877) has widely distributed in northwestern Eurasia. In 1973–1976, rotan populations were first detected in the basin of the Irtysh River: in Lake Peschanoe (Mertvoe) of Chelyabinsk oblast in the basin of the Tobol, a tributary of the Irtysh. In 1975, this fish was introduced to one of the ponds of the city of Chelyabinsk. Analysis of the spatiotemporal dynamics of findings shows that earlier records of the fish correlate with the network of roads, and later findings, with the river network. The results of a special inspection of isolated and floodplain water bodies (n = 77) performed within Chelyabinsk, Kurgan, Omsk, and Tyumen oblasts in 2006 indicate that at present this unwanted invader occurs in the major part of the Tobol basin. The northernmost finding of this species is from the floodplain of the Tobol in the vicinity of Tobolsk, from the Karachino oxbow (58°02′N, 68°10′E). Rotan has also been found at the Irtysh headwaters as well as in the basins of its other tributaries: Ishim and Om. The expansion of rotan threatens the floodplain water bodies of the middle Irtysh. This species carries diseases, competes with other fish species for food, and actively eats young fish of these species. Therefore, the data on rotan distribution in the tributaries of the Irtysh should be taken into account when analyzing the dynamics of the fish community of this river. Biology of rotan assumes its influence on the functioning of the natural foci of opisthorchiasis.     

Diversity of planktonic rotifer assemblages in the water bodies of the Gemenc floodplain (Duna-Dráva National Park,Hungary)

Gemenc floodplain lies between river kilometres km 1503 and 1469 of the Danube River; it is a part of the Danube-Dráva National Park in Hungary. The floodplain is one of the largest in Europe with an 18,000 hectares (Natura 2000) area, and various characteristic side arms and backwaters which lie completely in the floodplain. The area needs hydrological revitalization because of the sinking of the river bed caused by the regulation of the main arm at the end of the 19^th century. For this revitalization an exhaustive knowledge of the hydrobiological relationships among the different water bodies is required. The aims of this study were to explore connections among the hydrological events, physico-chemical parameters of the water bodies and the diversity of the planktonic rotifer assemblages. From 2002 to 2004, 56 planktonic and 15 tychoplanktonic rotifer taxa were found in the area. The diversity of rotifer assemblages was generally higher in the parapotamal water bodies than in the main arm and in the plesio/paleo-potamal water bodies, which could be explained by the intermediate disturbance hypothesis. The diversity, dominance, and evenness of the assemblages were correlated with water flow and water temperature. The most important factors influencing the diversity patterns were the occurrence of flow (retention time) as a disturbance, connectivity with the main arm, and habitat diversity of the given water body. Most of these factors are determined by the water level fluctuation of the main arm.     

The distribution of macroinvertebrate assemblages in a reach of the River Allier (France), in relation to riverbed characteristics

Investigations on large canalised rivers, for example the Danube, have shown that transported particulate matter, which is typically inorganic, is predominantly deposited in waters near the river’s main channel. This investigation deals with the lower section of the River Havel (NE Germany), a canalised lowland river with a very flat floodplain. This river is highly polluted by nutrients from urban areas (Berlin) and a long chain of river lakes produces high concentrations of phytoplankton. Due to the high proportion of planktogenic detritus, it was hypothesised that greater quantities of nutrient-rich fine particulate organic matter (FPOM) would be deposited in floodplain waters located further from the main channel than has been reported for large rivers. The total nutrient, P-binding metal (Fe, Al, and Mn), organic and inorganic carbon (TOC, TIC) contents of the upper organic sediment layer (0–4 cm) were analysed in samples collected from 48 floodplain water and river sites. The sediment bulk density, calculated on the basis of dry mass content and loss on ignition, was used to characterise the waters according to the impact of the river current. The results showed that the variability of total phosphorus (TP) was best explained by the variability of total iron (TFe, R² = 0.52). The floodplain water sediments could clearly be separated into two groups on the basis of the sediment particle size composition, and of the element ratios TOC:TP, TN:TP, primarily TFe:TP. The sediments from impounded river sections and from mouth sections of backwaters (approx. 100–200 m) were characterised by a high proportion particles from the 0.1 to 0.5 mm size fraction and by homogeneous, low TFe:TP, TOC:TP and TN:TP ratios. Sediments from distal sections of backwaters and of oxbow lakes tended to exhibit high element ratios with much higher variability. These results were interpreted as a spatially limited impact of the river on the floodplain water sediments. Contrary to expectation, the phosphorus bound in river seston was predominantly and very homogeneously deposited in the impounded river and mouth sections of backwaters. This implies that the inundation of the floodplain waters during spring floods seems to have no important material impact on the sediments in waters of low hydrological connectivity with the River Havel.     

Impact of the Gabcíkovo hydropower plant operation on planktonic copepods assemblages in the River Danube and its floodplain downstream of Bratislava

The structure of planktonic copepod assemblages in a by-passedDanube section, and in the adjacent floodplain water bodies,haschanged since the Gabcíkovo hydropower plant was putintooperation. The greatest change occurred in a former side arm(upstream of the village of Dobrohosí), which driedout.Changes were observed in the parapotamon-type side armssituatedbetween river km 1840 and 1820, fed artificially with waterfromthe head-race canal. The abundance of euplanktonic copepodspeciesdecreased, while that of tychoplanktonic species increased.Summerzooplankton biomass decreased. Definitive, but less extreme,changes occurred in the copepod assemblage in the open-waterareasof the former main channel of the Danube, and in theplesiopotamon-type side arms in the affected river section.These results suggest that the water supply provided to theby-passed floodplain is unable to maintain the previoushabitatdiversity, nor its planktonic assemblages speciesdiversity.     

The influence of floodplain habitat on the quantity and quality of riverine phytoplankton carbon produced during the flood season in San Francisco Estuary

Primary productivity, community respiration, chlorophyll a concentration, phytoplankton species composition, and environmental factors were compared in the Yolo Bypass floodplain and adjacent Sacramento River in order to determine if passage of Sacramento River through floodplain habitat enhanced the quantity and quality of phytoplankton carbon available to the aquatic food web and how primary productivity and phytoplankton species composition in these habitats were affected by environmental conditions during the flood season. Greater net primary productivity of Sacramento River water in the floodplain than the main river channel was associated with more frequent autotrophy and a higher P:R ratio, chlorophyll a concentration, and phytoplankton growth efficiency (α^B). Total irradiance and water temperature in the euphotic zone were positively correlated with net primary productivity in winter and early spring but negatively correlated with net primary productivity in the late spring and early summer in the floodplain. In contrast, net primary productivity was correlated with chlorophyll a concentration and streamflow in the Sacramento River. The flood pulse cycle was important for floodplain production because it facilitated the accumulation of chlorophyll a and wide diameter diatom and green algal cells during the drain phase. High chlorophyll a concentration and diatom and green algal biomass enabled the floodplain to export 14–37% of the combined floodplain plus river load of total, diatom and green algal biomass and wide diameter cells to the estuary downstream, even though it had only 3% of the river streamflow. The study suggested the quantity and quality of riverine phytoplankton biomass available to the aquatic food web could be enhanced by passing river water through a floodplain during the flood season.     

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

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.     

Zooplankton abundance and transport in a tropical white-water river

Zooplankton abundance and transport were studied in the Apure River of western Venuzuela over a 15-month period. Much of the lower portion of the drainage basin, which is an extensive savanna of low relief, is subject to seasonal flooding. Although more than 50 zooplankton species were recorded during the study, nine rotifer species accounted for more than 90% of total density (mean, 138 individuals · L^-1). Copepods were represented primarily by Mesocyclops decipiens. Most cladocerans were planktonic, but cladocerans were not abundant. Crustaceans comprised 46% of annual mean zooplankton biomass (1.9 gC·L^-1) but only 2% of zooplankton numbers. The annual transport of zooplankton biomass from the watershed was 29 300 kgC. Zooplankton abundance showed a strong, inverse relationship to discharge. Secondary branches (caños) of the braided channel appeared to be the primary source areas, but populations of some species also reproduced in the main channel at low water. As the river began to rise, caños were flushed and thus abruptly ceased to serve as source areas. Zooplankton transport fell sharply and remained low until the river inundated the floodplain. Secondary production in the floodplain was exported to the river as long as a connection existed. After drainage ceased from the floodplain, transport fell to very low levels until caños again became suitable habitat. Seasonal fluctuations in river level regulate the development of source areas suitable for zooplankton growth and control the export of plankton from the source areas. Plankton in the running waters reproduce only at low water.     

Dispersal,diversity and distribution patterns in pioneer vegetation: The role of river‐floodplain connectivity

Question: Are there changes in dispersal patterns in floodplain pioneer vegetation with effects on seedling number, species richness and species composition along a gradient of declining river‐floodplain connectivity? Location: Middle Elbe river floodplain, Germany. Methods: An experiment with five treatments was set up along a gradient of declining river‐floodplain connectivity, partitioning seedlings into three groups: (1) emerging solely from water dispersed seeds, (2) from wind/animal dispersed seeds and (3) from the soil diaspore bank. Two controls were established: without any manipulation and exclusion of all seeds. The results were compared with those of vegetation and soil sampling to evaluate the representativeness of the experimental sites in terms of species composition, diversity, seedling number and soil parameters. Results: Water dispersal and the soil diaspore bank were the major dispersal strategies shaping floodplain pioneer vegetation at the Middle Elbe river. The number of seedlings, species richness and the variation in species composition in these habitats depend on the degree of connectivity. The seedling number and species richness is highest in sites of permanent or almost permanent exchange with the main channel, where water dispersal additionally contributes to the number of seedlings grown from the soil seed bank. Conclusion: The results underline the importance of river‐floodplain ecotones as sink habitats for water‐dispersed seeds. Considering the strongly reduced river‐floodplain interactions due to dykes and other engineering structures, management strategies are necessary to improve connectivity and the renewal of fluvial land forms.     

Vegetation responses to environmental conditions in floodplain grasslands: Prerequisites for preserving plant species diversity

We studied plant species responses to environmental conditions in floodplain grasslands (alliance Cnidion) of the river Elbe (northern Germany) in order to describe relationships between species composition/diversity and underlying site gradients. We analysed a total area of 639 ha of floodplain grasslands, using 98 sampling plots. Vegetation responses to environmental conditions were examined by means of DCA. The relationships between species diversity and environmental conditions were examined using regression analyses. Our results show that species composition and species richness are mainly affected by present-day utilization, by the phosphate supply and by the productivity of stands. Species richness decreases significantly under grazing, with increasing phosphate supply and productivity of stands. Intermediate productivity levels, mowing and high inundation frequencies promote floodplain target species. N_inorg availability and base availability have only a minor impact on species composition/species diversity. Productivity is closely related to the amount of plant available phosphate. Low phosphate concentrations in the topsoil are particularly characteristic of Cnidium dubium floodplain meadows. Therefore, it is likely that in floodplain grasslands with low to intermediate productivity, phosphate is the most important factor influencing plant competition and plant species diversity. We hypothesize that an increased phosphate supply increases stand productivity and, hence, the competition for light, thus excluding weak competitors. This interpretation is supported by the present-day distribution of C. dubium floodplain meadows of the river Elbe: these are concentrated in the hybrid (between summer and winter dykes) and fossil floodplain (landward side of winter dykes), but have been replaced in the functional floodplain by grass-dominated plant communities as a result of a steady increase in the river water phosphate content. It is, therefore, only in the hybrid and fossil floodplain grasslands that species diversity may be preserved in the long term, as the P input is lower here than in the functional floodplain.     

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.     

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

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Modelling the longitudinal distribution,abundance, and habitat use of the giant freshwater shrimp (Macrobrachium spinipes) in a large intermittent,tropical Australian river to inform water resource policy

1. Water development threatens rivers and their biodiversity. Amphidromous shrimp are particularly vulnerable as they require migration between freshwater and estuaries to complete their life cycle. The Fitzroy River is a large tropical intermittent river undergoing water development that is home to the amphidromous shrimp Macrobrachium spinipes (cherabin), yet little is known about its habitat use and flow-ecology making it difficult to inform sustainable water-take.
2. We investigated habitat associations, distributional patterns suggestive of amphidromy, and the influence of water availability by sampling main channel and floodplain pools along a 350-km river length during 2 contrasting flow years. Applying a size-specific abundance model, we estimated abundance per size class, site, and year. We then predicted abundance at the landscape scale with remotely sensed water to reveal the impact of water availability on the meta-population.
3. Our model revealed that juveniles were in greatest abundance in downstream main channel pools, whereas adults were in greatest abundance in upstream floodplain pools. Abundance varied by year with lower numbers predicted in the low-flow year. Longitudinal and habitat patterns remained when our pool-level results were scaled to the landscape, and the positive relationship of abundance to wet-season flow was strengthened. The predominance of smaller cherabin in the lower reaches of the river provides indirect support for an estuarine nursery and amphidromous life history; however, small individuals observed in landlocked pools, during late dry season suggests possible within-river recruitment.
4. The importance of water development policies that protect wet-season flow and passage along the Fitzroy River is supported by this work. These types of policies are likely to be important for this and other amphidromous shrimp species across Australia, Southeast Asia and further afield. Further research detailing the species life history and describing flow–recruitment relationships will be important contributions to understanding this important taxonomic group and refining policies for current and future water resource development.

     

Fluxes of bacteria and organic matter into a blackwater river from river sediments and floodplain soils

1. The Ogeechee River, in south-eastern Georgia, U.S.A, is a blackwater river with an extensive floodplain that is inundated regularly during winter months. Previous studies have shown that low to moderate bacterial production rates cannot support the relatively high suspended bacteria concentrations observed (10^7?-10⁸ cells ml^?1), suggesting an allochthonous source of bacteria. We report the results of a combination of field and flume experiments which demonstrate that river sediments and floodplain soils are significant sources of suspended bacteria during seasonal flooding. Benthic bacteria are also entrained by normal discharges. There are sizeable fluxes of POC and DOC from river sediments and floodplain soils. 2. Bacterial, POC and DOC fluxes (14, 250, and 790 mg Cm^?2 h^?1, respectively) were substantial when water was percolated upward through floodplain soils. 3. Simulation of overland flow using a flume demonstrated further fluxes of bacteria and POC from floodplain soils (up to 61 and 10700 mg Cm^?2h^?1, respectively) and river sediments, but did not yield additional DOC from floodplain soils. 4. These laboratory results are supported by experiments in which we measured significant increases in concentrations of bacteria and DOC in a downstream direction in (i) the main river channel during a winter flood in 1986, and (ii) a floodplain slough (channel side-arm) which re-entered the main channel 800m from its initial divergence. Inputs of bacteria and DOC from the surrounding floodplain were estimated to be up to 3500 kg DOC h^?1, and 4000 kg bacteria Ch^?1 over a 50-km reach. 5. These previously unmeasured fluxes of organic carbon help to explain the high concentrations of suspended bacteria in the Ogeechee River.     

Impact of hydrology on the chemistry and phytoplankton development in floodplain lakes along the Lower Rhine and Meuse

The impact of hydrology (floods, seepage) on the chemistry of water and sediment in floodplain lakes was studied by a multivariate analysis (PCA) of physico-chemical parameters in 100 lakes within the floodplains in the lower reaches of the rivers Rhine and Meuse. In addition, seasonal fluctuations in water chemistry and chlorophyll-a development in the main channel of the Lower Rhine and five floodplain lakes along a flooding gradient were monitored. The species composition of the summer phytoplankton in these lakes was studied as well.At present very high levels of chloride, sodium, sulphate, phosphate and nitrate are found in the main channels of the rivers Rhine and Meuse, resulting from industrial, agricultural and domestic sewage. Together with the actual concentrations of major ions and nutrients in the main channel, the annual flood duration determines the physico-chemistry of the floodplain lakes. The river water influences the water chemistry of these lakes not only via inundations, but also via seepage. A comparison of recent and historical chemical data shows an increase over the years in the levels of chloride both in the main channel of the Lower Rhine and in seepage lakes along this river. Levels of alkalinity in floodplain lakes showed an inverse relationship with annual flood duration, because sulphur retention and alkalinization occurred in seepage waters and rarely-flooded lakes. The input of large quantities of nutrients (N, P) from the main channel has resulted, especially in frequently flooded lakes, in an increase in algal biomass and a shift in phytoplankton composition from a diatom dominated community towards a community dominated by chlorophytes and cyanobacteria.     

Effect of river straightening on the hydrochemical properties of floodplain lakes: Observations from the Łyna and Drwęca Rivers,N Poland

The aim of the paper is to present two independent approaches to the issue of preservation of former meander loops based on examples of hydrotechnical works undertaken along meandering sections of the Drw?ca and ?yna Rivers (N Poland) in the 20th century. Except for significant changes in morphology of both rivers (shortened lengths, decrease in sinuosities and increase in channel slopes), the direct effect of their straightening is numerous cut-off channels. Hydrotechnical works performed along the middle section of the ?yna River maintained the connections between the active river channel and downstream arms of former meander loops. Thanks to that connection, the new water bodies successfully function as biogeochemical filters in the river valley and create favorable conditions for biodiversity. The consequence of the channelization of the Drw?ca River is numerous cut-offs separated from the river channel and characterized by advanced processes of eutrophication.The water quality data collected within the period of 2004–2006 showed significant consequences of the performed works for the man-made ecosystems. The comparison of water quality in semi-lotic and lentic cut-offs in relation to water in their parent rivers showed statistical differences. Significantly high concentrations of orthophosphates, total phosphorus and sulphates as well as the lowest concentrations of nitrates characterized lentic environments along the Drw?ca River. Besides hydrological connectivity, an important factor responsible for transformation processes of the man-made ecosystems is hydrological regime. It influences ecological integrity in floodplain ecosystems during high water levels and largely controls their chemical characteristics and aeration. Flood disturbances for lentic water bodies may renew connectivity and reset succession, can hold up the trend towards terrestrialization and disappearance of man-made water bodies in the floodplain. However, in light of the conducted studies, the maintenance of the connection between the downstream arm of the cut-off channel and the river is highly recommended.     

Waterfowl of a subtropical African floodplain

Many water bodies in arid southern Africa are seasonal and waterfowl habitats become restricted during the dry winters. Nevertheless, some tropical species, such as the white-faced duck, are expanding their ranges into more temperate regions where summer habitats are available, thus increasing pressure on overwintering habitats. The subtropical Pongolo River floodplain is an important overwintering area for a diverse water-fowl community that includes 13 of the 16 species that breed in South Africa and one of the few regular migrants. Historical evidence suggests that numbers of white-faced duck, which represent over 70% of the floodplain community, have increased 10-fold over the last two decades. An abundant, nutritious food (Potamogeton crispus turions) attracts waterfowl to the Pongolo floodplain during winter and an increase in bird fat reserves suggests that they leave for summer breeding grounds in good condition. Unseasonal flood releases from an upstream reservoir may cause low food supplies in late spring before the onset of the summer rainy season when breeding habitats become available. This study emphasizes the need for a better understanding of the role of winter habitats in maintaining waterfowl populations.