Sources of organic carbon supporting the food web of an arid zone floodplain river

SUMMARY 1. Many Australian inland rivers are characterised by vast floodplains with a network of anastomosing channels that interconnect only during unpredictable flooding. For much of the time, however, rivers are reduced to a string of disconnected and highly turbid waterholes. Given these features, we predicted that aquatic primary production would be light-limited and the riverine food web would be dependent on terrestrial carbon from floodplain exchanges and direct riparian inputs.
2. To test these predictions, we measured rates of benthic primary production and respiration and sampled primary sources of organic carbon and consumers for stable isotope analysis in several river waterholes at four locations in the Cooper Creek system in central Australia.
3. A conspicuous band of filamentous algae was observed along the shallow littoral zone of the larger waterholes. Despite the high turbidity, benthic gross primary production in this narrow zone was very high (1.7–3.6 g C m⁻² day⁻¹); about two orders of magnitude greater than that measured in the main channel.
4. Stable carbon isotope analysis confirmed that the band of algae was the major source of energy for aquatic consumers, ultimately supporting large populations of crustaceans and fish. Variation in the stable carbon and nitrogen isotope signatures of consumers suggested that zooplankton was the other likely major source.
5. Existing ecosystem models of large rivers often emphasise the importance of longitudinal or lateral inputs of terrestrial organic matter as a source of organic carbon for aquatic consumers. Our data suggest that, despite the presence of large amounts of terrestrial carbon, there was no evidence of it being a significant contributor to the aquatic food web in this floodplain river system.     

Hysteresis effects in organic matter turnover in a tropical floodplain during a flood cycle

Tropical inland waters are increasingly recognized for their role in the global carbon cycle, but uncertainty about the effects of such systems on the transported organic matter remains. The seasonal interactions between river, floodplain, and vegetation result in highly dynamic systems, which can exhibit markedly different biogeochemical patterns throughout a flood cycle. In this study, we determined rates and governing processes of organic matter turnover. Multi-probes in the Barotse Plains, a pristine floodplain in the Upper Zambezi River (Zambia), provided a high-resolution data set over the course of a hydrological cycle. The concentrations of oxygen, carbon dioxide, dissolved organic carbon, and suspended particulate matter in the main channel showed clear hysteresis trends with expanding and receding water on the floodplain. Lower oxygen and suspended matter concentrations prevailed at longer travel times of water in the floodplain, while carbon dioxide and dissolved organic carbon concentrations were higher when the water spent more time on the floodplain. Maxima of particulate loads occurred before highest water levels, whereas the maximum in dissolved organic carbon load occurred during the transition of flooding and flood recession. Degradation of terrestrial organic matter occurred mainly on the floodplain at increased floodplain residence times. Our data suggest that floodplains become more intense hotspots at prolonged travel time of the flood pulse over the floodplain.     

The life history of Salicaceae living in the active zone of floodplains

1. Exposed riverine sediments are difficult substrata for seedling establishment because of extremes in the microclimate, poor soil conditions and frequent habitat turnover. Various species of willows and poplars (Salicaceae) appear to be particularly successful in colonising such sediments and are often dominant in floodplain habitats throughout the northern temperate zone.
2. In many Salicaceae regeneration seems to be adapted to regular disturbance by flooding. Efficient seed dispersal is achieved by the production of abundant seed in spring and early summer, which are dispersed by air and water. Seeds are short-lived and germinate immediately on moist surfaces. Seedling establishment is only possible if these surfaces stay moist and undisturbed for a sufficient period of time.
3. Larger plants of Salicaceae have exceptional mechanical properties, such as high bending stability, which enable them to withstand moderate floods. If uprooted, washed away or fragmented by more powerful floods these plants re-sprout vigorously.
4. While these life characteristics can be interpreted as adaptations to the floodplain environment, they may also cause a high genetic variability in populations of Salicaceae and predispose Salicaceae to hybridization. Thus, a feed back between adaptive life history characteristics and the evolutionary process is proposed.     

Activity profiles of bacterioplankton in a eutrophic river

1. The significance of microbial diversity in processing dissolved organic matter (DOM) is largely unknown. We investigated the range of functional diversity in the bacterioplankton from a eutrophic river by profiling extracellular enzyme activities (EEA) and substrate-induced respiration (SIR) patterns.
2. The EEA profiles consisted of assays for 21 hydrolases, measured using fluorogenic substrates arrayed on 96-well microplates. Commercially available BiOLOG^® GN and ECO plates, which contained 120 different substrates, were used for the SIR profiles.
3. The EEA data were more dynamic than the SIR. Five enzymes, leucine aminopeptidase, alkaline phosphatase, alanine aminopeptidase, arginine aminopeptidase and β-glucosidase, showed consistently high activity; ten others were ubiquitous at lower activity levels; the remainder were detected intermittently. The SIR data showed less temporal variability. With one exception (citrate), the 20 substrates that generated the largest responses were all saccharides or their derivatives.
4. The EEA and SIR data did not generally correlate. Both methods were effective for ordinating bacterioplankton although, unlike the SIR, the EEA ordination followed a clear temporal trajectory.
5. Because the SIR profiles are based on a culture response, whilst the EEA profiles measure activity of the extant community, the latter appear to be more directly linked to the mechanics of DOM processing.     

A River's Liver – Microbial Processes within the Hyporheic Zone of a Large Lowland River

Little is known on microbial activities in the sediments of large lowland rivers despite of their potentially high influence on biogeochemical budgets. Based on field measurements in a variety of sedimentary habitats typical for a large lowland river (Elbe, Germany), we present results on the abundance and production of sedimentary bacteria, the potential activity of a set of extracellular enzymes, and potential nitrification and denitrification rates. A diving bell was used to access the sediments in the central river channel, enabling us to sample down to 1 m sediment depth. Depth gradients of all measures of microbial activity were controlled by sediment structure, hydraulic conditions, as well as by the supply with organic carbon and nitrogen. Microbial heterotrophic activity was tightly coupled with the availability of carbon and nitrogen, whereas chemolithotrophic activity (nitrification rate) was related to the available surface area of particles. In the central bed of the river, bacterial production and extracellular enzyme activity remained high down to the deepest sediment layers investigated. Due to the large inner surface area and their connectivity with the surface water, the shifting sediments in the central channel of the river were microbially highly active There, vertically integrated bacterial production amounted to 0.95 g C m⁻³ h⁻¹, which was 2.9 to 5.5 times higher than in the nearshore habitats. We conclude that carbon and nitrogen cycling in the river is controlled by the live sediments of the central river channel, which thus represent a “liver function” in the river's metabolism.     

Managed flooding can augment the benefits of natural flooding for native wetland vegetation

Managed flooding is increasingly being used to maintain and restore the ecological values of floodplain wetlands. However, evidence for its effectiveness is sometimes inconsistent and water available for environmental purposes often limited. We experimentally inundated a floodplain wetland (or “billabong”) in late spring by pumping water from its adjacent creek, aiming to promote the native wetland flora and suppress terrestrial exotics. Vegetation was surveyed before (spring) and after (late summer) the managed flood in the experimental billabong and in three control billabongs. Floodplain water levels were continuously monitored. Wet conditions caused two of the control billabongs to also flood, but to a lesser extent than the experimental billabong. We therefore assessed vegetation changes relative to flooding duration. With increasing flooding duration, the cover of wetland vegetation (amphibious and aquatic species) increased and the cover of terrestrial and exotic vegetation decreased, with these effects largest in the deliberately flooded billabong. Flooding durations greater than 20 days generally resulted in increased cover of wetland plants and restricted the growth of terrestrial plants. Reinstatement of more appropriate flooding regimes can thus promote native wetland plants, while suppressing terrestrial exotic species. Our study also provides evidence for the use of modest water allocations to augment the benefits of natural flooding in the maintenance and restoration of native wetland plant communities.     

Regulation and Seasonal Dynamics of Extracellular Enzyme Activities in the Sediments of a Large Lowland River

We tested whether seasonal changes in the sources oforganic substances for microbial metabolism were reflected changes in the activities of five extracellular enzymes in the eighth order lowland River Elbe, Germany. Leucine aminopeptidase showed the highest activities in the water column and the sediments, followed by phosphatase > β-glucosidase > α-glucosidase > exo-1,4-β-glucanase. Individual enzymes exhibited characteristic seasonal dynamics, as indicated by their relative contribution to cumulative enzyme activity. Leucine aminopeptidase was significantly more active in spring and summer. In contrast, the carbohydrate-degrading enzymes peaked in autumn, and β-glucosidase activity peaked once again in winter. Thus, in sediments, the ratio of leucine aminopeptidase/β-glucosidase reached significant higher medians in spring and summer (5-cm depth: ratio 7.7; 20-cm depth: ratio 10.1) than in autumn and winter (5-cm depth: ratio 3.7, 20-cm depth: ratio 6.3). Therelative activity of phosphatase in the sediments was seasonally related to both the biomass of planktonic algae as well as to the high content of total particulate phosphorus in autumn and winter. Due to temporal shifts in organic matter supply and changes in the storage capacity of sediments, the seasonal peaks of enzyme activities in sediments exhibited a time lag of 2–3 months compared to that in the water column, along with a significant extension of peak width. Hence, our data show that the seasonal pattern of extracellular enzyme activities provides a sensitive approach to infer seasonal or temporary availability of organic matter in rivers from autochthonous and allochthonous sources. From the dynamics of individual enzyme activities, a consistent synoptic pattern of heterotrophic functioning in the studied river ecosystem could be derived. Our data support the revised riverine productivity model predicting that the metabolism of organic matter in high-order rivers is mainly fuelled by autochthonous production occurring in these reaches and riparian inputs.     

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.     

Input, movement and exchange of organic matter within a subtropical coastal black water river-flood plain system

SUMMARY. 1. Inputs, movements and exchanges of particulate organic matter were measured on two contrasting floodplains of the Ogeechee River, Georgia, U.S.A. A model, which incorporated measurements of standing crop, respiration, litterfall, inundation, and litter processing rates, was used to estimate annual exchanges of organic matter between the river and floodplains.
2. Annual litterfall was higher on the East floodplain than on the lower elevation West floodplain (902 v. 784 g ash-free-dry-mass [AFDM] m⁻²).
3. Experiments with tagged leaves and sticks demonstrated that litter was readily displaced during floods. The distance and direction of displacement varied within and between floodplains but tended to be higher closer to the river and was generally parallel to the river.
4. The model indicated that both floodplains lost organic matter to the river. The lower elevation floodplain (East) lost more organic matter to the river (208 g AFDM m⁻² year⁻¹) than did the higher elevation (West) floodplain (79g AFDM m⁻² year⁻¹).
5. Inputs of organic matter from the floodplain to the river exceeded the amount of litterfall typically entering heavily forested high gradient headwater streams (5.5 v. 0.4-0.6 kg AFDM m⁻² year⁻¹).
6. Floodplain organic matter inputs may exert a greater influence upon structure and function within these streams than do upstream inputs or primary production. Consequently, current conceptualizations of stream structure and function need to be modified to account for the effects of floodplain inputs on stream channel processes within large, low-gradient rivers.     

Seasonal and spatial distribution of dissolved and particulate organic carbon and bacteria in the bank of an impounding reservoir on the Enns River, Austria

1. Interstitial bacterial abundance, production and ectoenzyme activity were investigated over an annual cycle in an Austrian river when infiltration of oligotrophic river water into a river-bank was artificially enhanced. These microbial parameters were related to porewater chemistry and the concentration of particulate (POC) and dissolved organic carbon (DOC).
2. Porewater chemistry reflected the hydrodynamic mixing of infiltrating river water with riparian groundwater. Seasonal fluctuations in the microbial parameters resulted mainly from changes in temperature and organic matter supply. Seasonal change in porewater chemistry in the river-bank was detectable laterally only within the first metre of the sediment and decreased rapidly with increasing distance from the sediment–water interface.
3. The DOC concentration decreased only slightly during lateral transport through the aquifer, while total organic carbon (TOC) concentration as well as abundance and activity of interstitial bacteria were reduced by up to one order of magnitude within the top metre of the sediment. Retention of incoming particulate matter structured the lateral distribution pattern of TOC concentration. The POC and not the DOC pool was the main source of carbon for interstitial bacteria and, therefore, the quality of POC determines the distribution of microbial metabolism within the riparian zone.     

Persistence of microbial extracellular enzymes in soils under different temperatures and water availabilities

Microbial extracellular enzyme activity (EEA) is critical for the decomposition of organic matter in soils. Generally, EEA represents the limiting step governing soil organic matter mineralization. The high complexity of soil microbial communities and the heterogeneity of soils suggest potentially complex interactions between microorganisms (and their extracellular enzymes), organic matter, and physicochemical factors. Previous studies have reported the existence of maximum soil EEA at high temperatures although microorganisms thriving at high temperature represent a minority of soil microbial communities. To solve this paradox, we attempt to evaluate if soil extracellular enzymes from thermophiles could accumulate in soils. Methodology at this respect is scarce and an adapted protocol is proposed. Herein, the approach is to analyze the persistence of soil microbial extracellular enzymes at different temperatures and under a broad range of water availability. Results suggest that soil high‐temperature EEA presented longer persistence than enzymes with optimum activity at moderate temperature. Water availability influenced enzyme persistence, generally preserving for longer time the extracellular enzymes. These results suggest that high‐temperature extracellular enzymes could be naturally accumulated in soils. Thus, soils could contain a reservoir of enzymes allowing a quick response by soil microorganisms to changing conditions. This study suggests the existence of novel mechanisms of interaction among microorganisms, their enzymes and the soil environment with relevance at local and global levels.     

Fine Scale Patterns in Microbial Extracellular Enzyme Activity during Leaf Litter Decomposition in a Stream and its Floodplain

Microorganisms mediate the decomposition of leaf-litter through the release of extracellular enzymes. The surfaces of decomposing leaves are both chemically and physically heterogeneous, and spatial patterns in microbial enzyme activity on the litter surface should provide insights into fine-scale patterns of leaf-litter decomposition. Platanus occidentalis leaves were collected from the floodplain of a third-order stream in northern Mississippi, enclosed in individual litter bags, and placed in the stream channel and in the floodplain. Replicate leaves were collected approximately monthly over a 9-month period and assayed for spatial variation in microbial extracellular enzyme activity and rates of organic matter (OM) decomposition. Spatial variation in enzyme activity was measured by sampling 96 small discs (5-mm diameter) cut from each leaf. Discs were assayed for the activity of enzymes involved in lignin (oxidative enzymes) and cellulose (β-glucosidase, cellobiohydrolase) degradation. Rates of OM loss were greater in the stream than the floodplain. Activities of all enzymes displayed high variability in both environments, with severalfold differences across individual leaves, and replicate leaves varied greatly in their distribution of activities. Geostatistical analysis revealed no clear patterns in spatial distribution of activity over time or among replicates, and replicate leaves were highly variable. These results show that fine-scale spatial heterogeneity occurs on decomposing leaves, but the level of spatial variability varies among individual leaves at the measured spatial scales. This study is the first to use geostatistical analyses to analyze landscape patterns of microbial activity on decomposing leaf litter and in conjunction with studies of the microbial community composition and/or substrate characteristics, should provide key insights into the function of these processes.     

Degradation of softwood [14C lignin] lignocelluloses and its relation to the formation of humic substances in river and pond environments

The fate of lignin in water and sediment of the Garonne river (France) and of a pond in its floodplain was examined using specifically labeled [¹⁴C-lignin] lignocelluloses. No significant differences appeared in the mineralization rate of alder, poplar or willow [¹⁴C-lignin] in running water samples. Conversion of total radioactivity to ¹⁴CO₂ ranged between 18.7% and 24.4% after 120 days of incubation. Degree of ¹⁴C-labeled lignin mineralization in standing water and sediments was clearly lower, especially in submerged sediments, and was correlated with oxygen supply. After 60 days of incubation 3.3% to 7.9% of the ¹⁴C-labeled lignin was recovered in water samples as dissolved organic carbon originating from microbial metabolism. In water extracts from sediment the percentage of dissolved organic ¹⁴C was only 0.4% to 1.3% of the applied activity. In the humic fraction extracted from sediments it did not exceed 4.4% which was much lower than in soils. No significant difference appeared between river and pond conditions for humic substances formation.     

Extracellular Hydrolytic Enzyme Activities of the Heterotrophic Microbial Communities of the Rouge River: An Approach to Evaluate Ecosystem Response to Urbanization

The potential effects of urbanization on the bioavailability of dissolved organic carbon (DOC) were tested by determining the extracellular enzyme activities of the heterotrophic microbial communities of the Rouge River. The activities of 19 enzymes were monitored across two water samples (river water and groundwater) at different spatial and temporal scales. High phosphatase, esterase, and aminopeptidase activities was observed in site 9 (site most exposed to anthropogenic sources) showed higher concentrations of DOC compared to sites 1 and 8 (sites exposed to less anthropogenic sources), where moderate activities of diverse range of enzymes were observed. High relative contributions of phosphatase, esterase, and aminopeptidase activities to the overall enzyme activity as observed in site 9 stressed the increased importance of peptides as C source for heterotrophic communities and high in-stream carbon processing, which account for high nonspecific extracellular enzyme activities. In contrast, high contribution of glycosyl hydrolases occurred consistently across all sites, which highlights the significance of microbial detrital and plant biomass as carbon sources. Majority of the enzymes showed evidence of activity at various extents during spring and summer. However, higher activities of leucine aminopeptidase, valine aminopeptidase, β-glucosidase, and α-mannosidase were observed in the summer; and alkaline phosphatase and α-glucosidase in the spring. The results presented here suggest a shift in organic carbon bioavailability across all sites of contrasting urbanization, despite similarities in DOC concentrations. Hence, API ZYM technique can be used as an effective indicator of river water and groundwater system health across an urban gradient.     

Responses of billabong rotifer communities to inundation

Daily plankton collections were taken from a billabong of the River Murray for two weeks prior to inundation in March 1990, and continued for ten days after flooding. Quantitative responses of the plankton community and the component species were analysed against measured environmental variables and between species. Rotifers and copepod nauplii were the predominant net plankton (> 53 µm). Significant negative or positive responses to inundation were detected for most common taxa of 63 rotifer species recorded. A four-fold dilution from intrusion of river water masked rapid population increases. Opportunistic responses to inundation appear to be a survival strategy in the highly unpredictable billabong environment.     

Dormancy, dispersal and the survival of cyclopoid copepods (Cyclopoida, Copepoda) in a lowland floodplain

1. The survival of cyclopoid copepods was investigated in a floodplain for 2 years where flooding occurred during the cold season. The cyclopoid community was studied in three waterbodies with distinct hydroperiods: a permanent pond connected to the flooded area during inundation, a temporary pool that is part of the flooded area and an isolated temporary pool.
2. Field studies, including data obtained from samples of water, sediment and soil, showed the overall predominance of species with a summer diapause over those with a winter diapause or without diapause. Emergence of cyclopoid copepods at the onset of flooding, examined using emergence traps and data from recently filled or still isolated temporary pools, showed that some species can survive several months of drying.
3. The ability of the diapausing fourth copepodid stages of Cyclops strenuus and C. insignis , the two cyclopoids most abundant during winter and spring flooding, to survive terrestrial conditions was tested in laboratory experiments. Both species survived for several months, but rates differed among the species. A higher percentage of C. strenuus survived for a longer period, possibly explaining why this species was relatively more abundant in more temporary habitats.
4. Both dormancy and dispersal facilitated survival of cyclopoid copepods in transient habitats connected to each other during flood periods. Dormancy was the most important survival strategy, whereas dispersal could be more important following prolonged periods without flooding.     

Geomorphic dynamics of floodplains: ecological implications and a potential modelling strategy

1.  The dynamics of channel migration and floodplain renewal constitute an important control of the ecological diversity of river corridors. Restoration initiatives should therefore assess whether these dynamics must be reinstated in order to address the cause rather than the symptoms of floodplain biodiversity decline.
2.  Restoration of reach-scale dynamism in rivers where this is a natural behavioural process will restore smaller-scale geomorphological and sedimentological processes that encourage vegetation regeneration, but may require catchment-scale management of material flows.
3.  Channel dynamics depend on the style of river–floodplain interaction, and this may be summarised in qualitative, classificatory, sedimentological models of floodplain architecture that have been somewhat neglected in the ecological literature.
4.  One approach to the assessment of floodplain biodiversity and its restoration would be through the development of simulation models based on specified channel styles, and involving simplified hydrodynamics and successional changes. Such models, currently the subject of research as a spin-off from modelling studies of landscape evolution, would permit evaluation of the consequences for ecological diversity of implementing various management options that may affect the dynamics of channel migration.     

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.     

Hydrological connectivity, and the exchange of organic matter and nutrients in a dynamic river–floodplain system (Danube, Austria)

1. The relationship between hydrological connectivity, and the exchange processes of suspended sediments, organic matter and nutrients (NO₃-N) was investigated in a dynamically connected river–floodplain segment of the Danube over a 15-month period in 1995 and 1996 in the Alluvial Zone National Park, Austria. 2. Based on water level dynamics and water retention times, three phases of river–floodplain connectivity were identified: disconnection (phase I), seepage inflow (phase II) and upstream surface connection (phase III). The frequency of occurrence of these phases was 67.5%, 29.3% and 3.2%, respectively, during the study period. 3. A conceptual model is presented linking hydrological connectivity with ecological processes. Generally, the floodplain shifts from a closed and mainly biologically controlled ecosystem during phase I to an increasingly open and more hydrologically controlled system during phases II and III. Phase I, with internal processes dominating, is designated the ‘biotic interaction phase’. 4. Phase II, with massive nutrient inputs to the floodplain yet relatively high residence times, and therefore, high algal biomass, is classified as the ‘primary production phase’. This demonstrates that water level fluctuations well below bankfull may considerably enhance floodplain productivity. 5. Finally, since transport of particulate matter is mainly restricted to short flood pulses above bankfull level, phase III has been defined as the ‘transport phase’. 6. The floodplain served as a major sink for suspended sediments (250 mt ha^??1 year^??1), FPOM (96 mt ha^??1 year^??1), particulate organic carbon (POC; 2.9 mt ha^??1 year^??1) and nitrate-nitrogen (0.96 mt ha^??1 year^??1), but was a source for dissolved organic carbon (DOC; 240 kg ha^??1 year^??1), algal biomass (chlorophyll-a; 0.5 kg ha^??1 year^??1) and CPOM (21 kg ha^??1 year^??1). Considerable quantities of DOC and algal biomass were exported to the river channel during phase II, whereas particulate matter transport was largely restricted to the short floods of phase III. 7. The Danube Restoration Project will create a more gradual change between the individual phases by increasing hydrological connectivity between the river channel and the floodplain, and is predicted to enhance productivity by maintaining a balance between retention and export of nutrients and organic matter.     

Determination of Microbial Extracellular Enzyme Activity in Waters,Soils, and Sediments using High Throughput Microplate Assays

Much of the nutrient cycling and carbon processing in natural environments occurs through the activity of extracellular enzymes released by microorganisms. Thus, measurement of the activity of these extracellular enzymes can give insights into the rates of ecosystem level processes, such as organic matter decomposition or nitrogen and phosphorus mineralization. Assays of extracellular enzyme activity in environmental samples typically involve exposing the samples to artificial colorimetric or fluorometric substrates and tracking the rate of substrate hydrolysis. Here we describe microplate based methods for these procedures that allow the analysis of large numbers of samples within a short time frame. Samples are allowed to react with artificial substrates within 96-well microplates or deep well microplate blocks, and enzyme activity is subsequently determined by absorption or fluorescence of the resulting end product using a typical microplate reader or fluorometer. Such high throughput procedures not only facilitate comparisons between spatially separate sites or ecosystems, but also substantially reduce the cost of such assays by reducing overall reagent volumes needed per sample.