Assessing surface water–groundwater interactions in a complex river‐floodplain wetland‐isolated lake system

Floodplain systems are most often hydrologically complex settings characterized by highly variable surface water–groundwater interactions that are subjected to wide‐ranging wetting and drying over seasonal timeframes. This study used field methods, statistical analysis, and the Darcy's law approach to explore surface water–groundwater dynamics, interactions, and fluxes in a geographically complex river‐floodplain wetland‐isolated lake system (Poyang Lake, China). The floodplain system of Poyang Lake is affected by strongly seasonal shifts between dry and wet processes that cause marked changes in surface water and groundwater flow regimes. Results indicate that wetland groundwater is more sensitive to variations in river levels than the seasonal isolated lakes. In general, groundwater levels are lower than those of the isolated lakes but slightly higher than river levels. Statistical analysis indicates that the river hydrology plays a more significant role than the isolated lakes in controlling floodplain groundwater dynamics. Overall, the river shows gaining conditions and occasionally losing conditions with highly variable Darcy fluxes of up to +0.4 and ?0.2 m/day, respectively, whereas the isolated lakes are more likely to show slightly losing conditions (less than ?0.1 m/day). Although seasonal flux rates range from 7.5 to 48.2 m/day for surface water–groundwater interactions in the floodplain, the flux rates for river–groundwater interactions were around four to seven times higher than that of the isolated lake–groundwater interactions. The outcomes of this study have important implications for improving the understanding of the water resources, water quality, and ecosystem functioning for both the river and the lake.     

Seasonal variation in nutrient retention during inundation of a short‐hydroperiod floodplain

Floodplains are generally considered to be important locations for nutrient retention or inorganic‐to‐organic nutrient conversions in riverine ecosystems. However, little is known about nutrient processing in short‐hydroperiod floodplains or seasonal variation in floodplain nutrient retention. Therefore, we quantified the net uptake, release or transformation of nitrogen (N), phosphorus (P) and suspended sediment species during brief periods (1–2 days) of overbank flooding through a 250‐m floodplain flowpath on the fourth‐order Mattawoman Creek, Maryland U.S.A. Sampling occurred during a winter, two spring and a summer flood in this largely forested watershed with low nutrient and sediment loading. Concentrations of NO increased significantly in surface water flowing over the floodplain in three of the four floods, suggesting the floodplain was a source of NO. The upper portion of the floodplain flowpath consistently exported NH, most likely due to the hyporheic flushing of floodplain soil NH, which was then likely nitrified to NO in floodwaters. The floodplain was a sink for particulate organic P (POP) during two floods and particulate organic N and inorganic suspended sediment (ISS) during one flood. Large releases of all dissolved inorganic N and P species occurred following a snowmelt and subsequent cold winter flood. Although there was little consistency in most patterns of nutrient processing among the different floods, this floodplain, characterized by brief inundation, low residence time and low nutrient loading, behaved oppositely from the conceptual model for most floodplains in that it generally exported inorganic nutrients and imported organic nutrients. Published in 2007 by John Wiley & Sons, Ltd.     

Applicability of the flood‐pulse concept in a temperate floodplain river ecosystem: thermal and temporal components

Annual growth increments were calculated for blue catfish (Ictalurus furcatus) and flathead catfish (Pylodictis olivaris) from the lower Mississippi River (LMR) to assess hypothesized relationships between fish growth and floodplain inundation as predicted by the Flood‐Pulse Concept. Variation in catfish growth increment was high for all age classes of both species, and growth increments were not consistently related to various measures of floodplain inundation. However, relationships became stronger, and usually direct, when water temperature was integrated with area and duration of floodplain inundation. Relationships were significant for four of six age classes for blue catfish, a species known to utilize floodplain habitats. Though similar in direction, relationships were weaker for flathead catfish, which is considered a more riverine species. Our results indicate the Flood‐Pulse Concept applies more strongly to temperate floodplain‐river ecosystems when thermal aspects of flood pulses are considered. We recommend that future management of the LMR should consider ways to ‘recouple’ the annual flood and thermal cycles. An adaptive management approach will allow further determination of important processes affecting fisheries production in the LMR. Copyright © 2006 John Wiley & Sons, Ltd.     

Satellite remote sensing reveals impacts from dam‐associated hydrological changes on chlorophyll‐a in the world's largest desert lake

We present an approach that uses satellite products to derive models for predicting lake chlorophyll from environmental variables, and for investigating impacts of changing environmental flows. Lake Turkana, Kenya, is the world's largest desert lake, and environmental flows from the Omo River have been modified since 2015 by the Gibe III dam in Ethiopia. Using satellite remote sensing, we have evaluated the influence of these altered hydrological patterns on large‐scale lake phytoplankton concentrations for the first time. Prior to dam completion, strong seasonal cycles and large spatial gradients in chlorophyll have been observed, related to natural fluctuations in the Omo River's seasonal discharge. During this period, mean lake chlorophyll showed a strong relationship with both river inflows and lake levels. Empirical models were derived which considered multiple hydro‐climatic drivers, but the best model for predicting chlorophyll‐a was a simple model based on Omo River discharge. Application of this model to data for 2015–2016 estimated that during the filling of Gibe III annual mean Lake Turkana chlorophyll declined by 30%. Future water management scenarios based on Gibe III operations predict reduced seasonal chlorophyll‐a variability, while irrigation scenarios showed marked declines in chlorophyll‐a depending on the level of abstraction. These changes demonstrate how infrastructure developments such as dams can significantly alter lake primary production. Our remote sensing approach is easy to adapt to other lakes to understand how their phytoplankton dynamics may be affected by water management scenarios.     

Impact of a 100‐year flood on vegetation,benthic invertebrates,riparian fauna and large woody debris standing stock in an alpine floodplain

This paper investigates the impact of a 100‐year flood in May 1999 on community composition and large woody debris standing stock in an alpine floodplain (Isar, Germany). Detailed pre‐flood data sampled from 1993 to 1998 are compared with the situation directly after the flood. In those parts of the Isar floodplain mainly covered with pioneer vegetation prior to flooding, the coverage of unvegetated gravel bars increased by 22% following the flood. However, the flood did not remove larger amounts of older successional vegetation stages (willow thickets, floodplain forest). No significant changes in the benthic invertebrate fauna were recorded. The lowest densities of riparian ground beetles (Carabidae) within the study were recorded one month after the flood. Two months later, the ground beetle densities increased to the highest values ever recorded, indicating the ground beetle's high potential for recolonization. These results highlight the degree of resilience of both the aquatic and the riparian invertebrate fauna. The flood also caused a significant increase in large woody debris standing stock; in one section the number of logs increased tenfold and the volume increased by a factor of 20, leading to the assumption that most woody debris in alluvial flood‐plains is provided by catastrophic events. Copyright © 2004 John Wiley & Sons, Ltd.     

The effects of short‐term inundation on carbon dynamics,microbial community structure and microbial activity in floodplain soil

A mesocosm study was carried out to determine whether flooding affected soil microbial structure and function. Over a 24 day period soils were either flooded or remained dry and changes in the microbial community structure, microbial metabolic activity (determined by looking at soil respiration rates and exo‐enzyme activity), community level physiological profiles and soil carbon speciation were examined. Flooding induced significant changes in the microbial community structure and increased soil respiration and enzymatic degradation rates. Shifts occurred in carbon speciation after flooding and increases in ‘labile’ carbon fractions were observed. The duration of flooding was also important for carbon dynamics and microbial community structure. Based on the results of the mesocosm experiment a conceptual model of floodplain soils response to flooding over time was formulated. The findings of this mesocosm study illustrate the importance of flooding to floodplain soil functioning. Copyright © 2010 John Wiley & Sons, Ltd.     

Determining the effects of dams on subdaily variation in river flows at a whole‐basin scale

River regulation can alter the frequency and magnitude of subdaily flow variations causing major impacts on ecological structure and function. We developed an approach to quantify subdaily flow variation for multiple sites across a large watershed to assess the potential impacts of different dam operations (flood control, run‐of‐river hydropower and peaking hydropower) on natural communities. We used hourly flow data over a 9‐year period from 30 stream gages throughout the Connecticut River basin to calculate four metrics of subdaily flow variation and to compare sites downstream of dams with unregulated sites. Our objectives were to (1) determine the temporal scale of data needed to characterize subdaily variability; (2) compare the frequency of days with high subdaily flow variation downstream of dams and unregulated sites; (3) analyse the magnitude of subdaily variation at all sites and (4) identify individual sites that had subdaily variation significantly higher than unregulated locations. We found that estimates of flow variability based on daily mean flow data were not sufficient to characterize subdaily flow patterns. Alteration of subdaily flows was evident in the number of days natural ranges of variability were exceeded, rather than in the magnitude of subdaily variation, suggesting that all rivers may exhibit highly variable subdaily flows, but altered rivers exhibit this variability more frequently. Peaking hydropower facilities had the most highly altered subdaily flows; however, we observed significantly altered ranges of subdaily variability downstream of some flood‐control and run‐of‐river hydropower dams. Our analysis can be used to identify situations where dam operating procedures could be modified to reduce the level of hydrologic alteration. Copyright © 2009 John Wiley & Sons, Ltd.     

Unrestricted ramping rates and long‐term trends in the food web metrics of a boreal river

Long‐term monitoring of the food web of a regulated hydropeaking river was conducted to assess if previously documented effects of changing ramping rates (RRs) were maintained with the addition of 6 years of data. Using carbon and nitrogen stable isotope analyses, we hypothesized that: (1) macroinvertebrates and fish inhabiting areas below peaking hydrodams would be higher in δ¹⁵N and lower in δ¹³C due to increased flow velocity and the influence of light respired dissolved inorganic carbon, relative to those sampled from areas with a natural flow regime; (2) the increase in δ¹⁵N of macroinvertebrates would lead a shorter food web length in the regulated river, but δ¹³C and niche width would be similar between the restricted and unrestricted RR periods (i.e., the BACI analysis); and (3) isotopic metrics (e.g., δ¹³C, δ¹⁵N, niche width [SEA_B], and food chain length [Δ¹⁵N]) would correlate with variations in flow characteristics through time. Consistent with previous analysis conducted over a shorter time period, a shift toward higher δ¹⁵N values was observed for both fish and invertebrates, but, contrarily, only invertebrates (not fish) had a lower δ¹³C value downstream of the dam. Over the long term, the before‐after‐control‐impact analysis found no effect of RRs on any of the food web metrics, implying that the change in operation did not affect the river food web. However, analysis of the time series data indicated that flow metrics and trophic metrics were often correlated, including a negative effect of RR (invertebrates) and discharge (fish) on food chain length. This study illustrates the difficulty in detecting changes in food web structure and function under changing flow regime influenced by natural and anthropogenic effects. As such, this study highlights the need for considering large spatial and temporal scales to differentiate between confounding effects of climate, natural variability, and altered flow regimes on food webs in regulated rivers.     

Effects of flooding regime upon the decomposition of Eichhornia azurea (Sw.) Kunth measured on a tropical,flow‐regulated floodplain (Paraná River,Brazil)

Decomposition of macrophytes is an important process in river‐floodplain systems, especially in the Upper Paraná River floodplain, given that this ecosystem receives high inputs of detritus from this vegetation. Release of nutrients by decomposition is essential in this floodplain because it is located downstream from a reservoir chain where nutrients are being trapped. Water level fluctuations are considered one of the most important aspects that affect macrophyte decomposition. Anthropogenic alterations, such as the control of flooding regimes, observed in this floodplain, could change the dynamic of this process. To evaluate the influence of the hydrological cycle upon the decomposition of Eichhornia azurea (an aquatic macrophyte that has high biomass values in this ecosystem), litter bags with senescent leaves and petioles of this plant were submitted to four different flooding treatments, which differed in time of flooding and exposure to dry conditions. The decomposition rates and the detritus chemical composition (nitrogen and phosphorus concentrations) were measured over 113 days. There were significant effects of the flooding treatments and time upon all parameters. The materials that decomposed with flood conditions showed the greatest decomposition rates. The quality of the detritus seems to be highly linked with the flooding regime, showing highest releases of phosphorus in the submerged treatments. It was shown that floods, even the short duration ones, increase the decomposition velocity and the nutrient cycling relative to dry conditions. Thus, investigations that assess the nutrient budgets on the Upper Paraná River floodplain and the role of nutrients in its productivity should consider the detritus compartment and the effects of flood regimes upon its dynamics. Copyright © 2006 John Wiley & Sons, Ltd.     

The influence of hydro‐climatic and anthropogenic effects on the long‐term variation of commercial fisheries in a large floodplain river

This study deals with the effects of climate fluctuations and anthropogenic impacts on fisheries of Paraná River over the last 100 years. It is the first attempt to appraise the influence of hydro‐climatic and anthropogenic variables on the population changes of the most important inland fisheries of Argentina. Datasets covering more than eight decades (1935–2016) of a number of frequent and abundant commercial species inhabiting the main channel as well as the large floodplain of Paraná River were used. Our results suggest that fish catches and structure changed over time. Long and short‐term changes and reductions were closely related to fluctuations of 18 hydro‐climatic variables. Positive effects on the ichthyofauna were recorded during humid periods (1930–1940 and 1970–2000), when the frequency of large spring–summer floods increased. An increase in anthropic impacts (accounted for with nine variables) were recorded during the last two decades. We highlight the usefulness of the approach to support the management of the resources, ensuring sustainability of commercial fish assemblages and the long‐term conservation of biodiversity in big rivers.     

The effects of drying and re‐flooding on nutrient availability in ephemeral deflation basin lakes in western New South Wales,Australia

We examined the response of nutrient concentrations to the drying and re‐flooding of ephemeral deflation basin lakes (EDBL) in western New South Wales, Australia. As lakes dried total nitrogen (TN) and total phosphorus (TP) concentrations increased. TN concentrations increased more quickly and TP concentrations increased more slowly than could be attributed to evaporation alone. This suggested that additional nitrogen was being sequestered from the atmosphere or sediments and that some phosphorus loss to the sediments was occurring. Concentrations of nitrogen oxides (NOx) and filterable reactive phosphorus (FRP), however, declined as lakes dried, suggesting a tighter coupling of nutrient release and uptake mechanisms. Inorganic nutrient concentrations rose sharply in response to re‐flooding in all lakes. Evidence is provided to suggest that post‐flood nutrient pulses are the net result of both riverine inputs and sediment releases and that the relative significance of either may be influenced by regulation. Copyright © 2002 John Wiley & Sons, Ltd.     

The effects of impoundment and non‐native species on a river food web in Mexico's central plateau

Habitat modifications, non‐native species and other anthropogenic impacts have restructured fish communities in lotic ecosystems of central Mexico. Conservation of native fishes requires understanding of food web changes resulting from the introduction of non‐native species, flow alteration and other human impacts. Using δ¹³C and δ¹⁵N analysis of fishes and invertebrates we investigated the effects of non‐native species, and reservoirs on food webs of the Laja river ecosystem (Guanajuato, central Mexico). We estimated trophic position (TP), relative trophic niche and food web dispersion at 11 reservoir, river and tailwater sites. Reservoirs and non‐native fishes modified food webs in the Laja. Food web dispersion was greater in reservoir than in tailwater and river sites. Reservoir food webs had the greatest range of δ¹³C values, indicating a more diverse resource base compared to rivers. δ¹³C values increased with distance downstream from reservoirs, suggesting declining subsidies of river food webs by reservoir productivity. Stable isotopes revealed potential effects of non‐native fishes on native fishes via predation or competition. Non‐native Micropterus salmoides were top predators in the system. Non‐natives Cyprinus carpio, Oreochromis mossambicus and Carassius auratus exhibited lowest TP in the Laja but overlapped significantly with most native species, indicating potential for resource competition. Native Chirostoma jordani was the only species with a significantly different trophic niche from all other fish. Many rivers in central Mexico share similar anthropogenic impacts and similar biotas, such that food web patterns described here are likely indicative of other river systems in central Mexico. Copyright © 2008 John Wiley & Sons, Ltd.     

Modelling the effects of climate and flow regulation on ice‐affected backwater staging in a large northern river

In cold region environments, ice‐jam floods (IJFs) pose a severe risk to local communities, economies, and ecosystems. Previous studies have shown that both climate and regulation affect IJF probabilities, but their relative impacts are poorly understood. This study presents a probabilistic modelling framework that couples hydrologic and hydraulic models to assess the relative role of regulated and naturalized flows on ice‐affected backwater staging. The framework is evaluated at an IJF‐prone town on the Peace River in western Canada, which has been regulated since 1972. Naturalized flows were generated for the comparison, and ice‐affected backwater profiles were calculated along jams of varying length and location and for different combinations of model parameters and boundary conditions. Results show significant differences in backwater staging (~2 m for a return period of T = 1:10 year) between two study time periods (1973–1992 vs 1993–2012) as compared with two different hydraulic flow conditions (regulated vs naturalized), suggesting a larger role of climate than regulation in backwater staging. However, regulation was found to offset flood risk during the 1973–1992 period and exacerbate flood risk during the 1993–2012 period.     

Impact of short‐term regulation on hyporheic water quality in a boreal river

Water regulation may alter hydraulic head gradients with consequences for the exchange of water between the river and the hyporheic zone. The objective of this study was to investigate the effect of discharge on hyporheic water quality in a regulated Swedish boreal river during a 10‐day experimental period with a sequence of alternating high‐ and low‐flow episodes. A 250 m reach was instrumented with 28 piezometers placed at 150 and 300 mm below the river bed or below the mean groundwater level in the floodplain, and these piezometers were used to measure temperature, oxygen, electric conductivity and pH. High daily variation in air temperature during the first 3 days was transmitted vertically through the stream water into the hyporheic zone within hours. An oxygen saturation of 100% in the river water corresponded to 60–70% saturation at 150 mm depth and 30% at 300 mm depth. The hyporheic oxygen concentration at 150 mm depth decreased during the experimental period, falling into a range that is potentially harmful to incubating salmonid eggs. This was interpreted as a long‐term response to the overall regulation regime, rather than a response to short‐term water regulation during the experiment. Even though the effect of short‐term regulation on the quality of hyporheic water in the river bed was limited, there was a more pronounced effect on the quality of floodplain hyporheic water. Most of the driving forces for temporal variation of water quality in the river bed came vertically from the river water, rather than from the lateral exchange. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of solar UV‐B exclusion on the phytoplankton community in a sub‐tropical mountain reservoir: a mesocosm study

A mesocosm experiment was conducted over a 4 week period to investigate the response of a phytoplankton community in a mountain freshwater reservoir to solar UV‐B exclusion. The mesocosms were filled with water taken from the depth of 0.30 m in the reservoir, and exposed to near‐ambient solar UV‐B (+UV‐B) and solar UV‐B exclusion (?UV‐B). The resulting chlorophyll‐a (Chl‐a), carotenoids, soluble sugars, algal composition and algal abundance were analysed. The study results indicated that carotenoids, carbohydrates and species distribution were affected under the ?UV‐B treatment. The carotenoid concentration was generally higher under the +UV‐B treatment than under the ?UV‐B treatment. The sucrose and glucose concentrations were affected differently by the UV‐B treatments, although both sugars strongly decreased under the +UV‐B treatment by the end of the experiment. The sucrose and glucose concentrations exhibited similar distribution patterns to those of carotenoids for the +UV‐B treatment, suggesting that these sugars could be directly involved in secondary metabolism. In contrast, the fructose content did not exhibit significant differences between UV‐B treatments. The Chl‐a concentration exhibited a progressive decrease during the experiment for both UV‐B treatments. Although no significant impact of UV‐B exclusion on the Chl‐a concentration was observed in this study, a slightly higher concentration was observed for the ?UV‐B treatment. The total phytoplankton biovolume (BV) exhibited a pattern similar to that of the Chl‐a concentration. Analysis of the phytoplankton composition revealed 52 species. However, because some species and classes were extremely difficult to identify, only 46 species from seven classes (Cyanophyceae, Chlorophyceae, Bacillariophyceae, Euglenophyceae, Charophyceae, Dinophyceae, Xanthophyceae) were unambiguously identified and considered for the UV‐B treatments. Chlorophyceae, Bacillariophyceae and Cyanophyceae comprised about 78% of the identified species. Aulacoseira granulata and Cyclotella meneghiniana, followed by Ceratium hirundinella, were the most abundant species. Principal component analysis, using sampling periods and algal densities (PCA‐1), indicated a heterogeneous phytoplankton assemblage, whereas PCA‐2, with algal densities and biochemical variables (soluble carbohydrates, Chl‐a and carotenoids), indicated that biochemical parameters could provide qualitative information about La Angostura phytoplankton community responses to quality of the sunlight.     

Fundamental study on the effects of the keystone height and structure on sediment deposition in a high‐gradient channel

Step–pool structures are among the most important structures in riverbed morphology that can contribute to the stabilization of riverbeds and riverbanks, energy dissipation during floods, and head reduction function. Two concepts have been suggested regarding step–pool formation: the development of antidunes (antidune concept) and keystone concept. The latter suggests that a large boulder (keystone) incidentally left in a river channel can trap pebbles and build a step–pool structure. The ultimate goal of this study is to reveal the step–pool formation process and establish a design method based on the keystone concept. This paper describes the experimental results of the effects of the keystone arrangement on sediment deposition. Step–pool formation based on the keystone concept has previously been focused on the process of sediment deposition a short distance from a keystone. However, this study reveals that the obstruction of sediment supply due to the decreased flow velocity in the main river course is an important factor. When the arrangement interval of keystones is small, the amount of sediment supply downstream decreases because the flow velocity at the main flow channel between the keystones does not increase. In contrast, the flow velocity at the main river course increases between the keystones, and supplied sediment is transported to the most downstream section if the keystone arrangement interval is large enough to join the flow from the bank side of the keystones.     

The effects of flow regulation and climatic variability on obstructed drainage and reverse flow contribution in a Northern river–lake–Delta complex,Mackenzie basin headwaters

A distinctive hydrological feature of the Lake Athabasca–Peace–Athabasca Delta (LA‐PAD) complex is that flow in channels that drain the system reverses direction when stage on the Peace River exceeds that for the central lakes. This river's hydrology has experienced natural and human induced changes since 1968. This study investigates the importance of spring break‐up and open‐water induced outflow obstruction and reverse flow contributions to annual lake level maxima under natural (1960–1967), regulated (1976–2004) and naturalized (1976–1996) flow regimes. Obstructed and reverse flow events during spring break‐up were common prior to and following flow regulation, suggesting that natural climatic variability in source areas below the W.A.C. Bennett Dam exerted a strong influence on their occurrence. Antecedent hydrological conditions, such as fall freeze‐up lake level, break‐up magnitude, peak spring flow and initial open‐water lake level were significantly associated with annual lake level maxima. During the summer period, lake level was linked to sustained high flows on the Peace River. The river obstructed outflow and contributed reverse flow to the LA‐PAD in each year prior to 1968. Following regulation, however, more than half the years did not experience any open‐water obstruction and/or reversal, and those that did were characterized by smaller events. The average estimated duration of obstruction was more than two weeks shorter and reverse flow volume was reduced by ～90% under a regulated regime compared to a simulated naturalized flow regime. This implied a lowered potential for lateral lake expansion into the delta floodplain in some years. The regulated hydrology could produce large stormflow and high lake levels, but only under extreme climatic events in areas below the dam and/or human‐induced alterations to normal reservoir operation. Copyright © 2009 Crown in the right of Canada and John Wiley & Sons, Ltd.     

Effect of Spatial Heterogeneity on Zooplankton Diversity: A Multi‐Scale Habitat Approximation in a Floodplain Lake

Environmental heterogeneity is an essential quality of ecosystems as it has important implications in community structure. Macrophytes are a main component of environmental heterogeneity in floodplain lakes, and their dynamics are highly influenced by water level changes. In this context, we analysed at different spatial scales the relationship among hydrological variations, environmental heterogeneity associated to macrophytes and zooplankton regional diversity (γ diversity) in a South American floodplain lake adjacent to the Paraná River, and we also compared the local zooplankton diversity (α diversity) among the different environments that comprised the lake heterogeneity. At very low waters, the environmental heterogeneity was reduced as the lake was mainly limited to open water areas with low zooplankton diversity. At high waters, the profuse vegetation development (emergent and free‐floating), in mixed or homogeneous patches, determined a higher lake environmental heterogeneity with enhanced regional zooplankton diversity; littoral species increased over limnetic ones. Zooplankton α diversity was higher in environments with free‐floating macrophytes than in those without these plants. The structural complexity in the water column provided by plant roots would be closely related to the enhanced diversity found under free‐floating mats. This study contributes to the knowledge on the effects of strong water level variations on environmental heterogeneity, which is strongly associated to macrophytes and on zooplankton diversity, and highlights the role of free‐floating plants as diversity hosts and ‘key structures’ in floodplain lakes. Copyright © 2013 John Wiley & Sons, Ltd.