Gradient Of Hydrological Connectivity Research Articles

Chlorophyll a variations and responses to environmental stressors along hydrological connectivity gradients: Insights from a large floodplain lake

Understanding the key drivers of eutrophication in floodplain lakes has long been a challenge. In this study, the Chlorophyll a (Chla) variations and associated relationships with environmental stressors along the temporal hydrological connectivity gradient were investigated using a 11-year dataset in a large floodplain lake (Poyang Lake). A geostatistical method was firstly used to calculate the hydrological connectivity curves for each sampling campaign that was further classified by K-means technique. Linear mixed effect (LME) models were developed through the inclusion of the site as a random effect to identify the limiting factors of Chla variations. The results identified three clear hydrological connectivity variation patterns with remarkable connecting water area changes in Poyang Lake. Furthermore, hydrological connectivity changes exerted a great influence on environmental variables in Poyang Lake, with a decrease in nutrient concentrations as the hydrological connectivity enhanced. The Chla exhibited contrast variations with nutrient variables along the temporal hydrological connectivity gradient and generally depended on WT, DO, EC and TP, for the entire study period. Nevertheless, the relative roles of nutrient and non-nutrient variables in phytoplankton growth varied with different degrees of hydrological connectivity as confirmed by the LME models. In the low hydrological connectivity phase, the Chla dynamics were controlled only by water temperature with sufficient nutrients available. In the high hydrological connectivity phase, the synergistic influences of both nutrient and physical variables jointly limited the Chla dynamics. In addition, a significant increasing trend was observed for Chla variations from 2008 to 2018 in the HHC phase, which could largely be attributed to the elevated nutrient concentrations. This study confirmed the strong influences of hydrological connectivity on the nutrient and non-nutrient limitation of phytoplankton growth in floodplain lakes. The present study could provide new insights on the driving mechanisms underlying phytoplankton growth in floodplain lakes.

How hydrological connectivity regulates the plant recovery process in salt marshes

Abstract Designing effective restoration strategies is a priority in recovering salt marsh plants. Hydrological connectivity is a main driver underpinning the success of the plant recovery process and can regulate life‐history process‐based restoration strategies. However, the relationship between these is unclear. Plant recovery needs to go through a whole life‐history process, from seed to adult. Common restoration strategies are seed addition (SA) or seedling transplantation (ST), which start from seed germination and seedling growth stage. Besides these two strategies, another strategy starting from seed retention stage, microtopographic adjustment (MA), was designed to study the relationship with hydrological connectivity. A framework was also constructed to assess a gradient of hydrological connectivity between marsh plain and sea. We conducted several field experiments to test their relationships. The composite measurement of hydrological connectivity with five geomorphic variables can well represent the variation of environmental factors. Soil moisture, inundation frequency and sediment deposition were positively correlated, while soil salinity and hardness were negatively correlated with hydrological connectivity. The success of different restoration strategies varied with hydrological connectivity. MA showed a monotone decreasing trend, while SA and ST showed a unimodal trend as hydrological connectivity increased. Importantly, each strategy occupies a non‐overlapping optimum range along the hydrological connectivity gradient. There is low hydrological connectivity for MA (0–0.28), middle hydrological connectivity for SA (0.28–0.55) and high hydrological connectivity for ST (0.55–1). Synthesis and applications. Our findings expand the quantification of the hydrological environment beyond elevation, distance or other single index to include a range of elements of hydrological connectivity, thus illustrating the underlying mechanisms of hydrological connectivity which regulate restoration strategies based on different life stages. The results provide a reliable framework to assess hydrological connectivity and offer guidance to select the optimum restoration strategy under different hydrological connectivities or to regulate the hydrological connectivity variables (topography on marsh plain and morphology of tidal creeks) to relief stresses. These findings will be beneficial to ecological restoration and coastal management.

The seasonality of macroinvertebrate β diversity along the gradient of hydrological connectivity in a dynamic river-floodplain system

Anthropogenic disturbance to natural hydrological connectivity, both longitudinal, lateral, and vertical, is threatening the ecological integrity of the freshwater realm. River-floodplain system is particularly adversely affected by the reduction in lateral hydrological connectivity (LHC), representing one of major biodiversity hotspots under increasingly pressure. Many studies have demonstrated that LHC has great influence on the spatial variations of flora and fauna communities (i.e. spatial β diversity) through facilitating dispersal. However, to fully understand the impact of anthropogenic disturbances on biodiversity, we must also understand how ecological communities change over time (i.e. temporal β diversity, TBI) and the underlying processes. To evaluate the processes structuring ecological communities, we examined the macroinvertebrate TBI in habitats along the gradient of LHC for an entire hydrological cycle in West Dongting Lake, a Ramsar-listed floodplain wetland at the middle reach of Yangtze River. Our results showed that the total spatial β diversity fluctuated with water level and peaked at high-water phase, and LHC was the driving forcing affecting both species richness and abundance in all hydrological periods. In particular, species richness and abundance were highest in habitats with medium LHC levels for water-rising and high-water periods reflecting the intermediate disturbance hypothesis except for water-recessing, during which there was no clear pattern. While replacement determined β diversity in most sites at water-rising and high-water phases, the contribution of nestedness were high during water-withdrawing phase. From water-rising to high-water, macroinvertebrates from other habitats spread to the modified mudflats, which had the lowest LHC, along with the floods. During water-withdrawing period, β diversity and its turnover component of all habitats were low compared with other hydrological phases. Temporal β diversity analysis illustrated that the species dispersal was the main mechanism underlying the temporal and spatial variations in the observed community patterns. These findings demonstrated that hydrological connectivity was critical to maintaining the ecological integrity of river-floodplain ecosystems.

Are different benthic communities in Arctic delta lakes distinguishable along a hydrological connectivity gradient using a rapid bioassessment approach?

Aquatic habitats in the Canadian Arctic are expected to come under increasing stress due to projected effects of climate change. There is a need for community-based biomonitoring programs to observe and understand the effects of these stressors on the environment. Here we present results from a 5 year annual sampling program of benthic invertebrates from lakes in the Mackenzie Delta, Northwest Territories, using a rapid bioassessment protocol. Connectivity between the deltaic lakes and main channels is a major driver of lake function and is expected to be substantially impacted by climate change. Lakes were selected along a gradient of connectivity based on sill elevation above the river. Using multivariate analyses of community structure, we determined that benthic assemblages responded to differences in connection time among lakes. This response was detected using a coarse taxonomic level that could be applied by community groups or volunteers but was stronger when invertebrates were identified to the family and genus levels. A secondary gradient was observed that corresponded to productivity gradients in lakes that are isolated from the river during summer. We show that benthic assemblages have potential use as sensitive indicators of climate-mediated changes to the hydrology of lakes in the Mackenzie Delta.

Niche construction within riparian corridors. Part I: Exploring biogeomorphic feedback windows of three pioneer riparian species (Allier River, France)

Within riparian corridors, biotic-abiotic feedback mechanisms occur between woody vegetation strongly influenced by hydrogeomorphic constraints (e.g., sediment transport and deposition, shear stress, hydrological variability), fluvial landforms, and morphodynamics, which in turn are modulated by the established vegetation. During field investigations in spring 2015, we studied 16 alluvial bars (e.g., point and lateral bars) within the dynamic riparian corridor of the Allier River (France) to assess the aptitude of three pioneer riparian Salicaceae species (Populus nigra L., Salix purpurea L., and Salix alba L.) to establish and act as ecosystem engineers by trapping sediment and constructing fluvial landforms. Our aim is to empirically identify the preferential establishment area (EA; i.e., the local areas where species become established) and the preferential biogeomorphic feedback window (BFW; i.e., where and to what extent the species and geomorphology interact) of these three species on alluvial bars within a 20-km-long river reach. Our results show that the EA and BFW of all three species vary significantly along the longitudinal profile, i.e., upstream-downstream exposure on the alluvial bars, as well as transversally, i.e., the main hydrological connectivity gradient from the river channel toward the floodplain. In the present-day context of the Allier River, P. nigra is the most abundant species, appearing to act as the main engineer species affecting landform dynamics at the bar scale; S. purpurea is established and acts as an ecosystem engineer at locations on alluvial bars that are most exposed to hydrosedimentary flow dynamics, while S. alba is established on the bar tail close to secondary channels and affects the geomorphology in mixed patches along with P. nigra. Our study highlights the role of functional trait diversity of riparian engineer species in controlling the extent of fluvial landform construction along geomorphic gradients within riparian corridors exposed to frequent hydrogeomorphic disturbances.

Spatial variability of chromophoric dissolved organic matter in a large floodplain river: control factors and relations with phytoplankton during a low water period

AbstractChromophoric dissolved organic matter (CDOM) affects ecological processes in freshwater environments. Few studies assessed its spatial variability and relations with phytoplankton in floodplain rivers. Therefore, these topics were examined in a hydrological connectivity gradient in the Middle Paraná system. Absorption coefficients at 250 and 365 nm were measured to estimate CDOM concentration and molecular weight (MW), to find their explanatory limnological variables (Redundancy Analysis, RDA), and to assess their contribution to the explanation of phytoplankton structure (Canonical Correspondence Analysis, CCA). Conductivity and turbidity explained CDOM significantly. Increased conductivity from the main channel to floodplain lakes was associated with increased CDOM concentration. The lake indirectly connected to the river showed the highest turbidity associated with high‐MW‐CDOM, while the isolated lake showed the highest conductivity associated with low‐MW‐CDOM. Phytoplankton structure was significantly explained by conductivity, turbidity, and high‐MW‐CDOM (CCA). Environments directly connected to the river (with the lowest conductivity) were associated with diatoms. Phytoflagellates were associated with turbidity and high‐MW‐CDOM in the lake indirectly connected to the river, whereas Cyanobacteria were associated with conductivity and low‐MW‐CDOM in the isolated lake. The combined effect of physical factors and CDOM explained the highest fraction of species variation (partial CCA). As a conclusion: (1) CDOM increases as hydrological connectivity with the river decreases; (2) lake sediment resuspension is linked to an increase of high‐MW‐CDOM; (3) lake isolation from the river corresponds to an increase in low‐MW‐CDOM; and (4) CDOM explains phytoplankton structure significantly and should be considered as an important variable in studies of microalgal assembly. Copyright © 2015 John Wiley & Sons, Ltd.

Macrobenthos Diversity Response to Hydrological Connectivity Gradient

The abundant riparian wetlands supported by the complex river network in Pearl River Delta (PRD) are under an intensely anthropogenic disturbance that threatens the local ecological environment. This paper examined the benthic invertebrate diversity and biomass within the Pearl River through a field study. A fieldwork was conducted to assess a hydrological connectivity gradient among the main channel of the Pearl River. The responses of macroinvertebrate metrics to the connectivity gradient and the restoration history in the river channel were examined. The Shannon diversity index ranged from 0.75 to 2.08, which was the highest among most connectivity river channels. Statistical analyses (PCA and CCA) revealed that the benthic abundance in high hydrological connectivity riverine habitats is clearly distinct from the low connectivity habitats. Results demonstrated that the diversity and density of macrobenthos are determined by hydrological connectivity and time span of the restoration (river channels). Therefore, conservation and restoration efforts should focus on the macroinvertebrate response to the hydrological connectivity of the river channels in PRD.

Ecological validity of river-floodplain system assessment by planktonic crustacean survey (Branchiata: Branchiopoda)

Studies of the Slovakian Danube have indicated profound changes in the structures of several invertebrate assemblages in the section of the river bypassed by the Gabčíkovo hydroelectric project in 1992 and in adjacent floodplain water bodies. The present study investigates the relationship between hydrological connectivity and species diversity in the old main channel and adjacent left-bank water bodies, using resident cladocerans as indicators of the ecological integrity of the river-floodplain system. The work aimed to quantify cladoceran habitat preferences using habitat values (HV) and indication weights (IW) calculated from data accumulated during long-term monitoring of planktonic communities of the Danube floodplain (1,840.5-1,804rkm) and supplemented with data from the literature. Although not listed as an indicator group in the European Union Water Framework Directive, cladocerans are shown to be highly apposite in the ecological assessment of river-floodplain systems. Of 71 recorded cladoceran species, 19 taxa showed a preference for eupotamal habitats, 24 species preferred eupotamal B/parapotamal waters, and 28 cladocerans were found to prefer the plesiopotamal/paleopotamal habitat type. Identified habitat types follow a gradient of hydrological connectivity with the main river channel, ranging from the eupotamal to more or less isolated floodplain water bodies. A secondary aim was to use the calculated values to assess the ecological integrity of the river-floodplain system. The key element of this procedure is the calculation of the Floodplain Index (FI) (Chovanec et al. Large Rivers, 15(1-4), 169-185 2005) from the summation of the habitat values and indication weights of all species present at the sampling sites. Calculated index values indicate the extent of disruption to lateral connectivity in the floodplain area.

Modelling sediment deposition and phosphorus retention in a river floodplain

Phosphorus (P) is one of the major limiting nutrient in many freshwater ecosystems. During the last decade, attention has been focused on the fluxes of suspended sediment and particulate P through freshwater drainage systems because of severe eutrophication effects in aquatic ecosystems. Hence, the analysis and prediction of phosphorus and sediment dynamics constitute an important element for ecological conservation and restoration of freshwater ecosystems. In that sense, the development of a suitable prediction model is justified, and the present work is devoted to the validation and application of a predictive soluble reactive phosphorus (SRP) uptake and sedimentation models, to a real riparian system of the middle Ebro river floodplain. Both models are coupled to a fully distributed two-dimensional shallow-water flow numerical model. The SRP uptake model is validated using data from three field experiments. The model predictions show a good accuracy for SRP concentration, where the linear regressions between measured and calculated values of the three experiments were significant (r2 ≥ 0.62; p ≤ 0.05), and a Nash–Sutcliffe coefficient (E) that ranged from 0.54 to 0.62. The sedimentation model is validated using field data collected during two real flooding events within the same river reach. The comparison between calculated and measured sediment depositions showed a significant linear regression (p ≤ 0.05; r2 = 0.97) and an E that ranged from 0.63 to 0.78. Subsequently, the complete model that includes flow dynamics, solute transport, SRP uptake and sedimentation is used to simulate and analyse floodplain sediment deposition, river nutrient contribution and SRP uptake. According to this analysis, the main SRP uptake process appears to be the sediment sorption. The analysis also reveals the presence of a lateral gradient of hydrological connectivity that decreases with distance from the river and controls the river matter contribution to the floodplain. Copyright © 2014 John Wiley & Sons, Ltd.

Seasonal changes in caddis larvae assemblages in river-floodplain habitats along a hydrological connectivity gradient

In order to assess the impact of seasonality versus connectivity on the ecological quality of the Lower Rhine river-floodplain habitats, we studied the seasonal variation in diversity and species assemblages of caddis larvae by monthly sampling of the littoral zone of four water bodies over a lateral connectivity gradient. Seasonal variation in diversity showed a general pattern in these floodplain habitats and could be related to caddisfly life history and habitat preferences. Despite this seasonal variation, caddis larvae species assemblages in the water bodies appeared to be rather stable and could be firmly related to the position of their habitats over the connectivity gradient. The main channel (lotic habitat) was clearly separated from the lentic water bodies, and also the sparsely vegetated water bodies from the well-vegetated water bodies. It is concluded that the impact of connectivity, and related parameters such as stream velocity and vegetation coverage, outweighed the impact of season on the caddis larvae assemblages.

Mussel responses to flood pulse frequency: the importance of local habitat

Summary1. Understanding mechanisms behind the distribution of organisms along a gradient of hydrological connectivity is crucial for sustainable management of river–floodplain systems. We tested the hypothesis that frequency of flood pulses exerts a direct influence on the distribution of freshwater mussels (Unionoida) by creating a local environment that limits their fitness.2. Multiscale habitat analyses combined with transplant‐rearing experiments were carried out with a focus on abundance, presence/absence, survival rates and growth rates of mussels. Sixty‐nine floodplain waterbodies (FWBs) were surveyed within a 15‐km lowland segment of the Kiso River in Japan.3. The abundance of mussels significantly increased with increased frequency of inundation associated with flood pulses at the among‐FWB scale, while the probability of occurrence of mussels was negatively predicted by the amount of benthic organic matter at the within‐FWB scale.4. Field‐rearing experiments showed that survival rates were low and growth rates nearly zero in infrequently inundated FWBs (these FWBs had no naturally occurring resident mussels). In such FWBs, hypoxia (DO < 2 mg L−1) was frequently observed near the bottom when temperature was optimal for mussel growth (>15 °C).5. These findings demonstrated that flood pulse frequency was the most important factor in determining mussel distribution in FWBs because it directly limits mussels’ fitness by mediating local environmental factors, possibly dissolved oxygen (DO) levels. Successful restoration efforts for mussel habitat conservation should focus on processes that lead to improved local conditions.

Ecological responses of two pioneer species to a hydrological connectivity gradient in riparian forests of the lower Paraná River

The vegetation of the Parana River floodplain has physiognomic, structural and ecological characteristics that are distinct from those of the surrounding landscapes across the river (Forest, Chaco Savannahs, Pampean steppe). Few species are able to live in the alternately flooded and dry soil. The distribution of each species is strongly conditioned by the water regime in each area of the floodplain, and its location in the topographical gradient of the islands allows us to determine the possibility of colonising the bars and islands of the river course to determine their tolerance to hydrological variations and to foresee the changes that can take place as a result of disturbances in the hydrological regime. Here, we present the occurrence of willow (Salix humboldtiana Willd.) and South American alder (Tessaria integrifolia Ruiz et Pav.) in bars and islands of different topographies in a section of the Parana River downstream from the confluence of the Paraguay and Parana rivers. The results indicate that both species have similar niches in relation to hydrological fluctuations. However, willow was significantly more frequent at of 48.49 m a.s.l., while palo bobo reached its highest frequency at 49.29 m a.s.l. The difference between the modes of the distribution curves of each species was 0.80 m. Canopy trees such as willow and palo bobo are adapted to flooded soil conditions for 77.8 and 40% of their respective lives and survive with a long-lasting inundation phase (306 and 366 days, respectively). However, many trees in the Parana River floodplain died when the flood period extended for more than 1 year.

Influence of lateral gradients of hydrologic connectivity on trophic positions of fishes in the Upper Mississippi River

Summary1. Riverscapes consist of the main channel and lateral slackwater habitats along a gradient of hydrological connectivity from maximum connection in main channel habitats to minimum connection in backwaters. Spatiotemporal differences in water currents along this gradient produce dynamic habitat conditions that influence species diversity, population densities and trophic interactions of fishes.2. We examined the importance of lateral connectivity gradients for food web dynamics in the Upper Mississippi River during spring (high flow, moderately low temperatures) and summer (low flow, higher temperatures). We used literature information and gut contents analyses to determine feeding guilds and stable isotope analysis to estimate mean trophic position of local fish assemblages. During June and August 2006, we collected over 1000 tissue samples from four habitats (main channel, secondary channels, tertiary channels and backwaters) distributed within four hydrologic connectivity gradients.3. Mean trophic position differed among feeding guilds and seasons, with highest values in spring. Mean trophic position of fish assemblages, variability in trophic position and food chain length (maximum trophic position) of the two dominant piscivore species (Micropterus salmoides and M. dolomieu) in both seasons were significantly associated with habitat along the lateral connectivity gradient. Food chain length peaked in tertiary channels in both seasons, probably due to higher species diversity of prey at these habitats. We infer that food chain length and trophic position of fish assemblages were lower in backwater habitats in the summer mainly because of the use of alternative food sources in these habitats.4. A greater number of conspecifics exhibited significant among‐habitat variation in trophic position during the summer, indicating that low river stages can constrain fish movements in the Upper Mississippi River.5. Results of this study should provide a better understanding of the fundamental structure of large river ecosystems and an improved basis for river rehabilitation and management through knowledge of the importance of lateral complexity in rivers.

Large river floodplain restoration: predicting species richness and trait responses to the restoration of hydrological connectivity

Summary Floodplains are species‐rich environments often strongly impacted by human activities. In particular, the negative effects of progressive and rapid disconnection of secondary channels have led to restoration programmes and a growing interest in restoration ecology. Current restoration strategies in large river floodplains focus on the macroinvertebrate response related to the increases in lateral connectivity of the secondary channels. We constructed a framework to assess a gradient of hydrological connectivity among 13 secondary channels and the main channel of a large river, and we modelled the response of a set of macroinvertebrate metrics to this gradient. Comparisons between predicted and observed metrics in restored channels allowed us to measure the effect of an increase in the hydrological connectivity on the biological characteristics of macroinvertebrate assemblages. The pre‐restoration framework enabled a clear ordering of channels into three types according to levels of hydrological connectivity. Rarefied richness and species traits, responding to the connectivity gradient, showed a net difference between disconnected channels and the main river channel. We were able to highlight a predation–colonization trade‐off along the gradient of hydrological connectivity with a maximum colonization potential in the most connected channels. Post‐restoration sampling showed deviations of the restored channels from their expected ecological state. A large proportion of colonizers were favoured by the restoration operations and non‐native species occurred in the restored channels. Synthesis and applications. Macroinvertebrate biodiversity in large river floodplains is shaped by lateral hydrological connectivity. Increasing hydrological connectivity led to an increase in colonization rate. One year after restoration, the increase in lateral connectivity had shifted the restored sites away from the predicted state. This unpredictability is, in part, a consequence of the rapid colonization by non‐native species of new habitats created by the restoration measures. We recommend that floodplain‐scale restoration should focus on diversification of the hydrological connectivity of channels, thereby conserving a maximum of functional characteristics in macroinvertebrate communities.

Modelling the response of floodplain aquatic assemblages across the lateral hydrological connectivity gradient

Hydrological connectivity is one of the main controlling factors of habitats and aquatic assemblages on river floodplains. Nevertheless, the lack of universal measures of river–floodplain connectivity (i.e. the lateral hydrological connectivity, LHC) limits the comparison of the response of aquatic assemblages to hydrological connectivity and impedes the understanding of floodplain functioning across different systems. To address these needs, we tested the ability of six different LHC surrogates to model changes in richness, abundance and composition of aquatic assemblages across a Mediterranean floodplain (Ebro River, NE Spain). As shown by generalised additive models, LHC surrogates explained 15% to 65% of the richness and abundance of aquatic assemblages. Zooplankton, macroinvertebrates and phytoplankton showed overlapping peaks of richness at flood duration rates of 5, 15 and 30 days year–1 respectively. Redundancy analyses showed that LHC surrogates explained 17% to 37% of aquatic assemblage composition. Distance to the river and flood duration were the most important determinants of macroinvertebrate composition, whereas flood magnitude and water-level variability best accounted for the variance in zooplankton and phytoplankton compositions. Models based on LHC surrogates such as those presented here can help in predicting the consequences of restoration measures and may be useful in setting restoration goals for aquatic assemblages.

Aquatic macroinvertebrate response along a gradient of lateral connectivity in river floodplain channels

Large river floodplains potentially include the full range of freshwater ecosystems from permanently flowing channels to temporary pools and springs. Attempts to restore such complex systems require tools adapted to assess restoration success. In an analysis of invertebrate assemblages in the Rhône River floodplain (France), taxonomic-based indices (rarefied richness and assemblage composition) were compared with functional metrics using trait-based ratios as surrogates of ecosystem processes. Their ability to respond to a gradient of hydrological connectivity was assessed in 7 cut-off channels. The sampling design included 2 sites/channel (upstream and downstream), 4 randomly chosen sampling points (0.5 × 0.5-m quadrats)/site, and 2 sampling seasons (spring and summer). Water physicochemical and habitat variables were recorded when invertebrates were sampled. Environmental variables, including water conductance, [NH3-N], submerged vegetation cover, diversity of sediment grain size, and organic matter content of the sediment, were used to construct a synthetic variable describing the hydrological connectivity of each site with the main river channel. A quadratic regression of rarefied taxonomic richness and the connectivity gradient was not quite significant, but assemblage composition was strongly related to the gradient. Four of 8 trait-based metrics were correlated with the connectivity gradient. Values of metrics that are surrogates for top-down control of assemblage structure and habitat stability (based on functional feeding groups) declined along the gradient from disconnected sites to more connected sites. Values of metrics that are surrogates for voltinism and food supply for water-column-feeding fish increased with connectivity. Top-down control and voltinism surrogates suggested a decline in predator–prey relationships and lower habitat stability, respectively, in the more connected sites. Assemblage composition and some of the trait-based metrics were sensitive to a flood that occurred before one of the sampling dates. Some of the trait-based metrics showed potential for explaining floodplain invertebrate assemblages and for monitoring postrestoration conditions in floodplain water bodies. However, the metrics were developed initially for studies of lotic systems and their use in heterogeneous floodplain water bodies will require further investigation, e.g., delineation of reference conditions for trait-based metrics.

The Floodplain Index - a new approach for assessing the ecological status of river/floodplain-systems according to the EU Water Framework Directive.

A new method was developed to assess the ecological status of river/floodplain-systems. The approach is based on the requirements of biological assessment laid down in the EU Water Framework Directive, and, therefore, focuses on water bodies and their environs while neglecting truly terrestrial floodplain sections. The following indicator groups have been integrated: molluscs, caddisflies, dragonflies, amphibians, and fish. These groups were chosen to describe species associations representative of all types of floodplain waters along the gradient of hydrological connectivity. Key element of the procedure is the Floodplain Index, which is calculated for each site investigated on the basis of species-specific habitat values expressing the species' habitat preferences. Since species compositions vary according to the hydrological conditions, the water bodies of an investigation area can be characterized according to the index values. The distribution of the index values shows if and to which extent the degree of lateral connectivity is disturbed in the floodplain area. The assessment of the ecological status is based on a comparison between a river-type-specific reference condition and the status quo. The method is presented with an example from a Danube floodplain area in the north of Vienna (Austria).