Fritz Schiemer

Biodiversity of floodplain river ecosystems: ecotones and connectivity1

A high level of spatio-temporal heterogeneity makes riverine floodplains among the most species-rich environments known. Fluvial dynamics from flooding play a major role in maintaining a diversity of lentic, lotic and semi-aquatic habitat types, each represented by a diversity of successional stages. Ecotones (transition zones between adjacent patches) and connectivity (the strength of interactions across ecotones) are structural and functional elements that result from and contribute to the spatio-temporal dynamics of riverine ecosystems. In floodplain rivers, ecotones and their adjoining patches are arrayed in hierarchical series across a range of scales. At a coarse scale of resolution, fringing floodplains are themselves complex ecotones between river channels and uplands. At finer scales, patches of various types and sizes form habitat and microhabitat diversity patterns. A broad spatio-temporal perspective, including patterns and processes across scales, is needed in order to gain insight into riverine biodiversity. We propose a hierarchical framework for examining diversity patterns in floodplain rivers. Various river management schemes disrupt the interactions that structure ecotones and alter the connectivity across transition zones. Such disruptions occur both within and between hierarchical levels, invariably leading to reductions in biodiversity. Species richness data from the connected and disconnected floodplains of the Austrian Danube illustrate this clearly. In much of the world, species-rich riverine/floodplain environments exist only as isolated fragments across the landscape. In many large rivers, these islands of biodiversity are endangered ecosystems. The fluvial dynamics that formed them have been severely altered. Without ecologically sound restoration of disturbance regimes and connectivity, these remnants of biodiversity will proceed on unidirectional trajectories toward senescence, without rejuvenation. Principles of ecosystem management are necessary to sustain biodiversity in fragmented riverine floodplains. Copyright

Conservation by restoration: the management concept for a river‐floodplain system on the Danube River in Austria

1. One of the last remnants of a functional alluvial landscape on the Danube extends from Vienna to the Slovakian frontier. It is recognized as an ecosystem extremely worthy of protection and therefore has been designated as a National Park (‘Alluvial Zone National Park’). 2. However, surface connectivity has been reduced and floodplain habits have been fragmented. At present, lateral exchange processes of matter are restricted to short-term flood pulses, while most of the year backwater processes are de-coupled from the river system. 3. A very high species diversity is recorded for this section, with a high proportion of endangered species in all groups, ranging from 16% for riparian vascular plants to 100% for amphibians and reptiles. High diversity is mainly a result of the remaining spatial array of water bodies of different age across the river-floodplain complex (between-channel diversity). 4. A successful conservation strategy for this floodplain area requires a management scheme based on a solid conceptual foundation of the key processes in river-floodplain systems. Re-establishing hydrological dynamics is recognized as the most vital step, because other processes are influenced by the flow regime and resulting connectivity. Therefore, a large-scale pilot project has been developed for a segment of the free-flowing section to restore gradually the hydrological connectivity between the river and its floodplain. 5. The side-arm system will be reconnected to the main channel by lowering parts of the riverside embankments. After implementation, the side-arm system will be integrated with the flow regime of the river for more than half of an average year (at present: <8 days per year). 6. A key challenge in the evaluation of the effects of restoration is the development and testing of an appropriate monitoring scheme, which has to include a wide range of physical, chemical, geomorphic, and ecological parameters.

The Danube restoration project: species diversity patterns across connectivity gradients in the floodplain system

The relationship between hydrological connectivity and species diversity patterns (alpha and beta diversity) of macrophytes, molluscs, odonates and amphibians was investigated in a semi-natural floodplain segment in the ‘Alluvial Zone National Park’ of the Danube River in Austria. Based on environmental variables, we distinguished four major channel types (inflow channel, parapotamal, plesiopotamal and palaeopotamal) that reflected a lateral connectivity gradient. In addition, a longitudinal environmental gradient along the parapotamal channel was found. Connectivity, rather than the surface area of individual floodplain water bodies, explained local species richness. Species diversity patterns varied among taxa: the highest species richness values for molluscs occurred in the parapotamal channels, for odonates in the para- and plesiopotamal channels, for macrophytes in the plesiopotamal channels and for amphibians in the palaeopotamal channels. Within the parapotamal channels, the species richness of odonates and amphibians increased moving upstream. Beta diversity displayed an almost inverse relationship with alpha diversity, with highest average values in isolated and fragmented floodplain channels. Habitat fragmentation favoured the beta diversity of most groups, although connectivity favoured the beta diversity of amphibians. The highest proportion of endangered species (mainly rheophilic forms) was found in the parapotamal channels. It is concluded that preservation of the high diversity of this alluvial flood plain would be more fully realised by reconstitution of fluvial dynamics and the associated connectivity gradients, rather than by restoration strategies for individual groups or endangered species. Copyright

Restoration of floodplain rivers: The ‘Danube restoration project’

Restoration concepts for large river systems are currently in an early stage of development. Possibilities for, and constraints of, rehabilitation—both from a process-oriented view as well as from the nature conservation perspective—differ strongly according to the degree of anthropogenic changes. This requires precisely formulated amelioration goals. Well-conceived and thoroughly analysed case studies are necessary to better understand reversibility, direction and time scale of changes, and the sustainability of various scenarios. It is important to plan controllable set-ups with long-term monitoring in the pre- and post-implementation phases of restoration programmes. In order to achieve these goals, monitoring instruments have to be developed and calibrated that allow the definition and analysis of improvements. Finally, controlled restoration programmes offer the opportunity for large-scale experiments to test overall concepts and hypotheses and have an important heuristic value to improve the understanding of river ecology in general. Copyright

An ecophysiological study of some meiofauna species inhabiting a sandy beach at Bermuda

The dominant nematode and harpacticoid species inhabiting a sheltered beach at Bermuda were characterized by their vertical distribution in the sediment, by their tolerance of high temperature under oxic and anoxic conditions, and by their tolerance of extreme pH-values. In 4 species of nematodes the respiratory rate proved to be inversely proportional to the depth at which the species occurs, and directly proportional to the size of the buccal cavity. One species, the nematode Paramonhystera n.sp., is more temperature resistant at zero or near zero pO2 than at atmospheric oxygen pressure; it is the first marine metazoan in which it can be shown that a specific biological process is favourably affected by anoxic conditions if compared with the situation at normal pO2.

Fish as indicators for the assessment of the ecological integrity of large rivers

Fish communities in large rivers are characterized by a high diversity, which reflects the structural diversity and habitat richness of inshore zones and connected floodplains. The connectivity of the different habitat elements in a broad spatio-temporal context, i. e., at various scales from catchment to microhabitat — and their nestedness — define the fitness of fish species both on the individual level (e. g., growth performances) and population level (i. e., population structure, mortality, etc.). Relevant spatial scales can be the whole river course in the case of some anadromous migrators or the availability of complementary microhabitat elements, e. g., during the early ontogeny of a species. The significance of connectivity at various scales from whole river to local reach have to be evaluated based on the requirements, reaction norms, and ecological flexibility of individual species. Integrity has to be evaluated in several respects: (1) with regard to population genetics over extensive biogeographic areas and in long time scales; (2) with regard to supplementary habitats in the course of the life cycle of individual species with ontogenetic habitat shifts and specific requirements during the reproductive phase; (3) with regard to longitudinal and lateral transport and exchange processes determining local habitat conditions and the food supply for fish. Due to these dependencies the status and condition of the fish fauna is a critical sensor of integrity at different scales and thus a good monitoring tool especially with regard to river engineering. The structure of the fish population — in particular the relative abundance of specifically adapted floodplain species — provides information on the overall conditions. For a more detailed evaluation the local distribution pattern of the fish fauna, population structure, seasonality, and growth performances of the early life history stages can indicate the quality and extent of connectedness at a finer scale and can be used to define management and engineering criteria.

The Danube restoration project: functional aspects and planktonic productivity in the floodplain system

In Austria, the ‘Danube Restoration Project’ (DRP) was implemented to re-establish the connectivity between the Danube and its floodplain along a free flowing section downstream of Vienna. Before the restoration, the status quo of the ‘Regelsbrunn’ floodplain segment was investigated at different levels of abiotic, biotic and functional properties according to the spatio-temporal effects of hydrological connectivity. The present paper deals with temporal effects on hydrochemistry and on phytoand bacterioplankton in the side channel (parapotamon). A hydrological situation of the present state of short-termed and intensive flood pulses followed by a period of isolation was compared with a period of higher connectivity reflecting the situation after the restoration. Rising water levels in the river, establishing local surface inflows, resulted in periodic nutrient pulses, high particle load and eutrophication in the floodplain. With disconnection after a spate, the nitrate and soluble reactive phosphate (SRP) concentration decreased significantly during 1 month. Phytoplankton correlated negatively with SRP during the first week after a flood pulse, reflecting the dominance of phytoplankton in SRP uptake. Planktonic primary production was stimulated by the nutrient import at higher connectivity and could be controlled by a decrease in retention time, related to the aims of the DRP. The two investigated periods indicated that increasing retention time in the side channel resulted in a shift from primary production towards prevailing bacterial secondary production. Copyright

Metabolic studies on thiobiotic free-living nematodes and their symbiotic microorganisms.

The marine, free-living Stilbonematinae (Nematoda: Desmodoridae) are remarkable for the ectosymbiotic, prokaryotic microorganisms that populate their entire body surface. These nematodes occur in sulfidic sediments in the microoxic zone just above the sulfide maximum. Several facts point to a chemolithotrophic, sulfide oxidizing nature of the microorganisms. The oxygen uptake of three species was measured with and without their microbial coat using Cartesian and Gradient Diver microrespirometry in February 1989 at Carrie Bow Cay (Belize Barrier Reef). Symbiont-free stilbonematids exhibited constant and uniform oxygen uptake rates over several hours; rates which are significantly lower than those of oxyphilic nematodes. Freshly extracted stilbonematids, with intact bacterial coats, consumed significantly more oxygen than symbiont-free worms in the first 3 h of measurement. While the rates of aposymbiotic worms were more or less constant over time, the rates of symbiont-carrying worms exhibited a conspicuous drop during prolonged respiration. InStilbonema sp., symbiont carrying individuals kept under oxygenated conditions for more than 12 h had a respiration rate similar to those of aposymbiotic specimens. When such worms were re-incubated in sulfide-enriched seawater the respiration rate was significantly elevated. The possibility of “recharging” the oxygenated symbiosis system via sulfide-uptake is seen as an indication that storage of reduced sulfur compounds, or reserve substances synthetized in the presence of sulfide, play a decisive role in the metabolisms of the symbiotic bacteria. Migration of nematodes between sulfidic and oxidized sediment-layers are, most likely, the key to understanding the success of this nematode-bacteria symbiosis.

Land/Inland water ecotones : intermediate habitats critical for conservation and management

In the 20th century the utilisation of natural resources, the emission of pollutants and the degradation of landscape has increased beyond a critical level. Only during the last two decades has the predictive ability of environmental sciences gained strength and become the basis for modern conservation and environmental management procedures. This is especially true for aquatic ecology, where a series of new concepts and theories arose (e.g. the biomanipulation concept Shapiro et. al. (1975); role of land/water ecotones — Naiman et al. (1988); river continuum concept — Van-note et al. (1980); flood pulse concept — Junk et al. (1989)).

The oxygen consumption of a freshwater benthic nematode, Tobrilus gracilis (Bastian)

SummaryTobrilus gracilis (Bastian) was found to live predominantly in oxygen deficient mud layers of Neusiedlersee, Austria. The O2-consumption of all active life stages was measured with Cartesian diver respirometers. The respiration-body weight regression (R=aWb; in μl O2·10-3/individual·h and μg wet weight) was calculated to be R=0.522 W0.693. This suggests that the species is not obligatory anaerobic but can use oxygen when available. Furthermore, a respiration-body weight regression was calculated for four other nematodes from a wide range of habitats using literature data. The regression coefficients ranged from 0.68–0.85 but were not significantly different (P=0.05). The intercept of the y-axis (“a” in the regression equation), which indicates the metabolic rate of a species, ranged from 1.34–2.01 μl·10-3 O2/individual·h at 20°C. Covariance analysis was used to test the differences in the level of metabolic activity for Tobrilus gracilis, Aphelenchus avenae and Panagrolaimus rigidus. It could be shown, that oxygen consumption of Tobrilus is significantly lower compared with the other species and it is concluded that the metabolism of this species is partially anaerobic, even when oxygen is available.