(P 117-140) Flood Pulse.Qxp

117 THE FLOOD PULSE CONCEPT: NEW ASPECTS, APPROACHES AND APPLICATIONS - AN UPDATE Junk W.J. Wantzen K.M. Max-Planck-Institute for Limnology, Working Group Tropical Ecology, P.O. Box 165, 24302 Plön, Germany E-mail: [email protected] ABSTRACT The flood pulse concept (FPC), published in 1989, was based on the scientific experience of the authors and published data worldwide. Since then, knowledge on floodplains has increased considerably, creating a large database for testing the predictions of the concept. The FPC has proved to be an integrative approach for studying highly diverse and complex ecological processes in river-floodplain systems; however, the concept has been modified, extended and restricted by several authors. Major advances have been achieved through detailed studies on the effects of hydrology and hydrochemistry, climate, paleoclimate, biogeography, biodiversity and landscape ecology and also through wetland restoration and sustainable management of floodplains in different latitudes and continents. Discussions on floodplain ecology and management are greatly influenced by data obtained on flow pulses and connectivity, the Riverine Productivity Model and the Multiple Use Concept. This paper summarizes the predictions of the FPC, evaluates their value in the light of recent data and new concepts and discusses further developments in floodplain theory. <ul style="display: flex;"><li style="flex:1">118 </li><li style="flex:1">The flood pulse concept: New aspects, </li></ul>INTRODUCTION plain, where production and degradation of organic matter also takes place. Rivers and floodplain wetlands are among the most threatened ecosystems. For example, 77 percent of the water discharge of the 139 largest river systems in North America and Europe is affected by fragmentation of the river channels by dams and river regulation (Dynesius and Nilsson 1994). In recent reviews on wetlands, demographic trends, economic and political development, demand for hydroelectric energy and water, agriculture and animal ranching, eutrophication and pollution, fisheries, logging, recreation and ecotourism and invasion by exotic species have been identified as the most important current determinants for the development of rivers and floodplain wetlands (Tockner and Stanford 2002; Junk 2002). The global water crisis and the threat to aquatic organisms, especially riverine biota (Dudgeon 2000; Pringle 2001), increase the necessity to develop models that serve both science and policy. The flood pulse concept (FPC) (Junk, Bayley and Sparks 1989) was primarily designed as a scientific concept, but it also outlined some strategies for use, recently specified in Junk et al. (2000). Here, the impact of advances in river ecology on this and other contemporary concepts is critically analyzed. These characteristics are reflected for lakes in the “Seentypenlehre” (Lake typology), elaborated by Thienemann and Naumann between 1915 and 1935 (e.g. Thienemann 1925; Naumann 1932) and for streams and rivers in articles by Illies (1961a) and Illies and Botosaneanu (1963) on the differentiation into different zones. These zones, described in these early studies as rhithron and potamon with epi-, metaand hypo-subzones, were mainly characterized by abiotic patterns (current, temperature) and by the occurrence of distinct animal and plant communities that depend on a given set of these abiotic patterns. For example, epirithral communities are those typical of glacier outflows and that depend on low temperatures, high oxygen concentrations and fast current. In latitudinal comparisons, Illies (1961b) found evidence for a generality of this zonation in mountain streams worldwide - high-elevation streams in the tropics have communities similar to those of low-latitude coldwater streams. Later, Vannote et al. (1980) substituted this rather static view of river classification with the River Continuum Concept (RCC), which introduced a dynamic concept of continuously changing physical conditions and biological components along the river channel, especially regarding the allochthonous and autochthonous inputs and the processing of organic matter in the flowing water along the river continuum. The RCC predicts major changes in the load and quality of organic matter and the biota in the stream/river channel from the headwaters to the lower river courses. The high allochthonous input from riparian vegetation in the headwaters decreases with increasing channel width (increasing stream order). Autochthonous primary production in the headwaters is low because of shading by trees (P/R <1), increases in the middle reaches because of low water depth and high irradiation (P/R >1) and decreases again in the lower reaches because of high water depth and increased turbidity and turbulence (P/R <1). In contrast to the zonation concept, the RCC claims that occurring species are THEORETICAL BACKGROUND HISTORICAL DEVELOPMENT Limnologists classify inland waters as standing waters (lakes, ponds) and running waters (streams and rivers). Both system types receive allochthonous substances and produce autochthonous organic matter, both of which are metabolized and recycled. Standing waters, however, are closed systems that store inorganic and organic matter, circulate organic matter and dissolved inorganic substances in characteristic internal cycles in the lake basin and eventually deposit them in the sediments. These systems are characterized mainly by thermal and/or chemical stratification. Running waters are open systems that transport water and dissolved and suspended matter from the continents to the sea or to endorheic basins. This transport includes intermediate deposition and re-suspension of sediments in the river channel or in the connected flood- <ul style="display: flex;"><li style="flex:1">approaches and applications - an update </li><li style="flex:1">119 </li></ul>replaced continuously rather than in discrete stages. The percentage of shredders decreases and the number of collectors increases with increasing stream order because of decreasing input of coarse particulate organic material and an increasing amount of fine particles owing to the increasing level of processing. Headwater communities tend to optimize their use of allochthonous matter, whereas an organism living in the lower river reaches largely depend on the inefficiency of organisms living in the upper reaches to process organic material. The interplay of processing, storage and leakage is predicted to reduce the diversity of organic matter types and the maximization of energy utilization (i.e. adaptation to poorly degradable organic matter) along the river continuum. restrial ecology (Mitsch and Gosselink 2000). Floodplains are areas that are periodically inundated by the lateral overflow of rivers or lakes and/or by rainfall or groundwater; the biota responds to the flooding by morphological, anatomical, physiological, phenological and/or ethological adaptations and characteristic community structures are formed (Junk et al. 1989). Until the 1970s, floodplains were studied separately by different disciplines: limnologists studied floodplain lakes treating them as classical lakes, ecologists dealt with the terrestrial fauna and flora and hydrologists investigated water and sediment transport. An integrated approach was used by Welcomme (1979), who summarized data on floodplain fishery, limnology and hydrology and coupled fish production with the nutrient status of the parent rivers and the extent of flooding. The consequences of the fluctuating water level on fish have also been summarized by Lowe-McConnell (1975, 1987). Bayley (1980) pointed to limits in limnological theory with respect to fish production in river floodplains. Junk (1980) described the multiple land-water interactions of the Central Amazon River floodplain, analysed limnological concepts of rivers and lakes, pointed out a gap in limnological theory and described floodplains as specific ecosystems. The RCC further predicts that biodiversity of aquatic organisms is lower in the headwater regions and in the lower parts of the rivers and that highest diversity is found in the middle reaches of the streams, where the variability of temperature, riparian influence and flow are highest and allow numerous different taxa to find their thermal optima. One of the major constraints of the RCC is that it was originally based mostly on results from northern, temperate, low-order streams with dense tree canopies and steep gradients that flowed towards more-or-less-regulated rivers in long-term-managed areas. The hydrology of small streams is strongly influenced by local rainfall and is rather erratic. Flooding of small streams occurs only for short periods and is often altered by management of rivers in intensively used areas. Therefore, flooding events and floodplains received little or no attention in the first version of the concept, but were considered later (Minshall 1985; Sedell, Richey and Swanson 1989). During the first Large River Symposium in Toronto in 1986, the discussion on the applicability of the RCC to large river-floodplain systems led to the formulation of the Flood Pulse Concept (FPC) (Junk et al. 1989). This concept focuses on the lateral exchange of water, nutrients and organisms between the river channel (or a lake) and the connected floodplain. It considers the importance of the hydrology and hydrochemistry of the parent river, but focuses on their impact on the organisms and the specific processes in the floodplain. Periodic inundation and drought (flood pulse) is the driving force in the river-floodplain system. The floodplain is considered as an integral part of the system that is periodically coupled and decoupled from the parent river by the aquatic/terrestrial transition zone (ATTZ). The flood pulse can be monomodal Floodplains fall into the wetland category, which includes ecosystems at the interface of aquatic and terrestrial ecosystems and are therefore often called ecotones. However, large wetlands have to be considered as specific ecosystems with unique properties not adequately covered by present ecological paradigms and by limnology, estuarine ecology and ter- <ul style="display: flex;"><li style="flex:1">120 </li><li style="flex:1">The flood pulse concept: New aspects, </li></ul>FURTHER CONCEPTS IN RUNNING-WATER ECOLOGY or polymodal, predictable or unpredictable and with a high or low amplitude. Predictable pulsing favors the adaptation of organisms and increases primary production and efficiency of nutrient use. Various conceptual approaches independent of (or complementary to) the RCC and FPC were developed in the 1980s and 1990s on lotic ecosystem structure and functioning. Many early seminal papers dealt with the distribution of organisms within the lotic systems. A key aspect was the description of hyporheic zones in which important ecosystem processes occur. Early work by Schwoerbel (1961) on the distribution of benthic and stygal fauna in bed sediments was extended by the description of aquatic organisms far from the river channel area (Stanford and Ward 1988) and factors contributing to vertical distribution of the organisms (Bretschko and Leichtfried 1988) and by the understanding of organic matter dynamics in this zone (e.g. Williams 1989; Triska, Duff and Avanzino 1993). Ward (1989) included the function and occurrence of hyporheic zones in general stream theory by describing streams as four-dimensional systems. The FPC predicts that the nutrient status of the floodplain depends on the amount and quality of dissolved and suspended solids of the parent river; however, it includes the premise that internal processes of the floodplain and nutrient transfer mechanisms between the terrestrial and the aquatic phase strongly influence nutrient cycles, primary and secondary production and decomposition. At the same time, flooding is considered as a disturbance factor that leads to a regular setback of community development and maintains the system in an immature, but highly productive stage. Another tenet of the FPC is that in the riverfloodplain system, a large part of the primary and secondary production occurs in the floodplain, whereas the river is mainly the transport vehicle for water and dissolved and suspended matter. The river is also the refuge for aquatic organisms during low-water periods and serves as a route for active and passive dispersal. The “highway analogy” describing the river channel as a transport and migration corridor was used to visualize the different functions of the main river channel and its floodplain (Junk et al. 1989). The FPC was based on the personal experiences of the authors on the Amazon and Mississippi rivers but also on a vast literature about other river systems. Therefore, the concept was not restricted to large tropical rivers, as is sometimes cited (e.g. Benke et al. 2000), but was conceived as a general concept for large river-floodplain systems. The importance of stream hydraulics (Statzner and Higler 1986) and of disturbance (Resh et al. 1988) for the distribution for benthic organisms has shown that variation in water flow caused by climate and geomorphology can set limits to the generalizations of the RCC since flow conditions typical of upstream and downstream areas can change several times along the river course (“discontinuum”, Poole 2002). Consequently, the template provided by the habitat conditions (Southwood 1977) and its alignment with species traits (Townsend and Hildrew 1994; Resh et al. 1994) might be more important for the occurrence of a species than the position of the given site along the continuum. Since geomorphology is subject to noncontinuous local variations, the distribution of stream habitats appears as a mosaic (Pringle et al. 1988) of hierarchically ordinated and dynamic patches (Townsend 1989; Poole 2002). Various hierarchical concepts have been developed for riverine landscape patterns and their scale-dependent processes (e.g. Frissell et al. 1986; Townsend 1996; Petts and Amoros 1996; Poff 1997; Montgomery 1999; Ward, Malard and Tockner. 2001; Poole 2002). Mostly limnologists, ichthyologists and fisheries biologists study the ecology of floodplains and their organisms. They test the predictions of the FPC only for the aquatic phases of the system. However, it has to be stressed that the FPC covers the river-floodplain-system during the entire year and that its predictions are also valid for the terrestrial phases that are an integral part of the river-floodplain system. <ul style="display: flex;"><li style="flex:1">approaches and applications - an update </li><li style="flex:1">121 </li></ul>Further conceptual approaches have dealt with the production and processing of organic matter, such as the nutrient spiraling concept (Elwood et al. 1983; Pinay et al. 1999), or with human impacts, e.g. the interruption of natural flow pathways by dams [the Serial Discontinuity Concept (Ward and Stanford 1983a; Ward and Stanford 1995)]. As an alternative to the RCC, Montgomery (1999) proposed a multi-scale hypothesis in which spatial variability in geomorphic processes governs temporal patterns of disturbances that influence ecosystem structure and dynamics (Concept of Process Domains). Channel networks can be divided into discrete regions in which community structure and dynamics respond to distinctly different disturbance regimes. according to their hydrological signature (Richter, Baumgartener, Powell et al. 1996; Puckridge et al. 1998). Providing more detail on the type of flow variation, Puckridge et al. (1998) have stressed the general importance of water level variations even below the bankfull stage (flow pulses), which might have significant influence on the habitat size and characteristics. Irregular flood events, especially in streams (e.g. Winterbourn, Rounik and Cowie 1981) and arid zone rivers, have selected for resilient strategies of organisms to survive these events (Poff and Ward 1989; Lytle 2001) rather than to adaptations to profit from them (Junk et al. 1989). However, this view has recently been modified because in seasonal climates, the period of flash flood events can be predicted and because flash-flood events mobilize organic matter resources from stream wetlands (Wantzen and Junk 2000). EXTENSIONS OF THE FPC HYDROLOGY AND FLOW CHARACTERISTICS Tockner et al. (2000) extended the FPC by considering that flooding resets temperature diversity in isolated aquatic floodplain habitats. Thus, aquatic habitats within floodplains might have a much broader temperature range than the river itself, especially in rivers with wide and diversified beds, such as the alpine Fiume Tagliamento, where flow pulses occur frequently. The FPC was developed based upon data and long-term observations of neotropical (Amazon) and temperate zone (Mississippi) rivers. It provided a general outline and strengthened the premise that rivers and their floodplains have to be considered as one unit and therefore cannot be treated separately in ecological studies. The FPC has stimulated various studies on river-floodplain ecosystems in, e.g. Lower Rhine floodplain lakes (van den Brink et al. 1994), Missouri floodplain lakes (Knowlton and Jones 1997), the Danube (Tockner, Malard and Ward. 2000), the Murray-Darling (Humphries, King and Koehn 1999) and the Mississippi River (Sparks, Bayley and Kohlert 1990). Various studies in which the FPC has been applied and its tenets tested have lead to proposals for supplementation to the original conceptual framework. Unpredictable flooding and decoupling of the flood pulse from the temperature pulse leads to low temperatures during floods and high temperatures that trigger spawning of some fish species during low water level in some parts of the Murray Darling River system in Australia (Humphries et al. 1999, Low Flow Recruitment Hypothesis). The fact that water level changes influence riverine systems four-dimensionally in space and time (Ward 1989) is important. Rising water levels not only increase the wetted surface of the channel and eventually of the floodplain, but at the same time influence the exchange between groundwater and surface water either by allowing an up-welling of groundwater or by forcing a down-welling of the surface water into the aquifer vertically and laterally. The hyporheic zone serves as an interface between groundwater and surface water (Schwoerbel 1961). Similarly, floodplains Increasing knowledge on the hydrological characteristics of rivers has contributed much towards the understanding of their ecological processes. The hydrographs of individual rivers are influenced by a series of partly interacting factors, such as climate, gradients, landscape morphology, floodplain buffering and human impacts, which together cause very complex patterns. Several authors have identified different measures for identifying the hydrological variability of rivers and have provided tools for classifying rivers <ul style="display: flex;"><li style="flex:1">122 </li><li style="flex:1">The flood pulse concept: New aspects, </li></ul>act as interfaces for the interchange between the river mainstreams and their tributaries or surface runoff from rainwater. The flow direction of the interfaces is influenced by the fourth dimension, time, such that the recent and the past hydrological situations become decisive: elevated water levels can cause blocking or even backflow of the tributaries and groundwater outflows. In the northern Pantanal wetland, frequent changes in the flow direction occur in floodplain channels that connect water bodies that receive rain and river water, depending on the respective water level (Wantzen and Da Silva, unpublished data). In the southern areas of the Pantanal, high levels of river water block the tributaries after the rainy season; therefore, a large part of the inundation occurs after cessation of the rainfall (Hamilton, Sippel and Melack 1996). growth and where conditions for production in the floodplain are restricted by turbidity, timing of inundation during the winter and river regulation (e.g. of the Ohio River), the in-channel primary production can be substantially higher than floodplain production. River channels can support diverse and productive fish communities under these conditions (Galat and Zweimuller 2001; Dettmers et al. 2001); however, this is not the case in very large and turbid lowland rivers with a sandy, permanently moving bed load. In those rivers that show a predictable, sufficiently long and timely inundation, such as the Mississippi River, the exploitation of floodplain resources provides a “flood pulse advantage” Gutreuter et al. (1999) for floodplain fishes compared to pure riverine species (Bayley 1991). Stable isotope ratios of many floodplain fish species of the Pantanal wetland show seasonal variation, which indicates a large variability in carbon sources and the trophic level between seasons (Wantzen et al. 2002) (Figure. 1). ALLOCHTHONOUS AND AUTOCHTHONOUS PRODUCTION AND NUTRIENTS The FPC has focused on the productivity within the floodplain areas, in contrast to the RCC, which has focused on the import of more-or-less processed allochthonous matter from the upriver sections. Both concepts have been criticized by Thorp and Delong (1994) in the Riverine Productivity Model (RPM), which predicts that autochthonous production in the river channel and allochthonous inputs in the lower reaches provide a substantial portion of the organic carbon used by river animals. While analyzing the potential influence of the floodplain on the carbon budget in a channel site and a floodplain region of the Ohio River, Thorp et al. (1998) did not find significant differences in the isotopic C and N signatures, which indicated a low floodplain contribution during a shorttermed, unpredictable flood event at low water temperatures in wintertime. This, however, does not necessarily contradict the predictions of the FPC. Figure 1. Seasonal isotopic shifts in small omnivore floodplain fish species in the Pantanal wetland, Brazil. Filled circles: rainy season values; open triangles: dry season values. There is a general increase in d15N values from the wet to the dry season, which indicates more omnivorous feeding when the wetland is flooded and elevated carnivory during the dry season when the lake becomes confined to its basin (Modified after Wantzen et al. 2002, with permission). Depending on temperature, light, nutrients and substrate conditions, river channels can show a considerable autochthonous primary production, which fuels the riverine food web as shown for Rhine River (Friedrich and Mueller 1984). Especially in those rivers where these conditions are beneficial for algal

(P 117-140) Flood Pulse.Qxp

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