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, biodi- versity and landscape ecology and also through wetland restoration and sustainable management of flood- plains 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. 118 The flood pulse concept: New aspects, 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 These characteristics are reflected for lakes in of the water discharge of the 139 largest river systems the “Seentypenlehre” (Lake typology), elaborated by in North America and Europe is affected by fragmen- Thienemann and Naumann between 1915 and 1935 tation of the river channels by dams and river regula- (e.g. Thienemann 1925; Naumann 1932) and for tion (Dynesius and Nilsson 1994). In recent reviews on streams and rivers in articles by Illies (1961a) and wetlands, demographic trends, economic and political Illies and Botosaneanu (1963) on the differentiation development, demand for hydroelectric energy and into different zones. These zones, described in these water, agriculture and animal ranching, eutrophication early studies as rhithron and potamon with epi-, meta- and pollution, fisheries, logging, recreation and eco- and hypo-subzones, were mainly characterized by abi- tourism and invasion by exotic species have been iden- otic patterns (current, temperature) and by the occur- tified as the most important current determinants for rence of distinct animal and plant communities that the development of rivers and floodplain wetlands depend on a given set of these abiotic patterns. For (Tockner and Stanford 2002; Junk 2002). The global example, epirithral communities are those typical of water crisis and the threat to aquatic organisms, espe- glacier outflows and that depend on low temperatures, cially riverine biota (Dudgeon 2000; Pringle 2001), high oxygen concentrations and fast current. In latitu- increase the necessity to develop models that serve dinal comparisons, Illies (1961b) found evidence for a both science and policy. The flood pulse concept (FPC) generality of this zonation in mountain streams world- (Junk, Bayley and Sparks 1989) was primarily wide - high-elevation streams in the tropics have com- designed as a scientific concept, but it also outlined munities similar to those of low-latitude coldwater some strategies for use, recently specified in Junk et al. streams. (2000). Here, the impact of advances in river ecology on this and other contemporary concepts is critically Later, Vannote et al. (1980) substituted this analyzed. rather static view of river classification with the River THEORETICAL BACKGROUND Continuum Concept (RCC), which introduced a dynamic concept of continuously changing physical HISTORICAL DEVELOPMENT conditions and biological components along the river Limnologists classify inland waters as standing channel, especially regarding the allochthonous and waters (lakes, ponds) and running waters (streams and autochthonous inputs and the processing of organic rivers). Both system types receive allochthonous sub- matter in the flowing water along the river continuum. stances and produce autochthonous organic matter, The RCC predicts major changes in the load and qual- both of which are metabolized and recycled. Standing ity of organic matter and the biota in the stream/river waters, however, are closed systems that store inorgan- channel from the headwaters to the lower river cours- ic and organic matter, circulate organic matter and dis- es. The high allochthonous input from riparian vegeta- solved inorganic substances in characteristic internal tion in the headwaters decreases with increasing chan- cycles in the lake basin and eventually deposit them in nel width (increasing stream order). Autochthonous the sediments. These systems are characterized mainly primary production in the headwaters is low because of by thermal and/or chemical stratification. Running shading by trees (P/R <1), increases in the middle waters are open systems that transport water and dis- reaches because of low water depth and high irradia- solved and suspended matter from the continents to the tion (P/R >1) and decreases again in the lower reaches sea or to endorheic basins. This transport includes because of high water depth and increased turbidity intermediate deposition and re-suspension of sedi- and turbulence (P/R <1). In contrast to the zonation ments in the river channel or in the connected flood- concept, the RCC claims that occurring species are approaches and applications - an update 119 replaced continuously rather than in discrete stages. restrial ecology (Mitsch and Gosselink 2000). The percentage of shredders decreases and the number Floodplains are areas that are periodically inundated of collectors increases with increasing stream order by the lateral overflow of rivers or lakes and/or by because of decreasing input of coarse particulate rainfall or groundwater; the biota responds to the organic material and an increasing amount of fine par- flooding by morphological, anatomical, physiological, ticles owing to the increasing level of processing. phenological and/or ethological adaptations and char- Headwater communities tend to optimize their use of acteristic community structures are formed (Junk et al. allochthonous matter, whereas an organism living in 1989). the lower river reaches largely depend on the ineffi- ciency of organisms living in the upper reaches to Until the 1970s, floodplains were studied sepa- process organic material. The interplay of processing, rately by different disciplines: limnologists studied storage and leakage is predicted to reduce the diversi- floodplain lakes treating them as classical lakes, ecol- ty of organic matter types and the maximization of ogists dealt with the terrestrial fauna and flora and energy utilization (i.e. adaptation to poorly degradable hydrologists investigated water and sediment trans- organic matter) along the river continuum. port. An integrated approach was used by Welcomme (1979), who summarized data on floodplain fishery, The RCC further predicts that biodiversity of limnology and hydrology and coupled fish production aquatic organisms is lower in the headwater regions with the nutrient status of the parent rivers and the and in the lower parts of the rivers and that highest extent of flooding. The consequences of the fluctuating diversity is found in the middle reaches of the streams, water level on fish have also been summarized by where the variability of temperature, riparian influence Lowe-McConnell (1975, 1987). Bayley (1980) pointed and flow are highest and allow numerous different taxa to limits in limnological theory with respect to fish to find their thermal optima. production in river floodplains. Junk (1980) described the multiple land-water interactions of the Central One of the major constraints of the RCC is that Amazon River floodplain, analysed limnological con- it was originally based mostly on results from north- cepts of rivers and lakes, pointed out a gap in limno- ern, temperate, low-order streams with dense tree logical theory and described floodplains as specific canopies and steep gradients that flowed towards ecosystems. more-or-less-regulated rivers in long-term-managed areas. The hydrology of small streams is strongly During the first Large River Symposium in influenced by local rainfall and is rather erratic. Toronto in 1986, the discussion on the applicability of Flooding of small streams occurs only for short periods the RCC to large river-floodplain systems led to the and is often altered by management of rivers in inten- formulation of the Flood Pulse Concept (FPC) (Junk et sively used areas. Therefore, flooding events and al. 1989). This concept focuses on the lateral exchange floodplains received little or no attention in the first of water, nutrients and organisms between the river version of the concept, but were considered later channel (or a lake) and the connected floodplain. It (Minshall 1985; Sedell, Richey and Swanson 1989). considers the importance of the hydrology and hydro- chemistry of the parent river, but focuses on their Floodplains fall into the wetland category, impact on the organisms and the specific processes in which includes ecosystems at the interface of aquatic the floodplain. Periodic