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Abstract:

The formulation of the flood pulse paradigm by Junk et al. in 1989 formalised a shift in conceptual thinking about the ecology and dynamics of rivers and lakes with floodplains and recurrent flooding events. The integration of processes of the ‘Aquatic/Terrestrial Transition Zone’ with those of the river and/or lake provides a comprehensive framework to describe the processes and dynamics of rivers, lakes and their floodplains.A flood pulse is determined by a large number of characteristics of the flooding, including height, duration, number of peaks etc. Each of these features has implications for the organisms of the ecosystem, whose ability to survive or thrive depends on their tolerance or adaptations to the extreme conditions between which parts of the ecosystem oscillate. Typically, the exchange of energy and nutrients between the aquatic and terrestrial phases means that flood-pulsed ecosystems are highly productive.Effective conservation and durable management of flood-pulsed ecosystems requires explicit consideration of the flood pulse. The Tonle Sap lake in Cambodia is connected to the Mekong river through the Tonle Sap river, and its annual massive flood cycle has been known and exploited for many centuries. Monsoon-driven swelling of the Mekong reverses flow in the Tonle Sap river and pushes vast amounts of water and sediments up into the lake and floodplain, causing 5-month long flooding. The Tonle Sap ecosystem is highly productive, supporting the livelihoods of at least one million, mostly poor people. Over 6 million people live in the immediate vicinity of its floodplain. It is in many respects unique on Earth, and its biodiversity importance is globally acknowledged. Prior to the undertaking of this research, the knowledge of biological and physical characteristics of the Tonle Sap ecosystem was scant. The ecosystem is under imminent threat from the construction of hydropower dams on the Mekong, which will alter the hydrological cycle and the flood pulse. The flood pulse character of the Tonle Sap was confirmed by own data on limnology as well as by the dynamics and ecology of fish populations. Seasonal and diurnal changes in water quality consistent with the processes of a flood pulse were demonstrated in the lake and in the floodplain. Corresponding changes in the abundance and distribution of fishes were found, together with indications of adaptations to the timing and opportunities of the flood pulse. The historic and current indirect evidence of the ecosystem’s high productivity was examined, and a practical indicator of ecosystem productivity proposed. An innovative four-dimensional combined ecological-hydrodynamic model was developed, allowing to predict the production potential of the Tonle Sap ecosystem and its likely response to hydrological changes. Finally, an analysis was made of how the flood pulse determines how biodiversity conservation must be practiced in the Tonle Sap ecosystem.