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United Nations Environment Programme
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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


8. THE REGULATION AND CONTROL OF HYDROLOGIC AND BIOTIC PROCESSESS WITHIN RESERVOIRS FOR WATER QUALITY IMPROVEMENT

A. Introduction

From the ecohydrological point of view, a large dam within a river basin can be considered as the technical system modifying three of the major components of river basin integrity; namely:

  • river continuum processes, by changing the pattern of mineral, organic matter, and nutrient spiralling downstream;
  • floods pulses, by interrupting the passage of flood peaks downstream of the dam, thereby modifying environmental maintenance processes such as ecological succession, biodiversity, biological productivity of terrestrial and aquatic ecosystems, and sediment loads; and,
  • retention rates, by trapping minerals, organic matter, and nutrients, increasing the vulnerability of reservoir ecosystems by accumulating nutrients and pollutants from large river basins, thereby modifying nutrients and energy dynamics within the impounded stretch of river.

In the upper sections of free flowing rivers, nutrients are transfer downstream in a trophic spiral (Webster 1979, Newbold et al. 1981). The processes that create this trophic spiral, and govern its length and breadth, are dependent on allochthonous inputs, hydrology, and energy. In the floodplains of larger rivers, the periodic pulse of the hydrological cycle depends on the connectivity of the river channel and the floodplain, and, especially, of the ecosystems situated along the river floodplain that are highly dependent upon seasonal fluxes of dissolved organic matter and nutrients (Agostinho and Zalewski 1995).

Hypothetical model to predict the effects of large dams on river basin environments

As a consequence of recent progress in understanding the functioning of ecosystems, the inclusion of ecological theories in the resolution of environmental dilemmas is now a practicable alternative, and one that is being increasingly applied worldwide

Rivers, throughout the different regions of the globe, are one of the most diverse and dynamic components of the ecosystem. Ward and Stanford (1983), in the "intermediate disturbance hypothesis" noted the role of disturbances of varying intensities in shaping riverine biotic communities. This hypothesis has the potential to be used as a tool for predicting the positive or negative effects of large dams on biodiversity and bioproductivity within the basin landscape. According to this hypothesis, the highest productivity and biodiversity should appear within ecosystems where disturbances (hydraulic variability) occur at an intermediate level. Because reservoirs act to stabilise flooding patterns and reduce the downstream transport of organic matter, nutrients, and pollutants, dams in hydrologically variable environments, such as the Syrian semi-desert, will limit the occurrence of catastrophic floods and by retaining water and gradually releasing it. This gradual discharge should enhance instream and river valley biological productivity and downstream diversity (Figure 8.1). In contrast, dams situated within tropical floodplain systems, inundating areas of low slope, not only fail to generate significant amounts of electric energy, but, instead, by flooding large areas covered with high levels of plant biomass, create serious problems by enhancing methane emissions and changing the character of these highly diverse areas.

Fig. 8.1. The link between ecological theory (intermediate disturbance concept) and the optimisation of dam usage according to ecohydrological principles (lager image)

Ecohydrology as a tool for the restoration of eutrophic reservoirs

Freshwater management strategies for dams have been focused, up to now, on issues such as flood protection, drought relief, and energy generation. However, degraded water quality in reservoirs as a consequence of catchment development is an emerging problem. The problem is linked to increasing nutrient loads to these lakes from their watersheds, and exacerbated by nutrient retention within the reservoir and subsequent recirculation by the biota. The worst of these impacts is toxic cyanobacterial blooms. Their carcinogenic hepatotoxins are one of the most dangerous substances generated by algae in eutrophic reservoirs, according to Cood (2000).

Notwithstanding, every successful strategy should possess a second important component, that of amplifying opportunities for sustainable use and economic development (see Figure 1.3). This element of environmental management has usually been neglected. Thus, one of the important emerging opportunities in environmental management is establishing a resistance and resilience to stress among freshwater ecosystems by utilising natural ecosystem-level processes. These processes are dependent upon hydrological processes at the basin scale. Understanding this relationship begins with the integration of three dynamic components: the catchment, the water, and the biota. Collectively, these components form a "Platonian superorganism" Ecohydrology, which has been defined as the science of integrating hydrological processes with biological systems over varied spatial and temporal scales, can be used to create an holistic model of river systems at the basin scale. This level of integration, analogous to the "Platonian superorganism" implies management targets that include the maintenance of sustainability, as measured by biodiversity, water quality, and water quantity.

The key to implementing this concept of conservation and restoration of freshwater ecosystems is ecosystem biotechnologies, which are focused on the conversion and control of nutrient circulation at the ecosystem and landscape levels. The regulation of biological, biogeochemical, and hydrological processes are essential elements in the retention, transformation, self-purification, attenuation, elimination, sedimentation, dislocation, biofiltration, and recirculation of nutrients and energy (Figure 1.10).

The ecohydrological approach has been recently applied in a programme of restoration of an eutrophic lowland reservoir, which, despite toxic cyanobacterial blooms, has been supplying drinking water for a population of about 1 million people (Zalewski 1999). The annual phosphorus load, mostly from non-point source pollution, exceeds 8 g m-2 year-1. To achieve a mesotrophic state, with a transparency of 2.5 m, and to eliminate toxic algal blooms, it has been determined that the phosphorus load should be reduced to less than 1 g m-2 year-1. Such a reduction could be achieved by integrating classical technical methods of nutrient management with landscape ecology and recent advances in biogeochemistry and phytotechnology at the basin scale.

Classical restoration measures consisted of upgrading the sewage treatment plants in the catchment, transferring the purified sewage effluent from the upstream catchment to a discharge point below the reservoir, and dredging to minimise internal phosphorus loading from contaminated sediments. In addition, phytotechnological methods were applied, including the restoration of ecotones, the regulation of flow regimes to modify the pattern of nutrient supply and loading into reservoir, the restoration of the river channel to enhance self-purification and nutrient retention, and the conversion of the upper part of reservoir into wetland to enhance sedimentation and nutrient trapping. A third step was the control of biotic processes, such as enhancing zooplankton populations, regulating water levels in the reservoir to promote reproduction of predatory fishes, enhancing filtration by molluscs, and creating artificial reed beds in the littoral zone to stabilise and regulate the land and water biotic interactions.

These processes were quantitatively evaluated to document their effectiveness in controlling eutrophication, and the results were integrated into an holistic model for lake restoration. This model was utilised in a programme of adaptive management (Holling et al. 1994), which was implemented in consideration of the scale of the processes, and the potential antagonistic and synergistic effects between the steps enumerated above. In this programme, ecohydrology provided the scientific core, by utilising ecosystem properties in the management of freshwater the ecosystem for human use and mitigation of human impacts (Zalewski 2000). This implementation programme was based upon three criteria. It had to be environmentally sound, economically feasible, and socially acceptable.

 

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