The health of an estuary ecosystem is determined by the dominant physical, chemical, and biological processes, sketched in Figure 1, within the estuary, as well as the natural characteristics of, and the human activities conducted within the entire river catchment upstream. The robustness of an estuary depends on the rate at which water is flushed; the longer the residence time, the greater the water quality problems. Ecological integrity also depends on the rate at which fine sediments are sequestrated in the estuary (primarily in the tidal wetlands) or flushed out to sea, as well as the efficiency with which organic matter is processed within the water column. This commonly occurs through the bacterial loop sketched in Figure 1. There are other important relationships between the estuary and its tidal wetlands. The tidal wetlands export or import different particulate and dissolved nutrients, and they are also nursery grounds for fish and invertebrates.
Human activities within the river catchment are a major driving force affecting the health of the estuary, mainly as a result of excess nutrients and sediment, a change in natural river flows from human activities, as well as land clearing and overgrazing that increase soil erosion. Such activities modify the natural flows (e.g., dams), and increase the riverine nutrient load (e.g., sewage discharge, animal waste from agribusiness such as pig farms and cattle feedlots, and fertilizers leaching from farms). All of these activities and processes also degrade the tidal wetlands because of the exchange of water and mass between the estuary and the tidal wetlands. The pressure on tidal wetlands is further increased by dredging, land reclamation for industry, and urbanization. For instance, nearly all estuarine marshes have been 'reclaimed' in the Netherlands and in Japan.
When an estuary has been degraded, its restoration cannot be successful in the long term without addressing the issues that led to its degradation. If there are several such issues, then they must all be addressed, though possibly at a different intensity, because impacts are cumulative. Restoration efforts must focus on the whole ecosystem, principally (1) managing human activities in the whole river catchment, and (2) restoring habitats to arrive at an estuarine ecosystem that is able to absorb human stresses. Managing human activities necessitates maintaining river flows, principally minimum environmental flows and controlling the timing of river floods, and limiting the riverine export to the estuary of sediment, pollutants and nutrients, so as to enable the estuarine ecosystem to function naturally. Because of the feedbacks between the estuary and its fringing wetlands, restoring estuarine habitats is essential to restoring the ecosystem health. It is also a science-based technology that is still under development.
Figure 1 Habitat distribution in a typical estuary and sketch of the feedbacks between physical, chemical, and biological processes that control the health of the ecosystem. Saline wetlands can be mangroves and/or saltmarshes. Coral reefs can exist within the estuary nearthe river mouth and in coastal waters. Seagrass usually occur between the wetlands and the coral reefs.
In practice, this integrative management policy is not properly implemented anywhere worldwide. Nowhere in the world is there an effective mechanism enabling cooperation between water-resource managers dealing with hydrology, water supply, and hydroelectricity, city councils dealing with urbanization and waste disposal, fisheries managers dealing with commercial and recreational fisheries, and land-use managers dealing with industrial and agricultural developments within the whole river catchment. Management is still largely dictated by politics. Thus estuarine restoration is still a science-based engineering practice in its infancy. There are attempts to restore estuaries based on river catchment management as well as local estuarine habitat restoration efforts, such as for the Mersey and Thames rivers in the UK, the Rhine River in Europe, and the Chesapeake Bay in the USA, however all of these attempts have been hampered by politics. As a result, at best only a fraction of the ecological function of these estuaries has been restored. For instance macrophytes - and the biological communities of shrimp and fish that they support - cannot be restored in estuaries as long as activities in the catchment generate turbid waters.
In what follows we will describe the present techniques to restore habitats, mainly tidal wetlands, seagrass, and coral reefs.
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