14.4 MULTIPLE USES OF WATER AND THEIR IMPACTS

Changed inundation patterns and implications for fishes / Salinization and acidification
Pesticides and nutrient enrichment / Irrigation, parasites and human health

Water is needed to support the needs of both agriculture and wetlands throughout the basin, from upper watersheds of tributaries to sea level areas of the Mekong Delta. If water and land use decisions are made to meet the needs of agriculture alone, then wetlands potentially will be affected by water removal to supply irrigation needs, elimination or alteration by drainage and conversion to agriculture, and changes in water quality. If such decisions are made to meet the needs of wetlands alone, then improvements to income and living conditions in agricultural areas may be constrained.


Loss/transformation of wetlands and consequences for biodiversity--
Conversion of wetlands to agricultural land can result in the elimination of the wetland ecosystem and species that depend upon it. Human activity and noise that results from increased agricultural and other activity in areas near wetlands can create disturbances that transform the wetland so that the sensitive species no longer use it. Similarly, the wetland ecosystem may be transformed by physical changes that result from alteration to the wetland hydrology, and chemical changes that result from the presence of pesticides, fertilizers, silt and other unwanted compounds. Loss or transformation of the wetland will likely mean that the area is no longer suitable for some species and they will cease to use it. The loss of species will reduce biodiversity, and if some species are already exposed to greatly reduced habitat and numbers, will contribute to species extinction on a regional and possibly global scale.


Changed inundation patterns and implications for fishes--
Fish use the annual inundation of floodplains along the Mekong River and its tributaries for spawning and rearing, and have evolved to match critical stages of their life history to the timing and magnitude of seasonal floods. Actions that affect the annual hydrologic events, such as upstream irrigation, hydroelectric or flood control works, could disrupt fish life stage events causing reductions in abundance of some species.


Salinization and Acidification--
Salinity in parts of the Mekong River and its floodplain and tributaries can be altered in several ways. Currently, saline water from the sea extends tens of kilometers into delta areas of Vietnam, usually reaching maximum penetration during the dry season when flow in the river is at its minimum. A further reduction in flow resulting from upstream activity will likely cause a corresponding increase in the extent of penetration of salt water into the delta. Similarly, an alteration in the seasonal pattern of flow likely will cause a change in the timing of saltwater intrusion into the delta. Soils along the edge of the delta possess a gradation of salinities with salinity being relatively high in some areas adjacent to the coast with salinities becoming progressively lower at increasing distances from the coast. These patterns also could change in relation to long-term changes in the amount or timing of freshwater flow in the Mekong River.

Some soils, particularly in the delta area, have high levels of acidic sulphate and, when disturbed, have potential to increase the acidity of surrounding and downstream waters. Such a change in water conditions can seriously affect species that are not able to tolerate relatively high acidity. This problem often occurs in former or existing mangrove areas when surface vegetation are removed for agriculture or aquaculture ponds.


Pesticides and nutrient enrichment--
Pesticides that are commonly used in agriculture to control insects and other pests usually go beyond the place where they are intended for use and enter receiving waters where they can cause ecological damage. This can lead to serious damage if they enter small and fragile ecosystems such as wetlands or come into contact with sensitive species that are already few in number.

Nutrients can increase in river and wetland areas as a result of fertilizers commonly used in agriculture and from other sources such as domestic waste. Although nutrients are often not toxic, they can alter the ecological status of rivers and wetlands through a process called nutrient enrichment. This process may be beneficial for some species, but can be detrimental for others. Nutrient enrichment can lead to high abundance of pollution-tolerant plant and animal species, often leading to increased production of hardy species of fish. However, it also often causes a reduction in species diversity as more sensitive species avoid, or in some cases are killed off, due to the nutrient enrichment. This can occur when high nutrient concentrations cause reductions in oxygen levels.


Irrigation, parasites and human health--
Irrigation works can create environments that are suitable for proliferation of parasites, by creating seasonal water at locations and times it did not previously occur. New water areas may provide habitat for disease-carrying organisms (such as mosquitoes and snails) to complete their life cycle. The consequence can be an increase in parasite-related diseases among humans in the surrounding area.