Impact of wetland drainage on the environment

Riparian and floodplain wetlands are ecologically important parts of the Earth’s landscape. By virtue of their low-lying position in the landscape, they receive hydrologic and nutrient inputs from adjacent upland areas. Some of the environmental consequences of wetland drainage are:
1. Subsidence of Wetland Soils
Accumulation of the organic soils characteristically associated with wetlands can take hundreds or even thousands of years. The stability of these soils is highly dependent on hydrology as their formation is due to the historically extended hydroperiod. In some regions, wetland hydrology has been dramatically altered, resulting in substantial loss of organic soils (histosols). Histosols are naturally productive and contain large amounts of organic carbon, nitrogen, sulfur, and phosphorus. These soils are naturally poorly drained and have high water-holding capacities. Wetlands are often drained for conversion to other land uses, and the drainage water pumped into adjacent wetlands and aquatic systems.
In many areas, organic soils that formed as wetlands have been drained for agricultural use. A commonly occurring result of drainage of organic soils is a decrease in soil depth, commonly referred to as subsidence or oxidation.
2. Environmental Effects of Wetlands Drainage
Water-level drawdown or drainage of wetlands can produce major changes in soil physical, chemical, and biological properties. Organic soils in wetlands developed under flooded conditions where organic
matter accumulation exceeded its decomposition. Deposition and accumulation of plant material over time resulted in increases of soil depth. Maintenance of these organic soils then depends on the continuation of processes that led to its development.
3. Nutrient stability
Drainage of flooded soils has a significant impact on diffusion of oxygen and other gases in soil, which regulates nutrient stability and mobility. Drainage forces the soil to change from an anoxic (anaerobic) to oxic (aerobic) system. Most chemical processes then increase in rate, including oxygen diffusion into soil and changes in redox chemistry, which enhance rates of nutrient cycling and microbial activity. Drainage exposes more volume of the soil to oxygen and alters the conditions which led to development of wetland soils. Following drainage, oxygen is rapidly consumed and resupplied by the atmosphere, leading to more rapid chemical changes than those occurring under flooded conditions. Results such as increased decomposition of organic matter, greater nutrient mineralization, greater carbon dioxide production and emission, and altered nutrient retention and release are the effects of water withdrawals and the subsequent increase in exposure to oxygen.
4. Changes in Vegetation and Microorganisms
Since microbial activity is greater in aerobic than anaerobic soils, water withdrawals enhance microbial activity, which stimulates organic matter decomposition and nutrient generation. The major effect of drainage in the ecosystem occurs belowground, where organic matter decomposition by heterotrophic microorganisms is significantly greater under drained than flooded conditions. Thus, wetlands that sequestered C would release C back to the atmosphere upon drainage. Changes from flooded to drained conditions in soils stimulate changes from aquatic to terrestrial vegetation and decrease
the algal contribution to total primary production. Drainage of organic soils then has the effect of removing a source of net primary production (algae) and carbon sequestration.
5. Nutrients
As a consequence of soil oxidation, nutrients contained within organic matter are released. This in turn functions to provide microorganisms with a nutrient supply to continue further oxidation. Water withdrawals from wetlands stimulate organic matter decomposition in areas because drainage creates a larger aerobic zone in soil. These areas in wetlands exhibit enhanced decomposition and release metals and nutrients contained within organic matter, as well as increase dissolved organic matter concentrations. The ultimate fate of these mineralized components will depend on vegetation and environmental conditions. Some nutrients can be taken up by wetland vegetation. However, nutrients in wetland floodwater are mobile and may eventually be exported to rivers or lakes. When this happens, nutrient concentrations increase and microbial activity is stimulated, possibly leading to undesirable consequences such as eutrophication, algal blooms, and fish dieoffs.

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Frederick paul answered the question on February 11, 2019 at 13:59