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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
Freshwater Management Series No. 5

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


E. Nutrient trapping using wetland vegetation

The biologically available pool of suspended matter (e.g., simple, easily-degradable organic compounds), as well as the dissolved forms of mineral nutrients, may be assimilated by wetland vegetation during the process of primary production.

The most effective plants, in terms of biomass development and, hence, phosphorus assimilation in a given region, are those species present in pristine floodplain vegetation communities. However, the effectiveness of different species in trapping phosphorus can vary significantly. In a temperate climate, one of the most effective plants is the various species of willow (Figure 7.5).

To maximise the nutrient load reduction benefits of biofiltering systems, vegetation must be properly managed. To achieve the best results, the species composition should be adjusted to regional conditions. The occurrence and development of different species of plants depends on many abiotic factors, such as temperature and hydrology. Using these characteristics, it is possible to create habitats of different depths, and, hence, water levels and retention times, and, in this way, promote the development of particular plant species within the wetland.

The best results are achieved by creating areas with different types of land cover. This promotes wetlands with varying types of vegetation, adapted to a range of abiotic conditions, and increases the biodiversity of the area. Vegetation should be removed seasonally from the wetlands, or, as in the case of the willow, at regular intervals of 3 to 5 years. This helps to ensure that the vegetation maintains their peak growth rates and rates of phosphorus uptake. Removing the vegetation after the growing season also minimises the release of nutrients back into the water during the autumnal and winter periods of senescence (Figure 7.5).

Fig. 7.5. Efficiency of total phosphorus assimilation by six species of willow used to maximise nutrient trapping. The highest rate of phosphorus assimilation was observed during spring and summer. This suggests that willows should be harvested after growing season. (Zielinska 1998)

Figure 7.6 shows a management plan for wetland vegetation used to optimise nutrient assimilation by biota. The central portions of the area, characterised by the highest water levels (water flows into this area from the main river channel on more than 111 days per year), are covered with reeds, which are adapted to deeper and changing water levels. The portion of the wetland surrounding the reed bed is covered by a willow plantation, which has been divided into four patches, three of which, in turn, are regularly harvested every three years. The patch located closest to the river is not harvested and plays the same role as a natural ecotone buffer zone. The upper portion of the floodplain has been left for use as meadow and pasture.

F. Other methods to reduce phosphorus transport to reservoirs

In addition to treatment wetlands, other methods can be applied to reduce phosphorus loads to reservoirs from polluted rivers. Most of these relate to enhanced sedimentation of the transported materials in retention areas situated above the reservoir, and include the following systems:

  • Pre-basin systems that involve a system of curtains to reduce the energy of the inflowing water, allowing heavy materials to settle upstream of the reservoir inlet. This system requires maintenance in the form of periodic removal of the sediment (Aqua Technique 1997) (Figure 7.7).
  • Pre-reservoir systems that involve the use of a smaller weir upstream of a reservoir can eliminate up to 90% of the phosphorus load if the critical retention time is maximised. Periodic removal of the sediments from the pre-reservoir is also required (Twinch and Grobler 1986).
  • Riparian wetland systems that involve the protection or reconstruction of an ecotone with a intermediate degree of complexity upstream of the reservoir, utilising wetland vegetation resistant to water level fluctuations, can control non-point sourced pollution from the catchment of the reservoir.
  • Flow regulation systems that release epilimnetic waters and their associated phosphorus accumulated in dense phytoplankton blooms. The use of hypolimnetic outlets, in contrast, removes deoxygenated water with high concentrations of nutrients and sediment.

Fig. 7.6.
Enhanced absorption capacity created by increased vegetative biomass. The annual accumulation of P in the wetland increased more than three-fold after establishment of the willow plantation, resulting, also, in the possibility of sustainable bioenergy production (after Kiedrzynska. 2001, changed)

 

Fig. 7.7. Methods for phosphorus sedimentation and trapping in inflowing reservoir waters: A - pre-basin; B - pre-reservoir; and, C- polders. (Not shown: flow regulation.)

 

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