The enhancement of floral biodiversity in small scale constructed wetland treatment systems

  • Nicholas Steggall

Student thesis: Doctoral ThesisDoctor of Philosophy


Within the U.K. small-scale treatment wetlands are primarily constructed using a monoculture of Phragmites australis. This thesis investigates the potential for enhancing the biodiversity value of these wetlands by the inclusion of appropriate floral species.

Extensive literature reviews found that although there was a plethora of data for the design of constructed wetlands, there was a dearth of information on enhancing the biodiversity value of these wetlands. Three potential biodiversity enhancing species were identified which could be beneficial; purple loosestrife Lythrum salicaria, meadowsweet Filipendula ulmaria and water mint Mentha aquatica.

A microcosm study was undertaken to investigate the growth of these species, the interactions between them and with Phragmites australis. The two pollutants employed in these studies were nitrogen and salinity. A second parallel system was constructed where competition between the plants was restricted by installing root dividers.

The results of the microcosm study identified that selected species survived within all of the nutrient concentrations employed. The roots of the biodiversity enhancing species predominantly stayed within the upper humus layer of the wetland and so would not interfere with the subsurface flow of the wetland or the treatment potential of the Phragmites australis roots. The area coverage of the biodiversity enhancing species combined with the coverage and treatment potential of the Phragmites australis roots show that these species are suitable for growing within a small-scale constructed wetland at the tested nutrient concentrations. Fatalities were present within the salinity concentrations, therefore they can only be utilised at up to a limiting salinity concentration.

A field study was subsequently undertaken at operational sites to investigate the addition of biodiversity enhancing species into mature and newly restored reedbeds with mixed results.

Following the study, design principle recommendations are made for including biodiversity enhancing species within a small-scale treatment wetland systems within the U.K.
Date of Award20 Jun 2017
Original languageEnglish
SupervisorPeter D Hedges (Supervisor) & Philip A Davies (Supervisor)


  • biodiversity
  • enhancement
  • constructed wetland
  • phytoremediation
  • microcosm

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