Macrophyte zonation in stormwater wetlands: getting it right! A case study from subtropical Australia

In Australia stormwater wetlands are becoming an increasingly popular component of water sensitive urban design. However, they must be designed to cope with the dynamic nature of urban hydrology, in particular, fluctuations in water level. The concept of macrophyte zonation relies on a thorough understanding of the water regimes of different plant species. Water depth is crucial and the hydroperiod, i.e. duration and frequency of inundation, has a significant impact on the survival of wetland vegetation. The aim of this study was to investigate plant establishment in a newly constructed stormwater wetland in Brisbane, subtropical Australia. Changes in plant distribution and density have been monitored since 2001. Rainfall and water depth data enabled us to use a hydrologic model to predict the extent of inundation of the different macrophytes zones. The field survey showed macrophyte survival was poor with the complete loss of several species in marsh and ephemeral zones. The main reason for the lack of macrophyte establishment and survival was the extended periods of inundation (supported by the hydrologic model) and deeper water levels. Stormwater wetlands must be designed to ensure that ephemeral species are not permanently inundated or the preferred water depths in marsh zones are not exceeded for extended periods.