Macrophyte Establishment in Stormwater Wetlands: Coping with Flash Flooding and Fluctuating Water Levels in the Subtropics

Water depth is crucial to macrophyte establishment and survival. Water depth and the extent of flooding and drying determines the macrophyte types and wetland zones. The concept of zonation relies on a thorough understanding of the hydroperiod and depth distribution of different plant species. Stormwater wetlands are designed with shallow and deep macrophyte zones and deeper ponds however, problems of macrophyte establishment, survival and colonisation occur when aquatic plants growing in the ephemeral zone or shallow-marsh zone are inundated for prolonged periods. Problems may also occur during dry periods when water levels drop. Thus aquatic plants growing in stormwater wetlands are subjected to huge fluctuations in water depth. A study of two stormwater wetlands in Brisbane, Australia highlighted many of the problems associated with establishing macrophyte zones in a subtropical climate with intense storm events. Increased water velocity during storm events uprooted newly planted stock, caused scouring and erosion, and washed away topsoil. At Golden Pond Wetland, submerged species with poorly developed root systems were washed away, along with aquatic creepers. However, recolonisation and spread of these species was rapid. At Bridgewater Creek Wetland, extended periods of inundation during the first two years of establishment resulted in the complete loss of 6 of the 15 species planted and after 4 years, only 3 species remained. Clumps of Juncus (J. usitatus and J. kraussii) survived only in the most elevated ephemeral zone, while Bolboschoenus fluviatilis survived by spreading landward. Ducks and wading birds also contributed to the loss of soft stemmed species in the ephemeral zone by using these elevated islands as day- roosting sites. Clay substrate prevented the lateral spread of rhizomatous species. Potamogeton javanicus self colonized extending to100cm depth (peak biomass at 40cm) however, increased water levels due to a blocked outlet riser caused severe decline in this submerged species. Thus asset maintenance is also crucial to ensure correct water depths for macrophyte survival.