Long-term Impacts of Runnelling on an Intertidal Saltmarsh

The values of intertidal wetlands have long been recognised (Gosselink et al. 1974, Dale and Hulsman 1990). In Australia, legislation has been enacted to provide for protecting coastal wetlands. For example in Queensland, there are over 50 Acts of the State Parliament, which have application in coastal areas. The major players include the Department of Primary Industries (Fisheries) under the Fisheries Act (1994), and the Environment Protection Agency under various Acts ranging from the general Environmental Protection Act (1994), Marine Parks Act (1982) to the specific Moreton Bay Marine Park Act (1993). Intertidal wetlands also include breeding sites for the salt marsh mosquito Ochlerotatus vigilax (Skuse) that transmits diseases such as Ross River virus (RRV). Although it is not the only arbovirus disease in Australia, RRV is the most prevalent, with up to 8000 cases reported annually, and with the largest proportion reported in Queensland (Curran et al. 1997). In Queensland, the State Health Act (1937) requires mosquito control to be undertaken and places responsibility for this with local government. Control is usually carried out at the larval stage and often by applying larvicides to breeding sites. Although the larvicides used are considered to have very low non-target toxicity, their indirect and long-term effects are not well known (Dale and Hulsman 1990). Management to minimise salt marsh mosquito-borne disease risk to human populations needs have regard to conserving intertidal wetlands and (Dale 1993). An effective alternative to using repeated chemical or biochemical treatment is to modify the environment so that it is less hospitable for the mosquito. This need not lead to significant loss of wetland or wetland function. Dale and Hulsman (1990) identified an evolution from intervention in mosquito breeding habitats that involved large projects such as ditching or impoundment through smaller ones such as Open Marsh Water Management to minimal modification or runnelling (Hulsman et al. 1989). Runnelling was first implemented in Queensland in 1985 as a simple form of water management (Hulsman et al. 1989). Runnelling is best suited to sites with simple and relatively clearly defined water movement patterns and where the length of runnel is relatively short, so that flushing reaches well into the marsh. Runnels are shallow spoon-shaped channels less than 0.30m deep and 0.90 m wide. They connect isolated mosquito breeding pools to the tidal source resulting in more frequent flushing of the marsh. In coastal Australia, Potential Acid Sulfate Soils often underlie areas that are mosquito breeding habitats and there is a risk of creating an acid sulfate problem if the substrate is disturbed, as the environmental consequences may be severe (Soukup and Portnoy 1986). This has been shown to be a relatively low risk for runnelling (Saffigna and Dale 1999). The aim of the current study was to assess the relatively long-term environmental impacts of runnelling at the study site. The null hypothesis was that there was no significant difference between treatment (runnelling) and control for each of the variables. The research hypothesis was that there were significant differences.