Freshwater indicators and thresholds of concern
Project Leaders: Prof Richard Pearson, JCU & Prof Angela Arthington, GU
This project sought to measure and understand the ecological processes, linkages and interdependencies that govern the condition, health, trend and resilience of rivers and their floodplain wetlands in Great Barrier Reef catchments, as a basis for development of cost-effective biophysical monitoring tools to help sustain these habitats and their dependent environmental assets. Researchers produced a conceptual biophysical model that identified the appropriate indicators of wetland health, and probable thresholds of concern, in terms of contaminant concentrations, ecological processes and biodiversity. The major indicators identified are related to pressures including types of land use, general water quality and contaminants, hydrological regime and connectivity, channel and habitat structure, riparian vegetation condition and alien species of plants and fish. Stressor-response relationships along gradients of disturbance (supported by data from laboratory trials and the literature) have helped to identify ‘thresholds of concern’, i.e. points along each disturbance gradient where ecological changes of scientific or management concern become apparent. The most serious factors currently affecting health in Wet Tropics streams and wetlands are changes to habitats, including flow modification, invasion by exotic weeds and loss of riparian vegetation, which can cause major changes to waterway morphology, habitat complexity, food availability, gas exchange with the atmosphere and, therefore, biodiversity. Dry tropics streams and wetlands are impacted by similar influences but due to varying land uses and a dominance of cattle grazing, are generally more exposed to issues related to sedimentation. Many dry tropics rivers cease to flow in the dry season, contracting to isolated lagoons, which provide refugia for the biota. The gradient of flow regime from mid-Wet Tropics to Dry Tropics is very clearly reflected in their biodiversity, with even the smaller rivers of the Wet Tropics supporting many more fish species than the large Dry Tropics systems. Flow regime has a high influence on fish populations. The relationship between the structure and dynamics of the larval fish assemblage in lowland riverine Wet Tropics habitats and the underlying variability of the habitat and its condition are shaped primarily by the prevailing flow regime. The fauna in the wetlands of the Tully-Murray floodplain and in streams of the Russell-Mulgrave catchment is in moderate to good condition. It appears that the fauna is resistant to the immense changes in land use that have taken place across the floodplains in the last century, mostly because of perennial flows. Less well-flushed systems (e.g. on the Herbert floodplain) do not fare so well. Potential indicators of ecosystem health, such as abundance and diversity of zooplankton and benthic invertebrates, tend to correlate very closely with habitat changes, which themselves are quite straightforward to monitor (e.g., floods, riparian condition, alien species and weeds). Fish provide a robust indication of lagoon condition and, importantly, connectivity, while fish and especially invertebrates and are good indicators of condition in streams.
Report from the MTSRF Projects 3.7.3 and 3.7.4 Workshop held at CSIRO Davies Laboratory, Townsville, 19-20 April 2007, which summarises the presentations, discussion and recommendations made. The key objective of the workshop was to initiate the development of an integrated package of conceptual and quantitative models, supported by field-based research, to predict the key hydro-ecological functions in Wet Tropics rivers, wetlands and floodplains.