Review and conceptual models of agricultural impacts and water quality in waterways of the Great Barrier Reef catchment area

Adequate conceptual frameworks that link land use to water quality and ecosystem health are lacking for tropical and subtropical freshwater systems, so we review here extensive water-quality research undertaken in the Great Barrier Reef catchment area (GBRCA) and present conceptual models synthesising the dynamics of agricultural pollutants and their ecological effects. The seasonal flow regime defines the following key periods of water-quality risk over the annual hydrological cycle for diverse GBRCA ecosystems: initial ‘pre-flush’ flows during the transition from the dry to the wet season; early wet-season ‘first flush’ flows; peak wet-season flood flows; and sustained base flow or periods of disconnection during the dry season. The level of seasonal contrast varies from the perennial systems of the wet tropics to the intermittent systems of the dry tropics. Major water-quality stressors may be catchment scale (e.g. in streams draining broad-scale agriculture) or more localised (e.g. cattle access, irrigation tail water). Water-quality stressors such as ammonia toxicity and hypoxia (due to organic or nutrient run-off and enhanced plant productivity) are of low relevance to downstream GBR ecosystems but are major threats to fresh waters. Similarly, whereas high contaminant loads in wet-season floods present the highest water-quality risk to marine ecosystems, the greatest risk in fresh waters is often from acute contamination during early wet-season ‘pre-flush’ flows into lentic waters, or continuous input of contaminants over long periods of base flow. Because of differences in the nature of risk periods, water-quality threats and pollutant-delivery mechanisms, the benefits of different management options to improve water quality can also differ among freshwater habitats and between freshwater and marine environments.