Biodiversity values of remnant freshwater floodplain lagoons in agricultural catchments: evidence for fish of the Wet Tropics bioregion, northern Australia

1. Tropical floodplain wetlands are among the world's most threatened and poorly documented freshwater ecosystems. This paper describes patterns of fish diversity in remnant freshwater lagoons in relation to natural environmental gradients and impacts of agriculture in the Tully–Murray catchment, Queensland Wet Tropics bioregion (QWT), in north-eastern Australia.\ud 2. Floodplain lagoons supported 21 of 29 native fish species recorded from freshwater habitats of these rivers, including three species not typically found in main river channels or tributaries, and six that require access to saline areas for spawning or larval development. Eight species favoured lagoons during their early life history,\ud highlighting the vital role of these water bodies in providing nursery habitat.\ud 3. Assemblage composition differed with distance from the coast, position on the floodplain, water quality and habitat. Aquatic vegetation discriminated lagoons and habitat patches within lagoons, and fish species richness was lower in patches of exotic ponded-pasture grasses. \ud 4. Although the lagoons are surrounded by intensive agriculture, especially sugarcane plantations, they are in good ecological condition, largely because of retention of some riparian vegetation, and frequent flushing by high stream flows. They offer opportunities to conserve taxonomic and functional biodiversity that is at present poorly protected by terrestrial reserves focused mainly on forested uplands. Of the 21 native species recorded, only one has >20% of its QWT distribution protected in IUCN category II protected areas (National Parks), and nine species have