Environmental DNA highlights the influence of salinity and agricultural run-off on coastal fish assemblages in the Great Barrier Reef region

Agricultural run-off in Australia’s Mackay-Whitsunday region is a major source of nutrient and pesticide pollution to the coastal and inshore ecosystems of the Great Barrier Reef. While the effects of run-off are well documented for the region’s coral and seagrass habitats, the ecological impacts on estuaries, the direct recipients of run-off, are less known. This is particularly true for fish communities, which are shaped by the physico-chemical properties of the coastal waterways that vary greatly in tropical regions. To address this knowledge gap, we used environmental DNA (eDNA) metabarcoding to examine teleost and elasmobranch fish assemblages at four locations (three estuaries and a harbour) subjected to varying levels of agricultural run-off during a wet and dry season. Pesticide and nutrient concentrations were markedly lower during the sampled dry season. With the influx of freshwater and agricultural run-off during the wet season, teleost and elasmobranch taxa richness signifi...