Aquatic biota in hot water: thermal gradients in rheocrene hot spring discharges as analogues for the effects of climate warming

Hot springs are characterised by water temperatures above 36.7 °C. Temperature decreases with distance in flow away from spring vents; this natural gradient provides a unique opportunity to investigate the influence of water temperature on aquatic biota. This study investigated the relationship between water temperature and the aquatic invertebrates and benthic diatoms in outflows from a hot spring complex in tropical north Queensland, Australia. Water temperature ranged from 62.7 °C at the vents to 26.0 °C at the location furthest downstream. Richness of benthic diatoms and aquatic invertebrates increased linearly in response to decreasing temperature, with no species present in the hot vents. Multivariate analysis showed that both community assemblages had a response to the temperature gradient. A drop in aquatic invertebrate richness and a change in assemblage composition occurred between 40 °C and 42 °C, indicating a threshold at this temperature. The nearby Einasleigh River has experienced several contemporary peaks in water temperature over 40 °C, which corresponds to this threshold level. The relationships indicate that consistent increases in water temperature expected under climate change could decrease biological richness and precipitate changes in the aquatic invertebrate and benthic diatom taxa of tropical aquatic ecosystems.