Temperature extremes reduce seagrass growth and induce mortality

Extreme heating (up to 43 degrees C measured from five-year temperature records) occurs in shallow coastal seagrass meadows of the Great Barrier Reef at low tide. We measured effective quantum yield (phi(PSII)), growth, senescence and mortality in four tropical seagrasses to experimental short-duration (2.5 h) spikes in water temperature to 35 degrees C, 40 degrees C and 43 degrees C, for 6 days followed by one day at ambient temperature. Increasing temperature to 35 degrees C had positive effects on phi(PSII) (the magnitude varied between days and was highly correlated with PPFD), with no effects on growth or mortality. 40 degrees C represented a critical threshold as there were strong species differences and there was a large impact on growth and mortality. At 43 degrees C there was complete mortality after 2-3 days. These findings indicate that increasing duration (more days in a row) of thermal events above 40 degrees C is likely to affect the ecological function of tropical seagrass meadows. (C) 2014 Elsevier Ltd. All rights reserved.