Herbicides increase the vulnerability of corals to rising sea surface temperature

In order to examine the potential interactive pressures of local pollution and global climate change, we exposed corals and crustose coralline algae (CCA) to three agricultural photosystem II (PSII) herbicides at four temperatures (26-32 degrees C). The coral Acropora millepora was 3- to 10-fold more sensitive to the three herbicides than the CCA Neogoniolithon fosliei. While the photosynthesis of CCA was not affected by the herbicide concentrations used (< 1 mu g L-1), temperatures of 31 degrees C and 32 degrees C alone significantly inhibited photosynthetic efficiency (Delta F: F-m') and caused chronic photoinhibition (reduced F-v : F-m) and substantial bleaching. Environmentally relevant concentrations of each herbicide increased the negative effects of thermal stress on coral at 31 degrees C and 32 degrees C. Mixed model analyses of variance showed that the effects of elevated sea surface temperatures (SST) and herbicide on photosynthetic efficiency of coral symbionts were additive. Furthermore, the effect of either diuron or atrazine in combination with higher SST (31 degrees C and 32 degrees C) on chronic photoinhibition was distinctly greater than additive (synergistic). Reducing the herbicide concentration by 1 mu g L-1 diuron above 30 degrees C would protect photosynthetic efficiency by the equivalent of 1.8 degrees C and reduce chronic photoinhibition by the equivalent of a 1 degrees C reduction. Reduced water quality increases the vulnerability of corals to elevated SSTs, and effective management of local water quality can reduce negative effects of global stressors such as elevated SST.