Response of the eelgrass (Zostera marina L.) to the combined effects of high temperatures and the herbicide, atrazine

High temperatures, especially in the summer or during local heat waves, often coincide with high concentrations of herbicides in some estuarine areas during flood plumes. The combined effects of thermal stress (at 26-32 °C) and the herbicide atrazine (at concentrations of 0, 3 and 10 μg L−1) on eelgrass (Zostera marina L.) were studied in a 7-day exposure experiment. The results showed that both atrazine and temperatures above 30 °C inhibited the photosynthetic efficiency and energy conversion potential of the photosystem II (PsiI) as indicated by the effective quantum yield (ΔF/F'm) and the maximum yield (Fv/Fm), respectively. High temperatures (31 °C and 32 °C) decreased the Chl a content and stopped the leaf growth of eelgrass. Atrazine concentration had no effect on the photosynthetic pigments at all temperatures; however, decreased leaf growth rate was detected at 30 °C. A high level of agreement between the observed and predicted inhibition of photosynthesis by an Independent Action (IA) model demonstrates that the combined effects of high temperature and atrazine are more harmful to eelgrass than is a single pressure. The present study adds further evidence for seagrass protection and environmental management under the hypothesis that thermal stress combined with herbicides pose a greater threat to seagrasses.