Macroalgae are a valuable commodity to the marine ecosystem as they are primary producers, provide a three dimensional habitat for small herbivores and are a sink for atmospheric CO2. Water acidification and ocean warming caused by anthropogenic activities are proving detrimental to many marine flora and fauna. We subjected three species of macroalgae (Ecklonia radiata, Sargassum linearifolium, and Laurencia brongniartii) to future climate change conditions. This was achieved by investigating the combined effects of increased temperature (+5°C), increased pCO2 and decreased pH (-0.5 pH units) on the physiological and chemical ecology of the algae. We then conducted a series of feeding assays using local herbivores to identify the indirect effects of these treatment conditions on feeding rates. Each algae had a species specific response to the treatment conditions. For example, E. radiata became more palatable to herbivores and the C:N ratios were altered, dependent on the treatment. In increased temperatures, S. linearifolium bleached significantly and reduced in effective quantum yields. L. brongniartii was effected in all physiological and chemical aspects, with increases in bleaching, blade density, and C:N ratios and decreases in growth, maximum quantum yield, blade toughness, total phenolics and consumption by mesograzers. This study reflects the importance of finding the thresholds of stress that ecologically important macroalgae and their associated mesograzers have to the changing environment.
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