Determining the impact of future climate change on ecologically important macroalgae

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... [ view full abstract ]

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. Many studies have focused on the effect of ocean acidification and warming on calcifying organisms such as corals. However, little is known about the effect these environmental changes will have on the physical and chemical traits of fleshy macroalgae, and the subsequent impact on the feeding preferences of associated mesograzers. In this study, the impact of projected climate change on two algae: a brown Sargassum sp. and the kelp Ecklonia radiata were assessed. This was achieved by investigating the combined effects of increased temperature (+5°C), increased pCO2 and decreased pH (-0.5 pH units) on the nutrient content, frond toughness, growth and photosynthetic rates of the two algae species over a two week period. Additionally, the algae was presented to marine mesograzers for a period of 24 hours to determine whether the treatments altered the palatability of the algal food.
The Sargassum sp. displayed substantial bleaching when exposed to higher water temperatures. This bleaching significantly impacted the algae’s photosynthetic rate as demonstrated by a lower dark-adapted Fv/Fm in the warmer treatments. However, at elevated pCO2 the Sargassum sp. exhibited lower levels of bleaching and higher dark-adapted Fv/Fm, but not an increased growth rate which is often associated with increased CO2 concentrations. Bleaching was minimal in the kelp species and the photosynthetic rates were not altered by changes in pCO2 or temperature. Interestingly, there were no significant results in the feeding assays and growth for both algae species, showing a somewhat tolerance in both algae species to ocean acidification and temperature. This study reflects the importance of finding the thresholds of stress that ecologically important macroalgae and their associated mesograzers have to the changing environment.