Physiological responses of habitat-forming seaweeds to increasing temperatures

Kelps and fucoids are dominant habitat-forming seaweeds along temperate rocky coastlines. Here, we tested the physiological performance of a dominant kelp (Ecklonia radiata) and two fucoids (Scytothalia dorycarpa and Sargassum fallax), distributed along the southwest coast of Australia. Photosynthesis and respiration were measured against increments in temperature for seaweeds collected along a latitudinal gradient in ocean temperature from Kalbarri (warm) to Hamelin Bay (cool). We found a similar decrease in photosynthetic activity from cooler to warmer latitudes in all three species. Seaweeds collected from warmer locations had significantly lower chlorophyll a concentration compared to cooler locations which could explain the lower levels of photosynthetic activity at warmer latitudes. The Q₁₀ values for photosynthesis and respiration tended to decrease from cooler to warmer locations. For all species, the optimum temperature for net photosynthesis remained similar across the locations. However, within locations, the optimum temperature for S. fallax (25.2°C) was significantly higher than E. radiata (24.0°C) and S. dorycarpa (23.6°C). The reduction rates of net photosynthesis observed after optimum temperature showed the greatest variation among the species within and across locations. A thermal performance breadth analysis revealed a broader performance range for S. fallax (21.5–28.4°C) followed by E. radiata (21.2–26.5°C) and S. dorycarpa (21.4–25.8°C). These results highlight the differences in temperature sensitivity among the three species which help explain their current distributional patterns and have potential implications for future responses to future ocean warming.