Temperature-dependent photosynthesis in the intertidal alga Fucus gardneri and sensitivity to ongoing climate change

• Photosynthesis of F. gardneri peaks 2°C above warmest recorded seawater temperature
• Pnet model suggest temperature is not the limiting factor for southern limit
• Simulations demonstrate +1m tidal elevation threshold for F. gardneri O2 production

Understanding the photosynthetic responses of marine macroalgae to changes in their thermal environment is vital to characterizing the success of these habitat-forming primary producers in the face of climate change. We measured net photosynthesis in apical tips of Fucus gardneri collected from the intertidal zone of Friday Harbor, WA over a range of irradiance levels (0–1500 μmol photons m− 2 s− 1) at 10°, 14°, and 18 °C to determine levels of saturating irradiance. We then recorded net photosynthesis at saturating irradiance in tips exposed to seawater temperatures ranging from 6 to 22 °C, as well as dark respiration. Results show that F. gardneri at this location has a peak in net photosynthesis at 16–18 °C seawater temperature, with significant declines in net photosynthesis at 20° and 22 °C. Respiration showed a positive linear relationship with increasing seawater temperature. Using archived seawater temperature, irradiance, and tidal data, we produced a model of net photosynthesis over two years (October 2010–October 2012). Maximal seawater temperatures recorded at FHL rarely exceeded 14 °C, suggesting that an increase of + 2 and + 4 °C in seawater temperature would lead to increased net photosynthesis at this site. These results allowed us to develop a predictive model to forecast the net photosynthesis of F. gardneri at different intertidal elevations to explore the effects of seawater temperature and irradiance on net photosynthesis. We also examined the effects of the timing of high tide, testing the hypothesis that net photosynthesis will be highest at sites where submersion occurs during peak levels of irradiance. Our results suggest that as seawater temperatures increase (up to + 4 °C above ambient) F. gardneri below + 1 m MLLW tidal elevation will experience increases in net photosynthesis. These analyses also suggest that the effects of environmental change may depend in part on tidal regime, which determines the extent to which algae are submerged during times of day when irradiance is high.