Seasonal and depth related variation in the photosynthesis-irradiance response of Ecklonia radiata (Phaeophyta, Laminariales) at West Island, South Australia

The photosynthesis-irradiance response of Ecklonia radiata (C. Agardh) J. Agardh, a common kelp in the temperate southern hemisphere, was investigated in situ throughout the year and across a depth profile at West Island, South Australia. Temperature and irradiance environment altered throughout the year, varying at 3 in between 14-20degreesC and 279-705 mumol photons m(-2) s(-1). Photosynthetic capacity (P-m) varied throughout the year between 177-278 mumol O-2 g(-1) dry wt h(-1) at 3 m and 133-348 mumol O-2 g(-1) dry wt h(-1) at 10 in. The irradiance required for sub-saturation of photosynthesis (E-k) varied between 97-152 and 81-142 mumol photons m(-2) s(-1) for 3 m and 10 rn respectively, and the respiration rate varied between 15-36 and 13-20 mumol O-2 g(-1) dry wt h(-1) for 3 in and 10 m. A clear seasonal change in photokinetic parameters was detected and provided strong evidence for a seasonal acclimation response. During winter an increase in the efficiency of light utilisation at low irradiance (a) was accompanied by a decrease in both Ek and that required for photosynthetic compensation. P-m, also increased during the winter and autumn months and respiratory requirements decreased. These changes enable E. radiata to display an optimal photosynthetic performance throughout the year despite significant changes in the surrounding environment.