Acclimations of macroalgae as reflected in photosynthetic parameters derived from PAM fluorometry, and possible implications for abundance patterns

The presences of the common macroalgae Ulva sp. and Jania rubens vary between seasons along the Israeli Mediterranean intertidal zone. To reveal some of the potential acclimation mechanisms of these algae, we examined their photosynthetic traits during the year using rapid light curves (RLC) derived from pulse amplitude modulated (PAM) fluorometry. In addition, the relationships between those photosynthetic traits and the relative abundances of the two algae were investigated. Ulva sp. showed high maximal electron transport rates (ETR max ) (49.7–68.8  μ mol electrons m −2  s −1 ) and onsets of light saturation values (E k ) (75.8–85.6  μ mol photons m −2  s −1 ) in the winter months of December–February, while low values were found in the summer months of June–July (5.2–20.6  μ mol electrons m −2  s −1 and 6.0–23.7  μ mol photons m −2  s −1 , respectively). At noon time (during the highest irradiance of the day), the maximal effective quantum yield (Y 0 ) did not vary significantly during the year. These results indicate that seasonal irradiance influences the number of reaction centres per thallus area. Both algae showed depressions in Y 0 at noon (23.8% for Ulva sp. and 20.3% for Jania rubens ), indicating an efficient non‐photochemical quenching mechanism. A positive correlation between the relative abundance and the photosynthetic parameter Y 0 was found for Jania rubens when Y 0 was sampled in the morning or noon, indicating that growth rate and primary production for this alga can be estimated from RLCs. No such correlation was found for Ulva sp., indicating that, in addition to photosynthetic traits, the relative seasonal abundance of this alga is influenced by other factors such as grazing and/or catastrophic events.