Depth distribution of canopy‐forming algae of the order Fucales is related to their photosynthetic features

Photosynthetic features and dark respiration of different species of dominant canopy‐forming algae of the Order Fucales (Phaeophyceae) consistently change with depth. Photosynthetic efficiency (α) and light at compensation ( I c ) change according to the amount of light available at each depth, α increasing and I c decreasing with depth. However, the values of light at saturation ( I k ) do not show any depth pattern. Deep‐water species show higher photosynthesis at saturation ( P max ) and lower dark respiration ( R d ) than shallow water ones. This result contrasts with the expected reduction of P max in low light environments shown by terrestrial plants but is consistent with other studies performed in macroalgae, suggesting that P max and α show the same trend of variation in the marine environment. The positive correlation between P max and α cannot be explained by differences in thickness as all tested species show an extremely high morphological similarity. The increase in P max with depth is also coupled to an increase of N and P content in branches, which points to nutrient‐limited photosynthesis in shallow water species.