Photosynthetic activity of macroalgae along a bathymetric gradient: interspecific and seasonal variability

Marine macroalgae are unevenly distributed with depth, with substitutions of the dominant species along the bathymetric gradient. Since light shows a sharp decrease with depth and is a critical factor for algal growth, we hypothesize that species dominating at a certain depth are the best adapted to the light environment reaching that depth. To this end, we selected seven species of Mediterranean macroalgae distributed from 0 to 50 m depth and looked for their photosynthetic performances. Photosynthetic responses showed striking differences, with photosynthetic efficiencies increasing at increasing depths. However, neither oxygen production at high light levels nor respiration are related to depth but seem to respond to other factors such as thallus thickness or nutrient availability. Some species show high production values in agreement with their high productivity and fast growth, while others show low production values but high biomass and durability. Thus, the species that dominates at each light level seems to be the one that makes best use of the light environment to accomplish its life cycle.