Dynamics of productivity in naturally structured macroalgal assemblages: importance of canopy structure on light-use efficiency

Macroalgae are undoubtedly a major supplier of organic carbon to marine food webs, yet few studies have attempted to examine primary production in complex macroalgal communities. Many studies have tested the photosynthetic characteristics of single species with little attention given to the associated algal assemblages. The role of canopy structure and species diversity in these assemblages will help in understanding the role of biodiversity in enhancing ecosystem function. In the present study, photosynthesis-irradiance relationships (P-E curves) of intact intertidal algal assemblages showed no signs of saturation at high irradiance levels, as is typically seen in single-species curves. Canopy-forming species appear to play a vital role in enhancing overall assemblage net photosynthesis, particularly at high irradiances (>1500 mu mol m(-2) s(-1)), with the loss of canopy species causing up to a 50% decline in the photosynthetic rate of assemblages. There is also evidence that the mechanisms of light use are significantly different in intact assemblages because they do not act as the sum of their parts at high irradiance levels. Furthermore, with greater structural complexity, net photosynthesis of the assemblage is enhanced, particularly at high irradiance. The present study demonstrates the importance of using realistic species compositions and assemblage structure to identify the potential mechanisms enhancing function with increasing diversity. A better understanding of light-use efficiency within macroalgal communities underpins the role of biodiversity and community structure in these ecosystems and the potential effects of species loss on various forms of ecosystem function.