Canopy theory for photosynthetic production by macroalgal forests

BACKGROUND: Many ecosystem services associated with macroalgae depend on the biomass of a canopy-forming habitat. Studies of photosynthetic production in macroalgal forests have generally not considered the consequences of uneven distributions of resources: like the decline in light with depth in the canopy. The effects of shading mean that it is not possible to multiply up from incubations of single fronds or parts of fronds to estimate canopy photosynthesis. SCOPE: Canopy models split the vertical structure into discrete layers before estimating photosynthesis. Models can also approximate the canopy as a single surface (a 'big-leaf' model). I demonstrate these approaches and make a sensitivity analysis of a previously published multiple layer model parameterised for Fucus serratus. Optimality theory suggests that canopies will tend toward a state that is maximally productive. Applied to a big-frond model, optimality predicts peak biomass at different irradiance levels. KEY RESULTS: Sensitivity analysis identified canopy attenuation coefficient as the most important influence on net photosynthesis. Increasing biomass (as thallus area index: m2 thallus m-2 seabed) increases areal net photosynthesis, but with diminishing returns. Eventually, the respiratory cost of adding extra biomass starts to reduce net photosynthesis. A big-frond model could describe the best-performing simulated canopies, particularly if photosynthetic parameters are tuned to implicitly take account of processes that the big-frond approach omits. Peak biomass tracking of light climate suggests more pronounced seasonality at relatively high-latitude locations. CONCLUSIONS: The investigation advocates for a canopy approach to macroalgae: identifying understudied variables of interest. Better empirical definition of parameters can improve large-scale summaries of macroalgal net production that currently rely on spatially invariant means. Canopy theory can enrich the discussion of the functioning of algal habitats and can suggest hypotheses for investigation, such as the degree and consequences of changes in the seasonal amplitude of macroalgal biomass.