Addition of species abundance and performance predicts community primary production of macroalgae.

Understory plant assemblages are important sources of primary production in both terrestrial and marine environments, and they may exhibit different dynamics than their overstory counterparts. For example, production within dense upper canopies is typically light-limited by shading, whereas such canopy architecture effects are likely unimportant in low-light environments, such as those inhabited by sparser understory assemblages. In these assemblages, light saturation of understory production may be common as species become limited by their photosynthetic capacity, which is adapted to low-light levels. Here we show that a simple model relating species-specific light use relationships measured in the laboratory to biomass and light levels measured in nature accurately predicts community gross primary production (GPP) in a marine understory algal community. We validate the model by comparing GPP measured in situ in enclosed chambers with model estimates for the same incubations. Model estimates of GPP explained 70% of the variation in the measured estimates. The results show that GPP was accurately estimated by simple addition of the photosynthetic capacity of each species in the community based on their biomass and the available light. The difference between modeled and measured GPP did not show any relationship with community biomass or diversity, and the results suggest that diversity does not significantly affect productivity in this system. This type of model should be applicable in other environments where canopy architecture does not play a significant role in limiting photosynthesis.