Many terrestrial plant canopies regulate spatial patterns in leaf density and leaf inclination to distribute light

evenly between the photosynthetic tissue and to optimize light utilization efficiency. Sessile aquatic macrophytes,

however, cannot maintain the same well-defined three-dimensional structure because of the strong drag and shear

forces of moving water. This difference in canopy structure has been suggested to account for the three- to fivefold

higher gross production rates in terrestrial than aquatic communities. To evaluate the effect of community structure

in aquatic habitats, we combined a simple mechanistic model and empirical measurements on artificially structured

macroalgal communities (Ulva lactuca) with varying thallus absorptance and community density. Predicted and

measured values corresponded closely and revealed that gross production in high-light environments was markedly

enhanced by a vertical orientation of thalli when absorptance and community density were both high. This result

implies that aquatic macrophytes of high thallus absorptance and community density exposed to high light are

limited in attaining high gross production rates because of their inability to distribute photons evenly between the

photosynthetic tissues. As scattering and attenuation in the water column increase, the effect of thallus structure on

production declines and thin transparent macrophytes are more efficient at utilizing light than thick opaque macrophytes.

The results confirm that inefficient distribution of light can account for the low community production

rates in aquatic habitats and the depth distribution of form-functional groups of macroalgae with different canopy

structure.