We compared 190 photosynthesis-irradiance (P-E) experiments with single- and multispecies communities of

macroalgae and vascular plants from freshwater and marine habitats. We found a typical hyperbolic P-E relation

in all communities and no sign of photosaturation or photoinhibition of photosynthesis at the highest irradiances

of about 2,000 mmol m22 s21. Macrophyte communities displayed much higher maximum gross production

(GPmax), respiration, and light compensation point than separate phytoelements because of the multilayered

structure and extensive self-shading in the communities, whereas light use efficiency at low irradiance (a) was the

same. Although GPmax and a varied extensively among the 190 communities, their upper limits increased linearly

and predictably with community absorption reaching 26.3 mmol m22 s21 O2 and 0.090 mol mol21 photon at

100% absorption. The upper limit of a is close to a realistic limit of O2 of 0.10 mol mol21 photon. The upper limit

of GPmax, however, is markedly below the theoretically attainable 180 mol m22 s21 O2, reflecting a suboptimal

three-dimensional structure and light distribution. Indirect measures supported this explanation as GPmax

increased fourfold from communities with a very uneven to a more even light distribution. Photosynthetic

characteristics of communities are strongly influenced by plant density, absorption, and distribution of light and

cannot be interpreted from the photosynthetic behavior of phytoelements. Thus, many examples of carbon and

nutrient limitation in experiments with separate phytoelements may not withstand at the relevant ecological scale

of communities where light almost always constrains photosynthetic production.