Abstract
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 µmol m‐2 s‐1. 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 µmol m‐2 s‐1 O2 and 0.090 mol mol‐1 photon at 100% absorption. The upper limit of a is close to a realistic limit of O2 of 0.10 mol mol‐1 photon. The upper limit of GPmax, however, is markedly below the theoretically attainable 180 mol m‐2 s‐1 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.
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