Determination of photosynthetic production in plant communities is essential for evaluating plant growth rates and carbon fluxes in ecosystems, but it cannot easily be derived from the photosynthetic response of individual leaves or thalli, which has been the focus of virtually all previous aquatic studies. To evaluate the regulation of aquatic community production, we measured the photosynthetic production of thallus parts and entire communities of Fucus serratus (L.) of different density and spatial structure exposed to varying photon flux density and dissolved CO₂ concentration. Photosynthetic production in relation to photon flux density differed considerably between entire communities and individual thallus parts. No light saturation occurred in the communities, and all photosynthetic variables such as light compensation point, efficiency of light utilisation at low light and maximum production at high light depended on community density. Therefore, while the determination of the production of individual algal thalli is useful for evaluating differences in acclimatisation and adaptation between species and stands, it is not useful for evaluating production rates for entire plants and communities. Maximum gross production of F. serratus followed a hyperbolic curve at increasing community density, reaching a maximum of 30 µmol O₂ (m² ground area)²¹ s²¹ at a leaf area index (LAI) of 8 to 10. Maximum net community production in the light was attained at an LAI of 6 to 8, while the maximum in a natural day-night cycle is attained at an LAI of 5 to 6. Maximum gross production was about 3-fold lower in F. serratus communities than in terrestrial communities in forests, grasslands and crops. Manipulating F. serratus communities to a more vertical structure increased light penetration and gross production in the communities, implying that canopy structure had a profound influence on community production and that a non-optimal distribution of light is likely to be the main reason for the lower maximum gross production rates in aquatic than terrestrial plant communities.