Comparisons of individual and community photosynthetic production indicate light limitation in the shallow water macroalgal communities of the Northern Baltic Sea

Primary production is of special importance in ecology, since it fuels other trophic levels with energy and biomass. In aquatic ecosystems, almost all primary production is performed by algae. The primary production of aquatic macrophytes has been mainly quantified using detached phyto-elements (fronds and algal thalli) or whole plants, whereas the community level is usually ignored. In a field experiment we evaluated macroalgal photosynthesis at individual and community levels. We used natural communities dominated by the ephemeral green alga Cladophora glomerata (L.) Kutzing and the perennial brown alga Fucus vesiculosus L. Fucus vesiculosus had temporally stable and C. glomerata highly variable photosynthetic production. On a biomass basis, net and gross photosynthetic production was always higher at the individual level than at the community level. The ratio of individual photosynthetic production to community photosynthetic production increased with light intensities. The observed relationships give clear evidence that in shallow water, where light is supposedly plentiful, macroalgae are nevertheless strongly light-limited. Although photosynthetic production estimates at individual levels are traditionally extrapolated to the community level, such estimates largely overestimate reality and give a false impression of the potential photosynthetic production of macroalgae.