MEPS 287:65-75 (2005)  -  doi:10.3354/meps287065

ABSTRACT: Experiments on aquatic plant photosynthesis are often performed on spatial scales much smaller than natural systems. The photosynthesis-irradiance relationship (P-I curve) depends, however, on whether photosynthesis is measured on leaves, shoots, plants or entire communities, so that results cannot be reliably extrapolated from the limited spatial scale of experiments to the larger scale of natural systems. To evaluate how the photosynthetic response changes with spatial scale for aquatic plants, we measured the photosynthetic production as a function of irradiance for thallus pieces, single-species and multi-species communities of 4 different marine algae (Fucus serratus, Chordaria flagelliformis, Enteromorpha sp. and Ahnfeltia plicata). Thallus pieces followed the well known hyperbolic shape of P-I curves; that is, the community P-I curves approached linearity. Thallus pieces, thus, saturated at irradiances below 160 µmol photons m^-2 s^-1, while the communities started to saturate at values 4 times higher and never became fully saturated even at 1750 µmol photons m^-2 s^-1. This increasing linearisation of the P-I relationship for communities influenced the photosynthetic parameters, as maximum gross photosynthesis and compensation irradiances increased 3 to 8 times. The increasing linearisation of the P-I curves is attributed to the 3-dimensional structure of the communities, since communities with a low extinction per unit leaf area, and thus an even distribution of irradiance among thallus parts, used their potential photosynthesis capacity more efficiently. Although not significantly different, multi-species communities had a slightly more linear response and a higher gross photosynthesis than single-species communities, suggesting a more efficient distribution and photosynthetic use of light among the photosynthetic tissue. An increase in spatial scale from thallus pieces to communities decreased variation in photosynthetic rates between species, implying that differences between species are much smaller on natural scales than experiments on single thallus pieces suggest. The findings imply that experiments on the thallus scale should be avoided or treated with caution when predicting the ecological performance of plants in their natural habitat.

KEY WORDS: Community · Photosynthesis · Production · Macroalgae · Irradiance