Photosynthesis by benthic marine macroalgae makes an important contribution to the productivity of coastal seas. Quantification of photosynthesis and productivity of macroalgal assemblages is therefore important in understanding ecosystem functioning in coastal seas and providing realistic values for coastal productivity in global models. Estimates of macroalgal productivity are often based on the photosynthetic characteristics of thallus pieces or whole thalli, and not on those of communities.

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... Such methods may overestimate rates of productivity as they do not account for neighborhood shading effects that may reduce photosynthetic rates in macroalgal stands that typically have high densities. In order to determine whether productivity estimates based on individuals differ from those based on communities, a controlled laboratory experiment was conducted with 3 dominant sub-canopy macroalgal species (Cystophora scalaris, Xiphophora gladiata and Undaria pinnatifida) from southern New Zealand. Photosynthetic parameters (initial slope of the photosynthesis vs. irradiance [P-E] curve α, saturation irradiance E k , maximum rate of photosynthesis P max and darkrespiration R d ) were obtained via P-E experiments using a custom-built respirometry chamber for a range of densities that corresponded to the minimum, average and maximum densities of these species in the field. A 5 to 7-fold decrease in P max was observed when the density of the algal stand was above 1 ind. m -2 . R d and α were also lower in communities than for individuals. Results illustrate that estimates based on single specimens substantially overestimate productivity and we recommend that the densities used in experiments reflect those observed in the field. Resale or republication not permitted without written consent of the publisher Editorial responsibility: Matthias Seaman,