Photosynthetic activity and productivity of intertidal macroalgae: In situ measurements, from thallus to community scale

•In situ photosynthesis and productivity of intertidal macroalgae were compared.•The uppermost species had the highest thallus photosynthetic activity under emersion.•The lowermost species had the lowest individual gross productivity under emersion.•The low mid-shore zone had the highest community gross productivity under emersion.•Primary productivity variability was reduced when entire communities were considered. The photosynthetic activity and productivity of four dominant canopy intertidal macroalgae were measured under emersion and immersion, at saturating light levels (PAR>300μmolphotonsm−2s−1), and compared at two sites (eastern and western English Channel) in spring and summer. The photosynthetic activity of thalli was measured by the electron transport rate (ETR) using pulse-amplitude modulated (PAM) fluorescence and the productivity of individuals and of communities was measured by carbon fluxes in closed chambers. Under emersion, when thalli were still hydrated, the uppermost species Pelvetia canaliculata had higher photosynthetic activity (mean ETR between 327 and 460μmole−m−2s−1) and individual gross productivity (between 60 and 212μmolCgDW−1h−1) than the lowermost species Laminaria digitata (mean ETR between 24 and 53μmole−m−2s−1 and gross productivity between 2 and 38μmolCgDW−1h−1), whatever the site and season. P. canaliculata had higher ETR in air than underwater (averaged 146μmole−m−2s−1) and L. digitata had lower ETR in air than underwater (averaged 112μmole−m−2s−1), while they exhibited, respectively, 3 and 5 times higher gross productivity underwater. At the community scale, the low mid-shore zone of Fucus serratus had the highest mean gross productivity under emersion (47mmolCm−2h−1) while rates were higher for the uppermost than lowermost zone at the eastern site (average 20 and 6mmolCm−2h−1, respectively) and of the same order of magnitude for both zones at the western site (about 30mmolCm−2h−1). Finally, the variability of under emersion primary productivity among sites and seasons was reduced when the measurements were performed on entire communities compared to isolated individuals of the dominant species.