ALBERT

Description: Intertidal communities dominated by canopy-forming brown algae (Phaeophyceae) usually prove to be highly productive systems, based on short-term measurements. However, long-term metabolism (primary production and respiration) is sensitive to several factors acting on different time scales (e.g. tidal cycle, seasonality), making its assessment challenging. Here, we used mathematical modelling to investigate the metabolism of a Fucus serratus-dominated community on daily and annual time scales. This widespread community, which is usually characteristic of the low mid-intertidal level of temperate rocky shores, is submerged for approximately 83% of the time at our study site (Brittany, France). The model incorporated a large spectrum of physiological (e.g. estimates of primary production versus irradiance parameters) and environmental (e.g. temperature, incident irradiance, depth of the water column) parameters measured in situ. The model simulation predicted that net community primary production (NCP) peaks at 8.0 gC m-2 d-1 in late spring, when environmental conditions are most favorable. In contrast, during fall and winter, respiration frequently overcomes primary production, making the system heterotrophic on a daily basis. For the year as a whole, simulation predicted that the community acts as an autotrophic system, with its annual gross primary production amounting to ca. 1301 gC m-2 and annual respiration to ca. 899 gC m-2. According to this simulation, the annual NCP of our intertidal F. serratus community therefore reached 402 gC m-2, which is comparable to subtidal communities dominated by canopy-forming brown algae. Although the F. serratus community lives mostly underwater, it was particularly autotrophic during aerial exposures.