Metabolic activity of intertidal Fucus spp. communities: evidence for high aerial carbon fluxes displaying seasonal variability

Abstract

Semi-sheltered rocky shores are characterized by a typical succession of Phaeophycean-dominated communities where canopy-forming algae are assumed to be very productive. However, metabolic activity of whole communities, integrating the main interactions that occur in these assemblages, still needs to be investigated more deeply, especially when communities are emerged. Thus, the metabolism of the mid-intertidal Fucus vesiculosus and Fucus serratus communities was surveyed once a month throughout a year in the Bay of Morlaix (France). In situ measurements of primary production and respiration were taken at the onset of emersion periods, using benthic chambers coupled to CO2 infrared gas analyzers. Benthic measurements were taken successively on whole communities and after canopy removal, to assess the contribution of the canopies to the communities’ carbon fluxes. Concurrently, nitrogen and organic carbon contents of the two Fucus species were evaluated monthly. Both communities exhibited high metabolic activity year-round (community gross primary production: 400–1000 mgC m−2 h−1; community respiration: 100–500 mgC m−2 h−1), with canopies accounting for 77–97 % of the global CO2 fluxes. However, this relative canopies’ contribution to global CO2 fluxes did not follow seasonal fluctuations, whereas both metabolic activities and nitrogen and carbon contents did, confirming the importance of light and temperature in the regulation of metabolic processes. A nutrient limitation was also suggested to occur in summer, when seawater was nutrient-depleted. Finally, the F. vesiculosus community, located upper on the shore, exhibited lower photosynthetic activity than the F. serratus community, being consistent with the vertical zonation rules of intertidal habitats.