Owing to their productivity, kelps may be the main primary carbon source for consumers in coastal areas. Their contribution has often been compared to that of phytoplankton, but the potential involvement of the red algae associated with kelp forests has been overlooked. All these 3 primary sources have distinct life cycles and may contribute to the particulate organic matter pool differently according to season. In the present study, we characterised the trophic structure of a pristine Laminaria hyperborea forest off the coast of Brittany (France) in terms of the organic carbon biomass of the main primary producers and consumer trophic groups on 4 sampling dates over a 1 yr period. Senescence of many red algae species occurred during their resting period (i.e. before November), whereas the kelp regrowth period (i.e. before March) was accompanied by the fragmentation of old lamina. During these periods, when phytoplankton biomass is comparable, stable isotope analyses (delta C-13 and delta N-15) were conducted in an attempt to link structure to trophic functioning. By combining analyses of temporal variability in primary source and consumer isotopic ratios and using mixing models, we inferred changes in the trophic significance of macroalgae in the associated food web. Decaying kelp laminae were a major contributor to the particulate organic matter pool, and the fragmentation of old lamina promoted their contribution to the diet of deposit-and suspension-feeders in March. Growth of red algae enhanced direct grazer consumption in March, while their senescence contributed significantly to primary consumer diets in November via indirect consumption.