Top predators rely on carbon derived from giant kelp Macrocystis pyrifera

Net primary production (NPP) fundamentally shapes ecosystems, but the fate of NPP and its transfer through the food web varies depending on its appeal and availability to consumers. Kelp forests are complex and diverse marine ecosystems, and kelp biomass and NPP can vary widely due to processes including trophic cascades and consequent changes in grazing pressure, storm wave disturbance, and changes in ocean climate and nutrients. Kelp, like other foundation species, clearly modulates the physical environment in which species interact: dense canopies of giant kelp Macrocystis pyrifera shade the reef, shaping the benthic community, and dampen flow through the forest. Yet little quantitative work has investigated the trophic contribution of kelps to kelp forest food webs. Here, we show that giant kelp provides significant trophic support to fishes living in the canopy of the kelp forest. Carbon contributions from kelp, via mesograzer prey, relative to pelagic carbon sources, increased with increasing trophic level of fishes, showing that giant kelp is particularly important to top predators. These findings suggest that losses of giant kelp would have significant consequences for coastal ecosystems and have important implications for the conservation and management of temperate reef ecosystems.