Organic enrichment of submarine‐canyon and continental‐shelf benthic communities bymacroalgal drift imported from nearshore kelp forests

Remotely operated vehicle (ROV) surveys were conducted over a 3‐year period to evaluate the importance of macroalgal drift exported from nearshore forests of giant kelp ( Macrocystis pyrifera ) to adjacent submarine‐canyon and continental‐shelf benthic habitats. Abundant macroalgal drift was found in the Carmel Submarine Canyon (153– 454‐m depth) but was rare at Pt. Joe, a continental shelf habitat 9 km away (87–357‐m depth). Most of the drift parcels consisted of brown algae, but 10 other categories were found, including green and red algae and the surfgrass Phyllospadix spp. M. pyrifera drift accounted for as much as 20% of the density of drift parcels in the Carmel Submarine Canyon, and up to 50% of the drift parcels at Pt. Joe. From previously determined turnover rates of drift M. pyrifera and the standing stock estimates from the present study, we estimated that the rate of carbon flux associated with drift M. pyrifera to the benthos of the Cannel Submarine Canyon was 45.2 mg Cm −2 d −1 . Based on published estimates of carbon flux from vertically sinking particulate organic carbon at 400‐m depth in the nearby Monterey Submarine Canyon, and our estimates of kelp‐derived carbon flux, drift M. pyrifera can account for 20–83% of the total particulate organic carbon reaching the sea floor in the Carmel Submarine Canyon. We conclude that drift macrophytes provide significant enrichment of organic carbon to the benthos above that provided by vertically sinking particulate organic material. The extent to which this alters the structure of benthic communities compared to areas without organic enrichment by drift macrophytes remains unknown and awaits further study.