Floating Kelp Rafts as Indicators of Aggregation Zones within the San Juan Archipelago

Abstract

Bull kelp forests line Salish Sea coastlines, providing important ecosystem services to shallow subtidal habitats. Each winter, bull kelp forests are dislodged by winter storms, setting “kelp rafts” adrift on currents. Changing climate conditions threaten to strengthen winter storms in the Pacific Northwest, which could both increase annual kelp forest dislodgement rates and the abundance of kelp rafts. The passive drifting behavior of kelp rafts is like that of plankton. Within the San Juan Archipelago, kelp rafts are often observed in high densities at tidal fronts, making them indicators of planktonic aggregation zones. Documenting kelp raft distribution patterns may provide insight into important aggregation areas for plankton and forage fish, as well as the distribution of important resting substrate for seabirds. This study investigated kelp raft distribution and density throughout the San Juan Archipelago and attempted to isolate driving factors behind kelp raft density and distribution. Kelp raft density was found to be highly correlated with both tidal amplitude and current strength but was not significantly correlated with proximity to the nearest kelp forest. Overall, 13.1% of all observed kelp rafts were occupied by gulls.