Influence of Kelp Forest Biomass on Nearshore Currents

As part of a project focused on the coastal fisheries of Isla Natividad, an island on the Pacific coast of Baja California, Mexico, we conducted a 2-1/2 year study of flows at two sites within the island's kelp forests. At one site (Punta Prieta), currents are tidal, whereas at the other site (Morro Prieto), currents are weaker and may be more strongly influenced by wind forcing. Satellite estimates of the biomass of the giant kelp (Macrocystis pyrifera) for this period varied between 0 (no kelp) and 3 kg/m(2) (dense kelp forest), including a period in which kelp entirely was absent as a result of the 2014-2015 "Warm Blob" in the Eastern Pacific. During this natural "deforestation experiment", alongshore velocities at both sites when kelp was present were substantially weaker than when kelp was absent, with low-frequency alongshore currents attenuated more than higher frequency ones, behavior that was the same at both sites despite differences in forcing. The attenuation of cross-shore flows by kelp was less than alongshore flows; thus, residence times for water inside the kelp forest, which are primarily determined by cross-shore velocities, were only weakly affected by the presence or absence of kelp. The flow changes we observed in response to changes in kelp density are important to the biogeochemical functioning of the kelp forest in that slower flows imply longer residence times, and, are also ecologically relevant in that reduced tidal excursions may lead to more localized recruitment of planktonic larvae. Plain Language Summary Observations of flows were made during a two and one half year period when the state of kelp forests in the nearshore of an island off Baja California varied between lush and nonexistent. These measurements are used to show how density of kelp biomass affects currents in the nearshore of the island. Notably, the effects vary with the frequency with which the currents vary, and are much stronger for flows along the coast as opposed to cross-shore flows that control exchange water in the kelp forest with water offshore.