Bimodal spore release heights in the water column enhance local retention and population connectivity of bull kelp, <i>Nereocystis luetkeana</i>

Dispersal of reproductive propagules determines recruitment patterns and connectivity among populations and can influence how populations respond to major disturbance events. Dispersal distributions can depend on propagule release strategies. For instance, the bull kelp, <i>Nereocystis luetkeana</i>, can release propagules (spores) from two heights in the water column ("bimodal release"): at the water surface, directly from the reproductive tissues (sori) on the kelp's blades, and near the seafloor after the sori abscise and sink through the water column. <i>N. luetkeana</i> is a foundation species that occurs from central California to Alaska and is experiencing unprecedented levels of population declines near its southern range limit. We know little of the kelp's dispersal distributions, which could influence population recovery and restoration. Here, we quantify how bimodal spore release heights affect dispersal outcomes based on a numerical model specifically designed for <i>N. luetkeana</i>. The model incorporates oceanographic conditions typical of the species' coastal range and kelp biological traits. With bimodal release heights, 34% of spores are predicted to settle within 10 m of the parental alga and 60% are predicted to disperse beyond 100 m. As an annual species, bimodal release heights can facilitate the local regeneration of adults within a source kelp forest while also supporting connectivity among multiple forests within broader bull kelp metapopulations. To leverage this pattern of bimodal spore dispersal in bull kelp restoration management, directing resources toward strategically located focal populations that can seed other ones could amplify the scale of recovery.