Climate change impacts on the kelp life history cycle

Anthropogenic climate change will cause changes to the abundance, distribution, and survival of species in ecosystems worldwide. Kelps are foundation species that form the structure of temperate, marine ecosystems on coastlines worldwide. Kelps support highly productive communities that are ecologically and economically valuable, but are susceptible to the increases in environmental stressors associated with climate change. This susceptibility varies with life history stage, with macroscopic stages less sensitive to environmental stress than microscopic stages. I addressed the effects of climate change on the different life history stages of intertidal kelp from rocky shores on the Pacific coast of North America. I began with two studies on the interactive effects of multiple climate stressors on microscopic stages of kelp. Increasing temperature, CO₂, and UV caused mechanical and functional damage to zoospores during their motile phase, and caused further reductions in settlement, germination, and adhesion of the initial sessile phase of the life history cycle. Settlement style was also affected, with decreased time spent looking for suitable attachment locations and microenvironments, and overdispersion of spore settlement distribution, which has been shown to decrease fertilization rates and sporophyte abundance. In my final research chapter, I describe the effects of increases in frequency of extreme warming events on macroscopic juvenile and adult kelp sporophytes. I also manipulated adult density in situ to determine the stress ameliorating affect of neighbor proximity on both juvenile recruitment and seasonal adult growth along a vertical tidal gradient. Extreme warming treatments reduced recruitment and seasonal growth of adults in the upper shore when adult density was low and environmental stressors were not mitigated by neighboring individuals. All other treatment combinations showed slightly positive effects of warming on recruitment and adult size. I predict that the aforementioned population effects resulting from increases in frequency of extreme warming events will cause an overall reduction in this species’ habitable vertical space in the intertidal zone. The combined impacts of overall reductions in microscopic life history stages with decreasing recruitment and habitable space for the macroscopic life history stage indicate overall reductions in abundance of future populations of intertidal kelp species.Science, Faculty ofZoology, Department ofGraduat