| Summary: | As the oceans absorb excess heat and CO2 from the atmosphere, marine primary producers face significant changes to their abiotic environments and their biotic interactions with other species. Understanding the bottom-up consequences of these effects on marine food webs is essential to informing adaptive management plans that can sustain ecosystem and cultural services. In response to this need, this dissertation provides an in-depth consideration of the effects of global change on foundational macroalgal (seaweed) species in a poorly studied, yet highly productive region of our world’s oceans. To explore how seaweeds within seasonally dynamic giant kelp forest ecosystems will respond to ocean warming and acidification, I employ a variety of methods: year-round environmental monitoring using an in situ sensor array, monthly subtidal community surveys, and a series of manipulative experiments. I find that a complementary phenology of macroalgal production currently characterizes these communities, providing complex habitat and a nutritionally diverse energy supply to support higher trophic levels throughout the year. I also find that future ocean warming and acidification will lead to substantial shifts in the phenology, quantity and quality of macroalgal production in these systems. My results suggest that the giant kelp Macrocystis pyrifera may be relatively resilient to the effects of global change in future winter and summer seasons at high latitudes. In contrast, the calcifying coralline algae Bossiella orbigniana and Crusticorallina spp. and the understory kelps Hedophyllum nigripes and Neoagarum fimbriatum will experience a suite of negative impacts, especially in future winter conditions. The resulting indirect effects on macroalgal-supported coastal food webs will be profound, with projected reductions in habitat and seasonal food supply on rocky reefs. Coming at a time of heightened interest in seaweed production potential at high latitudes, this dissertation provides a comprehensive evaluation of the ...
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