| Summary: | Marine macroalgae are ubiquitous across coastal oceans worldwide and provide critical habitat and services for diverse assemblages of organisms as well as services including fisheries production, carbon sequestration, and nutrient cycling. The effects of climate change are altering multiple aspects of macroalgal community ecology including food web structure, organismal diversity, and community resilience. In this dissertation, I approach macroalgal community ecology on multiple scales, across broad geographic space, and through various analytical lenses to capture key insights into the functioning of macroalgal forests. In Chapter II I identify the unique fatty acid profiles and stable isotope content of 31 Antarctic macroalgae. While the phylogenetic differentiation driven by fatty acids has a stronger influence on distinguishing Antarctic macroalgae, the added dimension of stable isotopes can likely make the combination of the two approaches particularly powerful in the application of food web studies. In Chapter III I provide a summary of the effects of conservation areas on algal assemblages. I find that all targeted kelp species are observed across the marine reserves but their presence varies among sites and years. In Chapter IV I describe the feeding preferences of the sunflower seastar through meta-analysis and a cafeteria experiment. A total of 114 prey taxa are reported across all studies, with bivalves and urchins tending to be the primary observed prey items. This agrees with our cafeteria experiments that find Pycnopodia tended to prefer green and purple urchins, and mussels, although the quantity of each prey type consumed is highly variable. In Chapter V I provide evidence for non-consumptive effects of the sunflower seastar on grazing sea urchins. I find that the presence of a waterborne Pycnopodia cue reduces the grazing rate of fed urchins by 50% over short (~24 h) time scales. In contrast, starved urchins consume kelp and do not exhibit an escape response in the presence of a Pycnopodia cue. ...
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