Abstract
To offer insight into the effects of macroalgal assemblage shifts under potential environmental stressors, the studies presented in the present thesis investigated the trophic connections between common native and invasive brown macroalgae and amphipods on subtidal rocky reefs and sandy beaches of Otago, Aotearoa New Zealand. I sampled macroalgae and their associated epifauna to reveal trophic relationships using stable isotope analysis (SIA). Mixing models revealed that epifaunal amphipods (Maeridae, Paradexamine and Aoridae) were heavily reliant on macroalgal organic matter (~80%) rather than phytoplankton, and that Sargassum sinclairii and Undaria pinnatifida were directly grazed on by epifauna, whereas Carpophyllum flexuosum was not consumed. Sandhopper (Bellorchestia quoyana) populations had similar trophic relationships with wrack material as epifauna did with host algae, suggesting a heavy reliance on organic matter derived from macroalgae in beach wracks. Analysis of populations sampled from different sites and wrack species showed Durvillaea sp. to attract the largest sandhoppers in a beach with multiple drift algae species, while also supporting more biomass at a beach with only Durvillaea sp. making up beach wrack. No-choice grazing experiments also revealed that U. pinnatifida and M. pyrifera were both readily consumed by sandhoppers, while C. flexuosum was not consumed. However, differences arose in the timing of consumption, as U. pinnatifida was eaten immediately and degraded within 48 hours, while M. pyrifera degraded within 144 hours and was consumed exponentially starting at 48 hours. Analyses of phlorotannin concentrations revealed U. pinnatifida and Durvillaea sp. to have the lowest (near zero) concentrations, followed by M. pyrifera (low concentrations), S. sinclairii (medium-low concentrations), and C. flexuosum (high concentrations). Additionally, experimental degradation of blades and analysis of phlorotannin concentrations over time demonstrated that concentrations in C. flexuosum decayed exponentially, while no difference was observed for M. pyrifera over seven days. However, field observations confirmed that phlorotannin concentrations were lower in Durvillaea sp. and M. pyrifera wracks than in fresh material. Consequently, I attribute differences in consumption of blades by both epifauna and sandhoppers to the initial (i.e., live blade tissue) phlorotannin concentration of macroalgae. I argue that my results and future work suggested herein could be used to understand the consequences of community shifts in kelp beds and to inform ecosystem-based management of macroalgal-based ecosystems of Otago.