The composition and variation of epiphytic communities on Sydney seaweeds and the differing approaches of mitigation by two species of macroalga

Macroalgal epiphytic communities are critical components of marine ecosystems and can drive increased biodiversity. Understanding the epiphytic communities on macroalgae and the mechanisms that algae to mitigate the impact of epiphytes adds to understanding of the evolution and ecology of these communities. The interaction between the communities and the host also informs our understanding of life history traits, growth and resource allocation strategies of the host algae. I assessed the epiphytic communities on a range of macroalgae across several sites around the Sydney, NSW region. The type of algal host was the most significant factor in determining both fouling load and epiphytic community composition. The green alga Caulerpa filiformis J.Agardh and the brown alga Dilophus marginatus J.Agardh were selected for further study. Unexpectedly I recorded a decrease in fouling coverage on both D. marginatus and C. filiformis over the period of this study. A decrease in fouling coverage was correlated with warmer local temperatures for D. marginatus, but the inverse was true for C. filiformis. I found that juvenile sporophyte fronds of the heavily fouled D. marginatus were chemically defended against fouling. The efficacy of these defences was significantly less on extracts from fronds with visible sporangia. The in-situ coverage of epiphytic communities on juvenile and reproductive fronds was reflected in the efficacy of the surface extracts, highlighting the importance of ontogeny. Compounds from juvenile D. marginatus fronds should be further investigated as a source of anti-fouling compounds.C. filiformis is heavily defended against herbivory, but I found little evidence that this alga had any chemical defences against fouling. The strong relationship between damage and fouling and the observation of empty fronds adjacent to abscission zones suggests that epiphyte growth is mitigated by blade abandonment. This is the second recorded case of blade abandonment suggests that C. filiformis may engage in protoplasm resorption to recycle resources. This thesis has demonstrated that in situ fouling is a remarkably poor predictor of the presence of chemical defences. Ontogeny and morphology should be considered in designing bioassays to reveal complex defence strategies