Macroalgae are useful as model organisms because their simple physiology and modular growth allows their investments into growth, defence, and reproduction to be quantified. Ascophyllum nodosum is a dioecious perennial which responds clearly to environmental stressors, and Sargassum muticum is an invasive pseudo-annual which grows in discrete populations of known time-since-invasion. Two chapters investigate the occurrence of sexual dimorphism in A. nodosum under stress, and incorporate surveys, chemical analyses, and feeding trials to demonstrate that females invest more into sexual reproduction at sites where stressors impact juvenile mortality, and compensate by reducing investment into defence, leaving them more vulnerable to grazers. At sites where stressors impact mainly on adult mortality and performance with lower influence on juvenile mortality, limited sexual dimorphism is observed. Similar results have been demonstrated in terrestrial plants, but none have quantified responses to stress in such detail. The second two chapters test the Enemy Release Hypothesis, the Evolution of Increased Competitive Ability hypotheses, and the Biotic Resistance hypothesis using surveys, chemical analyses, and feeding trials with S. muticum and its associated grazers from populations established for different lengths of time, over 40 years. Grazers from older S. muticum were more likely to feed upon it in the laboratory, but surveys of chemical defences did not reflect any increased pressure over time in the field. Instead, S. muticum increased its defensive investment in the presence of a greater diversity of grazers, irrespective of the length of time those grazers had been exposed to it. Therefore although grazers can learn to consume S. muticum, they are still unlikely to do so in the field, such that many species must be present before at least some begin to impart top-down pressure upon it. Collectively these investigations demonstrate the high value of using macroalgae in wider ecology.