This thesis characterises the roles of indirect effects and temporal contingencies, which are events that have happened at some point in time and lead to one of many possible outcomes, on ecosystem level response to multiple potentially-interacting stressors. The assemblage response of a semi-natural marine food web to the interactive effects of warming, eutrophication and changing body size of a top predator was observed. The effects of minor nutrient enrichment are negligible and warming produces subtle responses, but the manipulation of top predator body size evokes a strong trophic cascade. This suggests that the long-term decreased body size effect of warming will be of greater ecological consequence than any short-term direct effects of higher temperature that may be experienced by individuals. The relative body size can explain much of the trophic interaction between predator and prey species. The absence of refugia down-shifts the size-structuring of the predator-prey relationship so that larger predators exert greater predation pressure on smaller prey. The effect of copper contamination on two successional stages of subtidal community assembly was examined. It was found that: (1) biofilms are sensitive to changes in copper concentration in the surrounding water column, irrespective of whether the changes involve exposure to or depuration from copper, and not just exposure to elevated copper concentrations per se and; (2) invertebrate assemblages respond to increased exposure to copper from the surrounding water column. This suggests that benthic invertebrate settlement is robust to the indirect effect of copper-induced changes in biofilm communities, but not to the direct toxic effect of copper itself. This work highlights the importance of temporal contingencies including legacy effects, ecological history and the coincidence of random events for identification of the mechanisms in community assembly
