The main objective of this thesis was to study the effects of crude oil extracts on intertidal sediment communities. The approach was to focus on community structure and investigate the impact of a specific perturbation, crude oil extracts, relating to hypothetical oil spill situations. Both the impact of different disturbances on the same benthic community (i.e. with uniform structure) and the impact of the same disturbance on differently structured benthic communities were investigated. The effects on number of species, distribution of individuals between species and diversity were analysed, as well as recovery mode (trajectory of the system in time) and species arrival. The effects of different levels of a disturbance on an intertidal community were assessed, using water soluble fractions of crude oil (WSF) and two different concentrations of formaldehyde. The WSF had very little effect on community structure and recovery of the benthic community was prompt. The effects of the different concentrations of formaldehyde on the benthic community were indistinguishable: both concentrations had dramatic immediate effects. The relation between complexity and stability in benthic communities was evaluated by first setting-up two communities of different complexity using the equilibrium theory of island biogeography. After these different communities were established they were subjected to the effects of oil and dispersant. The impact of this perturbation on the two communities with different complexity was analysed both in terms of structure (as above) and functional level, through the analysis of oxygen uptake in the different communities. Although there was an indication that the oil had a depressing effect on the structure of both community types, it was not clear which species were most affected by the treatment. Individual comparisons between the two differently-structured treated communities and their controls were undertaken.