The matrix influences direct and indirect effects of an anthropogenic disturbance on marine organisms.

The magnitude and direction of direct and indirect effects of disturbances can be context-dependent, with the matrix (surrounding habitat) in which populations are embedded either mitigating or worsening the impacts of disturbances. Chemical disturbances are particularly harmful and can affect organisms directly or indirectly. We used bleach, a common stressor in marine systems, to test hypotheses about direct and indirect effects of anthropogenic disturbances on intertidal grazers and the influence of the surrounding macro-algal matrix on such effects. We manipulated the contaminant, food (biofilm) and surrounding macro-algal matrix. Fewer limpets were found in contaminated areas. Bleach had a strong direct negative effect on limpets and caused a reduction in biofilm food, indirectly affecting limpets. This effect was strongest in the presence of macro-algal matrix. Anthropogenic disturbances can have major consequences via direct and indirect effects on key interacting species. We showed that such effects are, however, context-dependent. Capsule: Pollution is a major driver of biodiversity declines. We show that direct and indirect effects of contaminants on organisms depend on the context in which they occur.