Size variation within species as a result of individual growth and development over the life cycle is a ubiquitous feature of many aquatic organisms. We review the implications of this size variation for the dynamics of aquatic systems. Ontogenetic development results in differences in size dependent competitive abilities between differently sized individuals giving rise to cohort cycles that are qualitatively different from traditional predator prey cycles. Size-dependent interactions also mean that the type of interaction - competitive or predatory - changes over the life cycle as a result of an increase in size. At the intraspecific level, cannibalistic interactions may, depending on the life history characteristics of the cannibal, give rise to either equilibrium or cycles driven by a mixture of inter-cohort cannibalism and competition. In multispecies contexts, size variation and particularly food dependent growth lead to the presence of alternative states involving catastrophic collapses. These size-structured interactions have so far been mainly demonstrated for fish and cladocerans, but do have whole lake food web ramifications.
