Recent European Court rulings in the context of eutrophication viewed proliferation of a particular species of algae as a disturbance to the balance of aquatic ecosystems and the proliferation of one or more species as a cause of a reduction in other species. We discuss the scientific basis for this opinion in relation to the growth of marine primary producers and current debates about ecosystem stability. Opposing views in this debate are those of (a) the `balance of nature' paradigm, in which communities of organisms tend towards a stable climax composition, and (b) communities as dynamic systems that may be governed by `basins of attraction' in state space. We use data from the Irish Sea and Narragansett Bay, together with a review of the literature, to show that: the dynamics of temperate marine phytoplankton, with seasonal successions, corresponds more to (b) than to (a); the temporary dominance of any one species of micro-alga or cyanobacterium is part of the natural dynamics of phytoplankton communities and does not permanently impact on other species. Understanding the phytoplankton as a dynamic system suggests its status should not be assessed against a `climax' model and that eutrophication should be diagnosed from fundamental (nutrient-induced) perturbations of ecosystem state and function rather than from changes in fixed assemblages of species and thresholds of abundance. (C) 2012 Elsevier Ltd. All rights reserved.