Division, Agri-Food and Biosciences Institute, Newforge Lane, Belfast, BT9 5PX, United Kingdom 2 Scottish Association for Marine Science, Scottish Marine Institute,

Oban, Argyll, PA37 1QA, United Kingdom 3 Marine Scotland Marine Laboratory, PO Box 101, 375 Victoria Road,

Aberdeen, AB11 9DB, United Kingdom 4Department of Earth and Ocean Sciences, University of British Columbia,

Vancouver, BC, Canada V6T 1Z3 5 Centre for Environment, Fisheries & Aquaculture Science, Pakeeld Road,

Lowestoft, Suffolk, NR33 0HT, United Kingdom 6The Marine Institute, Rinville, Oranmore, Galway, Ireland

7Corresponding author, e-mail address: [email protected]

Abstract Anthropogenic nutrient enrichment of coastal waters is considered to be one reason for the putative worldwide increase in the occurrence of harmful algal blooms (HABs). This has led to the view that the occurrence of HABs diagnoses the undesirable consequence of anthropogenic nutrient enrichment and thus eutrophication. This view rests on a set of assumptions. In this paper, these assumptions and the arguments that have been made about the increase in HABs and their causes are reviewed. No consensus regarding the role of anthropogenic nutrients in stimulating the occurrence of HABs was found. Attempts to relate trends in HABs to enrichment are confounded by spatial and temporal variability in naturally occurring HABs; the human-mediated transport of HAB species; increased monitoring and reporting; variability in large-scale meteorological forcing; the in£uence of climate change. For large-biomass HABs, the hypothesis that nutrient enrichment can cause HABs is supported in some water bodies but not in others. The global evidence that enrichment brings about an increase in low-biomass HABs of toxin-producing species is more equivocal. It is concluded that evidence of a link in one coastal region should not be taken as evidence of a general linkage in other coastal regions. The occurrence of HABs and the abundance of HAB species should not be used to diagnose eutrophication unless a link to anthropogenic nutrient enrichment can be demonstrated.