Harmful algal blooms: How strong is the evidence that nutrient ratios and forms influence their occurrence?

Harmful algal blooms: How strong is the evidence that nutrient ratios and forms influence their occurrence?
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Journal Article
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Davidson K, Gowen RJ, Tett P, Bresnan E, Harrison PJ, McKinney A, Milligan S, Mills DK, Silke J, Crooks AM

Estuarine Coastal and Shelf Science
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phytoplankton, Harmful algae, Nutrient ratios, eutrophication, mesocosm, review


There is a perception that anthropogenically-driven changes in nutrient supply to coastal waters influencesthe abundance, frequency and toxicity of harmful algal blooms (HABs) through a change in theform or ratio of nutrient that limits phytoplankton growth. If nutrient concentrations are not limiting forgrowth, then ratios do not influence floristic composition. At non-limiting concentrations, evidence thatalteration of nitrogen: phosphorus (N:P) ratios has stimulated HABs is limited, and primarily based onhypothesised relationships in relatively few locations (in particular: Tolo Harbour Hong Kong and DutchCoastal Waters). In all cases, an unequivocal causal link between an increase in HABs (frequency,magnitude or duration) and change in N or P as the limiting nutrient is difficult to establish. The silicon(Si) limitation hypothesis is generally supported by experimental evidence and field data on the nuisanceflagellate Phaeocystis. We found little evidence that high N:Si ratios preferentially promote harmfuldinoflagellates over benign species. Laboratory studies demonstrate that nutrient ratios can influencetoxin production, but genus and species specific differences and environmental control make extrapolationof these data to the field difficult. Studies of the role of dissolved and particulate organic nutrientsin the growth of HAB species, while limited, demonstrate the potential for organic nutrients (especiallyorganic N) to support the growth of a range of HAB species. There is a clear need for better understandingof the role of mixotrophy in the formation of HABs and for studies of HAB and non-HAB species incompetition for environmentally realistic concentrations of organic nutrients

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