Scales characterising a high density thin layer of Dinophysis acuta Ehrenberg and its transport within a coastal jet

► High density sub-surface thin layer of Dinophysis acuta off south coast of Ireland. ► Transport of small scale patch within a coastal jet. ► Implications for modelling and prediction. An investigation into the distribution of Dinophysis spp. in coastal waters off the south coast of Ireland was carried out in July 2007. Dinophysis acuta was present as a sub surface layer containing up to 55,000cellsL−1. The population had a high percentage of viable cells (mean: 89%; median: 94%; n=24) with a high specific division rate (∼0.55d−1). The layer, of approximately 5m thickness, did not coincide with the fluorescence maximum and was present as a patch of horizontal dimension less than 10km×7km. Both conventional and towed undulating CTD used in conjunction with high vertical resolution sampling methods showed the patch of Dinophysis to move with a similar speed and direction as the coastal flow, which ran parallel to the coast in the form of a coastal jet with speed of the order of 6.5–7kmday−1. The implications of the alongshore transport of populations of harmful species in coastal jets for monitoring programmes and predictive models are discussed.