Phased cell division and other biological observations in Dinophysis populations in sub-surface layers off the southwest coast of Ireland.

The proportions of viable cells of Dinophysis spp. that were paired (dividing) and recently divided duringa cell cycle were measured on populations of D. acuta and D. acuminata observed off the south coast ofIreland in July 2007 and July 2009. Both species exhibited phased cell division in 2009 with maximum frequency of division (fmax) 2 h after sunrise. Different patterns of division (timing of fmax) were shown by D. acuta in 2007, when the population aggregated in a thin layer was transported by a coastal jet flow. High resolution (decimetre-scale) profiles within the thin layer showed large differences in the vertical distribution of biological properties (feeding status, mortality). Values of the specific growth rate m were compared to estimates derived in similar fashion from observations on Dinophysis populations elsewhere.  Different patterns exhibited by the same species in different regions may be attributed to adaptations to latitudinal differences (length of photoperiod). The question of whether phased cell division always occurs in Dinophysis populations, and the incorporation of the potential specific division rate into models of Dinophysis growth are discussed. Comprehensive field data sets demonstrate the impact of the results on the coherence of Dinophysis populations during their transport along the Irish coast in jet-like flows towards sites of intensive shellfish culture.