In February 1982 there was an intense bloom of dinoflagellates close inshore off West‐land, New Zealand, and dinoflagellates also formed a large proportion of the phytoplankton offshore. Diatoms were present only at the bottom of the photic zone inshore off Wanganui Bluff, where dissolved nitrate‐nitrogen levels were elevated. Most biomass and production parameters were at least twice as large as in winter except for offshore biomass which was larger in winter. Weighted average chlorophyllaconcentrations were negatively correlated with the depth of the mixed layer. The relative contribution which the vertical distribution of chlorophylla, the production per unit chlorophyllaat optimal light intensities, and the initial slope of the light saturation curve made to the relationship between water column production per unit chlorophyllaand incident solar irradiance in inshore and offshore waters is discussed. In the non‐senescent inshore populations dominated byProrocentrum micans, 91% of the variance in water column PBwas explained by variance in incident solar irradiance with a light utilisation index of 3.00 g C (g Chl.a)‐1E‐1m2.