Results obtained from short-term (8 h to 24 h intervals) measurements of physical, chemical and biological properties of the 70 m water column from an anchor station in the Bornholm Sea over a 10-day period are presented and discussed. Phytoplankton biomass concentration and production rates indicated that the spring bloom was in progress in this period. The onset of the spring bloom occurred prior to the advent of thermal stratification. Peak growth rates, accompanied by nutrient depletion and biomass accumulation in surface layers, were concomitant with calm weather and a cloudless sky after which a part of the population was observed to sink out of the water column unimpeded by the permanent halocline. Maximum sinking rates of the dominant species, Skeletonema costatum, ranged between 30 to 50 m per day during this event. The development of the spring bloom apparently takes place in a series of events during which periods of low production alternate with periods of high production and rapid sedimentation of parts of the population
