Spring is the season of most pronounced changes of biotic and abiotic parameters in boreal marine plankton communities. In order to establish a basis for the interpretation of trophic interactions and succession processes we monitored a set of ecological key parameters around the island of Helgoland (North Sea, German Bight) in 2004. We focussed our interest on the ability of copepods to utilize food during the development of the phytoplankton bloom when food quantity and quality are changing.Water temperature rose from 3 to 4°C in early March to more than 10°C at the end of May. Phytoplankton started to bloom in early April. The nitrogen content of seston increased concomitantly. The size of the dominant copepod species (Acartia clausii and Temora longicornis) increased from March to April. Their dry masses almost doubled. Variations in gross biochemical parameters and digestive enzyme activities were evident in both species. Grazing of females of either species on natural seston under in-situ conditions was selective.In spring 2004 phytoplankton bloomed comparatively late and may have been controlled by grazers. Food quantity and quality increased and enhanced the copepods physiological condition and reproductive success. The ability of copepods to efficiently utilize food was reflected by high digestive potentials. Species specific enzyme patterns indicate different traits of food utilization. The comprehensive knowledge about nutritional demands and catalytic potential may help to better understand the driving forces of food selection and, consequently, the succession of plankton communities.Acartia clausii, Temora longicornis, grazing, seston quality, digestive enzymes