Some investigators have proposed aspartate transcarbamylase (ATCase) activity as an overall index of mesozooplankton productivity. However, seasonal changes in mesozooplankton species composition have never been investigated as a possible source of variation in ATCase activity. In this study, we investigate mesozooplankton composition in terms of (i) developmental stages, (ii) species and developmental stages body mass and (iii) species composition, and their relationship to ATCase activity. In controlled laboratory conditions, ATCase activity variability was closely related to changes in somatic growth rate of the copepod Calanus helgolandicus, but was not related to changes in nucleic acid concentrations. It can be argued, however, that the activity of this enzyme is partially involved in copepod somatic productivity, and should be a good index of embryogenesis. In addition, changes in ATCase activity were not significantly influenced by variability in mesozooplankton biomass, when investigated both on inter- and intraspecific levels. Finally, when a complete seasonal cycle was investigated at a fixed station off Plymouth (English Channel), ATCase activity was not correlated with the abundance of any mesozooplankton species apart from copepodites and adult C. helgolandicus. Furthermore, ATCase activity measured both on mesozooplankton and female C. helgolandicus was significantly correlated (R-2 = 0.72, n = 33, P < 0.001) throughout the year, apart from April. At that particular time of the year, ATCase activity was in phase with the peak of abundance of copepod eggs and nauplii. It is suggested that mesozooplankton peaks of ATCase activity reflect two periods in the life history of the copepod: embryogenesis and terminal moult. We propose further experiments to test this hypothesis and to promote the development of molecular biomarkers in order to characterize specific zooplankton metabolic processes.