Sources of seasonal variability in mesozooplankton aspartate transcarbamylase activity in coastal waters off Plymouth, UK

Some investigators have proposed aspartate transcarbamylase (ATCase) activi ty as an overall index of mesozooplankton productivity. However, seasonal c hanges in mesozooplankton species composition have never been investigated as a possible source of variation in ATCase activity. In this study, we inv estigate mesozooplankton composition in terms of (i) developmental stages, (ii) species and developmental stages body mass and (iii) species compositi on, and their relationship to ATCase activity. In controlled laboratory con ditions, ATCase activity variability was closely related to changes in soma tic growth rate of the copepod Calanus helgolandicus, but was not related t o changes in nucleic acid concentrations. It can be argued, however, that t he activity of this enzyme is partially involved in copepod somatic product ivity, and should be a good index of embryogenesis. In addition, changes in ATCase activity were not significantly influenced by variability in mesozo oplankton biomass, when investigated both on inter- and intraspecific level s. Finally, when a complete seasonal cycle was investigated at a fixed stat ion off Plymouth (English Channel), ATCase activity was not correlated with the abundance of any mesozooplankton species apart from copepodites and ad ult C. helgolandicus. Furthermore, ATCase activity measured both on mesozoo plankton and female C. helgolandicus was significantly correlated (R-2 = 0. 72, n = 33, P < 0.001) throughout the year, apart from April. At that parti cular time of the year, ATCase activity was in phase with the peak of abund ance of copepod eggs and nauplii. It is suggested that mesozooplankton peak s 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 o rder to characterize specific zooplankton metabolic processes.