Estimates of carbohydrate assimilation within the mesozooplankton compartment are a fundamental concern in oceanography because they give a general expression of the biologically-regulated incorporation of metabolic fuel produced from inorganic nutrients by phytoplankton in the sea. A new tool is proposed for the assessment of such processes. It is based on the measurement of the activity of an enzyme, pyruvate kinase (PK), on samples of the whole mesozooplankton community. Here, the results are presented of an early attempt at such an implementation in the Bay of Biscay (north-east Atlantic) in springtime. A main area situated in front of an estuary (Gironde) has been extensively studied-the same sampling grid was covered on three occasions (two 20 day time lags between each). Strong mesozooplankton PK activities were related to high values of nitrates or chlorophyll a; the inverse was also true. A significant correlation has been found between thermal stratification and enzyme activity. On a broader scale, in the southern part of the Bay of Biscay continental shelf, the overall significant influence of de-salted water coming from rivers was revealed by spatial variations in mesozooplankton PK activities. The whole dataset suggests that this new tool should be potentially very useful in certain future studies, especially in fisheries oceanography which often requires characterization of mesoscale features.