The distribution of microplankton biomass and the metabolism of surface waters in the western Mediterranean are shown to be strongly associated with the circulation. Chlorophyll a and respiratory electron transport system (ETS) activity of microplankton were measured in surface waters during the Western Mediterranean Circulation Experiment in May and June 1986. Biological signatures, defined by satellite and surface ship measurements, defined complex circulation patterns of surface waters between the Alboran Basin and the Straits of Sicily. Close correlation was found between the distribution of in situ measured surface chlorophyll, ETS activity, ETS/Chl ratio (chlorophyll-specific ETS activity), salinity, and temperature with satellite-derived chlorophyll and surface temperature. High ETS/Chl ratios were found located in frontal regions of the Algerian Current and encircling a large anticyclonic eddy. Statistical correlations were performed on in situ and synoptic satellite data. Results show positive correlation between chlorophyll and ETS activity and ETS/Chl ratio and temperature. Excellent correlations were found between in situ chlorophyll and chlorophyll derived from the coastal zone color scanner. Improved statistical correlations were observed by separation of the surface waters based on their salinity into Modified Atlantic Water (MAW) (S37). MAW with higher chlorophyll and metabolic activity is shown to characterize the circulation of the Algerian Current. The differences in the statistical correlations of these water masses indicate that regional circulation significantly influences the biological functions of microplankton populations. ¿ American Geophysical Union 1990