A consistent hydrographical feature of the Catalan Balearic Sea is the presence of a doming of the isopycnals, approximately mid-way between the Catalan coast and the Balearic Islands. The Catalan and Balearic boundaries of this feature are marked by two fronts. In July 1983, high chlorophyll concentrations were found near the coast and in a deep chlorophyll maximum which extended across the frontal zones. The phytoplankton distribution was basically determined by the vertical stratification patterns of the water column. The distribution of oxygen and nutrients across the Catalan front, and the presence of relatively high diatom concentrations in the deep chlorophyll maximum of the frontal zone suggested the existence of intermittent fertilization events.