In order to study the importance of phytoplankton as a food source for benthic consumers, Kinsale harbour, a small estuary in the southwest of Ireland, was sampled on a monthly basis during the course of 1994. Nutrients, salinity, temperature, pigments (chlorophyll a and phaeopigments) and phytoplankton species composition were determined along longitudinal and vertical profiles. Based on salinity distributions, Kinsale harbour can be classified as a partially mixed estuary. River discharge and tidal mixing result in a strong estuarine circulation and an intensive exchange between the estuary and coastal waters. As a result, residence time is too low (< 1 day) for autochthonous phytoplankton populations to develop and phytoplankton observed in Kinsale harbour is predominantly advected from coastal waters. High concentrations of phaeopigments, exceeding those of chlorophyll a, were observed at the uppermost stations sampled, suggesting mass mortality and accumulation of coastal phytoplankton within the estuary. Possible causes for this phenomenon include osmotic stress or grazing by macrozooplankton within the estuary and the subsequent entrapment of senescent phytoplankton or faecal pellets in the baroclinic circulation near the salt wedge. It is suggested that in small estuaries, like Kinsale harbour, which are strongly influenced by river discharge, estuarine circulation induces the import of coastal phytoplankton into the estuary and provides an allochthonous carbon source to consumers within the estuary.