Import, mortality and accumulation of coastal phytoplankton in a partiallymixed estuary (Kinsale harbour, Ireland)

In order to study the importance of phytoplankton as a food source for bent hic consumers, Kinsale harbour, a small estuary in the southwest of Ireland , was sampled on a monthly basis during the course of 1994. Nutrients, sali nity, temperature, pigments (chlorophyll a and phaeopigments) and phytoplan kton species composition were determined along longitudinal and vertical pr ofiles. 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 au tochthonous phytoplankton populations to develop and phytoplankton observed in Kinsale harbour is predominantly advected from coastal waters. High con centrations of phaeopigments, exceeding those of chlorophyll a, were observ ed at the uppermost stations sampled, suggesting mass mortality and accumul ation of coastal phytoplankton within the estuary. Possible causes for this phenomenon include osmotic stress or grazing by macrozooplankton within th e estuary and the subsequent entrapment of senescent phytoplankton or faeca l pellets in the baroclinic circulation near the salt wedge. It is suggeste d that in small estuaries, like Kinsale harbour, which are strongly influen ced 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.