Temporal changes in the ciliate assemblage and consecutive estimates of their grazing effect during the course of a Heterocapsa circularisquama bloom

Abstract

Temporal changes in ciliate assemblages during the course of a bloom of the harmful microalga Heterocapsa circularisquama (Dinophyceae) were investigated and consecutive estimates of species-specific maximum grazing losses were analyzed from August to September 1998 at a site in western Hiroshima Bay, the Seto Inland Sea of Japan. Temporal increases of the H. circularisquama mean concentration in the water column were observed twice (25–29 August and 7–10 September) with the maximum concentration (ca. 4000 cells mL–1) being recorded on 25 August. The main ciliate genera during the bloom were Favella, Tontonia, Eutintinnus, Tintinnopsis and Amphorellopsis. Increases of Favella and Tontonia were observed when the concentration of H. circularisquama ranged from 260 to 1170 cells mL–1. Total maximum grazing loss estimated from the abundance and ingestion rate of each ciliate species on H. circularisquama ranged from 1 to 75% standing stock removed d–1 of the H. circularisquama concentration. High grazing losses mainly due to the genera Favella and Tontonia occurred during the period when the H. circularisquama concentration was decreasing. These results suggest that grazing by ciliate assemblages can influence the population dynamics of H. circularisquama despite the potentially toxic nature of the phytoplankter.