Production of Oikopleura dioica (Appendicularia) following a picoplankton ‘bloom’ in a eutrophic coastal area

The ecological importance of the appendicularian Oikupleura dioica as a picoplankton predator and a metazoan secondary producer was assessed in summer 1995 in the Seto Inland Sea, Japan. Just after the collapse of a red tide of Gymnodinium mikimotoi, the abundances of bacteria and picocyanobacteria increased rapidly and attained values of 4.6 × 106 and 5.6 × 105 ml−1, respectively. Concurrent with a subsequent sharp decrease in picoplankton abundance, the abundance and biomass of O.dloica increased drastically, and reached levels of 57 individuals 1−1 and 12 μg C 1−1 respectively. During this period, the carbon-based growth rate of O.dioica was estimated by a bottle incubation experiment The value was very high (1.66 day−1) for a metazoan and its production was comparable with that of copepods reported previously in the Seto Inland Sea in summer. A carbon budget analysis indicates that ingesting picoplankters as main food sources, O.dioica grew rapidly and its biomass attained the high value. Furthermore, the sharp decrease in picoplankton abundance was at least partly attributable to the ingestion by O.dioica.