Seasonal changes of planktonic polychaete larvae and chlorophyll a concentration in Onagawa Bay, northeastern Japan

Abstract

Seasonal changes of abundance and composition of planktonic polychaete larvae were investigated monthly from January 2007 to December 2009 in Onagawa Bay, northeastern Japan. Larvae belonging to 18 families were identified: Spionidae, Serpulidae, Nephtyidae, Magelonidae, Phyllodocidae, Polynoidae, Syllidae, Capitellidae, Nereididae, Terebellidae, Arenicolidae, Chaetopteridae, Oweniidae, Pectinariidae, Glyceridae, Dorvilleidae, Sabellidae, and Lumbrineridae. The density of polychaete larvae varied from 18 to 6901 ind m−3. Spionid larvae occurred throughout the year, being the dominant family throughout the year, comprising 88.7% of the total, with those belonging to genus Polydora dominant during winter to spring and Pseudopolydora during summer to autumn. Larvae belonging to the Serpulidae, Magelonidae, Nephtyidae, Phyllodocidae, and Polynoidae tended to be frequent in summer and autumn. The close timing between phytoplankton blooms and the production of planktonic polychaete larvae is discussed, and it is noted that most planktonic polychaete larvae tended to synchronize with summer phytoplankton increases and autumn blooms in the near-surface water, but not with spring blooms. One possible explanation is the diversity of food during summer to autumn in Onagawa Bay. Only larvae belonging to the genus Polydora were synchronized with spring phytoplankton blooms, perhaps reflecting their northern biogeographic origin.