Feeding by the cyclopoid copepod Oithona similis on the microplankton assemblage in the Oyashio region during spring

Onboard incubation experiments were conducted to examine the feeding rates and selectivity of the cyclopoid copepod Oithona similis on natural assemblages of microplankton during the spring bloom in the Oyashio region. O. similis cleared ciliates at higher rates (2.8–11.9 mL copepod 1 d ) than dinoflagellates (1.4 mL copepod 1 d ), with no detectable ingestion of diatoms. Clearance increased with ciliate size, reaching maximum rates at a size of 35 mm equivalent spherical diameter. The total ingestion rates of protozooplankton (ciliates and dinoflagellates) by O. similis ranged between 24.8 and 41.5 ngC copepod 1 d , which exceeded by about 2–3 times the metabolic expenditure of the copepod. Applying the ingestion rates to abundance data of O. similis, we estimated that the copepod population removed each day 0.1–0.4% of the protozooplankton standing stock. These results indicate that protozooplankton, especially aloricate ciliates, are an important food resource available to O. similis even under bloom conditions in the Oyashio region, though the predation by the copepod did not have a significant impact on protozooplankton assemblages.