Post-spring-bloom feeding rates of krill Euphausia pacifica (Euphausiacea) in the Oyashio region, western North Pacific

Abstract

Krill (Euphausia pacifica) play an important role in the transfer of energy from lower to higher trophic levels, and are considered to be key species in the Oyashio ecosystem. However, detailed information on their feeding ecology is limited for the Oyashio region. Here onboard feeding experiments were performed to determine post-spring-bloom feeding rates of E. pacifica in the Oyashio region. Total ingestion rates ranged from 1.6 to 4.6 µg C krill−1 h−1, corresponding to 1.4%–5.3% of body carbon. Prey of size 30–40 µm equivalent spherical diameter (ESD) contributed substantially (43.3%–48.5%) to ingested carbon. Of 20 prey categories, ingestion rates of the most abundant prey, nano-sized cryptophyta-like flagellates (based on cell number), were highest. However, ingestion rates of naked ciliates (based on carbon biomass) were consistently high, while those of other pennate diatoms, cryptophyta-like flagellates, thecate dinoflagellates >50 µm, and copepod nauplii were less frequently high. Ingestion rates of other diatoms, and thecate and athecate dinoflagellates, were generally low. Post-spring-bloom E. pacifica predates more microzooplankton than phytoplankton. Ciliate clearance rates were higher than predicted by the Ivlev model; however, experimental design likely influences naked ciliate grazing rates, as most previous experiments on E. pacifica were conducted in smaller bottle volumes. Population grazing rates of 0.7–24.7 mg C m−2 d−1 corresponded to 0.2%–7.1% of primary production in the post-spring-bloom period, indicating E. pacifica plays an important role in planktonic food web energy flow during post-bloom periods.