Contribution of auto- and heterotrophic protozoa to the diet of copepods in the Ulleung Basin, East Sea/Japan Sea

We estimated the ingestion rates and prey selectivity of two calanoid copepods, Calanus sinicus and Neocalanus plumchrus, on auto- and heterotrophic protozoa in the Ulleung Basin in the East Sea/Japan Sea. Both copepod species preferentially ingested auto- and heterotrophic protozoa. The heterotrophic protozoa comprised between 42.9 and 86.7% of the total carbon ration ingested by the copepods, although they comprised on average 21.5% of the total available prey carbon. In particular, ciliates comprised the major dietary component for the copepods in the study area. The copepod consumption of ciliates was on average 38.2% of the total daily carbon ration of auto- and heterotrophic protozoa. Auto- and heterotrophic protozoa in the 20–50 μm size were the fraction that was most actively consumed by copepods and they comprised on average 45.1% of the total carbon ration ingested. In all experiments, the copepods selected the ciliate and heterotrophic dinoflagellate (HDF) groups over the other protozoa. Our results indicate that the copepod feeding regime was influenced by the composition and size of the potential food and that, in this study area, feeding behavior of copepods may control the populations of ciliates and HDFs that are larger than 10 μm in size. Thus, the selective feeding patterns and higher grazing pressure of copepods on heterotrophic protozoa, compared with autotrophic protozoa, indicate a trophic coupling between copepods and the microbial food web in the Ulleung Basin.