Predation and respiration by the small cyclopoid copepod Oithona similis: How important is feeding on ciliates and heterotrophic flagellates?

Feeding on natural plankton populations and respiration of the small cyclopoid copepod Oithona similis were measured during the warm season in Buzzards Bay, Massachusetts, USA. Although O. similis did not significantly ingest small autotrophic and heterotrophic flagellates (2-8 mu m), this copepod actively fed on >10 mu m particles, including autotrophic/heterotrophic (dino)flagellates and ciliates, with clearance rates of 0.03-0.38 ml animal(-1) h(-1). The clearance rates increased with the prey size. O. similis also fed on copepod nauplii (mainly composed of the N1 stage of Acartia tonsa) with a clearance rate of 0.16 ml animal(-1) h(-1). Daily carbon ration from the combination of these food items averaged 148 ng C animal(-1) day(-1) (41% of body C day(-1)), with ciliates and heterotrophic dinoflagellates being the main food source (similar to 69% of total carbon ration). Respiration rates were 0.20-0.23 mu l O-2 animal(-1) day(-1). Assuming a respiratory quotient of 0.8 and digestion efficiency of 0.7, the carbon requirement for respiration was calculated to be 125-143 ng C animal(-1) day(-1), close to the daily carbon ration estimated above. We conclude that predation on ciliates and heterotrophic dinoflagellates was important for O. similis to sustain its population in our study area during the warm season.