Clearance rates on natural assemblages of aloricate, phagotrophic ciliates (20 to 100 pm) were measured for calanoid copepods in Oregon, USA, coastal waters. Experiments were conducted prior to and throughout the 1991 upwelling season in an effort to determine the trophic significance of cihates as copepod prey in a system often dominated by large phytoplankton. Copepods cleared ciliates at higher rates than they cleared phytoplankton, except during upwelling-induced diatom blooms. Clearance rates on ciliates (nll copepod-' h-') were measured for Calanus pacificus (12.6 to 32.4), Pseudocalanus sp. (4.8 to 7.4) and Centropages abdominalis (1.2 to 7.1). The highest clearance rates on chates were measured when the phytoplankton standing stock was < 5 pg chl a I-' No detectable clearance of ciliates was measured for Acartja longiremis or C. pacificus during upwelling bloom events (45 pg chl a I-'). Ciliate biomass ranged from 3 to 32 pg C I-' and was significantly less than phytoplankton biomass (100 to 2200 pg C I-') on all dates. During non-upwelling months and between diatom blooms, ciliates contributed 16 to 100% of the estimated carbon ingested by copepods. Calculated ingestion of ciliate carbon (pg C copepod-' d-') averaged 6.5 (e2.4) for C. pacifjcus, 4.6 (* 1.1) for Pseudocalanus sp. and 0.7 (k0.4) for C. abdominalis. Ingestion of ciliates alone provided enough carbon to meet the basic respiration requirements of C. pacificus and Pseudocalanus sp. in January and February respectively. Estimates of predation, based on average copepod densities, indicate 25 to 45 % of the phagotrophic ciliate population is cleared from surface waters per day.
ABSTRACT:We compared specific feeding rates of ciliates and thecate and gymnodinoid dinoflagellates on the natural food spectrum applying a dual-label isotope method (Daro 1978, Helgolander wiss Meeresunters 31:241-248; Lessard & Swift 1985, Mar Biol 87:289-296) in coastal waters off Oregon, USA. Ciliates had the highest size-specific clearance rates on heterotrophic food sources with an average of 3.9 X 10' body volumes (BV) h-'. Gymnodinoid dinoflagellates strongly selected for phototrophic food sources and showed the highest size-specific clearance rates o n phototrophs with an average of 2.2 X 105 BV h-' The range of clearance rates on photosynthetic organisms in gymnodinoids overlapped with that in the c~llates. We also present evidence for bacterivory in gymnod~noid dinoflagellates with a mean of 2.4 X 104 BV h-', comparable to bacterivory by thecate dinoflagellates, but more than an order of magnitude lower than the mean found for the ciliates studied. Our data indicate that in gymnodinoid dinoflagellates, small cells have higher size-specific clearance rates than larger cells. The composition as well as the size distribution of the grazer populations need to be considered when comparing the grazing impact of ciliates and dinoflagellates on prey populations in the field.
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