ON THE TROPHIC FATE OF PHAEOCYSTIS-POUCHETII (HARIOT) .3. FUNCTIONAL-RESPONSES IN GRAZING DEMONSTRATED ON JUVENILE STAGES OF CALANUS-FINMARCHICUS (COPEPODA) FED DIATOMS AND PHAEOCYSTIS

The objective of this study was to quantify the functional response in feeding rate in the various developmental stages of Calanus finmarchicus to different concentrations of the diatoms Thalassiosira nordenskioeldii and Porosira glacialis, and the haptophysean Phaeocystis pouchetii. Grazing of copepodite stage I-V C.finmarchicus was measured using two different approaches. Feeding rates were obtained from either incubation experiments, estimating the rate of removal of particles from suspension, or by quantifying the turnover rate of the plant pigments in the gut. Clearance as a function of algal concentration (1-30 μg plant pigment 1-1) was described in juvenile stages of C.finmarchicus fed the diatoms T.nordenskioeldii [∼20 μm equivalent spherical diameter (ESD)], P.glacialis (∼40 μm ESD), and two size categories (30-100 μm and >100 μm ESD) of the gelatinous alga P.pouchetii. When the copepodite stages were fed T.nordenskioeldii, the gut content of plant pigments was in general higher than when fed P.glacialis. Rates obtained were variable when the same copepodite stages were offered the two size categories of P.pouchetii, but within the same order of magnitude as those obtained for the larger diatom. At unialgal diets, diatoms were more readily consumed than the larger size fraction among colonies of P.pouchetii by copepodite stage I-III C.finmarchicus. But given an appropriate prey size, C.finmarchicus grazed both diatoms and colonies of gelatinous algae at equal rates. A linear relationship between gut content and food concentrations <10 μg chlorophyll 1-1 was found. This indicates that the ingestion rate in C.finmarchicus is directly proportional to the ambient food concentration during the most productive period in May and June in high latitudes irrespective of algal species present. © 1990 Oxford University Press.