On the trophic fate of Phaeocystis pouchetii (Harlot). III. 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 responsein feeding rate in the various developmental stages of Calanusfinmarchicus to different concentrations of the diatoms Thalassiosiranordenskioeldii and Porosira glacialis, and the haptophyseanPhaeocystis pouchetii. Grazing of copepodite stage I–VC.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 inthe gut. Clearance as a function of algal concentration (1–30µg plant pigment 1^–1) was described in juvenilestages of C.finmarchicus fed the diatoms T.nordenskioeldii [20µm equivalent spherical diameter (ESD)], P.glacialis (40µm ESD), and two size categories (30–100 µmand >100 µm ESD) of the gelatinous alga P.pouchetii.When the copepodite stages were fed T.nordenskioeldii, the gutcontent of plant pigments was in general higher than when fedP.glacialis. Rates obtained were variable when the same copepoditestages were offered the two size categories of P.pouchetii,but within the same order of magnitude as those obtained forthe larger diatom. At unialgal diets, diatoms were more readilyconsumed than the larger size fraction among colonies of P.pouchetiiby copepodite stage I–III C.finmarchicus. But given anappropriate prey size, C.finmarchicus grazed both diatoms andcolonies of gelatinous algae at equal rates. A linear relationshipbetween gut content and food concentrations <10 µgchlorophyll 1^–1 was found. This indicates that the ingestionrate in C.finmarchicus is directly proportional to the ambientfood concentration during the most productive period in Mayand June in high latitudes irrespective of algal species present. ¹Present address: Marine Biological Laboratory, University ofCopenhagen, Strandpromenaden 5, DK-3000 Helsingør, Denmark²Present address: Greater Copenhagen Council, Gl. KøgeLandevej 1–3, DK-2550 Valby, Denmark