Predation by copepods upon natural populations of Phaeocystis pouchetii as a function of the physiological state of the prey

Confl~chng data have been previously presented on the ablllty of copepods to prey upon the prymneslophyte Phaeocystls pouchetu Whde some have suggested that gelatinous colonies of thls species con tan blochemlcal substances that prevent t h e ~ r consumption others have shown that both slngle cells and colon~es of P pouchetu can serve as an excellent food source The present study presents data from feedlng expenments uslng 4 specles of copepods and natural samples of phytoplankton prey from a south-north transect dunng May 1989 in the Barents Sea Natural phytoplankton contalned P pouchefn colonies m assoclatlon w t h varylng amounts of diatoms Along the transect these colonies vaned from h~gh ly fluorescent and healthy In the north to weakly fluorescent In the south Results of expenments uslng both image analysis and radiotracer techniques indicate that diatoms were actlvely preyed upon In all expenments w t h long-chain-formlng species as the preferred food Predahon upon P pouchefn colon~es was dependent upon the physiological condition of the colonies Healthy colonies were not consumed, while suscephble colon~es were consumed at rates 2 to 10 tlmes those for chain-forrmng dlatoms The selective predalon descnbed here has important impl~cations for specles composition in Archc waters