ON THE TROPHIC FATE OF PHAEOCYSTIS-POUCHETTI (HARRIOT) .5. TROPHIC RELATIONSHIPS BETWEEN PHAEOCYSTIS AND ZOOPLANKTON - AN ASSESSMENT OF METHODS AND SIZE DEPENDENCE

A laboratory study was conducted using natural populations of Phaeocys tis pouchetii and co-occurring crustacean zooplankton from northern No rwegian fjords. Phytoplankton communities dominated by colonial and so litary cells of Phaeocystis were incubated with added zooplankton span ning five orders of magnitude in biomass. including the euphausiids Th ysanoessa spp. and the calanoid copepods Calantis hyperboreus Kroyer ( CV), C.finmarchicus Gunnerus (CIV-V) and Acartia longiremis Liljeborg (adult females). Grazing was determined from changes in concentrations of four size classes of chlorophyll a. Phaeocystis cells and colonies . These were compared to zooplankton grazing on cultured and naturally occurring diatoms. Clearance and optimum prey size range were both pr oportional to grazer size. Diatoms were grazed more than Phaeocystis b y euphausiids and A. longiremis. Comparison of clearance estimated fro m colony disappearance to those from changes in chlorophyll a and Phae ocystis cell number indicated that the feeding/swimming activities of Thysanoessa spp. and Calanus spp. resulted in fragmentation of colonie s into smaller size classes. In particular, Thysanoessa spp. redistrib uted and consumed colonies in the size range >500-20 mum, Calanus spp. redistributed and grazed those <500 mum. and A.longiremis did not gra ze or break Phaeocystis. The colony breakage by Thysanoessa spp. resul ted in increases in Phaeocystis cells in the <20 mum size fractions re lative to controls. Measurements of Phaeocystis cell abundance and siz e using an image-analysis system showed distinct differences in cell s ize between colony cells and solitary cells, which provided further ev idence of colony disruption independent of ingestion. The data indicat e that changes in colony abundance are an inadequate methodological to ol and that multiple techniques are required to assess the trophic imp ortance of Phaeocystis in zooplankton diets. Moreover, the role of Pha eocystis in planktonic food webs is highly dependent upon a continual match between the size structure of Phaeocystis populations and the co llection efficiency of co-occurring zooplankton.