On the trophic fate of Phaeocystis pouchetti (Harriot). V. Trophic relationships between Phaeocystis and zooplankton: an assessment of methods and size dependence

Abstract

A laboratory study was conducted using natural populations of Phaeocystis pouchetii and co-occurring crustacean zooplankton from northern Norwegian fjords. Phytoplankton communities dominated by colonial and solitary cells of Phaeocystis were incubated with added zooplankton spanning five orders of magnitude in biomass, including the euphausiids Thysanoessa spp. and the calanoid copepods Calanus 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 proportional to grazer size. Diatoms were grazed more than Phaeocystis by euphausiids and A.longiremis. Comparison of clearance estimated from colony disappearance to those from changes in chlorophyll a and Phaeocystis cell number indicated that the feeding/swimming activities of Thysanoessa spp. and Calanus spp. resulted in fragmentation of colonies into smaller size classes. In particular, Thysanoessa spp. redistributed and consumed colonies in the size range >500-20μm). Calanus spp. redistributed and grazed those <500 μm, and A.longiremis did not graze or break Phaeocystis. The colony breakage by Thysanoessa spp. resulted in increases in Phaeocystis cells in the <20 μm size fractions relative to controls. Measurements of Phaeocystis cell abundance and size using an image-analysis system showed distinct differences in cell size between colony cells and solitary cells, which provided further evidence of colony disruption independent of ingestion. The data indicate that changes in colony abundance are an inadequate methodological tool and that multiple techniques are required to assess the trophic importance of Phaeocystis in zooplankton diets. Moreover, the role of Phaeocystis in planktonic food webs is highly dependent upon a continual match between the size structure of Phaeocystis populations and the collection efficiency of co-occurring zooplankton.