Zooplankton feeding ecology: does a diet of Phaeocystis support good copepod grazing, survival, egg production and egg hatching success?

The bloom-forming alga Phaeocystis is ingested by a variety of zooplankton grazers, but is thought to be a poor source of food. We examined copepod grazing on solitary Phaeocystis cells by adult females of Temora stylifera, and survival, fecal pellet production, egg production and egg hatching success in Calanus helgolandicus and T. stylifera over periods of 15 consecutive days. Phaeocystis cell concentrations were high (1.2-3.6 X 10 4 cells ml -1 for C. helgolandicus and 2.5-7.9 × 10 4 cells ml -1 for T. stylifera), but within the range of maxima recorded for natural blooms. Both copepods survived well and continuously produced fecal pellets (indicating continuous grazing) on a diet of Phaeocystis. However, egg production rates for both copepods were low, even though hatching success of the few eggs produced was high. Clearance rates for T stylifera were higher than for most previous measurements of other copepods feeding on Phaeocystis solitary cells at lower cell concentrations. We conclude that even though copepods feed well upon Phaeocystis, resulting poor fecundity on this diet may inhibit copepod population increases during blooms, thereby contributing to the perpetuation of blooms. However, the high egg hatching success on this diet argues against Phaeocystis containing chemical compounds that act as mitotic inhibitors reducing copepod egg viability, such as those found in some other phytoplankters.