Plankton development and trophic transfer in seawater enclosures with nutrients and Phaeocystis pouchetii added

In high latitude planktonic ecosystems where the prymnesiophyte alga Phaeocystis pouchetii is often the dominant primary producer, its importance in structuring planktonic food webs is well known. In this study we investigated how the base of the planktonic food web responds to a P. pouchetii colony bloom in controlled mesocosm systems with natural water enclosed in situ in a West Norwegian fjord. Similar large (11 m 3 ) mesocosm studies were conducted in 2 successive years and the dynamics of various components of the planktonic food web from viruses to mesozooplankton investigated. In 2002 (4 to 24 March), 3 mesocosms comprising a control containing only fjord water; another with added nitrate (N) and phosphate (P) in Redfield ratios; and a third with added N, P, and cultured solitary cells of P. pouchetii, were monitored through a spring bloom cycle. In 2003 (27 Feb- ruary to 2 April) a similar set of mesocosms were established, but cultured P. pouchetii was not added. As expected, during both years, addition of N and P without addition of silicate resulted in an initial small diatom bloom followed by a colonial bloom of P. pouchetii (600 to 800 µg C l -1 ). However, the hypothesis that addition of solitary cells of P. pouchetii would enhance subsequent colony blooms was not supported. Interestingly, despite the large production of Phaeocystis colonial material, little if any was transferred to the grazing food web, as evidenced by non-significant effects on the biomass of micro- and mesozooplankton in fertilized mesocoms. Separate experiments utilizing material from the mesocosms showed that colonies formed from solitary cells at rates that required only ca. 1% con- version efficiencies. The results are discussed from the perspective of future research still required to understand the impact of life cycle changes of this enigmatic phytoplankter on surrounding ecosystems.