PROTIST HERBIVORY IN THE OREGON UPWELLING SYSTEM

We investigated the role of phagotrophic protists in the Oregon upwell ing system over a 2 yr period by determining protist abundance, and by measuring grazing and phytoplankton growth rates with the seawater-di lution method. Off Oregon (USA), upwelling occurs predominantly during the summer months as episodic upwelling events which give rise to phy toplankton blooms. Abundance of phagotrophic protists, grouped into ci liates (mostly choreotrich forms), thecate and gymnodinoid dinoflagell ates and nonpigmented nanoflagellates, was lowest in freshly upwelled water (3 mug C 1(-1)) and highest during the late stages of the phytop lankton blooms (61 mug C 1(-1)). Overall rates of herbivory ranged fro m 0 to 0.6 d-1 during the upwelling season which represented 0 to 120% of the phytoplankton production. Highest phytoplankton growth rates w ere measured during bloom periods in the upwelling season (0.67 to 1.5 8 d-1) when phytoplankton biomass was highest (10.7 to 55.3 mug chl a 1(-1)). During bloom periods, phagotrophic protists utilized 16 to 52% of phytoplankton production, or 612 to 1413 mug C 1(-1) d-1. Gymnodin oid dinoflagellates were most abundant during the upwelling season, of ten exceeding the biovolume of choreotrich ciliates and of thecate din oflagellates. Rates of herbivory were closely linked to the abundance of this group of dinoflagellates. The abundance of large-celled gymnod inoids (> 20 mum) and large phytoplankton (> 20 mum, mostly diatoms) c ovaried, indicating that the bloom-forming diatoms were being utilized as a food source. Thus, the microbial food web is an integral part of the trophic interactions in coastal upwelling systems including large phytoplankton cells as prey of protist grazers.