PROTIST HERBIVORY IN THE OREGON UPWELLING SYSTEM

We investigated the role of phagotrophic protists in the Oregon upwelling system over a 2 yr period by determining protist abundance, and by measuring grazing and phytoplankton growth rates with the seawater-dilution method. Off Oregon (USA), upwelling occurs predominantly during the summer months as episodic upwelling events which give rise to phytoplankton blooms. Abundance of phagotrophic protists, grouped into ciliates (mostly choreotrich forms), thecate and gymnodinoid dinoflagellates and nonpigmented nanoflagellates, was lowest in freshly upwelled water (3 mug C 1(-1)) and highest during the late stages of the phytoplankton blooms (61 mug C 1(-1)). Overall rates of herbivory ranged from 0 to 0.6 d-1 during the upwelling season which represented 0 to 120% of the phytoplankton production. Highest phytoplankton growth rates were measured during bloom periods in the upwelling season (0.67 to 1.58 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. Gymnodinoid dinoflagellates were most abundant during the upwelling season, often exceeding the biovolume of choreotrich ciliates and of thecate dinoflagellates. Rates of herbivory were closely linked to the abundance of this group of dinoflagellates. The abundance of large-celled gymnodinoids (> 20 mum) and large phytoplankton (> 20 mum, mostly diatoms) covaried, 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.