This project, part of the Northeast Pacific GLOBEC Long Term Observation Project (NEPGLOBEC- LTOP), constitutes the first multi-year study of phytoplankton variability in the Oregon coastal environment. The work divides into two studies: analysis of interseasonal change and analysis of detailed changes within the summer upwelling period. In the first study, I found that the majority of variability in phytoplankton biomass in this system is due to changes in the abundance of chain forming diatoms, particularly diatoms of the genera Chaetoceros and Skeletonema. The abundance of nanoflagellates (<10 μm) remains constant across the shelf. They dominate the phytoplankton community in the offshore stations and larger cells, usually diatoms, are added inshore. Cyanobacteria, though numerically abundant, never comprise more than approximately 10% of phytoplankton biomass. Variation within the summer phytoplankton bloom over the shelf was analyzed based on results from the four NEP-GLOBEC-LTOP summer cruises between August 1998 and July 2001. Nutrient concentrations during the summer upwelling blooms show a linear decrease with increasing temperature from 8 to 12° C. Over this range of temperatures, total Chl a and phytoplankton biomass increase, though not significantly, and the % Chl a >10 μm remains high (>50 %). Above 12° C, when inorganic nutrients are depleted or greatly reduced, total Chl a and % Chl a >10 μm decrease. The diatom genera Chaetoceros and Skeletonema are also responsible for the majority of the variability in phytoplankton stock during the summer phytoplankton bloom. Dinoflagellates, although present, are consistently less abundant than diatoms.