Pacific oyster Crassostrea gigas spat were reared in an outdoor seawater pond where artificial upwelling and the addition of agricultural fertilizers were used separately and together to culture phytoplankton. Spat reared in a nearby fiord served as controls to see if shell and meat growth was accelerated in the nutrient enriched habitat. The phytoplankton taxa in the pond was continually changing throughout each 2 week assay, as did the cell abundance. In the fiord phytoflagellates dominated the summer growth assays. In the nutrient enriched pond phytoflagellates and diatom co-existed. In the pond phytoplankton were 2 to 21 times more abundant than in the fiord.  Neither phytoplankton abundance nor their taxa present in the pond were predictably related to the fertilization method. In 5 of the 6 growth assays oyster spat reared in the fertilized pond had more rapid tissue weight gains than those reared in the fiord, and in 4 of 6 trials the phytoplankton community in the fertilized pond supported larger increases in shell length. In the fertilized pond the shells of oyster spat grew in an average of 0.1 mm.d for all of the growth assays, vs 0.06 mm.d for spat reared in the fiord.