The relative survival (as net treatment mortality - red line) in bioassays comparing development to D-stage larvae of fertilized Pacific oyster eggs in artificial and in normally treated hatchery seawater during the period May to July, 1977 (A) and 1978 (B). The horizontal black line, equivalent to a net treatment mortality of zero, denotes equality in survival in both the tested seawater and the control medium. The blooming of the colonial flagellate, PhaeocyZstis pouchetti, (as colonies per ml) and the number of bacteria colonies (cfus - colony forming units - as thousands per ml) growing on TCBS agar in samples taken from the adjacent coastal waters are superimposed. Adapted from Utting and Helm (1985) including previously unpublished data