Yellow water in La Jolla Bay, California, July 1980. I. A bloom of the dinoflagellate, Gymnodinium flavum Kofoid & Swezy

During the latter half of July, 1980, a bloom of Gymnodinium flavum Kofoid & Swezy caused water discoloration in La Jolla Bay, California. This naked dinoflagellate dominated the phytoplankton, numeri- cally and by volume, and was found in concentrations as high as 6.15 × 10 3 cells · ml −1. It was most abundant near the surface above a shallow (5–10 m), sharp thermocline and a nitracline at 10 m. Near the end of July, the depth of maximum phytoplankton abundance descended and water discoloration was no longer visible at the surface even though areal concentrations of G. flavum did not decrease. Concurrent with changes in the vertical distribution of the phytoplankton, warm, nutrient-depleted water moved into the area and nitrate availability in the upper 20 m of the water column was drastically reduced. Measure- ments of the chemical composition of the phytoplankton do not, however, indicate progressive nutrient stress during the period of environmental change. We conclude that shoaling of the thermocline and nitracline associated with apparent upwelling were conducive to development of the bloom and that advection of warmer water from offshore led to disappearance of yellow surface water from the bay.