Approximately 90 Km2 of Chesapeake Bay contiguous with the Severn, South, Rhode and West Rivers were surveyed by in vivo chlorophyll fluorescence and captured samples following a large pulse of rainfall in summer, 1971. The growth and subsequent dissipation of blooms containing chlorophyll a concentrations up to 40 x pre-bloom values were completed within 21 days. A distinction is made between the blooms produced by nutrient pulses and dinoflagellate blooms normally observed in the fall. In the former, there is a complete change of phytoplankton relative species composition. The latter is a phototactic segregation of species already existing within the water column. The methodology is presented and a mathematical description is attempted.