Selection for a red tide organism: Physiological responses to the physical environment1,2

The dinoflagellate Prorocentrum mariae‐lebouriae has specific physiological characteristics that allow it to participate in a subsurface transport from the southern Chesapeake Bay to the northern bay, where it upwells and forms red tides. A particular growth rate dependence both on temperature and salinity restricts its year‐round distribution to the high‐salinity southern bay. At summer temperatures, increased tolerance to low salinities allows rapid growth in the low‐salinity waters of the northern bay. Positive phototaxis is proposed to act in conjunction with downwelling convergence at a frontal region to form the initial subsurface concentration maximum or lens. Repression of positive phototaxis at a salinity interface appears to prevent the subsurface concentrations from crossing the sharp halocline, retaining the lens population in northward‐flowing bottom waters. Prorocentrum increases its pigment concentration and retains its photosynthetic capacity at the extremely low light intensities encountered during the 200‐km northward transport. In nutrient‐poor surface waters in summer, Prorocentrum migrates at night to the higher nutrient pycnocline region. The winter‐spring phasing of the streamflows in both northern and southern bays may be used to predict the degree of Prorocentrum blooming in the northern bay in summer.