• Jane C. Gallagher

    1. Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island 02881
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    • 2

      Biology Dept., City College of the City University of New York, Convent Ave. at 138th St., New York, N.Y. 10031.

  • 1

    Accepted: 29 April 1980.

  • This paper represents a portion of a dissertation submitted in partial fulfillment of the Ph.D. degree at the University of Rhode Island. I thank by thesis committee, particularly Paul E. Hargraves, T. J. Smayda, and Lynda S. Murphy, for providing facilities and data used in this study. This research was supported by an N.S.F. Pre-doctoral Fellowship to the author, by N.S.F. Grant OCE 7708947 to P.E.H. and J.C.G., and by PSC-BHE Grant 13058 to J.C.G. Gertrude Fisher drafted the figures.


During a two year period 457 clones of the diatom Skeletonema costatum were isolated prior to and during the summer-fall and winter-spring blooms of this species in Narragansett Bay, R.I. Their allozyme banding patterns were examined for 5 enzyme loci. Genotypic frequencies indicated that the winter bloom populations were genetically different from the prevalent summer bloom populations of the same species. Genetic differences between seasonal blooms are as great as those found between species of terrestrial organisms, but are not accompanied by morphological variation. Although blooms have distinct prevalent forms, they are not genetically homogeneous. No single clone is ever representative of all populations of S. costatum. The dynamics of these allochronic populations appear to be governed by a form of cyclic natural selection, and are probably a regular feature of the cycles of abundance of this species in this area. These results cast doubt on some of the assumptions often made in the “autecological approach” to phytoplankton ecology. This study comprises the first quantitative examination of the population genetics of a microalga.