POPULATION DYNAMICS OF THE SPORE-FORMING DIATOM LEPTOCYLINDRUS DANICUS IN NARRAGANSETT BAY. RHODE ISLAND1

A field study to assess resting spore importance in long-and short-term population dynamics of Leptocylindrus danicus ( Ehr.) Gran was carried out, based on the observation that germinating spores produce only vegelative cells of maximum diameter. From May through November 1978 . L. danicus was present in lower Narragansett Bay, but resting spores were not found. May and June bioassays suggest that Bay water could not support sexuality (hence spore formation) in L. danicus during this period. Mean valve diameter decreased abruptly in early June. as did cell number. When mean valve diameter increased abruptly in August, with no resting spores seen, 70 additional clonal isolates showed that one clone had the "normal" sexual and spore-forming life cycle while 69 clones had an "alternate" life cycle lacking sperm and spore formation. Valvar fine structure of the two types differed with respect to the presence or absence of a subcentral valvar pore, and a new subspecific taxon is here distinguished: variety apora. Examination of the early June data indicates the replacement of one population by another. Populations of L. danicus may be sporadically introduced into Narragansett Bay from Gulf Stream rings .