Macroalgal blooms occur worldwide and have the potential to cause severe ecological and
economic damage. Narragansett Bay, RI is a eutrophic system that experiences summer
macroalgal blooms composed mostly of Ulva compressa and Ulva rigida, which have
biphasic life cycles with separate haploid and diploid phases. In this study, we used flow
cytometry to assess ploidy levels of U. compressa and U. rigida populations from five sites
in Narragansett Bay, RI, USA, to assess the relative contribution of both phases to bloom
formation. Both haploid gametophytes and diploid sporophytes were present for both species.
Sites ranged from a relative overabundance of gametophytes to a relative overabundance
of sporophytes, compared to the null model prediction of 2 gametophytes: 1
sporophyte. We found significant differences in cell area between ploidy levels for each species,
with sporophyte cells significantly larger than gametophyte cells in U. compressa and
U. rigida.We found no differences in relative growth rate between ploidy levels for each species.
Our results indicate the presence of both phases of each of the two dominant bloom
forming species throughout the bloom season, and represent one of the first studies of in
situ Ulva life cycle dynamics.