LIFE CYCLE DYNAMICS OF THE HARMFUL BLOOM FORMING MACROALGAE ULVA SPP. IN NARRAGANSETT BAY, RI

Abstract

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. All Ulva species have isomorphic, biphasic life cycles, and the relative contribution of the haploid and diploid life history stages to bloom formation is poorly understood. 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. 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 to 1 sporophyte. We also assessed growth rates and measured cell sizes to investigate potential differences between life history phases. We found no significant differences in growth rate between ploidy levels for either species. Sporophyte cells were significantly larger than gametophyte cells in U. compressa. 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.