The influence of invertebrate herbivores on macroalgal blooms

Macroalgal blooms are a serious threat to coastal ecosystems around the world and, as such, their causes and consequences have been extensively studied. However, many questions remain about the role herbivores play in limiting bloom severity and persistence. Previous studies on herbivore-bloom dynamics have found their relationship to be complex and highly dependent on the algal species, herbivore species, and environmental factors present in the system. Consequently, the work described in this dissertation aimed to 1) determine the dominant macroalgal species within Narragansett Bay blooms and 2) assess the impacts of herbivores on the biomass of these bloom species across a range of bloom-impacted sites. Macroalgal bloom composition was determined through extensive surveys at four Narragansett Bay sites representing a gradient of bloom severity. These surveys assessed algal species richness and percent cover as well as the biomass of distromatic Ulva species. Ulva species were targeted because previous work indicated distromatic Ulva was a major contributor to Narragansett Bay blooms. We found that bloom biomass was dominated by two Ulva species: U. compressa and U. rigida. In addition, we observed spatial and temporal differences in Ulva and total macroalgal diversity between bloom-impacted sites, indicating that bloom composition can be radically different between similar sites within close proximity. The mud snail, Ilyanassa obsoleta, is numerically dominant at many RI bloom sites; therefore its impacts on Ulva biomass were intensively studied. Initial experiments indicated that rather than decreasing Ulva spp. biomass, I. obsoleta facilitated Ulva growth through the consumption of microalgal fouling organisms. Subsequent experiments examined I. obsoleta\u27s impact on U. compressa and U. rigida separately. These experiments determined that both bloom-forming species were facilitated by I. obsoleta, however the primary facilitative mechanism differed. U. compressa experienced increased growth due to I. obsoleta nitrogenous wastes, while U. rigida was facilitated by I. obsoleta removal of fouling organisms. This facilitative pathway has the potential to further increase Ulva growth, exacerbating Ulva bloom events. Additional herbivore impacts were assessed through mesocosm feeding assays and in situ herbivore exclusion experiments. Five invertebrates were confirmed as Ulva consumers; two of these herbivores (Panopeus herbstii and Gammarus mucronatus) preferentially consumed U. compressa to U. rigida . In addition, in situ herbivore exclusion experiments determined herbivory can limit Ulva growth, however this impact varied amongst the study sites, months, years, and Ulva species. Moreover, we were unable to consistently attribute grazing at our field sites to any one herbivore taxon or group of taxa. This research highlights the complexities of herbivore-bloom interactions and demonstrates that herbivore impacts within bloom events are dependent on the identity of both the blooming macroalga and herbivore species