SPECIES COMPOSITION AND CONTROLS OF ULVOID ALGAL BLOOMS IN WASHINGTON STATE

Nelson, T. A. Blakely Island Field Station, Seattle Pacific University, Seattle, WA 98119‐1997, USA Blooms of green macroalgae can devastate important finfish and shellfish habitats. In earlier work, blooms in the San Juan Islands, Washington State were shown to be composed primarily of Ulva fenestrata in the intertidal and Ulvaria obscura subtidally. Ulva is more desiccation‐tolerant than Ulvaria, allowing it to dominate the intertidal. Ulvaria does not have a growth or photo‐synthetic advantage when grown subtidally. This study examined ulvoid algal blooms throughout Washington State to see if the intertidal versus subtidal distribution of these two species was consistent over a larger geographical scale. We also tested grazer preferences using a suite of herbivores to see if top‐down effects might allow Ulvaria to dominate the subtidal. All tested sites in Puget Sound had the highest relative Ulvaria biomass in the subtidal and the highest relative Ulva biomass in the intertidal. Ulvaria was completely absent from sites in Hood Canal, Grays Harbor, and Willapa Bay. At several sites Enteromorpha spp. contributed significantly to the ulvoid bloom. Grazing preferences were examined with two species of gastropod (Lacuna variegata and Lacuna vincta), an urchin (Strongylocentrotus droebachiensis), and an isopod (Idotea woesnesenskii). All tested grazers are more active in the subtidal and preferred Ulva to Ulvaria. In conclusion, Ulvaria dominates the intertidal and Ulva the subtidal whenever the two co‐occur. While not the only plausible explanation, it appears that subtidal grazers may allow Ulvaria to dominate the subtidal.