Macroalgal blooms present a growing threat to coastal systems worldwide. By sequestering light and nutrients, reducing dissolved oxygen, and changing habitat structure, fast growing macroalgae can harm foundation species and alter trophic interactions in ecosystems ranging from estuaries to coral reefs. Other evidence shows some macroalgae contribute dissolved organic carbon and nutrients to adjacent plants and sediments that may benefit communities, so the net effect of these blooms over time can be unclear. Here we report results of ongoing experiments on the short- and long-term dynamics and consequences of seasonal green algal blooms (Ulva sp.) in the seagrass community of Bodega Bay, CA. In the field, we are manipulating Ulva biomass in 3-meter wide enclosures (removal, ambient, double ambient) to determine effects on seagrass and associated epifauna and flora. Over the previous 3 summers, Ulva abundance varied over several orders of magnitude. Only in summer 2006, when Ulva abundance was greater than 4 kg/m2 (over double that of the two previous years), did Ulva decrease seagrass density and growth. Six months after the bloom, seagrass density in the Ulva treatments remains lower than the removal treatment. In mesocosm experiments exploring the effects of Ulva on trophic interactions, a common herbivore suffered 60% less predation by fish in seagrass beds when Ulva was present. The predation refuge provided by Ulva may allow these herbivores to reach high densities observed in the field in the late summer, which may eventually speed the decline of the bloom in the late fall months.