Eutrophication-induced macroalgal blooms can impact marine communities, yet little is known about this phenomenon in the northeastern Pacific region. Changes in ulvoid biomass and water quality parameters (nutrients, transparency, temperature, salinity, and oxygen concentrations) were monitored at 6 sites on Blakely Island, Washington State, USA for two years. Observed ulvoid biomass varied from 0 to 441 g dw m −2 . Biomass peaked in summer and autumn and was greatest at sites with the highest water-column nitrogen concentrations. Intertidal ulvoid communities were dominated by Ulva fenestrata Postels et Ruprecht while Ulvaria obscura (Kützing) Gayral dominated the subtidal zone near Blakely Island. In contrast with these observations, prior quantitative studies describe Ulvaria as rare or ephemeral. Fifteen additional sites were examined for species composition patterns. Similar patterns were seen at sites in Puget Sound, while Enteromorpha spp. were more common and U . obscura absent at sites in Hood Canal, Willapa Bay, and Grays Harbor. Green tides and other macroalgal blooms are typically reported to be monospecific, while in the present study two or more species are in close association, albeit at different tidal elevations. Enteromorpha spp. and Ulva spp. are known as important components of green tides, but relatively little is known about the biology of Ulvaria . Substantial variation in biomass on a fine geographic scale indicates the need for fine scale monitoring of the green tide phenomenon in the northeastern Pacific region.