Ulva fenestrata and Ulvaria obscura are major constituents of ulvoid macroalgal blooms in the northeastern Pacific. These blooms have been associated with a variety of environmental problems and are thought to be caused by increased nitrogen concentration. We grew these algae at several nutrient concentrations in incubator and seawater table experiments. Growth rate and the concentrations of DMSP, C, H, and N in the tissues were determined for both kinds of experiments. Incubator experiments alone were used to measure nutrient uptake rate, chlorophyll a and b content, and protein content (as determined by far-UV and bicinchoninic acid (BCA) techniques). Increasing nutrient concentration caused significantly more growth in the seawater table experiment but only a trend toward greater growth in the incubator. There was no significant interspecific difference in growth rate in the seawater table experiment, but Ulva grew significantly faster than Ulvaria in the incubator experiment. Tissue nitrogen increased significantly with increasing nutrient content in the growth medium in the incubator experiment and showed a similar (albeit not significant) trend in the seawater table experiment. Ulvaria had higher tissue nitrogen, protein, and chlorophyll a concentrations than Ulva in all experiments. The ratio of chlorophyll b:a was also higher in Ulvaria than in Ulva. Far-UV and BCA techniques yielded equivalent protein concentration in Ulva but protein concentration determined by BCA was 2.5 times higher in Ulvaria than when determined by far-UV. Nutrient uptake rates were similar between both species. DMSP concentration was not affected significantly by nutrient regime, but [DMSP] was higher in Ulvaria than in Ulva. These results help explain previous observations of these species. Ulvaria generally grows more slowly than Ulva because it requires more nitrogen to make chlorophyll and protein but has similar nitrogen uptake rates. Ulvaria is also shade adapted, while Ulva can acclimate to various conditions.

Key words: eutrophication, green tide, Ulva fenestrata, Ulvaceae, Ulvaria obscura