Toxic properties are rarely associated with macroalgal blooms. A few studies have demonstrated inhibition of invertebrate larval development by extracts from Ulva spp. and we have observed substantial discoloration of the seawater in small bays following mass desiccation-induced mortality of Ulvaria obscura. We examined the effects of extracts from Ulva fenestrata and Ulvaria obscura on enzyme systems, larval oyster development, Fucus gardneri zygote development, epiphytic diatom growth, and growth of Ulva and Ulvaria. Extracts from Ulvaria inhibited trypsin activity, but not pepsin or amyloglucosidase activity when tested at pH optimized for each enzyme system (pH 8, 2, and 5.5, respectively). When tested at neutral to slightly alkaline pH (7 or 8), Ulvaria extracts also inhibited amyloglucosidase activity. "Aged" extract caused greater enzyme inhibition than fresh extract. Maximum inhibition occurred in 6 hour old extract. Oyster larval development was inhibited by extracts from both algae, although the visible effects were much greater when the larvae were exposed to extracts of Ulvaria. Fucus zygote development was inhibited by extracts of both species, although the effect was measurably greater in extracts from Ulvaria. Epiphytic diatom growth and growth of Ulva and Ulvaria were inhibited by extracts from both species. Thus, extracts from both Ulva fenestrata and Ulvaria obscura have allelopathic properties, although the impacts are more widespread for Ulvaria extracts. These properties could alter competitive interactions by inhibiting germination or development of algal and animal competitors. Multiple toxins are probably involved, since the two species tested cause visibly different responses.

Key words: enzyme inhibition, invertebrate, toxin, Ulva fenestrata, Ulvaceae, Ulvaria obscura