ARE "GREEN TIDES" HARMFUL ALGAL BLOOMS? TOXIC PROPERTIES OF WATER-SOLUBLE EXTRACTS FROM TWO BLOOM-FORMING MACROALGAE, ULVA FENESTRATA AND ULVARIA OBSCURA (ULVOPHYCEAE)

Toxic properties are more often associated with microalgal blooms than with macroalgal blooms, although herbivore deterrents are well known in macroalgae, and inhibition of invertebrate larval development by extracts from Ulva spp. has been suggested. This study was prompted from our observation that substantial discoloration of seawater occurred in small bays after mass desiccation‐induced mortality of Ulvaria obscura (Kützing) Gayral. We examined the effects of extracts from Ulva fenestrata Postels et Ruprecht and Ulvaria obscura on Fucus gardneri Silva zygote development, growth of Ulva and Ulvaria, epiphytic algal accumulation, and oyster larval development. Fucus zygote development was inhibited by extracts from both species, although the effects of Ulvaria extracts were significantly greater. Epiphytic algal accumulation and the growth of Ulva and Ulvaria were inhibited by extracts from both species. Oyster larval development was arrested by the presence of extracts from each species. We conclude that extracts from both Ulva fenestrata and Ulvaria obscura have allelopathic properties. The effects are more widespread and occur at lower concentrations for extracts from Ulvaria than Ulva. These properties could alter competitive interactions by inhibiting germination or development of algae and invertebrates.