Seabird guano is recognized as a powerful fertilizer and its entry into the intertidal zone of seabird breeding islands off the west coast of South Africa is hypothesized to cause enhanced intertidal algal production. Intertidal nutrient levels and algal production were measured monthly on the shores of seabird islands and on nearby mainland shores where no aggregabons of seabirds occur. The shores of seabird islands were characterized by significantly higher levels of nutrients and significantly enhanced algal production. Nutrient enrichment, in conjunction with the predatory activity of African black oystercatchers on ~nterbdal herb~vorous hmpets, leads to the formation of extensive, permanent mats of foliose algae on the shores of seabird breeding islands. Mussel spat, polychaetes and small crustaceans settle in the algal mats and provide food for small shorebirds. The structures of intertidal algal, invertebrate and avlan communities on the shores of seabird islands are significantly altered by the activities of guano-producing seabirds and predatory shorebirds.

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