A field experiment demonstrating the influence of seabird guano on intertidal primary production.

Nutrient concentrations in intertidal rockpools on the shores of seabird breeding islands off the west coast of South Africa are significantly higher than concentrations in similar pools on mainland shores. This is due to the presence of terrestrial and upper intertidal guano deposits which are washed through the intertidal zone. Growth rates of intertidal macroalgae are significantly faster on the rocky shores of seabird breeding islands than on mainland rocky shores, and the extensive, permanent algal mats that develop on island shores are absent from mainland shores. The hypothesis that guano run-off enhances intertidal algal growth was tested experimentally at a site on the west coast of South Africa where no seabird colonies exist. Seawater, artificially enriched with guano, was trickled down an intertidal rocky slope during a period of two months. Algal settlement and growth on artificial surfaces were significantly enhanced by the guano treatment when compared with controls. The conclusion is that increased intertidal primary production on seabird islands, and the consequent modification of intertidal community structure, are due in part to nutrients which are introduced into the intertidal zone by run-off from the guano cap and by direct deposition of guano in the high-shore region by roosting birds