The relative importance of herbivore-induced effects on productivity of crustose coralline algae: Sea urchin grazing and nitrogen excretion

In Hong Kong, the sea urchin, Anthocidaris crassispina (A. Agassiz, 1863), is an important grazer in low-shore rock pools, which are dominated by crustose coralline algae (CCA > 70% cover). The pools are emersed for an average of ∼ 6 h during daytime low tides, at which time the pool water becomes nitrogen enriched, primarily due to urchin excretion. This nitrogen enhancement potentially provides an episodic, additional, nitrogen source for the CCA. Factorial laboratory experiments were conducted to examine whether this nitrogen source contributed to CCA primary productivity and the relative importance of sea urchin induced effects (grazing vs. nitrogen regeneration). Urchin grazing significantly stimulated area-specific CCA productivity, although the CCA showed no response to nitrogen excretion, suggesting these algae were not nitrogen limited. This grazing enhanced CCA productivity apparently facilitates CCA to quickly occupy newly released, small, patches (cm) of bare surface and suggests that top-down (grazing) effects are more important then bottom-up (nitrogen) effects in controlling the dominance of CCA in low-shore, tropical, rock pools.