Crustose coralline algae can suppress macroalgal growth and recruitment on Hawaiian coral reefs

Crustose coralline algae are important components of tropical reef communities because they promote successful settlement by corals and contribute to solidification of the reef framework. We show experimentally that crustose coralline algae are also capable of suppressing the growth and recruitment potential of an abundant Hawaiian reef macroalga, Ulva fasciata. When mixed communities of crustose coralline algae were absent, relative growth rates of U. fasciata increased by 54.6%. When experimental nutrient additions were used to induce algal spore release, effective recruitment of U. fasciata approached zero only when crustose coralline algae were present. Mixed communities of crustose coralline algae are thus capable of limiting the local abundance of already-established macroalgae by reducing both their growth rate and recruitment success. This experimental observation was confirmed by field surveys. Because crustose coralline species also induce settlement and metamorphosis in a large number of scleractinian coral species, their abundance and species composition are expected to affect the (future) abundance of macroalgae and corals, which are often used to characterize degraded and 'healthy' reefs, respectively.