Effect of macroalgal competition on growth and survival of juvenile Caribbean corals

Lobophora variegata and Dictyota pulchella are dominant algal components on coral reefs across the Caribbean, but the mechanisms and outcomes of spatial competition between these algae and scleractinian corals are poorly understood. ln this study the effects on growth and mortality of juvenile corals by 2 forms of algal competition, shading and abrasion, were investigated. The growth of small Agaricia spp. (< 20 mm diameter) was monitored over a 14 mo period on shallow forereefs in Roatan, Honduras. Experimental manipulations of algal shading and algal contact with the periphery of colonies were conducted in isolation from the effects of grazing through the use of exclusion cages. Shading by L. variegata caused an overall loss of coral tissue and significantly increased colony mortality rates from 0 to 50% in 6 mo. The presence of L. variegata around the periphery of a coral colony significantly reduced the overall growth of juvenile corals, decreasing the growth rate to 60% of that of control corals, but had no detectable effect on mortality. Shading by D. pulchella resulted in 99 % growth inhibition (i.e. to just 1 % of the growth rate of control corals). Peripheral contact with D. pulchella (without shading) also retarded coral growth rates but to a lesser extent: to 31 % of that of controls. A synthetic alga made to mimic the action of D. pulchella abrasion caused a similar reduction in growth rate to actual D. pulchella, suggesting that the reduction in coral growth occurred because of physical mechanisms rather than allelochemical inhibition. The severe inhibition of colony growth caused by the proximity of D. pulchella or L. variegata may extend a coral's period of vulnerability to whole colony mortality. Based on the monthly mortality rate observed in uncaged control corals of 0.035 0.135 (SE), peripheral contact with D. pulchella could decrease the survivorship of corals reaching a 3 cm diameter from 29 to < 2 %. Peripheral contact with L. variegata could likewise decrease cohort survival to 11 %. The ability of these common macroalgae to reduce the survivorship of juvenile corals through interference competition could contribute to the perpetuating dominance of macroalgae on many Caribbean reefs.