Effects of the benthic cyanobacterium Lyngbya majuscula on larval recruitment of the reef corals Acropora surculosa and Pocillopora damicornis

Abstract

Coral reef degradation resulting from a multitude of stressors is occurring on a global scale (Hoegh-Guldberg 1999; Hughes et al. 2003). The outcome of reef degradation is often a decrease in live coral, followed by the proliferation of algae (termed a phase shift) (Hughes 1994; McCook 1999). Successful coral recruitment is critical to the regeneration of reefs, and recruitment failuremay be a major reason that reefs are not recovering from phase shifts (Hughes and Connell 1999; Hughes and Tanner 2000). When reefs become dominated by macroalgae, larval access to suitable settling habitat is decreased (McCook et al. 2001), and sediments trapped within algal turfs may reduce survival of coral spat (Sato 1985). Rates of coral recruitment are inversely correlated to algal biomass (Birkeland 1977; Rogers et al. 1984). Interactions between benthic cyanobacteria and corals have not been examined experimentally to date, but recent studies have documented cyanobacterial blooms on coral reefs in Guam (Nagle and Paul 1998; Thacker and Paul 2001), atoll islands across Micronesia and the Northwest Hawaiian Islands (especially near iron shipwrecks, J. Maragos, personal communication), and off Broward county, Florida (authors’ personal observations). Also, cyanobacteria are often dominant in the turfs that first colonize dead coral skeleton after mortality caused by coral bleaching (Diaz-Pulido and McCook 2002). The environmental factors that contribute to cyanobacterial bloom formation in the benthic marine environment have not been definitively identified, but low wave action (Thacker and Paul 2001), phosphate levels (Kuffner and Paul 2001), and iron bioavailability (J. Maragos, personal communication) may be worth examining in the field. There could be other mechanisms for reductions in coral recruitment rates besides the preemption of space by weedy primary producers. Many benthic species of algae (Hay et al. 1987; Schmitt et al. 1995) and cyanobacteria (Nagle and Paul 1998; 1999; Pennings et al. 1997) produce toxic secondary metabolites that act as anti-herbivory and anti-fouling agents (see Paul et al. 2001 for review). Lyngbya majuscula, in particular, has been shown to produce three bioactive compounds (malyngolide, malyngamide A and malyngamide B) that deter feeding in juvenile parrotfishes (Thacker et al. 1997). Given the generalized toxicity and multi-function nature of many secondary metabolites produced by marine algae and cyanobacteria (Paul et al. 2001), it would not be surprising if these or other chemicals perform allelopathic roles as well. In this study we examine the effects of Lyngbya majuscula on the larvae of two species of corals, Acropora surculosa (broadcast spawner) and Pocillopora damicornis (brooder), to test whether the presence of Lyngbya majuscula has a negative effect on coral larval survival or recruitment beyond levels that would be expected due to space preemption.