Coral recruitment is a key process in the maintenance and recovery of coral reef ecosystems. While intense competition between coral and algae is often assumed on reefs that have undergone phase shifts from coral to algal dominance, data examining the competitive interactions involved, particularly during the larval and immediate post-settlement stage, are scarce. Using a series of field and outdoor seawater table experiments, we tested the hypothesis that common species of macroalgae and cyanobacteria inhibit coral recruitment. We examined the effects of Lyngbya spp., Dictyota spp., Lobophora variegata (J. V. Lamouroux) Womersley, and Chondrophycus poiteaui (J. V. Lamouroux) Nam (formerly Laurencia poiteaui) on the recruitment success of Porites astreoides larvae. All species but C. poiteaui caused either recruitment inhibition or avoidance behavior in P. astreoides larvae, while L. confervoides and D. menstrualis significantly increased mortality rates of P. astreoides recruits. We also tested the effect of some of these macrophytes on larvae of the gorgonian octocoral Briareum asbestinum. Exposure to Lyngbya majuscula reduced survival and recruitment in the octocoral larvae. Our results provide evidence that algae and cyanobacteria use tactics beyond space occupation to inhibit coral recruitment. On reefs experiencing phase shifts or temporary algal blooms, the restocking of adult coral populations may be slowed due to recruitment inhibition, thereby perpetuating reduced coral cover and limiting coral community recovery.

Competition between corals and macroalgae is often assumed to occur on reefs, especially those that have undergone shifts from coral to algal dominance; however, data examining these competitive interactions, especially during the early life-history stages of corals, are scarce. We conducted a series of field and outdoor seawater-table experiments to test the hypothesis that allelopathy (chemical inhibition) mediates interactions between 2 common brown macroalgae, Dictyota pulchella and D. pinnatifida, and the coral Porites astreoides at different life-history stages of the coral. D. pinnatifida significantly reduced larval survival and larval recruitment. The extracts of both D. pinnatifida and D. pulchella significantly reduced larval survival, and the extract of D. pulchella also negatively influenced larval recruitment. There was no measurable effect of the crude extracts from Dictyota spp. on the photophysiology of adult corals. Our results provide evidence that these Dictyota species chemically compete with P. astreoides by negatively affecting larval settlement and recruitment as well as the survival of larvae and new recruits. Macroalgae may perpetuate their dominance on degraded reefs by chemically inhibiting the process of coral recruitment.KEY WORDS: Allelopathy · Coral-algal interactions · Dictyota · Chemical defense · Phase shift Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 426: [161][162][163][164][165][166][167][168][169][170] 2011 Thacker et al. 1998). Habitat selection, differential growth rates and physical or chemical defenses may be important in mediating competitive interactions among species (Jackson & Buss 1975, Connell et al. 1997, Thacker et al. 1998, Knowlton & Jackson 2001, Ritson-Williams et al. 2009, Rasher & Hay 2010.Macroalgae are known to negatively influence the settlement and recruitment of coral larvae (Kuffner & Paul 2004, Birrell et al. 2005, Kuffner et al. 2006, Birrell et al. 2008a,b, Diaz-Pulido et al. 2010) and the survival and growth of juvenile corals (Birkeland 1977, Van Moorsel 1985, Box & Mumby 2007, Ritson-Williams et al. 2009). Dictyota spp., brown macroalgae common on reefs in the Florida Keys and throughout the Caribbean, caused recruitment inhibition and avoidance behavior in larvae of the coral Porites astreoides and increased the mortality rates of recruits (Kuffner et al. 2006). Dictyota spp. inhibited larval recruitment through unknown competitive mechanisms that exceeded the effects of space occupation alone. Allelochemical effects could account for this, because Dictyota spp. are known to chemically inhibit the settlement of invertebrate larvae (Schmitt et al. 1995, Walters et al. 1996 and can cause bleaching and death of adult corals in direct contact with the algae (Rasher & Hay 2010). Dictyota spp. are known to be rich in terpenoid natural products that can function as chemical defenses against generalist herbivores (Hay 1996, Paul et al. 2001, Vallim et al. 2005. To better determine the me...

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Microscale pigment adjustments to a tropical photosynthetically active radiation and ultraviolet (UV) environment by the intertidal turf algae Ahnfeltiopsis concinna (J. Ag.) Silva et DeCew and Laurencia mcdermidiae (J. Ag) Abbott were promoted by thalli densities that self‐shade the under story portions of the same diminutive axes. Tissues of A. concinna from canopy microsites had significantly reduced levels of phycoerythrin, phycocyanin, and allophycocyanin compared to tissues from understory microsites of the same axes. Tissues of L. mcdermidiae from canopy microsites had reduced levels of only phycoerythrin compared to tissues from understory microsites. These alterations coupled with enhanced levels of carotenoid and UV‐absorbing compounds in tissues from canopy compared to tissues from understory microsites indicated a pattern of remarkably sensitive photoacclimation over the ≤10‐cm axes of these turf‐forming rhodophytes. Microscale variation in the in vivo UV absorbance capabilities for turfs of A. concinna and L. mcdermidiae was directly related to the amount of extractable UV‐absorbing compounds. An in vivo absorbance signature at ∼345 nm appears to provide a method to quickly and accurately gauge the potential UV‐shielding capacity of primary producers even at remarkably fine ecological scales. The capacity for highly responsive biochemical adjustments that result in marked canopy–understory distinctions coupled with a turf morphology may be crucial for macroalgal tolerance of physiological stresses associated with tropical intertidal zones. This responsive capacity allows for enhanced photoprotective mechanisms in tissues from canopy microsites while optimizing irradiance capture in deeply shaded tissues from understory microsites < 10 cm away.

Parameters of photosynthesis vs. irradiance curves varied markedly between tissues from microsites along the < 10‐cm axes of the tropical intertidal red algae Ahnfeltiopsis concinna (J. Ag.) Silva et DeCew and Laurencia mcdermidiae (J. Ag.) Abbott. Differences in photosynthetic performance between tissues from canopy and understory microsites indicates that L. mcdermidiae exhibited an expected sun‐to‐shade acclimation but over the space of < 10 cm. Respiration, Ic, Ik, and Pmax values were significantly lower in tissues from the understory relative to tissues from the canopy of L. mcdermidiae, while photosystem I (PS I) sizes (PSU I) were significantly higher in tissues from understory microsites. Quantum efficiency was unchanged. Ahnfeltiopsis concinna, in contrast, exhibited higher α in tissues from understory rather than canopy microsites. The values of Pmax for tissues from the canopy of A. concinna were not higher than the understory, while PSU O2(PS II size) of tissues from canopy microsites were unusually higher than those of understory microsites. These characteristics suggest a high degree of irradiance stress in tissues from the canopy of A. concinna, the highest tidal alga in Hawaiian coastal zones. Acclimation to high photosynthetically active radiation and ultraviolet irradiance levels especially in tropical regions appears to be an essential mechanism(s) for stress resistance and persistence of intertidal algae. Algal turfs acclimate at microscales in part fostered by their dense stands that create sharp irradiance gradients as well as adjust physiologically to canopy irradiance levels as mechanisms for optimal photosynthetic performance and stress tolerance.