An ecophysiological investigation of the effects of macroalgae on juvenile corals and larvae on coral reefs in the Pacific and Caribbean

On macroalgal-dominated reefs, the continuation of coral populations is dependent on successful recruitment and post-settlement success, both of which may be challenged by the presence of macroalgae. This study considers how the environment, particularly macroalgal abundance, contributes to the growth and survival of juvenile corals in two regions in the tropics: the South Pacific and the Caribbean. To determine how juvenile corals and coral larvae are affected by macroalgae in the back reef of Moorea, surveys and manipulative experiments were used to test the hypotheses that proximity to macroalgae with and without contact, or cover of macroalgae, will impact the survival and growth of early life stages of corals. Survival of Pocillopora damicornis larvae did not differ when they were incubated in situ adjacent to coral, macroalgae, or rock. Growth of juvenile colonies of massive Porites spp. and Pocillopora spp. were unaffected by centimeter-scale proximity to macroalgae. Additionally, growth was not affected by cover of macroalgae in 4-m2 plots, or by cages, which protected coral from macroalgal abrasion. Caged corals tended to grow faster, although this was not significant, and I hypothesize that this was related to protection from fish predation on lower cover plots, and from algae on higher cover plots. Macroalgae may not decrease Porites and Pocillopora growth through chemical effects, but factors including macroalgal cover may have indirect effects on the fish community that adversely affect exposed coral colonies. In St. John, US Virgin Islands, 12 sites distributed between White Point and Cabritte were selected and analyzed for benthic cover, herbivore abundance, rugosity, and rock type. Surveys of juvenile corals were conducted to determine whether the abundance and distribution of juvenile colonies within three types of microhabitats were affected by site characteristics, including macroalgae cover, and herbivore abundance. While the characteristics analyzed at the site level explained 75.9% of the variation between sites, these characteristics were not predictive of juvenile microhabitat distribution; ~ 75% of juvenile corals were found on exposed habitats at every site. While juvenile coral distribution among microhabitats was not related to site variation, survival rates within microhabitats may still vary among these sites.

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