MEPS 577:67-77 (2017)  -  DOI: https://doi.org/10.3354/meps12249

ABSTRACT: To evaluate the effects of ocean acidification (OA) and algal presence on the early life-history stages of corals, we conducted an aquarium study that examined the isolated and combined effects of reduced pH (pH 8.10 vs. 7.85) and contact with the alga Stypopodium zonale on the survival, settlement, and post-settlement growth of larvae from the brooding coral Porites astreoides. Two settlement substrates, biofilmed tiles and the crustose coralline alga (CCA) Hydrolithon boergesenii, were initially incubated for 12 d in separate tanks under a factorial combination of low pH and S. zonale contact, and then subjected to a series of settlement assays. Across both substrate types, S. zonale presence significantly reduced coral settlement. Low pH imposed relatively minor effects; however, there was a significant interaction between pH and S. zonale presence for settlement on the CCA substrate, such that low pH exacerbated the negative effects of S. zonale. Post-settlement growth for 2 wk was unaffected by either S. zonale presence or low pH on either substrate. While our results demonstrate that S. zonale contact likely remains a dominant threat to larval survival and settlement, in certain cases, OA may amplify the negative effects of S. zonale presence, highlighting the need to consider multiple factors in studies aimed at assessing the future health of coral reef ecosystems.

KEY WORDS: Coral-algal interactions · Climate change · Reef recruitment · Porites astreoides · Stypopodium zonale · Hydrolithon boergesenii