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Marine Biology Graduate Student Association

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Heather Hillard

Heather Hillard

Phycology Lab
Department of Biology
18111 Nordhoff Street
Northridge, CA 91330-8303


B.S. Biology, The College of William and Mary, VA

Research Interests

Coral reef ecology, ocean acidification, coral—algal interactions, anthropogenic disturbances, marine conservation

M.S. Thesis

The combined and potentially interactive effects of macroalgae and ocean acidification on corals

Ocean acidification (OA), the decrease in the pH and carbonate ion concentration due to the uptake of anthropogenic CO2, threatens coral reefs worldwide as studies predict the decline of important calcifying organisms, such as corals, and the proliferation of non-calcifiers, such as fleshy macroalgae. For shallow corals reefs, the carbonate chemistry partly depends on the benthic community composition and associated metabolic processes. On the island of Moorea, French Polynesia, large diel fluctuations in pH were recorded directly behind the reef crest, which is dominated by the macroalga Sargassum pacificum. The pH fluctuations potentially were driven by algal primary production. In addition to the macroalgae, a diverse community of corals and other invertebrates inhabit the understory of the Sargassum thickets blanketing the reef crest and parts of the back reef. My research focuses on how these organisms, specifically scleractinian corals, survive in a macroalgal-dominated environment and how macroalgae may ameliorate or exacerbate the effects of elevated CO2 conditions on coral growth.

Through combined mesocosm and field experiments, I hope to determine: (1) if macroalgae, such as Sargassum pacificum, can have an indirect positive effect on coral calcification by increasing the pH through photosynthesis, (2) whether or not algal shading will outweigh any indirect positive impacts and lastly, (3) if the presence of macroalgae will exacerbate the negative effects of OA on corals. Previous studies describe how macroalgae can harm corals from shading, abrasion, allelopathy, overgrowth, and altering the microbial communities; however, few studies explore any potential positive, indirect impacts from a change in carbonate chemistry. My research emphasizes the importance of considering not only the effects of OA on reef corals but also how these effects may interact with macroalgae as macroalgal communities are expected to expand with increasing human disturbance. I hope my research will help inform managers and conservationists as the future recovery and persistence of coral dominated reefs may depend on the ability of corals to survive in association with macroalgae.

Caption: The coral reef surrounding Moorea is composed of a diverse benthic community with the reef crest dominated by fleshy macroalgae (top) and portions of the back reef covered by a variety of corals (middle). Plates suspended with organisms on them (lower).

algal-dominated reefcoral-dominated reefdiver with suspended plates