Publication Date

2013-05-08

Availability

Embargoed

Embargo Period

2014-10-30

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine Biology and Fisheries (Marine)

Date of Defense

2013-04-09

First Committee Member

Chris Langdon

Second Committee Member

Joan Kleypas

Third Committee Member

Diego Lirman

Fourth Committee Member

Andrew C Baker

Fifth Committee Member

Martin Grosell

Abstract

Increasing atmospheric carbon dioxide (pCO2) dissolves in the ocean, decreasing the calcium carbonate saturation state (Ωaragonite) and creating conditions unfavorable for calcification (G) in reef-building corals. Understanding the effects of ocean acidification on coral reefs requires a robust description of the relationship between Ω and calcification (Ω-G) at both the reef scale and at the organismal scale. To evaluate the Ω-G relationship on the reef, we conducted repeat surveys across 200 km of the Florida Reef Tract over a 2 year period. Results showed that net community calcification switches from positive in the summer to negative in the winter, indicating net dissolution and revealing that the reef tract is currently straddling the tipping point between reef growth and loss. To evaluate the Ω-G relationship at the organismal scale, we grew Acropora cervicornis under six CO2 levels, 2-3 times more than typically achieved in laboratory settings. The associated Ω stretched from current levels to highly undersaturated seawater, creating a robust test of linearity of the Ω-G relationship. Our results show that the Ω-G relationship is linear and maintained even in highly undersaturated seawater. The effect of pCO2 on calcification was also strongly mediated by heterotrophy, which significantly alleviated the effect of ocean acidification at all pCO2 levels. The obligate symbiont, Symbiodinium microadriaticum, also showed a significant response to increasing pCO2 and declined in density in both corals in the laboratory (A. cervicornis), and in corals at natural CO2 vents in the South Pacific (Acropora millepora and Pocillopora damicornis). This dissertation demonstrates that while heterotrophy can offset a significant portion of the negative effects of ocean acidification, coral calcification still declines in direct proportion to reductions in Ω. At the community level, evidence is presented that calcification on the Florida Reef Tract exists near the threshold for net carbonate accretion. Thus, while corals can utilize heterotrophic sources of nutrients to alleviate the effects of ocean acidification, it is unlikely to translate into net reef growth in the natural environment, where net annual dissolution is already occurring.

Keywords

coral calcification; dissolution; ocean acidification; heterotrophy; symbiosis; net ecosystem calcification

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