As the surface ocean equilibrates with rising atmospheric pCO2, the pH of surface seawater is decreasing with potentially negative impacts to coral calcification and coral reef ecosystems. This dissertation is composed of 4 individual studies that explore the impacts of ocean acidification on community reef development, coral calcification rates, and the acclimatization potential of corals to decreasing seawater pH. This is accomplished through in-situ field investigations on a tropical coral reef and laboratory experiments on temperate solitary corals.
In Chapters II-IV, I present findings from field investigations at Puerto Morelos, Mexico concerning the impact of in-situ declines in saturation state (Ωarag) on a reef community. Chapter II is a survey of the impact of saturation state on coral species richness, abundance, and colony size. I observe that while corals are often found in under-saturated waters, species richness, number of individuals, and colony size all decrease with decreasing saturation state. The study concludes that impacts of ocean acidification vary widely by species and geographic distribution, but that overall coral coverage will decline significantly in the 21st century.
Chapter III explores the calcification rates of Porites astreoides corals in low and under-saturated waters and compares them to rates of colonies growing in control zones approximately 10m away. I conclude that decreases in saturation state are associated with significant declines in coral calcification, driven mainly by decreasing density of the skeletal material. Additionally, decreasing saturation state was associated with significant increases in the rate of bioerosion by boring organisms.
In Chapter IV, I address how ocean acidification may impact a reef ecosystem through a year-long recruitment experiment. I deploy limestone tiles in both low saturation and control zones and recover them at 3, 6, and 14 month intervals. Tiles in low saturation zones have up to 70% less coverage of calcifying organisms, coincident with an increase in fleshy algal coverage. Crustose and upright coralline algae are up to 90% less abundant on low saturation tiles after 14 months, despite their ability to establish on the tiles. These findings indicate that calcifying organisms, while physiologically tolerant of low saturation, are outcompeted by fleshy algae under ocean acidification conditions.
In Chapter V, I explore laboratory experiments on a temperate scleractinian coral, Balanophyllia elegans, to address how decreasing pH and level of nutrition impact coral calcification. In these experiments, I manipulate pCO2 (410, 770, and 1220 μatm) and feeding frequency (3 days vs. 21 days) in a closed seawater system to address the energetic requirements of calcification in corals without the aid of the symbiotic dinoflagellate, zooxanthellae. Planulation rates were affected by food level but not pCO2, while juvenile mortality was highest under high pCO2 (1220 µatm) and low food (21 day intervals). While net calcification was positive even at 1220 µatm (~3 times current atmospheric pCO2), overall calcification declined by ~25-45%, and skeletal density declined by ~35-45% as pCO2 increased from 410 to 1220 µatm. Aragonite crystal morphology changed at high pCO2, becoming significantly shorter but not wider at 1220 µatm.
Combined, these chapters suggest that the response of organisms to ocean acidification will be highly species-specific, complex, and will depend on multiple factors, such as community interactions and feeding amount. There is, however, overwhelming evidence suggesting that coral calcification and reef accretion will decline significantly over the 21st century.