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Ocean acidification is a pervasive threat to coral reef ecosystems, and our understanding of the ecological processes driving patterns in tropical benthic community development in conditions of acidification is limited. We deployed limestone recruitment tiles in low aragonite saturation (Ωarag) waters during an in-situ field experiment at Puerto Morelos, Mexico, and compared them to tiles placed in control zones over a 14-month investigation. The early stages of succession showed relatively little difference in coverage of calcifying organisms between the low Ωarag and control zones. However, after 14 months of development, tiles from the low Ωarag zones had up to 70% less cover of calcifying organisms coincident with 42% more fleshy algae than the controls. The percent cover of biofilm and turf algae was also significantly greater in the low Ωarag zones, while the number of key grazing taxa remained constant. We hypothesize that fleshy algae have a competitive edge over the primary calcified space holders, coralline algae, and that acidification leads to altered competitive dynamics between various taxa. We suggest that as acidification impacts reefs in the future, there will be a shift in community assemblages away from upright and crustose coralline algae toward more fleshy algae and turf, established in the early stages of succession.

The effect of Ocean Acidification (OA) on marine biota is quasi-predictable at best. While perturbation studies, in the form of incubations under elevated pCO(2), reveal sensitivities and responses of individual species, one missing link in the OA story results from a chronic lack of pH data specific to a given species' natural habitat. Here, we present a compilation of continuous, high-resolution time series of upper ocean pH, collected using autonomous sensors, over a variety of ecosystems ranging from polar to tropical, open-ocean to coastal, kelp forest to coral reef. These observations reveal a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units. The nature of the observed variability was also highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes. These biome-specific pH signatures disclose current levels of exposure to both high and low dissolved CO(2), often demonstrating that resident organisms are already experiencing pH regimes that are not predicted until 2100. Our data provide a first step toward crystallizing the biophysical link between environmental history of pH exposure and physiological resilience of marine organisms to fluctuations in seawater CO(2). Knowledge of this spatial and temporal variation in seawater chemistry allows us to improve the design of OA experiments: we can test organisms with a priori expectations of their tolerance guardrails, based on their natural range of exposure. Such hypothesis-testing will provide a deeper understanding of the effects of OA. Both intuitively simple to understand and powerfully informative, these and similar comparative time series can help guide management efforts to identify areas of marine habitat that can serve as refugia to acidification as well as areas that are particularly vulnerable to future ocean change.

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 21 st 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 pCO 2, 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.

Ocean acidification is a pervasive threat to coral reef ecosystems, and our understanding of the ecological processes driving patterns in tropical benthic community development in conditions of acidification is limited. We deployed limestone recruitment tiles in low aragonite saturation ([omega]arag) waters during an in-situ field experiment at Puerto Morelos, Mexico, and compared them to tiles placed in control zones over a 14-month investigation. The early stages of succession showed relatively little difference in coverage of calcifying organisms between the low [omega]arag and control zones. However, after 14 months of development, tiles from the low [omega]arag zones had up to 70% less cover of calcifying organisms coincident with 42% more fleshy algae than the controls. The percent cover of biofilm and turf algae was also significantly greater in the low [omega]arag zones, while the number of key grazing taxa remained constant. We hypothesize that fleshy algae have a competitive edge over the primary calcified space holders, coralline algae, and that acidification leads to altered competitive dynamics between various taxa. We suggest that as acidification impacts reefs in the future, there will be a shift in community assemblages away from upright and crustose coralline algae toward more fleshy algae and turf, established in the early stages of succession.