Abstract:

Ocean acidification (OA) will result in lower calcification rates for numerous marine taxa, including many species of corals which create important reef habitat. Seawater carbonate chemistry fluctuates over cycles ranging from days to seasons, often driven by biological processes such as respiration and photosynthesis. The magnitude of diel fluctuations varies spatially and may become more pronounced in the future due to OA. Due to technical constraints, OA experiments that incorporate diel variability into treatments are few in number. As a result, the degree to which coral reef organisms are influenced by ambient daily carbonate chemistry variability is poorly understood. Here we describe an experiment conducted in a novel seawater system which can independently manipulate carbonate chemistry in 16 separate aquaria, in real time, allowing precise control of the mean and magnitude of pH oscillations while minimizing pseudoreplication. Five genotypes of the threatened Caribbean coral Acropora cervicornis were subjected to a total of five pH treatments, 7.80 ± 0.20, 7.80 ± 0.10, and 7.80 ± 0.00, as well as 8.05 ± 0.10 and 8.05 ± 0.00. Those corals exposed to variable contemporary conditions (8.05 ± 0.10) calcified faster than those in current and future static treatment levels, which did not significantly differ from each other. Variable contemporary pH also resulted in faster growth rates than highly variable future conditions (7.80 ± 0.20), but were not significantly different than future conditions with the same ±0.10 diel pH oscillation. These findings support the importance of incorporating diel variability into OA experiments and suggest that more variable natural ecosystems may yield higher calcification rates for corals.

Abstract:

The persistence of coral reef frameworks requires that calcium carbonate (CaCO₃) production by corals and other calcifiers outpaces CaCO₃ loss via physical, chemical, and biological erosion. Coral bleaching causes declines in CaCO₃ production, but this varies with bleaching severity and the species impacted. We conducted census-based CaCO₃ budget surveys using the established ReefBudget approach at Cheeca Rocks, an inshore patch reef in the Florida Keys, annually from 2012 to 2016. This site experienced warm-water bleaching in 2011, 2014, and 2015. In 2017, we obtained cores of the dominant calcifying coral at this site, Orbicella faveolata, to understand how calcification rates were impacted by bleaching and how they affected the reef-wide CaCO₃ budget. Bleaching depressed O. faveolata growth and the decline of this one species led to an overestimation of mean (± std. error) reef-wide CaCO₃ production by + 0.68 (± 0.167) to + 1.11 (± 0.236) kg m⁻² year⁻¹ when using the static ReefBudget coral growth inputs. During non-bleaching years, the ReefBudget inputs slightly underestimated gross production by − 0.10 (± 0.022) to − 0.43 (± 0.100) kg m⁻² year⁻¹. Carbonate production declined after the first year of back-to-back bleaching in 2014, but then increased after 2015 to values greater than the initial surveys in 2012. Cheeca Rocks is an outlier in the Caribbean and Florida Keys in terms of coral cover, carbonate production, and abundance of O. faveolata, which is threatened under the Endangered Species Act. Given the resilience of this site to repeated bleaching events, it may deserve special management attention.

Abstract:

Sea-level rise (SLR) is predicted to elevate water depths above coral reefs and to increase coastal wave exposure as ecological degradation limits vertical reef growth, but projections lack data on interactions between local rates of reef growth and sea level rise. Here we calculate the vertical growth potential of more than 200 tropical western Atlantic and Indian Ocean reefs, and compare these against recent and projected rates of SLR under different Representative Concentration Pathway (RCP) scenarios. Although many reefs retain accretion rates close to recent SLR trends, few will have the capacity to track SLR projections under RCP4.5 scenarios without sustained ecological recovery, and under RCP8.5 scenarios most reefs are predicted to experience mean water depth increases of more than 0.5 m by 2100. Coral cover strongly predicts reef capacity to track SLR, but threshold cover levels that will be necessary to prevent submergence are well above those observed on most reefs. Urgent action is thus needed to mitigate climate, sea-level and future ecological changes in order to limit the magnitude of future reef submergence.

Abstract:

Experiments have demonstrated that ocean acidification (OA) conditions projected to occur by the end of the century will slow the calcification of numerous coral species and accelerate the biological erosion of reef habitats (bioerosion). Microborers, which bore holes less than 100 μm diameter, are one of the most pervasive agents of bioerosion and are present throughout all calcium carbonate substrates within the reef environment. The response of diverse reef functional groups to OA is known from real-world ecosystems but, to date, our understanding of the relationship between ocean pH and carbonate dissolution by microborers is limited to controlled laboratory experiments. Here we examine the settlement of microborers to pure mineral calcium carbonate substrates (calcite) along a natural pH gradient at a volcanically acidified reef at Maug, Commonwealth of the Northern Mariana Islands (CNMI). Colonization of pioneer microborers was higher in the lower pH waters near the vent field. Depth of microborer penetration was highly variable both among and within sites (4.2–195.5 μm) over the short duration of the study (3 mo.), and no clear relationship to increasing CO₂ was observed. Calculated rates of biogenic dissolution, however, were highest at the two sites closer to the vent and were not significantly different from each other. These data represent the first evidence of OA-enhancement of microboring flora colonization in newly available substrates and provide further evidence that microborers, especially bioeroding chlorophytes, respond positively to low pH. The accelerated breakdown and dissolution of reef framework structures with OA will likely lead to declines in structural complexity and integrity, as well as possible loss of essential habitat.

Abstract:

Ocean acidification (OA) impacts the physiology of diverse marine taxa; among them corals that create complex reef framework structures. Biological processes operating on coral reef frameworks remain largely unknown from naturally high-carbon-dioxide (CO₂) ecosystems. For the first time, we independently quantified the response of multiple functional groups instrumental in the construction and erosion of these frameworks (accretion, macroboring, microboring, and grazing) along natural OA gradients. We deployed blocks of dead coral skeleton for roughly 2 years at two reefs in Papua New Guinea, each experiencing volcanically enriched CO₂, and employed high-resolution micro-computed tomography (micro-CT) to create three-dimensional models of changing skeletal structure. OA conditions were correlated with decreased calcification and increased macroboring, primarily by annelids, representing a group of bioeroders not previously known to respond to OA. Incubation of these blocks, using the alkalinity anomaly methodology, revealed a switch from net calcification to net dissolution at a pH of roughly 7.8, within Intergovernmental Panel on Climate Change's (IPCC) predictions for global ocean waters by the end of the century. Together these data represent the first comprehensive experimental study of bioerosion and calcification from a naturally high-CO₂ reef ecosystem, where the processes of accelerated erosion and depressed calcification have combined to alter the permanence of this essential framework habitat.

Abstract:

Rising anthropogenic CO₂ in the atmosphere is accompanied by an increase in oceanic CO₂ and a concomitant decline in seawater pH. This phenomenon, known as ocean acidification (OA), has been experimentally shown to impact the biology and ecology of numerous animals and plants, most notably those that precipitate calcium carbonate skeletons, such as reef-building corals. Volcanically acidified water at Maug, Commonwealth of the Northern Mariana Islands (CNMI) is equivalent to near-future predictions for what coral reef ecosystems will experience worldwide due to OA. We provide the first chemical and ecological assessment of this unique site and show that acidification-related stress significantly influences the abundance and diversity of coral reef taxa, leading to the often-predicted shift from a coral to an algae-dominated state. This study provides field evidence that acidification can lead to macroalgae dominance on reefs.