ALBERT

Description: Ocean acidification and coastal nutrient enrichment threaten the persistence of nearshore ecosystems, yet little is known about their combined effects on marine organisms. Here, we show that elevated nitrogen concentrations, in the forms of nitrite + nitrate and ammonium, offset the negative effects of near-future OA on calcification of the reef-building crustose coralline alga, Porolithon onkodes. Projected near-future pCO2 levels (~850 µatm) decreased P. onkodes calcification by 30% relative to ambient conditions. Conversely, nitrogen enrichment increased P. onkodes calcification by 90% and 130% in ambient and high pCO2 treatments, respectively, relative to ambient controls. pCO2 and nitrogen enrichment interactively affected instantaneous photophysiology. Relative electron transport rates (rETR) were highest in high pCO2 and high nitrogen conditions. Nitrogen enrichment alone increased concentrations of the photosynthetic pigments chlorophyll a, phycocyanin and phycoerythrin by ~80-450%, regardless of pCO2. These results demonstrate that nutrient enrichment can mediate organismal responses to OA, which has far-reaching implications for nearshore coral reefs that experience persistent or episodic nutrient enrichment via eutrophication or consumer excretions. Multi-stressor OA experiments increasingly are becoming important for improving our ability to predict the response of marine organisms and coral reefs to simultaneously occurring local and global change stressors.

Description: The objective of this study was to investigate whether a tipping point exists in the calcification responses of coral reef calcifiers to CO2. We compared the effects of six partial pressures of CO2 (PCO2) from 28 Pa to 210 Pa on the net calcification of four corals (Acropora pulchra, Porites rus, Pocillopora damicornis, and Pavona cactus), and four calcified algae (Hydrolithon onkodes, Lithophyllum flavescens, Halimeda macroloba, and Halimeda minima). After 2 weeks of acclimation in a common environment, organisms were incubated in 12 aquaria for 2 weeks at the targeted PCO2 levels and net calcification was quantified. All eight species calcified at the highest PCO2 in which the calcium carbonate aragonite saturation state was ~1. Calcification decreased linearly as a function of increasing partial PCO2 in three corals and three algae. Overall, the decrease in net calcification as a function of decreasing pH was ~10% when ambient PCO2 (39 Pa) was doubled. The calcification responses of P. damicornis and H. macroloba were unaffected by increasing PCO2. These results are inconsistent with the notion that coral reefs will be affected by rising PCO2 in a response characterized by a tipping point. Instead, our findings combined among taxa suggest a gradual decline in calcification will occur, but this general response includes specific cases of complete resistance to rising PCO2. Together our results suggest that the overall response of coral reef communities to ocean acidification will be monotonic and inversely proportional to PCO2, with reef-wide responses dependent on the species composition of calcifying taxa.

Description: Ocean acidification (OA) poses a severe threat to tropical coral reefs, yet much of what is know about these effects comes from individual corals and algae incubated in isolation under high pCO2. Studies of similar effects on coral reef communities are scarce. To investigate the response of coral reef communities to OA, we used large outdoor flumes in which communities composed of calcified algae, corals, and sediment were combined to match the percentage cover of benthic communities in the shallow back reef of Moorea, French Polynesia. Reef communities in the flumes were exposed to ambient (400 matm) and high pCO2 (1300 matm) for 8 weeks, and calcification rates measured for the constructed communities including the sediments. Community calcification was reduced by 59% under high pCO2, with sediment dissolution explaining ~ 50% of this decrease; net calcification of corals and calcified algae remained positive but was reduced by 29% under elevated pCO2. These results show that, despite the capacity of coral reef calcifiers to maintain positive net accretion of calcium carbonate under OA conditions, reef communities might transition to net dissolution as pCO2 increases, particularly at night, due to enhanced sediment dissolution.

Description: Ocean acidification (OA) is a major threat to coral reefs, which are built by calcareous species. However, long-term assessments of the impacts of OA are scarce, limiting the understanding of the capacity of corals and coralline algae to acclimatize to high partial pressure of carbon dioxide (pCO2) levels. Species-specific sensitivities to OA are influenced by its impacts on chemistry within the calcifying fluid (CF). Here, we investigate the capacity of multiple coral and calcifying macroalgal species to acclimatize to elevated pCO2 by determining their chemistry in the CF during a year-long experiment. We found no evidence of acclimatization to elevated pCO2 across any of the tested taxa. The effects of increasing seawater pCO2 on the CF chemistry were rapid and persisted until the end of the experiment. Our results show that acclimatization of the CF chemistry does not occur within one year, which confirms the threat of OA for future reef accretion and ecological function.

Description: Coral reefs are threatened by ocean acidification (OA), which depresses net calcification of corals, calcified algae, and coral reef communities. These effects have been quantified for many organisms, but most experiments last weeks-to-months, and do not test for effects on community structure. Here, the effects of OA on back reef communities from Mo'orea, French Polynesia (17.492 S, 149.826 W), were tested from 12 November 2015 to 16 November 2016 in outdoor flumes maintained at mean pCO2 levels of 364 µatm, 564 µatm, 761 µatm, and 1067 µatm. The communities consisted of four corals and two calcified algae, with change in mass (Gnet, a combination of gross accretion and dissolution) and percent cover recorded monthly. For massive Porites and Montipora spp., Gnet differed among treatments, and at 1067 µatm (relative to ambient) was reduced and still positive; for Porolithon onkodes, all of which died, Gnet was negative at high pCO2, revealing dissolution (sample sizes were too small for analysis of Gnet for other taxa). Growth rates (% cover month−1) were unaffected by pCO2 for Montipora spp., P. rus, Pocillopora verrucosa, and Lithophyllum kotschyanum, but were depressed for massive Porites at 564 µatm. Multivariate community structure changed among seasons, and the variation under all elevated pCO2 treatments differed from that recorded at 364 µatm, and was greatest under 564 µatm and 761 µatm pCO2. Temporal variation in multivariate community structure could not be attributed solely to the effects of OA on the chemical and physical properties of seawater. Together, these results suggest that coral reef community structure may be more resilient to OA than suggested by the negative effects of high pCO2 on Gnet of their component organisms.

Description: Central to evaluating the effects of ocean acidification (OA) on coral reefs is understanding how calcification is affected by the dissolution of CO2 in sea water, which causes declines in carbonate ion concentration [CO3]2- and increases in bicarbonate ion concentration [HCO3]-. To address this topic, we manipulated [CO3]2- and [HCO3]- to test the effects on calcification of the coral Porites rus and the alga Hydrolithon onkodes, measured from the start to the end of a 15-day incubation, as well as in the day and night. [CO3]2- played a significant role in light and dark calcification of P. rus, whereas [HCO3]- mainly affected calcification in the light. Both [CO3]2- and [HCO3]- had a significant effect on the calcification of H. onkodes, but the strongest relationship was found with [CO3]2-. Our results show that the negative effect of declining [CO3]2- on the calcification of corals and algae can be partly mitigated by the use of [HCO3]- for calcification and perhaps photosynthesis. These results add empirical support to two conceptual models that can form a template for further research to account for the calcification response of corals and crustose coralline algae to OA.

Description: Recently, it has been suggested that there are conditions under which some coral species appear to be resistant to the effects of ocean acidification. To test if such resistance can be explained by environmental factors such as light and food availability, the present study investigated the effect of 3 feeding regimes crossed with 2 light levels on the response of the coral Porites rus to 2 levels of pCO2 at 28 °C. After 1, 2, and 3 weeks of incubation under experimental conditions, none of the factors-including pCO2-significantly affected area-normalized calcification and biomass-normalized calcification. Biomass also was unaffected during the first 2 weeks, but after 3 weeks, corals that were fed had more biomass per unit area than starved corals. These results suggest that P. rus is resistant to short-term exposure to high pCO2, regardless of food availability and light intensity. P. rus might therefore represent a model system for exploring the genetic basis of tolerance to OA.

Description: To identify the properties of taxa sensitive and resistant to ocean acidification (OA), we tested the hypothesis that coral reef calcifiers differ in their sensitivity to OA as predictable outcomes of functional group alliances determined by conspicuous traits. We contrasted functional groups of eight corals and eight calcifying algae defined by morphology in corals and algae, skeletal structure in corals, spatial location of calcification in algae, and growth rate in corals and algae. The responses of calcification to OA were unrelated to morphology and skeletal structure in corals; they were, however, affected by growth rate in corals and algae (fast calcifiers were more sensitive than slow calcifiers), and by the site of calcification and morphology in algae. Species assemblages characterized by fast growth, and for algae, also cell-wall calcification, are likely to be ecological losers in the future ocean. This shift in relative success will affect the relative and absolute species abundances as well as the goods and services provided by coral reefs.

Description: Ocean acidification (OA) and its associated decline in calcium carbonate saturation states is one of the major threats that tropical coral reefs face this century. Previous studies of the effect of OA on coral reef calcifiers have described a wide variety of outcomes for studies using comparable partial pressure of CO2 (pCO2) ranges, suggesting that key questions remain unresolved. One unresolved hypothesis posits that heterogeneity in the response of reef calcifiers to high pCO2 is a result of regional-scale variation in the responses to OA. To test this hypothesis, we incubated two coral taxa (Pocillopora damicornis and massive Porites) and two calcified algae (Porolithon onkodes and Halimeda macroloba) under 400, 700 and 1000 µatm pCO2 levels in experiments in Moorea (French Polynesia), Hawaii (USA) and Okinawa (Japan), where environmental conditions differ. Both corals and H. macroloba were insensitive to OA at all three locations, while the effects of OA on P. onkodes were location-specific. In Moorea and Hawaii, calcification of P. onkodes was depressed by high pCO2, but for specimens in Okinawa, there was no effect of OA. Using a study of large geographical scale, we show that resistance to OA of some reef species is a constitutive character expressed across the Pacific.

Description: The threat represented by ocean acidification (OA) for coral reefs has received considerable attention because of the sensitivity of calcifiers to changing seawater carbonate chemistry. However, most studies have focused on the organismic response of calcification to OA, and only a few have addressed community-level effects, or investigated parameters other than calcification, such as photosynthesis. Light (photosynthetically active radiation, PAR) is a driver of biological processes on coral reefs, and the possibility that these processes might be perturbed by OA has important implications for community function. Here we investigate how CO2 enrichment affects the relationships between PAR and community net O2 production (Pnet), and between PAR and community net calcification (Gnet), using experiments on three coral communities constructed to match (i) the back reef of Mo'orea, French Polynesia, (ii) the fore reef of Mo'orea, and (iii) the back reef of O'ahu, Hawaii. The results were used to test the hypothesis that OA affects the relationship between Pnet and Gnet. For the three communities tested, pCO2 did not affect the Pnet-PAR relationship, but it affected the intercept of the hyperbolic tangent curve fitting the Gnet-PAR relationship for both reef communities in Mo'orea (but not in O'ahu). For the three communities, the slopes of the linear relationships between Pnet and Gnet were not affected by OA, although the intercepts were depressed by the inhibitory effect of high pCO2 on Gnet. Our result indicates that OA can modify the balance between net calcification and net photosynthesis of reef communities by depressing community calcification, but without affecting community photosynthesis.