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1. Upwelling, climate change, and the shifting geography of coral reef development

   https://doi.org/10.1038/s41598-023-28489-0  

   Rodriguez-Ruano, Victor ; Toth, Lauren T. ; Enochs, Ian C. ; Randall, Carly J. ; Aronson, Richard B. ( February 2023 , Scientific Reports)

   The eastern tropical Pacific is oceanographically unfavorable for coral-reef development. Nevertheless, reefs have persisted there for the last 7000 years. Rates of vertical accretion during the Holocene have been similar in the strong-upwelling Gulf of Panamá (GoP) and the adjacent, weak-upwelling Gulf of Chiriquí (GoC); however, seasonal upwelling in the GoP exacerbated a climate-driven hiatus in reef development in the late Holocene. The situation is now reversed and seasonal upwelling in the GoP currently buffers thermal stress, creating a refuge for coral growth. We developed carbonate budget models to project the capacity of reefs in both gulfs to keep up with future sea-level rise. On average, the GoP had significantly higher net carbonate production rates than the GoC. With an estimated contemporary reef-accretion potential (RAP) of 5.5 mm year⁻¹, reefs in the GoP are projected to be able to keep up with sea-level rise if CO₂emissions are reduced, but not under current emissions trajectories. With an estimated RAP of just 0.3 mm year⁻¹, reefs in the GoC are likely already unable to keep up with contemporary sea-level rise in Panamá (1.4 mm year⁻¹). Whereas the GoP has the potential to support functional reefs in the near-term, our study indicates that their long-term persistence will depend on reduction of greenhouse gases.

    

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2. Decadal-scale variation in coral calcification on coral-depleted Caribbean reefs

   https://doi.org/10.3354/meps14345  

   Edmunds, PJ ; Perry, CT ( June 2023 , Marine Ecology Progress Series)

   Over recent decades, many Caribbean reefs have transitioned to states where stony corals are no longer spatially dominant. The community dynamics culminating in this outcome are well known, but its functional implications remain incompletely understood. Here we used annual surveys from 1992 to 2019 to describe coral communities at 6 sites off St. John, US Virgin Islands, and explored how their ecological dynamics interact with their capacity to sustain estimated coral community calcification (G, kg CaCO 3 m -2 yr -1 ). These communities had low coral cover (≤4.4%), but they changed through small and incremental events that summed to a slight decline in coral cover and changes in species assemblages favoring biotic homogenization and weedy species. Estimated coral G remained low, between 0.3 and 1.3 kg CaCO 3 m -2 yr -1 (8.2-35.6 mmol CaCO 3 m -2 d -1 ), but it differed among sites and years. The dominant contributors to G were Siderastrea siderea (1 site), Porites astreoides (1 site), and Orbicella spp. (4 sites), but higher G only occurred where Orbicella spp. remained relatively common; G dramatically declined at 1 site when the abundance of this genus decreased. These results suggest that some coral-depleted reefs may maintain low G that could be sufficient to avoid transitions into net negative budget states, provided that biological and physical erosion and dissolution of CaCO 3 (not recorded here) are minimal. Further mortalities of the few coral species remaining on these reefs through disturbances like stony coral tissue loss disease would compromise this delicate production-erosion balance, and likely see transitions of such reefs into negative carbonate budget states. 

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3. Using B isotopes and B/Ca in corals from low saturation springs to constrain calcification mechanisms

   https://doi.org/10.1038/s41467-019-11519-9  

   Wall, M. ; Fietzke, J. ; Crook, E. D. ; Paytan, A. ( August 2019 , Nature Communications)

   Ocean acidification is expected to negatively impact calcifying organisms, yet we lack understanding of their acclimation potential in the natural environment. Here we measured geochemical proxies (δ¹¹B and B/Ca) inPorites astreoidescorals that have been growing for their entire life under low aragonite saturation (Ω_sw: 0.77–1.85). This allowed us to assess the ability of these corals to manipulate the chemical conditions at the site of calcification (Ω_cf), and hence their potential to acclimate to changing Ω_sw. We show that lifelong exposure to low Ω_swdid not enable the corals to acclimate and reach similar Ω_cfas corals grown under ambient conditions. The lower Ω_cfat the site of calcification can explain a large proportion of the decreasingP. astreoidescalcification rates at low Ω_sw. The naturally elevated seawater dissolved inorganic carbon concentration at this study site shed light on how different carbonate chemistry parameters affect calcification conditions in corals.

    

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4. Effects of year‐long exposure to elevated pCO2 on the metabolism of back reef and fore reef communities

   https://doi.org/10.1002/lno.12504  

   Edmunds, Peter J. ; Doo, Steve S. ; Carpenter, Robert C. ( January 2024 , Limnology and Oceanography)

   The implications of ocean acidification are acute for calcifying organisms, notably tropical reef corals, for which accretion generally is depressed and dissolution enhanced at reduced seawater pH. We describe year‐long experiments in which back reef and fore reef (17‐m depth) communities from Moorea, French Polynesia, were incubated outdoors under pCO₂regimes reflecting endpoints of representative concentration pathways (RCPs) expected by the end the century. Incubations were completed in three to four flumes (5.0 × 0.3 m, 500 L) in which seawater was refreshed and circulated at 0.1 m s⁻¹, and the response of the communities was evaluated monthly by measurements of net community calcification (NCC) and net community productivity (NCP). For both communities, NCC (but not NCP) was affected by treatments and time, with NCC declining with increasing pCO₂, and for the fore reef, becoming negative (i.e., dissolution was occurring) at the highest pCO₂(1067–1433μatm, RCP8.5). There was scant evidence of community adjustment to reduce the negative effects of ocean acidification, and inhibition of NCC intensified in the back reef as the abundance of massivePoritesspp. declined. These results highlight the risks of dissolution under ocean acidification for coral reefs and suggest these effects will be most acute in fore reef habitats. Without signs of amelioration of the negative effects of ocean acidification during year‐long experiments, it is reasonable to expect that the future of coral reefs in acidic seas can be predicted from their current known susceptibility to ocean acidification.

    

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5. Coccolithophore Growth and Calcification in an Acidified Ocean: Insights From Community Earth System Model Simulations

   https://doi.org/10.1029/2018MS001483  

   Krumhardt, K. M. ; Lovenduski, N. S. ; Long, M. C. ; Levy, M. ; Lindsay, K. ; Moore, J. K. ; Nissen, C. ( May 2019 , Journal of Advances in Modeling Earth Systems)

   Anthropogenic CO₂emissions are inundating the upper ocean, acidifying the water, and altering the habitat for marine phytoplankton. These changes are thought to be particularly influential for calcifying phytoplankton, namely, coccolithophores. Coccolithophores are widespread and account for a substantial portion of open ocean calcification; changes in their abundance, distribution, or level of calcification could have far‐reaching ecological and biogeochemical impacts. Here, we isolate the effects of increasing CO₂on coccolithophores using an explicit coccolithophore phytoplankton functional type parameterization in the Community Earth System Model. Coccolithophore growth and calcification are sensitive to changing aqueous CO₂. While holding circulation constant, we demonstrate that increasing CO₂concentrations cause coccolithophores in most areas to decrease calcium carbonate production relative to growth. However, several oceanic regions show large increases in calcification, such the North Atlantic, Western Pacific, and parts of the Southern Ocean, due to an alleviation of carbon limitation for coccolithophore growth. Global annual calcification is 6% higher under present‐day CO₂levels relative to preindustrial CO₂(1.5 compared to 1.4 Pg C/year). However, under 900 μatm CO₂, global annual calcification is 11% lower than under preindustrial CO₂levels (1.2 Pg C/year). Large portions of the ocean show greatly decreased coccolithophore calcification relative to growth, resulting in significant regional carbon export and air‐sea CO₂exchange feedbacks. Our study implies that coccolithophores become more abundant but less calcified as CO₂increases with a tipping point in global calcification (changing from increasing to decreasing calcification relative to preindustrial) at approximately ∼600 μatm CO₂.

    

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