Experiments have shown that ocean acidification due to rising atmospheric carbon dioxide concentrations has deleterious effects on the performance of many marine organisms.
However, few studies have addressed the long-term consequences of ocean acidification for marine ecosystems.
The study investigated the effects of natural in situ exposure to elevated seawater pCO2 on tropical coral reef communities, coral growth, recruitment, seagrasses, and sedimentary properties around three cool volcanic CO2 seeps in Papua New Guinea.
As pH declined from 8.1 to 7.8 (the change expected if atmospheric CO2 concentrations increase from 390 to 750 ppm), some organisms benefited, but many more lost out.
At reduced pH, the study observed reductions in coral diversity, recruitment, and abundances of structurally complex framework builders, and shifts in competitive interactions between taxa.
However, coral cover remained constant between pH 8.1 and ~7.8, as massive Porites corals established dominance over structural corals, despite low rates of calcification.
Reef development ceased below pH 7.7, and the empirical data confirm model predictions that ocean acidification, together with temperature stress, will likely lead to severely reduced diversity, structural complexity, and resilience of Indo-Pacific coral reefs within this century.
Coral community changes:
- Coral cover doubled, but structurally complex corals (branching, foliose, tabulate) declined three-fold.
- Taxonomic richness of hard corals reduced by 39%.
- Cover of fleshy macroalgae doubled and seagrass increased eight-fold.
- Cover of crustose coralline algae and other red calcareous algae reduced seven-fold.
- Cover and richness of soft corals and sponges significantly reduced.
- Density and taxonomic richness of hard and soft coral juveniles declined significantly.
Coral calcification:
- Calcification rates of massive Porites corals were 30% lower than expected over the past 12 years.
- Calcification decline in Porites and other corals is likely due to temperature stress and/or ocean acidification.
- Massive Porites were less frequent near intense CO2 vents with pH < 7.7, showing sensitivity to low pH.
Seagrass and epiphyte responses:
- Seagrass communities had higher shoot densities and biomass but reduced diversity near intense CO2 vents.
- Calcareous epiphytes on seagrass blades were nearly absent near the vents.
Sediment composition:
- Sediments at high CO2 sites were almost devoid of inorganic carbon, calcareous biota, and their remains.
The figure shows the response ratios (high pCO2/low pCO2) for various reef communities and associated biotic changes, with significant differences at the 5% level indicated by error bars not including the value 1.0.
The panels include:
a. Reef communities including hard and soft corals
b. Juvenile corals, skeletal properties, tissue thickness, and macrobioeroder densities in Porites and Pocillopora
c. Seagrass shoot density, below-ground biomass, diversity, epibiont cover, and foraminifera densities
d. Sediment properties and associated calcifying biota
Organic carbon, nitrogen, and siliceous spicules did not change along the pH gradient, but total seawater alkalinity was elevated by ~50 μEq/kg, suggesting continued net carbonate dissolution.
The more sparsely seeping Esa'Ala high pCO2 site had lower inorganic carbon content (5%) compared to the controls (6-10%), and many foraminifera tests were corroded or pitted.
The observed ecological changes along the pH gradient at Upa-Upasina Reef are presented in Figure 3, showing progressive declines in various reef biota with decreasing pH.
The implications of these changes are severe, including reduced habitat availability and quality, decreased coral recovery and community resilience, impaired larval recruitment, and increased susceptibility to macroalgal cover and bioeroder densities.
Natural limitations exist in using the Milne Bay CO2 seeps as proxies, as they are surrounded by areas with ambient pH, providing larval supply, and experience respite from low pH during windy periods.
The study examines the effects of high and low pCO2 (ocean acidification) on coral reef ecosystems in the Milne Bay region of Papua New Guinea.
The data suggests that tropical coral reefs can still exist at a seawater pH of 7.8 (750 ppm pCO2), but with significant losses in biodiversity, structural complexity, and resilience.
As pH declines from 8.1 to 7.8, the reef community shifts from fast-growing, structurally complex corals to slow-growing, structurally simple massive Porites corals, maintaining overall coral cover.
Reef development ceases at a pH of 7.7 (980 ppm pCO2), suggesting this is a terminal threshold for any form of coral reef development.
The study provides in-situ measurements of various biotic responses, including coral calcification rates, tissue thickness, bioerosion, and symbiotic algae composition.
Seagrass communities also showed changes in shoot density, below-ground biomass, and epiphytic cover under high pCO2 conditions.
The study highlights the need for rapid transition to low CO2 emissions to minimize the risk of profound losses of coral reef ecosystem functions and services due to both climate change and ocean acidification.
The rate of atmospheric CO2 increase continues to accelerate.
An INGENT PharU DGGIE unit was used for 15 h at 75 V to process samples.
Representative bands from polyacrylamide gels with a 35-63% denaturant gradient (formamide and urea) were re-amplified, sequenced, and compared with sequences in the public library.
Seagrass shoot density and species composition (dominated by Cymodocea rotundata and Cymodocea serrata) were assessed in fifteen 400 cm2 quadrats at <1 m depth at the high and low pCO2 sites of Upa-Upasina and Esa'Ala.
Below-ground biomass of seagrasses was harvested, dried at 60°C, and weighed.
Calcareous epiphytic cover and foraminiferal densities (Marginopora vertebralis) on seagrass blades were analyzed using photography and in situ measurements at the high and low pCO2 sites.
Sediment geochemistry and foraminiferal assemblages in the upper 1 cm sediment layer were assessed along the benthic transects and seagrass sites (20 samples in total).
Sediments were wet-sieved (63 μm mesh), dried, weighed, sorted (200 foraminifera per sample), and the abundances of siliceous, calcareous, and echinoid spicules, Halimeda segments, and small gastropods were estimated.
Total carbon, organic carbon, and nitrogen were analyzed in dried and ground sediment samples.
The main points from the given information are:
Inorganic carbon measurements were made using a Shimadzu TOC-V Analyzer, calibrated with the certified reference MESS-1 and two in-house standards.
Statistical methods:
- Generalized additive models (GAMs) were used to predict pH values from field observations along 10-m sections of transects and spatial grids of the three reefs.
- The degree of smoothness of the spatial smoothers was selected by cross-validation.
- The GAMs explained 44%, 74%, and 59% of the deviance in the pH data for Upa-Upasina, Dobu, and Earlik reefs, respectively.
Ratios of observed variables for known to high pCO2 sites were estimated using generalized linear models (GLMs):
- The GLMs used a log link function to constrain the estimated ratios to be non-negative.
- The variance of the ratios was assumed to be proportional to the mean.
- The models included the effects of the reefs, and the reported ratios represent weighted averages across the three reefs.
GLMs were also used to assess changes in biota along the pH gradient at the Upa-Upasina reef.
All statistical analyses were performed using the R software.
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