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Planktonic foraminiferal area density as a proxy for carbonate ion concentration: a calibration study using the Cariaco Basin Ocean Time Series

Biweekly sediment trap samples and concurrent hydrographic measurements collected between March 2005 and October 2008 from the Cariaco Basin, Venezuela are used to assess the relationship between [CO₃^2-] and the area densities (ρ_A) of two species of planktonic foraminifera (Globigerinoides ruber (pink) and Globigerinoides sacculifer). Calcification temperatures were calculated for each sample using species-appropriate oxygen isotope (δ¹⁸O) temperature equations that were then compared to monthly temperature profiles taken at the study site in order to determine calcification depth. Ambient [CO₃^2-] was determined for these calcification depths using alkalinity, pH, temperature, salinity, and nutrient concentration measurements taken during monthly hydrographic cruises. ρ_A, which is representative of calcification efficiency, is determined by dividing individual foraminiferal shell weights (± 0.43 µg) by their associated silhouette areas and taking the sample average. The results of this study show a strong correlation between ρ_A and ambient [CO₃^2-] for both G. ruber and G. sacculifer (R² = 0.89 and 0.86 respectively), confirming that [CO₃^2-] has a pronounced effect on the calcification of these species. Though the ρ_A for both species reveal a highly significant (p < 0.001) relationship with ambient [CO₃^2-], linear regression reveals that the extent to which [CO₃^2-] influences foraminiferal calcification is species-specific. Hierarchical regression analyses indicate that other environmental parameters (temperature and [PO₄^3-]) do not confound the use of G. ruber and G. sacculifer ρ_A as a predictor for [CO₃^2-]. This study suggests that G. ruber and G. sacculifer ρ_A can be used as reliable proxies for past surface ocean [CO₃^2-].

Variability and trends of ocean acidification in the Southern California Current System: a timeseries from Santa Monica Bay

We investigate the temporal variability and trends of pH and of the aragonite saturation state, Ω_arag, in the southern California Current System on the basis of a 6 year timeseries from Santa Monica Bay, using bi-weekly observations of dissolved inorganic carbon and combined calculated and measured alkalinity. Median values of pH and Ω_arag in the upper 20 m are comparable to observations from the subtropical gyres, but the temporal variability is at least a factor of 5 larger, primarily driven by short-term upwelling events and mesoscale processes. Ω_arag and pH decrease rapidly with depth, such that the saturation horizon is reached already at 130 m, on average, but it occasionally shoals to as low as 30 m. No statistically significant linear trends emerge in the upper 100 m, but Ω_arag and pH decrease, on average, at rates of -0.009 ± 0.006 yr^-1 and -0.004 ± 0.003 yr^-1 in the 100 to 250 m depth range. These are somewhat larger, but not statistically different from the expected trends based on the recent increase in atmospheric CO₂. About half of the variability in the deseasonalized data can be explained by the El Niño Southern Oscillation (ENSO), with warm phases (El Niño) being associated with above normal pH and Ω_arag. The observed variability and trend in Ω_arag and pH is well captured by a multiple linear regression model on the basis of a small number of readily observable independent variables. This permits the estimation of these variables for related sites in the region.

Ocean acidification session at AGU fall meeting

OS021: The Carbonate System Chemistry of Coastal Ecosystems: Physical, Chemical and Biological Drivers

Ocean acidification (OA) is changing the carbonate system chemistry of the ocean with potentially profound impacts on marine organisms and ecosystems. While the chemistry of OA is well understood, physical processes and biological feedbacks complicate efforts to project future conditions. This is especially true for the coastal ocean where carbonate chemistry is strongly affected by complex circulation patterns, high rates of biogeochemical cycling and terrestrial input. This session highlights observational and modeling studies of the chemical, physical and biological processes that control carbonate chemistry of coastal environments from near shore to continental shelf, including coral reefs.

Session on ocean acidification at the EUR-OCEANS hot topics conference – A changing ocean

This EUR-OCEANS Conference will be hosted by PLOCAN in Gran Canaria, Spain, 6-8 November 2013. The conference aims at showing progress and perspectives on selected ‘hot topics’ in Marine Science. Over the past 5 years, the EUR-OCEANS Consortium has identified, supported and promoted a number of such topics, notably by funding foresight workshops, flagship programs and other types of activities. One of the hot topics is “Ocean deoxygenation and acidification”.

UN General Assembly set to explore impacts of ocean acidification

The United Nations General Assembly will begin today its consideration of the impacts that the increasing acidification of the world’s oceans will have on the marine environment and on people.

This year’s informal Consultative Process on oceans, which opens today and runs through 20 June, will provide a forum for countries to discuss the challenges posed by rising ocean acidity due to increased carbon emissions from human activities.

Photosynthate translocation increases in response to low seawater pH in a coral–dinoflagellate symbiosis (update)

This study has examined the effect of low seawater pH values (induced by an increased CO₂ partial pressure) on the rates of photosynthesis, as well as on the carbon budget and carbon translocation in the scleractinian coral species Stylophora pistillata, using a new model based on ¹³C labelling of the photosynthetic products. Symbiont photosynthesis contributes to a large part of the carbon acquisition in tropical coral species, and it is thus important to know how environmental changes affect this carbon acquisition and allocation. For this purpose, nubbins of S. pistillata were maintained for six months at two pH_Ts (8.1 and 7.2, by bubbling seawater with CO₂). The lowest pH value was used to tackle how seawater pH impacts the carbon budget of a scleractinian coral. Rates of photosynthesis and respiration of the symbiotic association and of isolated symbionts were assessed at each pH. The fate of ¹³C photosynthates was then followed in the symbionts and the coral host for 48 h. Nubbins maintained at pH_T 7.2 presented a lower areal symbiont concentration, and lower areal rates of gross photosynthesis and carbon incorporation compared to nubbins maintained at pH_T 8.1. The total carbon acquisition was thus lower under low pH. However, the total percentage of carbon translocated to the host as well as the amount of carbon translocated per symbiont cell were significantly higher under pH_T 7.2 than under pH_T 8.1 (70% at pH_T 7.2 vs. 60% at pH_T 8.1), such that the total amount of photosynthetic carbon received by the coral host was equivalent under both pHs (5.5 to 6.1 μg C cm⁻² h⁻¹). Although the carbon budget of the host was unchanged, symbionts acquired less carbon for their own needs (0.6 compared to 1.8 μg C cm⁻² h⁻¹), explaining the overall decrease in symbiont concentration at low pH. In the long term, such decrease in symbiont concentration might severely affect the carbon budget of the symbiotic association.