The uptake of anthropogenic CO2 by the ocean decreases seawater pH and carbonate mineral aragonite saturation state (Ωarag), a process known as Ocean Acidification (OA). This can be detrimental to marine organisms and ecosystems1, 2. The Arctic Ocean is particularly sensitive to climate change3 and aragonite is expected to become undersaturated (Ωarag < 1) there sooner than in other oceans4. However, the extent and expansion rate of OA in this region are still unknown. Here we show that, between the 1990s and 2010, low Ωarag waters have expanded northwards at least 5°, to 85° N, and deepened 100 m, to 250 m depth. Data from trans-western Arctic Ocean cruises show that Ωarag < 1 water has increased in the upper 250 m from 5% to 31% of the total area north of 70° N. Tracer data and model simulations suggest that increased Pacific Winter Water transport, driven by an anomalous circulation pattern and sea-ice retreat, is primarily responsible for the expansion, although local carbon recycling and anthropogenic CO2 uptake have also contributed. These results indicate more rapid acidification is occurring in the Arctic Ocean than the Pacific and Atlantic oceans5, 6, 7, 8, with the western Arctic Ocean the first open-ocean region with large-scale expansion of ‘acidified’ water directly observed in the upper water column.
Posts Tagged 'regionalmodeling'
Tags: Arctic, chemistry, field, modeling, regionalmodeling
Development and application of foraminiferal carbonate system proxies to quantify ocean acidification in the California CurrentPublished 22 February 2017 Science Leave a Comment
Tags: biological response, calcification, chemistry, field, methods, modelling, North Atlantic, protists, regionalmodeling, sediment
The oceanic uptake of anthropogenic carbon has mitigated climate change, but has also resulted in a global average 0.1 decline in surface ocean pH over 20th century known as ocean acidification. The parallel reduction in carbonate ion concentration ([CO32-]) and the saturation state of seawater (Ω) has caused many major calcium carbonate-secreting organisms such as planktonic foraminifera to exhibit impaired calcification. We develop proxy calibrations and down core records that use calcification and geochemical characteristics of planktonic foraminifera as proxies for the marine carbonate system. This study focuses specifically on the surface ocean chemistry of the California Current Ecosystem (CCE), which has been identified as a region of rapidly progressing ocean acidification due to natural upwelling processes and the low buffering capacity of these waters. The calibration portion of this study uses marine sediments collected by the Santa Barbara Basin (SBB), California sediment-trapping program located in the central region of the CCE. We calibrate the relationships of Globigerina bulloides calcification intensity to [CO3 2-] and the B/Ca ratios of G. bulloides, Neogloboquadrina dutertrei and Neogloboquadrina incompta shells to Ω calcite using in situ measurements and model simulations of these independent variables. By applying these proxy methods to down core, our records from the SBB indicate a 20% reduction in foraminiferal calcification since ~1900, translating to a 35% decline in [CO 32-] in the CCE over this period. Our high-resolution calcification record also reveals a substantial interannual to decadal modulation of ocean acidification in the CCE related to the sign of Pacific Decadal Oscillation and El Niño Southern Oscillation. In the future we can expect these climatic modes to both enhance and moderate anthropogenic ocean acidification. Based on our historic record, we predict that if atmospheric CO2 reaches 540 ppm by the year 2100 as predicted by a conservative CO3 pathway, [CO32-] will experience a net reduction of 55%, resulting in at least a 30% reduction in calcification of planktonic foraminifera that will likely be mirrored by other adversely affected marine calcifiers.
Eutrophication-induced acidification of coastal waters in the northern Gulf of Mexico: Insights into origin and processes from a coupled physical-biogeochemical modelPublished 2 February 2017 Science Leave a Comment
Tags: biogeochemistry, chemistry, modeling, North Atlantic, regionalmodeling
Nutrient inputs from the Mississippi/Atchafalaya River system into the northern Gulf of Mexico promote high phytoplankton production and lead to high respiration rates. Respiration coupled with water column stratification results in seasonal summer hypoxia in bottom waters on the shelf. In addition to consuming oxygen, respiration produces carbon dioxide (CO2), thus lowering the pH and acidifying bottom waters. Here we present a high-resolution biogeochemical model simulating this eutrophication-driven acidification and investigate the dominant underlying processes. The model shows the recurring development of an extended area of acidified bottom waters in summer on the northern Gulf of Mexico shelf that coincides with hypoxic waters. Not reported before, acidified waters are confined to a thin bottom boundary layer where the production of CO2 by benthic metabolic processes is dominant. Despite a reduced saturation state, acidified waters remain supersaturated with respect to aragonite.
CO2 seawater acidification by CCS-simulated leakage: Kinetic modelling of Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediment using pH-static leaching testsPublished 13 January 2017 Science Leave a Comment
Tags: chemistry, laboratory, modeling, North Atlantic, regionalmodeling, sediment
A modified pH-dependent leaching test with continuous pH control that employed CO2 to acidify a seawater-sediment mixture is used to address Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediments under the influence of acidification processes. Long-term (480 h) leaching experiments at pH values of 7.0, 6.5 and 6.0 are performed. The different evolutionary patterns of the redox potential and Fe release at pH = 6 with respect to the other pH values shows the need to assess the influence of the initial Fe content in seawater upon elemental release. Hence, assays at pH = 6.0 are conducted using natural seawater with Fe concentrations between 9.02 and 153 μg/L. A set of in-series reactions for trace elements, Fe and other ions associated with Fe is proposed to model a Fe/multi-ion-dependent mechanism for trace metal release. The maximum concentration of each contaminant that can be released from the sediment and the kinetic parameters of the proposed model are completed for the studied pH values, for good consistency between the experimental and simulated mobilisation of each studied element.
Risks of ocean acidification in the California Current food web and fisheries: ecosystem model projectionsPublished 13 January 2017 Science Leave a Comment
Tags: biological response, BRcommunity, communitymodeling, fisheries, modeling, North Pacific, regionalmodeling
The benefits and ecosystem services that humans derive from the oceans are threatened by numerous global change stressors, one of which is ocean acidification. Here, we describe the effects of ocean acidification on an upwelling system that already experiences inherently low pH conditions, the California Current. We used an end-to-end ecosystem model (Atlantis), forced by downscaled global climate models and informed by a meta-analysis of the pH sensitivities of local taxa, to investigate the direct and indirect effects of future pH on biomass and fisheries revenues. Our model projects a 0.2-unit drop in pH during the summer upwelling season from 2013 to 2063, which results in wide-ranging magnitudes of effects across guilds and functional groups. The most dramatic direct effects of future pH may be expected on epibenthic invertebrates (crabs, shrimps, benthic grazers, benthic detritivores, bivalves), and strong indirect effects expected on some demersal fish, sharks, and epibenthic invertebrates (Dungeness crab) because they consume species known to be sensitive to changing pH. The model’s pelagic community, including marine mammals and seabirds, was much less influenced by future pH. Some functional groups were less affected to changing pH in the model than might be expected from experimental studies in the empirical literature due to high population productivity (e.g., copepods, pteropods). Model results suggest strong effects of reduced pH on nearshore state-managed invertebrate fisheries, but modest effects on the groundfish fishery because individual groundfish species exhibited diverse responses to changing pH. Our results provide a set of projections that generally support and build upon previous findings and set the stage for hypotheses to guide future modeling and experimental analysis on the effects of OA on marine ecosystems and fisheries.
Assessing the effects of ocean acidification in the Northeast US using an end-to-end marine ecosystem modelPublished 9 January 2017 Science Leave a Comment
Tags: biological response, BRcommunity, communitymodeling, methods, modeling, North Atlantic, regionalmodeling
The effects of ocean acidification on living marine resources present serious challenges for managers of these resources. An understanding of the ecosystem consequences of ocean acidification is required to assess tradeoffs among ecosystem components (e.g. fishery yield, protected species conservation, sensitive habitat) and adaptations to this perturbation. We used a marine ecosystem model for the Northeast US continental shelf to address direct and indirect effects of species responses to ocean acidification. Focusing on upper trophic level groups that are primary targets of fishing activity, we projected changes for systemic ecological and fisheries indicators. We modeled effects of ocean acidification as either fixed changes in mortality rate or production for select species groups over twenty years. Biomass and fishery yield of species groups that were modeled to have direct acidification impacts and groups that were not directly impacted both declined, due to both increased mortality/decreased growth and a decrease in availability of food for groups that prey on shelled invertebrates. Our analyses show that food web consequences of ocean acidification can extend beyond groups thought most vulnerable, and to fishery yield and ecosystem structure. However, the magnitude and precise nature of ocean acidification effects depend on understanding likely species’ responses to decrease in pH. While predicting the effects of ocean acidification is difficult, the potential impacts on ecosystem structure and function need to be evaluated now to provide scientists and managers preliminary assessments for planning and priority setting. Scenario analysis using simulation models like ours provides a framework for testing hypotheses about ecosystem consequences of acidification, and for integrating results of experiments and monitoring.
Tags: Arctic, biogeochemistry, chemistry, field, modeling, regionalmodeling
Understanding the physical and biogeochemical processes that control CO2 and dissolved oxygen (DO) dynamics in the Arctic Ocean (AO) is crucial for predicting future air-sea CO2 fluxes and ocean acidification. Past studies have primarily been conducted on the AO continental shelves during low-ice periods and we lack information on gas dynamics in the deep AO basins where ice typically inhibits contact with the atmosphere. To study these gas dynamics, in situ time-series data have been collected in the Canada Basin during late summer to autumn of 2012. Partial pressure of CO2 (pCO2), DO concentration, temperature, salinity, and chlorophyll-a fluorescence (Chl-a) were measured in the upper ocean in a range of sea ice states by two drifting instrument systems. Although the two systems were on average only 222 km apart, they experienced considerably different ice cover and external forcings during the 40-50 d periods when data were collected. The pCO2 levels at both locations were well below atmospheric saturation whereas DO was almost always slightly supersaturated. Modeling results suggest that air-sea gas exchange, net community production (NCP) and horizontal gradients were the main sources of pCO2 and DO variability in the sparsely ice-covered AO. In areas more densely covered by sea ice, horizontal gradients were the dominant source of variability, with no significant NCP in the surface mixed layer. If the AO reaches equilibrium with atmospheric CO2 as ice cover continues to decrease, aragonite saturation will drop from a present mean of 1.00 ± 0.02 to 0.86 ± 0.01.