Posts Tagged 'modeling'

Environmental controls on modern scleractinian coral and reef-scale calcification

Modern reef-building corals sustain a wide range of ecosystem services because of their ability to build calcium carbonate reef systems. The influence of environmental variables on coral calcification rates has been extensively studied, but our understanding of their relative importance is limited by the absence of in situ observations and the ability to decouple the interactions between different properties. We show that temperature is the primary driver of coral colony (Porites astreoides and Diploria labyrinthiformis) and reef-scale calcification rates over a 2-year monitoring period from the Bermuda coral reef. On the basis of multimodel climate simulations (Coupled Model Intercomparison Project Phase 5) and assuming sufficient coral nutrition, our results suggest that P. astreoides and D. labyrinthiformis coral calcification rates in Bermuda could increase throughout the 21st century as a result of gradual warming predicted under a minimum CO2 emissions pathway [representative concentration pathway (RCP) 2.6] with positive 21st-century calcification rates potentially maintained under a reduced CO2 emissions pathway (RCP 4.5). These results highlight the potential benefits of rapid reductions in global anthropogenic CO2 emissions for 21st-century Bermuda coral reefs and the ecosystem services they provide.

Continue reading ‘Environmental controls on modern scleractinian coral and reef-scale calcification’

Reef-scale modeling of coral calcification responses to ocean acidification and sea-level rise

To predict coral responses to future environmental changes at the reef scale, the coral polyp model (Nakamura et al. in Coral Reefs 32:779–794, 2013), which reconstructs coral responses to ocean acidification, flow conditions and other factors, was incorporated into a reef-scale three-dimensional hydrodynamic-biogeochemical model. This coupled reef-scale model was compared to observations from the Shiraho fringing reef, Ishigaki Island, Japan, where the model accurately reconstructed spatiotemporal variation in reef hydrodynamic and geochemical parameters. The simulated coral calcification rate exhibited high spatial variation, with lower calcification rates in the nearshore and stagnant water areas due to isolation of the inner reef at low tide, and higher rates on the offshore side of the inner reef flat. When water is stagnant, bottom shear stress is low at night and thus oxygen diffusion rate from ambient water to the inside of the coral polyp limits respiration rate. Thus, calcification decreases because of the link between respiration and calcification. A scenario analysis was conducted using the reef-scale model with several pCO2 and sea-level conditions based on IPCC (Climate change 2013: the physical science basis. Contribution of working group I to the fifth assessment report of the intergovernmental panel on climate change, Cambridge University Press, Cambridge, 2013) scenarios. The simulation indicated that the coral calcification rate decreases with increasing pCO2. On the other hand, sea-level rise increases the calcification rate, particularly in the nearshore and the areas where water is stagnant at low tide under present conditions, as mass exchange, especially oxygen exchange at night, is enhanced between the corals and their ambient seawater due to the reduced stagnant period. When both pCO2 increase and sea-level rise occur concurrently, the calcification rate generally decreases due to the effects of ocean acidification. However, the calcification rate in some inner-reef areas will increase because the positive effects of sea-level rise offset the negative effects of ocean acidification, and total calcification rate will be positive only under the best-case scenario (RCP 2.6).

Continue reading ‘Reef-scale modeling of coral calcification responses to ocean acidification and sea-level rise’

Coccolithophore growth and calcification in a changing ocean

Coccolithophores are the most abundant calcifying phytoplankton in the ocean. These tiny primary producers have an important role in the global carbon cycle, substantially contributing to global ocean calcification, ballasting organic matter to the deep sea, forming part of the marine food web base, and influencing ocean-atmosphere CO2 exchange. Despite these important impacts, coccolithophores are not explicitly simulated in most marine ecosystem models and, therefore, their impacts on carbon cycling are not represented in most Earth system models. Here, we compile field and laboratory data to synthesize overarching, across-species relationships between environmental conditions and coccolithophore growth rates and relative calcification (reported as a ratio of particulate inorganic carbon to particulate organic carbon in coccolithophore biomass, PIC/POC). We apply our relationships in a generalized coccolithophore model, estimating current surface ocean coccolithophore growth rates and relative calcification, and projecting how these may change over the 21st century using output from the Community Earth System Model large ensemble. We find that average increases in sea surface temperature of ∼2-3 °C leads to faster coccolithophore growth rates globally ( >10% increase) and increased calcification at high latitudes. Roughly an ubiquitous doubling of surface ocean pCO2 by the end of the century has the potential to moderately stimulate coccolithophore growth rates, but leads to reduced calcification ( ∼25% decrease). Decreasing nutrient availability (from warming-induced increases in stratification) produces increases in relative calcification, but leads to ∼25% slower growth rates. With all drivers combined, we observe decreases in calcification and growth in most low and mid latitude regions, with possible increases in both of these responses in most high latitude regions. Major limitations of our coccolithophore model stem from a lack of conclusive physiological responses to changes in irradiance (we do not include light limitation in our model), and a lack of physiological data for major coccolithophore species. Species within the Umbellosphaera genus, for example, are dominant in mid to low latitude regions where we predict some of the largest decreases in coccolithophore growth rate and calcification.

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Ship emissions and the use of current air cleaning technology: contributions to air pollution and acidification in the Baltic Sea

The shipping sector is a significant contributor to emissions of air pollutants in marine and coastal regions. In order to achieve sustainable shipping, primarily through new regulations and techniques, greater knowledge of dispersion and deposition of air pollutants is required. Regional model calculations of the dispersion and concentration of sulfur, nitrogen, and particulate matter, as well as deposition of oxidized sulfur and nitrogen from the international maritime sector in the Baltic Sea and the North Sea, have been made for the years 2011 to 2013. The contribution from shipping is highest along shipping lanes and near large ports for concentration and dry deposition. Sulfur is the most important pollutant coupled to shipping. The contribution of both SO2 concentration and dry deposition of sulfur represented up to 80 % of the total in some regions. WHO guidelines for annual concentrations were not trespassed for any analysed pollutant, other than PM2.5 in the Netherlands, Belgium, and central Poland. However, due to the resolution of the numerical model, 50 km  ×  50 km, there may be higher concentrations locally close to intense shipping lanes. Wet deposition is more spread and less sensitive to model resolution. The contribution of wet deposition of sulfur and nitrogen from shipping was up to 30 % of the total wet deposition. Comparison of simulated to measured concentration at two coastal stations close to shipping lanes showed some underestimations and missed maximums, probably due to resolution of the model and underestimated ship emissions.

A change in regulation for maximum sulfur content in maritime fuel, in 2015 from 1 to 0.1 %, decreases the atmospheric sulfur concentration and deposition significantly. However, due to costs related to refining, the cleaning of exhausts through scrubbers has become a possible economic solution. Open-loop scrubbers meet the air quality criteria but their consequences for the marine environment are largely unknown. The resulting potential of future acidification in the Baltic Sea, both from atmospheric deposition and from scrubber water along the shipping lanes, based on different assumptions about sulfur content in fuel, scrubber usage, and increased shipping density has been assessed. The increase in deposition for different shipping and scrubber scenarios differs for the basins in the Baltic Sea, with highest potential of acidification in the southern basins with high traffic. The proportion of ocean-acidifying sulfur from ships increases when taking scrubber water into account and the major reason for increasing acidifying nitrogen from ships is increasing ship traffic. Also, with the implementation of emission control for nitrogen, the effect of scrubbers on acidification is evident. This study also generates a database of shipping and scrubber scenarios for atmospheric deposition and scrubber exhaust from the period 2011 to 2050.

Continue reading ‘Ship emissions and the use of current air cleaning technology: contributions to air pollution and acidification in the Baltic Sea’

The potential future contribution of shipping to acidification of the Baltic Sea

International regulation of the emission of acidic sulphur and nitrogen oxides from commercial shipping has focused on the risks to human health, with little attention paid to the consequences for the marine environment. The introduction of stricter regulations in northern Europe has led to substantial investment in scrubbers that absorb the sulphur oxides in a counterflow of seawater. This paper examines the consequences of smokestack and scrubber release of acidic oxides in the Baltic Sea according to a range of scenarios for the coming decades. While shipping is projected to become a major source of strong acid deposition to the Baltic Sea by 2050, the long-term effect on the pH and alkalinity is projected to be significantly smaller than estimated from previous scoping studies. A significant contribution to this difference is the efficient export of surface water acidification to the North Sea on a timescale of 15–20 years.

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The importance of freshwater to spatial variability of aragonite saturation state in the Gulf of Alaska

High latitude and subpolar regions like the Gulf of Alaska (GOA) are more vulnerable than equatorial regions to rising carbon dioxide (CO2) levels, in part due to local processes that amplify the global signal. Recent field observations have shown that the shelf of the GOA is currently experiencing seasonal corrosive events (carbonate mineral saturation states Ω, Ω<1), including suppressed Ω in response to ocean acidification as well as local processes like increased low alkalinity glacial melt water discharge. While the glacial discharge mainly influences the inner shelf, on the outer shelf, upwelling brings corrosive waters from the deep GOA. In this work, we develop a high-resolution model for carbon dynamics in the GOA, identify regions of high variability of Ω, and test the sensitivity of those regions to changes in the chemistry of glacial melt water discharge. Results indicate the importance of this climatically sensitive and relatively unconstrained regional freshwater forcing for Ω variability in the nearshore. The increase was nearly linear at 0.002 Ω per 100 µmol/kg increase in alkalinity in the freshwater runoff. We find that the local winds, biological processes, and freshwater forcing all contribute to the spatial distribution of Ω and identify which of these three is highly correlated to the variability in Ω. Given that the timing and magnitude of these processes will likely change during the next few decades, it is critical to elucidate the effect of local processes on the background ocean acidification signal using robust models, such as the one described here.

Continue reading ‘The importance of freshwater to spatial variability of aragonite saturation state in the Gulf of Alaska’

Bottom water acidification and warming on the Western Eurasian Arctic shelves: dynamical downscaling projections

The impacts of oceanic CO2 uptake and global warming on the surface ocean environment have received substantial attention, but few studies have focused on shelf bottom water, despite its importance as habitat for benthic organisms and demersal fisheries such as cod. We used a downscaling ocean biogeochemical model to project bottom water acidification and warming on the western Eurasian Arctic shelves. A model hindcast produced 14‒18 year acidification trends that were largely consistent with observational estimates at stations in the Iceland and Irminger seas. Projections under SRES A1B scenario revealed a rapid and spatially-variable decline in bottom pH by 0.10‒0.20 units over 50 years (2.5–97.5% quantiles) at depths 50–500 m on the Norwegian, Barents, Kara, and East Greenland shelves. Bottom water undersaturation with respect to aragonite occurred over the entire Kara shelf by 2040 and over most of the Barents and East Greenland shelves by 2070. Shelf acidification was predominantly driven by the accumulation of anthropogenic CO2, and was concurrent with warming of 0.1–2.7°C over 50 years. These combined perturbations will act as significant multistressors on the Barents and Kara shelves. Future studies should aim to improve the resolution of shelf bottom processes in models, and should consider the Kara Sea and Russian shelves as possible bellwethers of shelf acidification.

Continue reading ‘Bottom water acidification and warming on the Western Eurasian Arctic shelves: dynamical downscaling projections’

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Ocean acidification in the IPCC AR5 WG II

OUP book