Posts Tagged 'regionalmodeling'

Modeling the variability, trends and future changes in ocean acidification in the Humboldt Current System

The largest buffer against climate change is the oceanic sink of anthropogenic CO2. However, this important ecosystem service to humanity leads to the reduction of pH and the saturation state of the biologically relevant calcium carbonate minerals aragonite and calcite  (Ωarag and Ωcalc). This process, known as anthropogenic ocean acidification, is a major marine ecosystem stressor and has negative impacts that range from reduced calcification to changes in population dynamics of important fisheries. Some of the most productive regions of the world, the Eastern Boundary Upwelling Systems (EBUS), are also among the most vulnerable to become undersaturated in the next decades due to the natural occurrence of low pH and Ω values at shallow depths and the projected further uptake of anthropogenic CO2. While extensive research on ocean acidification has been conducted in the California Current upwelling System, little is known about the dynamics of the marine carbonate chemistry in the much more productive Humboldt Current Upwelling System (HumCS), in the west coast of South America. To fill this gap, I used the high resolution Regional Ocean Modelling System (ROMS) and two different ecosystem models to study the progression of ocean acidification in the HumCS and the natural fluctuations superimposed to this anthropogenic perturbation. Results from a preindustrial simulation (year 1870) show that even then, pH and Ωarag values in the HumCS were respectively ⇠ 0.3 and 1 units lower than the preindustrial global average. The simulated evolution of ocean acidification to the end of the century showed that the continuous uptake of anthropogenic CO2 will push the nearshore off Peru to even lower values, from a present-day pH of 7.8 and Ωarag of 1.8 to year-round undersaturated conditions in year 2090 in at least 60 % of the top layer of the water column in the nearshore off Peru. Aragonite undersaturation in the following decades is already a committed change regardless of the amount of carbon emitted to the atmosphere in the future. However, a striking difference arises between following a “high CO2 emissions” scenario (RCP8.5, pCO2 values 840 μatm by year 2090) or a “low CO2 emissions” scenario (RCP2.6, pCO2 values of 428 μatm). In the former, water corrosive to calcite, a less soluble form of calcium carbonate than aragonite, will be found in the first 15 km off Peru and will potentially impact a larger range of calcifying organisms. On the other hand, this can be avoided the RCP2.6 scenario is followed, and strong CO2 mitigation measures are established and executed. In the high CO2 emissions scenario, an overall decrease of 0.9 ± 0.1 units in Ωarag from present day to the end of the century is projected in the nearshore off Peru, and a similar trend in the nearshore off Chile. On top of this long-term trend, natural climate variability off Peru can lead to strong year-to-year variations in the progression of ocean acidification. The largest contribution to Ωarag variability in the HumCS is on the interannual timescale, mainly forced by remotely forced tropical oscillations (e.g. El Ni˜no/Southern Oscillation) but also by local and regional phenomena (e.g., El Ni˜no costero). Analysis from a hindcast simulation for the period of 1979-2016 reveals that under present day conditions, the magnitude of such variability is comparable to the anthropogenic trend. Interannual changes in Ω are mainly driven by variations in the thermocline structure and wind patterns. Off Peru, the deepening of the thermocline associated with warm, El Ni˜no-like events, is translated into an increase in Ωarag in the surface layer of 0.4 units, while a shallower thermocline driven by cold interannual events (e.g., La Ni˜na) leads to a decrease in Ωarag values of 0.3 units. These natural interannual variations account for ⇠ 30 to and 40 % of the magnitude of the expected anthropogenic change, potentially bringing forward of delaying by some decades the pervasive appearance of aragonite undersaturated waters in the surface layer of the most productive EBUS of the Pacific: the Humboldt Current.

Continue reading ‘Modeling the variability, trends and future changes in ocean acidification in the Humboldt Current System’

Environmental controls on pteropod biogeography along the Western Antarctic Peninsula

Pteropods are abundant zooplankton in the Western Antarctic Peninsula (WAP) and important grazers of phytoplankton and prey for higher trophic levels. We analyzed long‐term (1993–2017) trends in summer (January–February) abundance of WAP pteropods in relation to environmental controls (sea ice, sea surface temperature, climate indices, phytoplankton biomass and productivity, and carbonate chemistry) and interspecies dynamics using general linear models. There was no overall directional trend in abundance of thecosomes, Limacina helicina antarctica and Clio pyramidata, throughout the entire WAP, although L. antarctica abundance increased in the slope region and C. pyramidata abundance increased in the South. High L. antarctica abundance was strongly tied to a negative Multivariate El Niño Southern Oscillation Index the previous year. C. pyramidata abundance was best explained by early sea ice retreat 1‐yr prior. Abundance of the gymnosome species, Clione antarctica and Spongiobranchaea australis, increased over the time series, particularly in the slope region. Gymnosome abundance was positively influenced by abundance of their prey, L. antarctica, during the same season, and late sea ice advance 2‐yr prior. These trends indicate a shorter ice season promotes longer periods of open water in spring/summer favoring all pteropod species. Weak relationships were found between pteropod abundance and carbonate chemistry, and no long‐term trend in carbonate parameters was detected. These factors indicate ocean acidification is not presently influencing WAP pteropod abundance. Pteropods are responsive to the considerable environmental variability on both temporal and spatial scales—key for predicting future effects of climate change on regional carbon cycling and plankton trophic interactions.

Continue reading ‘Environmental controls on pteropod biogeography along the Western Antarctic Peninsula’

Bioeconomic analysis of the impact of ocean acidification associated with low recruitment of Isostichopus badionotus and implications for adaptive fishery management in the north of the Yucatan Peninsula, Mexico

The impact that ocean acidification (OA) could generate in the fisheries of Isostichopus badionotus at the north of the Yucatan Peninsulta, Mexico, was analysed by reducing the value of a parameter of the Beverton-Holt recruitment function, in accordance with the acidification scenarios of the Intergovermental Panel Panel on Climate Change (IPCC). The behaviour of the stock and the resulting fishery were analysed in a bioeconomic model structured by age, taking into account different market prices and fishing efforts. The results were compared in decision matrices that used the MiniMax and MaxMin criteria to determine the management strategy that best reduced the impact of  acidification. The largest stock reduction occurred during the first years of exploitation (B10>B15/BO) and all the variables that were considered did stabilize with time, reaching bioeconomic equilibrium. The worst scenario for not considering acidification occurred with low market prices, while the increase in price decreased the exploitation rate. The recruitment reduction determined the maximum effort that should have been applied; under such conditions it is recommended to operate an effort of 137 boats, considering the best market price.

Continue reading ‘Bioeconomic analysis of the impact of ocean acidification associated with low recruitment of Isostichopus badionotus and implications for adaptive fishery management in the north of the Yucatan Peninsula, Mexico’

Modelling the mechanisms and drivers of the spatiotemporal variability of pCO2 and air-sea CO2 fluxes in the Northern Humboldt Current System

Highlights

• Air-sea CO2 fluxes follow the upwelling intensity throughout the year.
• Circulation is the dominant mechanism driving variability in the nearshore area.
• Biology and solubility effects partially damp upwelling-driven pCO2 variability.
• High coastal pCO2 values are due to a spatiotemporal decoupling between circulation and biology.
pCO2 is more sensitive to changes in dissolved inorganic carbon and temperature.

Abstract

We use a coupled physical-biogeochemical model to investigate the drivers and mechanisms responsible for the spatiotemporal variability of the partial pressure of carbon dioxide in seawater (pCO2) and associated air-sea CO2 fluxes in the Northern Humboldt Current System (NHCS). Simulated pCO2 is in good agreement with available observations with an average absolute error of, approximately, 24 μatm. The highly productive upwelling region, 300 km from the shore and between 5°S-17°S, is shown to be a strong CO2 source with an averaged flux of 5.60  ±  2.94 mol C m−2 yr−1, which represents an integrated carbon flux of 0.028  ±  0.015 Pg C yr−1 . Through a series of model experiments we show that the high pCO2 is primarily the result of coastal upwelling, which is incompletely compensated by biology. Specifically, the supply of dissolved inorganic carbon (DIC)-rich waters to the surface pushes pCO2 up to levels around 1100 μatm. Even though biological production is high, it reduces pCO2 only by about 300 μatm. We show that this relatively low degree of biological compensation, which implies an inefficient biological pump in the nearshore domain, results from a spatiotemporal decoupling between the counteracting effects of biological production and the transport and mixing of DIC. The contribution of the outgassing and the processes affecting CO2 solubility, associated with the seasonal cycle of heating and cooling, are minor. Across the whole domain, the balance of mechanisms is similar, but with smaller amplitudes. We demonstrate that seawater pCO2 is more sensitive to changes in DIC and sea surface temperature, while alkalinity plays a minor role.

Continue reading ‘Modelling the mechanisms and drivers of the spatiotemporal variability of pCO2 and air-sea CO2 fluxes in the Northern Humboldt Current System’

Rapid warming and salinity changes in the Gulf of Maine alter surface ocean carbonate parameters and hide ocean acidification

A profound warming event in the Gulf of Maine during the last decade has caused sea surface temperatures to rise to levels exceeding any earlier observations recorded in the region over the last 150 years. This event dramatically affected CO2 solubility and, in turn, the status of the sea surface carbonate system. When combined with the concomitant increase in sea surface salinity and assumed rapid equilibration of carbon dioxide across the air sea interface, thermodynamic forcing partially mitigated the effects of ocean acidification for pH, while raising the saturation index of aragonite (ΩARΩAR ) by an average of 0.14 U. Although the recent event is categorically extreme, we find that carbonate system parameters also respond to interannual and decadal variability in temperature and salinity, and that such phenomena can mask the expression of ocean acidification caused by increasing atmospheric carbon dioxide. An analysis of a 34-year salinity and SST time series (1981–2014) shows instances of 5–10 years anomalies in temperature and salinity that perturb the carbonate system to an extent greater than that expected from ocean acidification. Because such conditions are not uncommon in our time series, it is critical to understand processes controlling the carbonate system and how ecosystems with calcifying organisms respond to its rapidly changing conditions. It is also imperative that regional and global models used to estimate carbonate system trends carefully resolve variations in the physical processes that control CO2 concentrations in the surface ocean on timescales from episodic events to decades and longer.

Continue reading ‘Rapid warming and salinity changes in the Gulf of Maine alter surface ocean carbonate parameters and hide ocean acidification’

Assessment of paleo-ocean pH records from boron isotope ratio in the Pacific and Atlantic ocean corals: Role of anthropogenic CO2 forcing and oceanographic factors to pH variability

Boron isotopes (δ11B) records from tropical ocean corals have been used to reconstruct paleo-pH of ocean for the past several decades to few centuries which are comparable to the resolution of instrumental records. In most of the studies, attempts have been made to decipher the role of anthropogenic CO2 forcing to recent trend of ocean acidification based on δ11B derived paleo-pH records. However, such attempts in past were often hindered by limited knowledge of oceanographic factors that contributed to past pH variability and changes. In this study, we have evaluated pH records reconstructed using δ11B records from the Pacific and the Atlantic Oceans corals and investigated major forcing factors that contributed to sub annual-decadal scale pH variability and changes since the industrial era ~1850AD.

To the best of our knowledge, total eight δ11B records from the Pacific and two from the Atlantic Oceans are available in published literatures. The compilations of these records show large variability; range between 26.27–20.82‰ which corresponds to pH range 8.40–7.63 respectively. Our investigation of pH records from the Pacific ocean based on principal component analysis (PCA) reveals that atmospheric CO2 can explains maximum up to ~26% of the total pH variability during 1950–2004AD, followed by the ocean-climate oscillations (i.e. ENSO and PDO) driven oceanographic factors up to ~17%. The remaining large variability (~57%) could not be explained by above forcing factors and hence we invoke possible influence of metabolic processes of corals and/or changes in micro-environments within the reefs which are often neglected in interpreting paleo-pH records. Therefore, we highlight the need for detailed investigation in future studies to understand about the exact mechanism, processes/factors that controlled boron isotope fractionations in coral reef environments. Further, our investigation reveals that amplitude of the ENSO driven pH variability shows fivefold increase during 1980–2000AD compared to the previous three decades (1950–1980AD). This observation is consistent with the historical records of global coral bleaching events and therefore underscores role of ENSO driven environmental stress responsible for coral bleaching events. Considering model based projections of increasing frequency and amplitude of extreme ENSO events in the backdrop of recent global warming, bleaching events are likely to increase in the next decades/centuries.

Continue reading ‘Assessment of paleo-ocean pH records from boron isotope ratio in the Pacific and Atlantic ocean corals: Role of anthropogenic CO2 forcing and oceanographic factors to pH variability’

Seasonal net ecosystem metabolism of the near-shore reef system in La Parguera, Puerto Rico

Changes in ocean chemistry as a direct response to rising atmospheric carbon dioxide (CO2) concentrations is causing a reduction of pH in the surface ocean. While the dynamics and trends in carbonate chemistry are reasonably constrained for open ocean waters, the ways in which ocean acidification (OA) manifests within the shallow near-shore waters, where coral reefs reside, is less understood. Constraining near-reef variability in carbonate chemistry and net ecosystem metabolic processes across diel, seasonal, and annual scales is important in evaluating potential biogeochemical thresholds of OA that could result in ecological community changes. The OA Test-Bed at La Parguera Marine Reserve in Puerto Rico provides long-term carbonate chemistry observations at high-temporal resolution within a Caribbean near-shore coral reef ecosystem. A 1-D model was developed using the carbon mass balance approach to yield information about net ecosystem production and calcification processes occurring in the water column adjacent to the reef. We present results of nine years of sustained monitoring at the Enrique mid-shelf forereef, which provides for the characterization of temporal dynamics in carbonate chemistry and net ecosystem metabolic processes encompassing near-shore and upstream locations. Results indicate that net heterotrophy and net dissolution dominate over most of the year, while net autotrophic conditions coupled with calcification dominated from only January to mid-April. The average carbonate dissolution rate observed during summer is estimated at −2.19g CaCO3m−2 day−1 and net community dissolution persists 76% of the seasonal year despite the water column remaining super-saturated with respect to aragonite. This corresponds to −0.62 kg CaCO3m−2 year−1, classifying the Enrique fore-reef and off-reef areas in a net dissolutional state. The combination of thermodynamically-driven depressed aragonite saturation state and high rates of respiration during the summer cause conditions that jeopardize the most soluble carbonate minerals and the free energy in the system for calcification. These data suggest that the reef area and associated ecosystems upstream of the sampling location are experiencing a net loss of CaCO3, possibly compromising coral ecosystem health and reef accretion processes necessary for maintenance as sea level increases. Resiliency from other climate-scale stressors including rising sea surface temperatures and coral bleaching is likely to be compromised in a system exhibiting net carbonate loss.

Continue reading ‘Seasonal net ecosystem metabolism of the near-shore reef system in La Parguera, Puerto Rico’


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OA-ICC HIGHLIGHTS

Ocean acidification in the IPCC AR5 WG II

OUP book