Posts Tagged 'regionalmodeling'



Controls on carbonate system dynamics in a coastal plain estuary: a modelling study

The study of acidification in Chesapeake Bay is challenged by the complex spatial and temporal patterns of estuarine carbonate chemistry driven by highly variable freshwater and nutrient inputs. A new module was developed within an existing coupled hydrodynamic‐biogeochemical model to understand the underlying processes controlling variations in the carbonate system. We present a validation of the model against a diversity of field observations, which demonstrated the model’s ability to reproduce large‐scale carbonate chemistry dynamics of Chesapeake Bay. Analysis of model results revealed that hypoxia and acidification were observed to co‐occur in mid‐bay bottom waters and seasonal cycles in these metrics were regulated by aerobic respiration and vertical mixing. Calcium carbonate dissolution was an important buffering mechanism for pH changes in late summer, leading to stable or slightly higher pH values in this season despite persistent hypoxic conditions. Model results indicate a strong spatial gradient in air‐sea CO2 fluxes, where the heterotrophic upper bay was a strong CO2source to atmosphere, the mid bay was a net sink with much higher rates of net photosynthesis, and the lower bay was in a balanced condition. Scenario analysis revealed that reductions in riverine nutrient loading will decrease the acid water volume (pH <7.5) as a consequence of reduced organic matter generation and subsequent respiration, while bay‐wide dissolved inorganic carbon (DIC) increased and pH declined under scenarios of continuous anthropogenic CO2 emission. This analysis underscores the complexity of carbonate system dynamics in a productive coastal plain estuary with large salinity gradients.

Continue reading ‘Controls on carbonate system dynamics in a coastal plain estuary: a modelling study’

In-situ incubation of a coral patch for community-scale assessment of metabolic and chemical processes on a reef slope

Anthropogenic pressures threaten the health of coral reefs globally. Some of these pressures directly affect coral functioning, while others are indirect, for example by promoting the capacity of bioeroders to dissolve coral aragonite. To assess the coral reef status, it is necessary to validate community-scale measurements of metabolic and geochemical processes in the field, by determining fluxes from enclosed coral reef patches. Here, we investigate diurnal trends of carbonate chemistry, dissolved organic carbon, oxygen, and nutrients on a 20 m deep coral reef patch offshore from the island of Saba, Dutch Caribbean by means of tent incubations. The obtained trends are related to benthic carbon fluxes by quantifying net community calcification (NCC) and net community production (NCP). The relatively strong currents and swell-induced near-bottom surge at this location caused minor seawater exchange between the incubated reef and ambient water. Employing a compensating interpretive model, the exchange is used to our advantage as it maintains reasonably ventilated conditions, which conceivably prevents metabolic arrest during incubation periods of multiple hours. No diurnal trends in carbonate chemistry were detected and all net diurnal rates of production were strongly skewed towards respiration suggesting net heterotrophy in all incubations. The NCC inferred from our incubations ranges from −0.2 to 1.4 mmol CaCO3 m−2 h−1 (−0.2 to 1.2 kg CaCO3 m−2 year−1) and NCP varies from −9 to −21.7 mmol m−2 h−1 (net respiration). When comparing to the consensus-based ReefBudget approach, the estimated NCC rate for the incubated full planar area (0.36 kg CaCO3 m−2 year−1) was lower, but still within range of the different NCC inferred from our incubations. Field trials indicate that the tent-based incubation as presented here, coupled with an appropriate interpretive model, is an effective tool to investigate, in situ, the state of coral reef patches even when located in a relatively hydrodynamic environment.

Continue reading ‘In-situ incubation of a coral patch for community-scale assessment of metabolic and chemical processes on a reef slope’

Modelling seawater carbonate chemistry in shellfish aquaculture regions: Insights into CO2 release associated with shell formation and growth

Highlights

• The role of CaCO3 shell production in CO2 release during shelled-mollusc cultivation at aquaculture installations is dependent on a variety of biotic and abiotic factors.
• Carbon sequestration through CaCO3 formation as a by-product of mollusc aquaculture may be included in carbon trading schemes in the future.
• Regional differences in the marine carbonate system can alter the amount of CO2 released per unit CaCO3 formation by a farm mussel.
• Through carbonate chemistry modelling, we show that calcification in identical mussel farms in the Baltic sea would produce 33% more CO2 per g of CaCO3 than in Southern Portugal, with Galician (3% more than Southern Portugal) and Scottish sites (10% more than Southern Portugal) falling in between. This trend is shown to be largely due to differences in abiotic factors such as water temperature and salinity that broadly correspond to latitudinal position, and has important implications for regional scale planning of aquaculture sites in relation to the potential for carbon trading.

Abstract

Mollusc aquaculture is a high-value industry that is increasing production rapidly in Europe and across the globe. In recent years, there has been discussion of the potential wide-ranging environmental benefits of this form of food production. One aspect of mollusc aquaculture that has received scrutiny is the production of calcareous shells (CaCO3). Mollusc shell growth has sometimes been described as a sink for atmospheric CO2, as it locks away carbon in solid mineral form. However, more rigorous carbonate chemistry modelling, including concurrent changes in seawater pCO2, pH, dissolved inorganic carbon, and total alkalinity, shows that calcification is a net CO2 source to the atmosphere. Combined with discussions about whether mollusc respiration should be included in carbon footprint modelling, this suggests that greater in-depth understanding is required before shellfish aquaculture can be included in carbon trading schemes and footprint calculations. Here, we show that regional differences in the marine carbonate system can alter the amount of CO2 released per unit CaCO3 formation. Our carbonate chemistry modelling shows that a coastal mussel farm in southern Portugal releases up to ~0.290 g of CO2 per g of CaCO3 shell formed. In comparison, an identical farm in the coastal Baltic Sea would produce up to 33% more CO2 per g of CaCO3 (~0.385 g-CO2·(g-CaCO3)−1). This spatial variability should therefore also be considered if mollusc aquaculture is to be included in future carbon trading schemes, and in planning future expansion of production across the industry.

Continue reading ‘Modelling seawater carbonate chemistry in shellfish aquaculture regions: Insights into CO2 release associated with shell formation and growth’

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’


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Ocean acidification in the IPCC AR5 WG II

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