The chicken embryo bioassay as a tool to assess the possible toxic effects of persistent organic pollutants (POPs): a study with special reference to thiamine deficiency, EROD induction and the bursa of Fabricius

Since the introduction of man made chemicals into the environment, the reproductive success of bird species all over the world has been compromised. Since the ban on the use of several chemicals, pollution has decreased, but residues still remain, and reproductive and physiological are still reported to be associated with contaminants. In the Baltic Sea, one of the most contaminated areas in the world, fish suffer from massive mortality during the early life stages. These fish contain high levels of contaminants and low levels of thiamine; thus, it has been hypothesised that contaminants induce thiamine deficiency. As fish eating birds occupy the same position in the food chain as the affected fish, one of the aims of the presented research was to establish the role of contaminant induced thiamine deficiency in bird embryotoxicity. Although the contamination levels in Baltic guillemots have decreased since the 1960s, the birds still contain appreciable amounts of contaminants. The second aim of the research was therefore to assess the potential toxic effects of these contaminants, at the current levels, in these birds.Several experiments were conducted to study both hypotheses, using an optimised chicken embryo bioassay. The thesis describes the development and validation of the bioassay, as well as its use in studies on the effects of contaminants on embryonal transketolase activity, which is a biomarker for thiamine deficiency. In addition, extracts from Baltic guillemots as well as from oystercatcher eggs from the Zeehavenkanaal in the Netherlands (which is contaminated with hexachlorobenzene) were tested for their toxic potencies. None of the selected model compounds or extracts resulted in decreased transketolase activity, indicating that contaminant induced thiamine deficiency probably does not play a role in avian embryotoxicity. Levels of PCBs in guillemot extracts were high enough to enhance the incidence of malformed embryos in fish eating birds. Both extracts resulted in decreased lymphocyte density in the bursa of Fabricius, and increased activity of EROD. Since EROD induction was found to be a very sensitive biomarker for exposure to contaminants that bind to the Ah receptor, this biomarker was studied during embryonic development; it was found to be a conservative predictor for toxicity in birds. Results were discussed in the framework of species differences in sensitivity towards toxic actions of chemicals. It was concluded that under the present contamination levels, toxic effects could not be excluded in Baltic guillemots and oystercatchers from the Zeehavenkanaal

Model uncertainty in financial markets:Long run risk and parameter uncertainty

Uncertainty surrounding key parameters of financial markets, such as the in- flation and equity risk premium, constitute a major risk for institutional investors with long investment horizons. Hedging the investors’ inflation exposure can be challenging due to the lack of domestic inflation-linked securities. I show that inflation hedging investors can benefit from holding bonds that are linked to inflation in foreign countries. Investors can further improve their inflation hedge by incorporating the long term interactions between his own inflation exposure and the foreign inflation measures. Focusing on the major inflation-linked security markets, I find an increase of the inflation risk premium over the financial crisis in the UK, whereas in the US it decreased. Since the parameter uncertainty of these estimates is large, and increased over the financial crisis in both the UK and US markets, I present a framework in which investors can quantify and integrate it in their long term investment decisions. Finally, I demonstrate that the difficulty of estimating the equity risk premium is the largest source of parameter uncertainty in defined contribution pension contracts. I introduce a methodology to take parameter uncertainty into account, so that participants can set contributions that reflect the uncertainty about their replacement rate at retirement. Overall, this thesis demonstrates robust methods to incorporate the effects of parameter uncertainty and contributes to the literature on how parameter uncertainty of financial models can substantially affect the investors’ investment risk

Addressing the Grid-size Sensitivity Issue in Large-eddy Simulations of Stable Boundary Layers

In this study, we have identified certain fundamental limitations of a mixing length parameterization used in a popular turbulent kinetic energy-based subgrid-scale model. Replacing this parameterization with a more physically realistic one significantly improves the overall quality of the large-eddy simulations (LESs) of stable boundary layers. For the range of grid sizes considered here (specifically, 1 m -- 12.5 m), the revision dramatically reduces the grid-size sensitivity of the simulations. Most importantly, the revised scheme allows us to reliably estimate the first- and second-order statistics of a well-known LES intercomparison case, even with a coarse grid-size of O(10 m)

Surface Energy Balance and Turbulence Characteristics Observed at the SHEBA Ice Camp During FIRE III

The Institute for Marine and Atmospheric Research Utrecht (IMAU) participated in the FIRE III (First ISCCP Regional Experiment, ISCCP International Satellite Cloud Climatology Project) experiment in May 1998. In this paper we describe surface layer measurements performed on the sea ice at the SHEBA (Surface Heat and Energy Balance of the Arctic ocean) camp and compare these with measurements collected above a grasscovered surface in Cabauw, the Netherlands. The observations consist of both highfrequency turbulence measurements and mean-profile measurements of wind, temperature and humidity. In addition, we measured the upward and downward components of both the longwave and shortwave radiation, and the snow and ice temperatures in the upper 40 cm. The observations give a detailed picture of all components of the energy balance of the Arctic sea-ice surface. The turbulence measurements are used to study the surface layer scaling of the turbulence variables in the stable boundary layer. More specifically, we showed that the integral length scale of the vertical velocity fluctuations serves as the relevant turbulence length scale. The monthly-averaged energy balance of the Arctic sea-ice was dominated by radiative fluxes, whereas, the sensible and latent heat flux and the energy flux into the surface were rather small. A detailed inspection of the diurnal variations in the turbulent fluxes however indicates that although the monthly-averaged values are small, the hourlyaveraged values for these fluxes are significant in the surface energy balance

Analogies between Mass-Flux and Reynolds-Averaged Equations

In many large-scale models mass-flux parameterizations are applied to prognose the effect of cumulus cloud convection on the large-scale environment. Key parameters in the mass-flux equations are the lateral entrainment and detrainment rates. The physical meaning of these parameters is that they quantify the mixing rate of mass across the thermal boundaries between the cloud and its environment. The prognostic equations for the updraft and downdraft value of a conserved variable are used to derive a prognostic variance equation in the mass-flux approach. The analogy between this equation and the Reynolds-averaged variance equation is discussed. It is demonstrated that the prognostic variance equation formulated in mass-flux variables contains a gradient-production, transport, and dissipative term. In the latter term, the sum of the lateral entrainment and detrainment rates represents an inverse timescale of the dissipation. Steady-state solutions of the variance equations are discussed. Expressions for the fractional entrainment and detrainment coefficients are derived. Also, solutions for the vertical flux of an arbitrary conserved variable are presented. For convection in which the updraft fraction equals the downdraft fraction, the vertical flux of the scalar flows down the local mean gradient. The turbulent mixing coefficient is given by the ratio of the vertical mass flux and the sum of the fractional entrainment and detrainment coefficients. For an arbitrary updraft fraction, however, flux correction terms are part of the solution. It is shown that for a convective boundary layer these correction terms can account for countergradient transport, which is illustrated from large eddy simulation results. In the cumulus convection limit the vertical flux flows down the cloud gradient. It is concluded that in the mass-flux approach the turbulent mixing coefficients, and the correction terms that arise from the transport term, are very similar to closures applied to the Reynolds-averaged equations

Observed Lagrangian Transition of Stratocumulus into Cumulus during ASTEX: Mean State and Turbulence Structure

Aircraft measurements made during the "First Lagrangian" of the Atlantic Stratocumulus Transition Experiment (ASTEX) between 12 and 14 June 1992 are presented. During this Lagrangian experiment an air mass was followed that was advected southward by the mean wind. Five aircraft flights were undertaken to observe the transition of a stratocumulus cloud deck to thin and broken stratocumulus clouds penetrated by cumulus from below. From the horizontal aircraft legs the boundary layer mean structure, microphysics, turbulence structure, and entrainment were analyzed. The vertical profiles of the vertical velocity skewness are shown to illustrate the transition of a cloudy boundary layer predominantly driven by longwave radiative cooling at the cloud top to one driven mainly by convection due to an unstable surface stratification and cumulus clouds. During the last flight before the stratocumulus deck was observed to be broken and replaced by cumuli, the total water flux, the virtual potential temperature flux, and the vertical velocity variance in the stratocumulus cloud layer were found significantly larger compared with the previous flights. To analyze the cloud-top stability the mean jumps of conserved variables across the inversion were determined from porpoising runs through the cloud top. These jumps were compared with cloud-top entrainment instability criteria discussed in the literature. It is suggested that enhanced entrainment of dry air is a key mechanism in the stratocumulus-cumulus transition

Computer-Supported Cooperative Education and Development in Southern Africa

In addition to all the political upheavals, South Africa is still encumbered by educational problems which cannot easily be addressed only by means of the traditional remedies of spending more money, building more schools and training more teachers. Bold and imaginative solutions, such as the proper and coordinated use of information technology, should be considered as complementary strategies

On the control of acute rodent malaria infections by innate immunity

Does specific immunity, innate immunity or resource (red blood cell) limitation control the first peak of the blood-stage parasite in acute rodent malaria infections? Since mice deficient in specific immunity exhibit similar initial dynamics as wild-type mice it is generally viewed that the initial control of parasite is due to either limitation of resources (RBC) or innate immune responses. There are conflicting views on the roles of these two mechanisms as there is experimental evidence supporting both these hypotheses. While mathematical models based on RBC limitation are capable of describing the dynamics of primary infections, it was not clear whether a model incorporating the key features of innate immunity would be able to do the same. We examine the conditions under which a model incorporating parasite and innate immunity can describe data from acute <i>Plasmodium chabaudi</i> infections in mice. We find that innate immune response must decay slowly if the parasite density is to fall rather than equilibrate. Further, we show that within this framework the differences in the dynamics of two parasite strains are best ascribed to differences in susceptibility to innate immunity, rather than differences in the strains' growth rates or their propensity to elicit innate immunity. We suggest that further work is required to determine if innate immunity or resource limitation control acute malaria infections in mice