1,720,976 research outputs found
A numerical investigation for dating 210Pbex and 137Cs vertical profiles in a coastal area: The Eastern Ligurian Sea, Italy
No full text210Pb and 137Cs vertical profiles in coastal sediments are studied using a sedimentation-mixing model. The cores were sampled in a complex coastal area characterized by the presence of different impact sources and environmental uses (riverine inputs, commercial and military harbour, marine protected area) in the Eastern Ligurian Sea, Italy. The analysis of accumulation and dispersion processes is performed using a numerical advection and diffusion model, in terms of the independent variables - time and mass depth. The flux of 210Pb is considered in steady state while the time dependent input flux of 137Cs is estimated from the concentration of this radionuclide in seawater, starting from observed data from 1960 to date. Differently from the atmospheric fallout distribution, this input function contains, at least partly, the contribution that still continues to reach the sediment in the last 25 years as a result of coastal and riverine input. The analysis highlights some features obscured in experimental data, and allows comparison of the effects of different scenarios. The specific effect of a pulsed input is discussed by analysing the effects of the Chernobyl event. The effects of mass flux in non–steady state are also considered: we observe that since the pulsed inputs in 137Cs are now too old, a strong superficial mixing and a time-variable flux produce similar profiles for both radionuclides. Hence, the general environmental knowledge of the area remains the main instrument to fully define the active processes in some cores
Modelling of predator--prey trophic interactions. Part II. Three trophic levels
No full textA general class of lumped parameter models describing the local dynamics of a tri-trophic chain in a controlled
environment is analyzed in detail. The trophic functions characterizing the interactions are defined only by some
properties and allow us to treat both prey-dependent and ratio-dependent models in a unified manner. Conditions for
existence and stability of extinction and coexistence equilibrium states are determined. Some peculiar aspects of the dynamics of the system depending on the bioecological parameters are presented, with particular attention to bistability situations, limit cycles and chaotic behaviours
Dispersion processes and residence times in a semi-enclosed basin with recirculating gyres: An application to the Tyrrhenian Sea
No full textIn this paper we investigate the dispersion of a tracer in a semi-enclosed basin characterized by a steady flow with recirculations. In particular, we focus on the macroscopic behavior of the system, characterized by the total concentration of the tracer in the basin, C(t), and by its residence time T. As a case study, the circulation in an idealized basin mirroring some of the major characteristics of the Tyrrhenian Sea is considered, with a northward current connecting the inflow and the outflow regions of the basin, and with a main cyclonic gyre in the northern part of it. Numerical simulations are performed from several release points in the basin and for two different values of the diffusivity coefficient K. Two independent models for dispersion are used, an Eulerian and a Lagrangian one, allowing us to validate both the basic formalism and the numerical results. The experiments show that the macroscopic properties of dispersion are largely influenced by the presence of the main gyre, while they do not depend strongly on K in the considered range. Namely, after a first phase which depends on the initial conditions, the tracer tends to be concentrated in the region of the northern gyre, and this influences the trend of C(t) versus time and the value T. For almost all the simulations, the decrease of C(t) in time can be approximated by an exponential decay, indicating a constant probability of tracer escaping the basin. The e-folding timescale of the decay is the inverse of the principal eigenvalue of the advection-diffusion operator, and it can be computed a priori knowing the flow field; this allows us to compute also an a priori estimate for the residence time T. Only when the initial release is very close to the outflow, is the initial decay of C(t) distinctly different from an exponential, and a more detailed analysis is necessary. The basic results appear generalizable to a number of other similar systems with recirculations
Modelling of predator-prey trophic interactions. Part I. Two trophic levels
No full textA class of lumped parameter models to describe the local dynamics in a controlled environment of a two-trophic chain
is considered. The class is characterized by a trophic function (functional response of predator to the abundance of prey)
depending on the ratio of prey biomass x and a linear function of predator biomass y: f(qx/[(1−ρ)k +ρy] ),
where q
is the efficiency of the predation process, k is a reference biomass, and ρ (0 ≤ ρ ≤ 1) specifies the predation
model. The trophic function is defined only by some properties determining its shape. A stability analysis of the models
has been performed by taking the parameters q and ρ as bifurcation parameters: the regions in the (ρ,q) plane of
existence and stability of nonnegative equilibrium states and limit cycles are determined. This analysis shows that the
behaviour of the models is qualitatively similar for 0 ≤ ρ< 1 (in particular the null state is always a saddle point),
while the value ρ = 1 gives rise to some kind of structural instability of the system (in particular the null state becomes
an attractor for sufficiently high predation efficiency)
Red coral population dynamics: stability analysis and numerical simulation of time evolution of perturbed states. In:
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