Search

Seagrasses are key structural elements in coastal ecosystems, and studying how temperature affects these species is crucial to anticipate the implications of global warming. In this work, we use an empirically-based numerical model to study the combined dynamics of {\it Posidonia oceanica} and {\it Cymodocea nodosa} and their resilience to sea warming. The model is parametrised using seagrass growth rates measured at the Western Mediterranean Sea. Under favorable growth conditions, our simulations predict the emergence of a coexistence region at the front between mono-specific meadows. This region can be characterised by its width and local shoot densities, which are found to depend on the coupling parameter between {\it Posidonia oceanica} and {\it Cymodocea nodosa} species. Such regions have been empirically observed in Ses Olles de Son Saura (Balearic Islands, Western Mediterranean Sea). A comparison between the field measurements at the study site with the model predictions has been used to fit the value of the coupling parameter. Field data also relates the width of the coexistence region to the average length of {\it Posidonia oceanica} leaves at the front. Remarkably, a linear relationship is found between the coupling parameter and the leaf length. In the presence of sea warming, the model predicts an exponential decay in the population of {\it Posidonia oceanica}, which is highly sensitive to temperature. This behaviour is a direct consequence of the clonal nature of the plant and can be characterised by the model parameters. Considering a scenario of high greenhouse emissions, our model forecasts that {\it Posidonia oceanica} meadows will lose 70$\%$ of their population by the year $2050$. {\it Cymodocea nodosa}, with higher thermal resilience, acts as an opportunistic species conquering the space left by the degraded {\it Posidonia oceanica}.

Seagrasses provide multiple ecosystem services and act as intense carbon sinks in coastal regions around the globe but are threatened by multiple anthropogenic pressures, leading to enhanced seagrass mortality that reflects in the spatial self-organization of the meadows. Spontaneous spatial vegetation patterns appear in such different ecosystems as drylands, peatlands, salt marshes, or seagrass meadows, and the mechanisms behind this phenomenon are still an open question in many cases. Here, we report on the formation of vegetation traveling pulses creating complex spatiotemporal patterns and rings in Mediterranean seagrass meadows. We show that these structures emerge due to an excitable behavior resulting from the coupled dynamics of vegetation and porewater hydrogen sulfide, toxic to seagrass, in the sediment. The resulting spatiotemporal patterns resemble those formed in other physical, chemical, and biological excitable media, but on a much larger scale. Based on theory, we derive a model that reproduces the observed seascapes and predicts the annihilation of these circular structures as they collide, a distinctive feature of excitable pulses. We show also that the patterns in field images and the empirically resolved radial profiles of vegetation density and sediment sulfide concentration across the structures are consistent with predictions from the theoretical model, which shows these structures to have diagnostic value, acting as a harbinger of the terminal state of the seagrass meadows prior to their collapse.

The Mediterranean Sea is warming more rapidly than the global average, leading to dire consequences for its inhabiting organisms. Seagrasses are key structural elements in coastal ecosystems, and studying how temperature affects these species is crucial to anticipate the implications of global warming. In this work, we use an empirically based numerical model to study the combined dynamics of Posidonia oceanica and Cymodocea nodosa and their resilience to sea warming. The model is parametrised using seagrass growth rates measured in the Western Mediterranean Sea. Under favourable growth conditions, our simulations predict the emergence of a coexistence region at the front between monospecific meadows. This region can be characterised by its width and local shoot densities, which depend on the coupling parameter between P. oceanica and C. nodosa. Such regions have been empirically observed in Ses Olles de Son Saura (Balearic Islands). A comparison between the field measurements at the study site with the model predictions was used to fit the value of the coupling parameter. Field data also relate the width of the coexistence region to the average length of P. oceanica leaves at the front. Remarkably, a linear relationship was found between the coupling parameter and leaf length. In the presence of sea warming, the model predicts an exponential decay in the population of P. oceanica, which is highly sensitive to temperature. This behaviour is a direct consequence of the clonal nature of the plant and can be characterised by the model parameters. Considering a scenario of high greenhouse emissions, our model forecasts that P. oceanica meadows will experience a 70% population decline by the year 2050. C. nodosa, with higher thermal resilience, acts as an opportunistic species and will colonise the space left by the degraded P. oceanica.

Code and data for the article "A mathematical model for inter-specific interactions in seagrasses" by Llabrés, Eva; Mayol, Elvira; Marbà, Núria; Sintes, Tomàs, published in OIKOS with doi:10.1111/oik.09296.

Seagrasses are vital organisms in coastal waters, and the drastic demise of their population in the last decades has worrying implications for marine ecosystems. Spatial models for seagrass meadows provide a mathematical framework to study their dynamical processes and emergent collective behavior. These models are crucial to predict the response of seagrasses to different global warming scenarios, analyze the resilience of existing seagrass distributions, and optimize restoration strategies. In this article, we propose a model that includes interactions among different species based on the clonal growth of seagrasses. We present a theoretical analysis of the model considering the specific case of the seagrass–seaweed interaction between Cymodocea nodosa and Caulerpa prolifera. Our simulations successfully reproduce field observations of shoot densities in mixed meadows in the Ebro River Delta in the Mediterranean Sea. Besides, the proposed model allows us to investigate the possible underlying mechanisms that mediate the interaction among the two macrophytes.

The behaviour of supramolecular brushes, whose filaments are composed by sequences of mag- netic and non-magnetic colloidal particles, has been studied using Langevin dynamics simula- tions. Two types of brushes have been considered: sticky or Stockmayer brushes (SB) and non- sticky magnetic brushes (NSB). In both cases, the microstructure and the collective behaviour have been analysed for a wide range of magnetic field strengths including the zero-field case, and negative fields. Results show that, for the same magnetic content, SB placed in a magnetic field present an extensibility up to two times larger than NSB. The analysis of the microstructure of SB at zero field shows that magnetic particles belonging to different filaments in the brush self-organize into ring and chain aggregates, while magnetic colloids in NSB mainly remain in a non-aggregated state. Clustering among magnetic particles belonging to different filaments is ob- served to gradually fade away as the magnetic content of SB filaments increases towards 100%. Under an external field, SB are observed to form chains, threads and sheets depending on the magnetic content and the applied field strength. Chain-like clusters in SB are observed to de- crease in size as the magnetic content in filaments increases. A non-monotonic field dependence is observed for the average size of these clusters. In spite of the very different microstructure, both NSB and SB are observed to have a very similar magnetization, especially in high strength fields.

In this master thesis we have analysed, by means of numerical simulations, the collective effects of an ensemble of voltage-gated ion channels. The membrane potential and the number of open channels has been computed for different values of the injected current, the total number of channels and the capacitance time, which is proportional to the capacitance of the membrane. We have followed two approaches: i) using Monte Carlo simulations to mimic the two-state kinetics with known transition rates between open and close states, and ii) solving a set of two differential equations, a deterministic one for the membrane potential and a Langevin type equation for the number of open channels. We have measured the normalized voltage autocorrelations and the power spectral density that have been compared with experimental results.

Extensive Langevin dynamics simulations are used to characterize the adsorption transition of a flexible magnetic filament grafted onto an attractive planar surface. Our results identify different structural transitions at different ratios of the thermal energy to the surface attraction strength: filament straightening, adsorption, and the magnetic flux closure. The adsorption temperature of a magnetic filament is found to be higher in comparison to an equivalent nonmagnetic chain. The adsorption has been also investigated under the application of a static homogeneous external magnetic field. We found that the strength and the orientation of the field can be used to control the adsorption process, providing a precise switching mechanism. Interestingly, we have observed that the characteristic field strength and tilt angle at the adsorption point are related by a simple power law.

In this master thesis we have presented the results of extensive computer simulations concerning the clonal growth dynamics of seagrass species under different frameworks. We have implemented a microscopic description of the growth process that is controlled by a few set of growth rules and parameters. This modelling is able to provide a full description of the patch growth, including the spatial distribution of shoots and apices, the overall biomass production and the distribution of ramets, that are valuables quantities in order to compare with observational data. We have considered the growth of a single species that has been used as a model test and compared with already existing results. Following, the growth of two seagrass species coexisting in the same region of space has been studied. We have focused on the most common Mediterranean species: Cymodocea nodosa and Posidonia oceanica. In both cases, the inclusion of local interacting term has been considered and its effect has been evaluated. We have proposed a method to implement the cross-interaction among species.

In the present work magnetic brushes under flow conditions and confined inside narrow slits have been studied using Langevin dynamics simulations. It has been observed that the structural properties of these confined magnetic brushes can be tuned via the application of an external magnetic field, and this control can be exerted with a relatively low content of magnetic colloidal particles in the filaments that form the brushes (20% in the present study). The potential of these brushes to perform a separation process of a size-bidispersed mixture of free non-magnetic colloidal particles flowing through the slit has also been explored. Numerical results show that it is possible to induce a two-fold effect on the bidispersed particle flow: A lateral separation of the two types of flowing colloidal particles and an enhancement of the differences in their velocities. These two features are key elements sought in separation processes and could be very relevant in the design of new chromatographic columns and microfluid separation devices.

We report for the first time the complex formation between cationic like-charged polyelectrolytes and surfactant systems. The mechanism of attraction has been investigated through extensive course-grained molecular dynamic simulations and atomic force microscopy (AFM) experiments. The addition of a tetravalent salt is found to play a crucial role in the polyelectrolyte backbone properties. We found extended structures at low and high salt concentration values, whereas, for intermediate ones, the chains condensate. The combination of elements with the same charge has been investigated in three different regimes depending on the tetrasalt concentration: before condensation; at the condensation point and during the re-expansion. The mixture of polyelectrolytes and surfactants in salt free is also studied and compared to the mixture in a tetrasalt solution.

Practical applications of polymer brush-like systems rely on a clear understanding of their internal structure. In the case of magnetic nanoparticle filament brushes, the competition between bonding and nonbonding interactions—including long range magnetic dipole–dipole interactions—makes the microstructure of these polymer brush-like systems rather complex. On the other hand, the same interactions open up the possibility to manipulate the meso- and macroscopic responses of these systems by applying external magnetic fields or by changing the background temperature. In this study, we put forward an approach to extract information about the internal structure of a magnetic filament brush from scattering experiments. Our method is based on the mapping of the scattering profiles to the information about the internal equilibrium configurations of the brushes obtained from computer simulations. We show that the structure of the magnetic filament brush is strongly anisotropic in the direction perpendicular to the grafting surface, especially at low temperatures and external fields. This makes slice-by-slice scattering measurements a technique very useful for the study of such systems.

Short-scale interactions yield large-scale vegetation patterns that, in turn, shape ecosystem function across landscapes. Fairy circles, which are circular patches bare of vegetation within otherwise continuous landscapes, are characteristic features of semiarid grasslands. We report the occurrence of submarine fairy circle seascapes in seagrass meadows and propose a simple model that reproduces the diversity of seascapes observed in these ecosystems as emerging from plant interactions within the meadow. These seascapes include two extreme cases, a continuous meadow and a bare landscape, along with intermediate states that range from the occurrence of persistent but isolated fairy circles, or solitons, to seascapes with multiple fairy circles, banded vegetation, and “leopard skin” patterns consisting of bare seascapes dotted with plant patches. The model predicts that these intermediate seascapes extending across kilometers emerge as a consequence of local demographic imbalances along with facilitative and competitive interactions among the plants with a characteristic spatial scale of 20 to 30 m, consistent with known drivers of seagrass performance. The model, which can be extended to clonal growth plants in other landscapes showing fairy rings, reveals that the different seascapes observed hold diagnostic power as to the proximity of seagrass meadows to extinction points that can be used to identify ecosystems at risks.

We present an extensive numerical study of the behaviour of a filament made of ferromagnetic colloidal particles subjected to the simultaneous action of a fluid flow and a stationary external magnetic field perpendicular to the flow lines. We found that in the presence of a shear flow, the tumbling motion observed at zero field is strongly inhibited when the external magnetic field is applied. The field is able to stabilise the filament with a well defined degree of alignment that depends on the balance between hydrodynamic and magnetic torques. In addition, for a Poiseuille flow, it has been found that the initial position has a long lasting influence on the behaviour of the magnetic filament when the external field is applied.

Vascularization of embryonic organs or tumours starts from a primitive lattice of capillaries that, in the first stages of development, are remodelled into branched arteries and veins upon perfusion. In this Master Thesis we investigate a theoretical model of vascular development that reproduces the main results in two and three dimensional patterns generated in the yolk sac based on the main characteristic features of vascular morphogenesis: i) the disconnection of side branches from main branches; ii) the reconnection of dangling sprouts and, iii) the plastic extension of the interstitial tissues. The (fluid) mechanical forces that are known to play a major role in arterial patterning are implemented numerically, and they predict the appearance of realistic patterns.

In the present work we use Langevin dynamics computer simulations to understand how the presence of a constant external magnetic field modifies the conformational phase diagram of magnetic filaments in the limit of infinite dilution. We have considered the filaments immersed in either a good (non-sticky filaments) or a poor (Stockmayer polymers) solvent. It has been found that in the presence of an applied field, filaments turn out to be much more susceptible to parameters such as temperature and solvent conditions. Filaments owe this increased susceptibility to the fact that the external magnetic field tends to level the free energy landscape as compared to the zero-field case. The field induces equalization in the free energy of competing conformational states that were separated by large energy differences in the zero-field limit. In this new scenario multistability arises, and manifests itself in the existence of broad regions in the phase diagram where two or more equilibrium configurations coexist. The existence of multistability greatly enhances the possibility of tuning the properties of the filament.

We present a theoretical study on the design of a supramolecular magnetoresponsive coating. The coating is formed by a relatively dense array of supracolloidal magnetic filaments grafted to a surface in a polymer brush-like arrangement. In order to determine and optimise the properties of the magnetic filament brush, we perform extensive computer simulations with a coarse-grained model that takes into account the correlations between the magnetic moments of the particles and the backbone crosslinks. We show that the self-assembly of magnetic beads from neighbouring filaments defines the equilibrium structural properties of the complete brush. In order to control this self-assembly, we highlight two external stimuli that can lead to significant effects: temperature of the system and an externally applied magnetic field. Our study reveals self-assembly scenarios inherently driven by the crosslinking and grafting constraints. Finally, we explain the mechanisms of structural changeovers in the magnetic filament brushes and confirm the possibility of controlling them by changing the temperature or the intensity of an external magnetic field.