Multi-scale variability in phytoplankton populations of the North Atlantic basin: from eddies to global change

The Continuous Plankton Recorder (CPR) survey has been deployed since 1931 to describe and analyze plankton variability in the North Atlantic and North Sea. This survey measures the presence and abundance of 437 phytoplankton and zooplankton taxa and provides an assessment of phytoplankton biomass, the Phytoplankton Colour Index (VCI). The diatoms and dinoflagellates are the two main phytoplankton groups identified by the CPR survey. The first part of this work provides insights into the space-time dynamics of phytoplankton communities through an analysis of diatom and dinoflagellate populations in the whole North Atlantic basin. Because the North Atlantic Ocean includes many different biotopes, the second part focuses on the mesoscale variability of phytoplankton species. The long-term fluctuations of the phytoplankton species are studied in the NW and in the NE Atlantic, the two best sampled areas of the CPR survey. In the NE Atlantic, the aim is to determine the contribution of the diatoms and dinoflagellates to the PCI, their fluctuations over 45 years of sampling and their geographical variations. Because local variability in environmental conditions is thought to play a dominant role in temporal fluctuations of phytoplankton biomass, the next part takes advantage to define small areas around the British Isles. This allowed me to study more precisely the processes influencing the long-term variation of phytoplankton assemblages. The North Atlantic Current transports water across the Northern basin of the Atlantic Ocean, along the shelf of Ireland and form the Norwegian current which corresponds to the inflow of oceanic waters into the Norwegian Sea and the North Sea. In this highly hydrodynamic region attention is focused on the fluctuations of plankton species in relation to the currents. The aim of this part is therefore to investigate the fluctuations of phytoplankton biomass, diatoms and dinoflagellates, their geographical distribution and abundance within the area and their relationship with physical processes. The intense hydrodynamic activity observed in the Northwestern Atlantic Shelves Province (NWCS) makes this region especially intriguing from the point of view of physical-biological interactions. The relationship between spatial and temporal structures of eddies (via Sea Surface Heights) and chlorophyll a (from the Sea-viewing Wide Field-of-view Sensor, Sea WiFS) was assessed along the Gulf Stream axis. In particular, the physical structures identified were followed and compared with phytoplankton distribution. In addition, the impact of the LSW changing flow along the Scotian Shelf and the influence of Gulf Stream rings along the George Bank is determined. This work demonstrated that changes are occurring in pelagic ecosystems at different temporal and spatial scales. These changes have been illustrated by the spatial variability induced by eddies and/or currents but also by the regional variability of the hydro-climatic processes, influencing in different ways Sea Surface Temperature, wind-regimes and mixing of local environments. Several different aspects of the North Atlantic Oscillations impact on pelagic ecosystems have been highlighted. In the northeast Atlantic, NA0 fluctuations imply changes in (i) SST in northern Europe, (ii) wind regimes, (iii) Atlantic Water inflow into the North Sea. In contrast, in the northwest Atlantic, the variations of NAO imply changes in (i) SST on the Scotian Shelf, (ii) coastal currents, and (iii) inflow of Labrador Sea Slope Water (LSSW) towards the Scotian Shelf and Georges Bank. These changes in environmental process impact phytoplankton production, abundance, spatial distribution, community structure phenology and ultimately would impact trophodynamics processes. It is, however, still difficult to explain unambiguously all the mechanisms that are involved in the control of the observed patterns

Influence of nitrogen deposition on carbon dynamics in peatlands

The impact of high levels of nitrogen deposition from the atmosphere (primarily from the combustion of fossil fuels and transportation) on soil carbon fluxes and carbon sequestration pathways in peatlands are uncertain and limit our understanding of its consequence on peatlands’ role as global carbon sinks. An alteration in peatlands’ natural carbon accumulation process could result in the increased release of CO2 into the atmosphere, potentially increasing the greenhouse effect and contributing to climate change. Recent studies in forest soils have shown that high concentrations of inorganic nitrogen accelerate the activity of key soil enzymes involved in the degradation of easily decomposable litter (low lignin content) but slow down the decomposition of lignin abundant litter. Peatland soils are adapted to slow rates of nitrogen mineralization; therefore increasing the nitrogen supply in these environments may have an even deeper impact on litter quality, potential litter decomposability, and overall carbon storage capacity. The aim of this study is to use plant wax biomarkers as proxies of vegetational change in litter profiles. An alkane and alcohol profile database of peatland vegetation is currently being characterized for comparison with samples taken from the Whim Moss experimental site (Edinburgh) where different levels of nitrogen has been added to peatland soil since 2002. A temporal study combining the MicroResp technique (community level physiological profiles) and enzyme activity assays is considered to look at the effect of litter compositional changes on soil microbial diversity and biological activity. To better understand how nitrogen deposition in peatland soil affects the mechanisms controlling carbon storage, the incorporation of stable isotope labelling (13C) would allow direct determination of the fate of carbon into the different carbon pools and better pin-point the changes in litter composition

Computationally Efficient Electromagnetic Transient Power System Studies using Bayesian Optimization

The power system of the future will be governed by complex interactions and non-linear phenomena, that should be studied more and more through computationally expensive software simulations. To solve the abovementioned problems, power system engineers face problems with following characteristics: (i) a computationally expensive simulator, (ii) non-linear functions to optimize and (iii) lack of abundance of data. Existing optimization settings involving EMT-type simulations have been developed, but mainly use a deterministic model and optimizer, which may be computationally inefficient and do not guarantee finding a global optimum. In this paper, an automation framework based on Bayesian Optimization is introduced, and applied to two case studies. It is found that the framework has the potential to reduce computational effort, outperform deterministic optimizers and is applicable to a multitude of problems. Nevertheless, it was found that the output of the Bayesian Optimization depends on the number of evaluations used for initialization, and in addition, careful selection of surrogate models, which should be subject to future investigation

Sensitivity of groundwater recharge using climatic analogues and HYDRUS-1D

The sensitivity of groundwater recharge to different climate conditions was simulated using the approach of climatic analogue stations, i.e. stations presently experiencing climatic conditions corresponding to a possible future climate state. The study was conducted in the context of a safety assessment of a future near-surface disposal facility for low and intermediate level short-lived radioactive waste in Belgium; this includes estimation of groundwater recharge for the next millennia. Groundwater recharge was simulated using the Richards based soil water balance model HYDRUS-1D and meteorological time series from analogue stations. This study used four analogue stations for a warmer subtropical climate with changes of average annual precipitation and potential evapotranspiration from −42% to +5% and from +8% to +82%, respectively, compared to the present-day climate. Resulting water balance calculations yielded a change in groundwater recharge ranging from a decrease of 72% to an increase of 3% for the four different analogue stations. The Gijon analogue station (Northern Spain), considered as the most representative for the near future climate state in the study area, shows an increase of 3% of groundwater recharge for a 5% increase of annual precipitation. Calculations for a colder (tundra) climate showed a change in groundwater recharge ranging from a decrease of 97% to an increase of 32% for four different analogue stations, with an annual precipitation change from −69% to −14% compared to the present-day climate

Structure of phytoplankton (Continuous Plankton Recorder and SeaWiFS) and impact of climate in the Northwest Atlantic Shelves

International audienceAll marine organisms are affected to some extent by the movement and thermal properties of oceanic currents. However phytoplankton, because of its small size is most directly coupled to the physical environment. The intense hydrodynamic activity observed in the Northwest Atlantic Shelves Province makes this region especially intriguing from the point of view of physical-biological interactions. In the present work, remote sensed data of Sea Surface Height (SSH) anomalies, Sea-surface chlorophyll a concentrations (SeaWiFS), and Sea Surface Temperature (SST) are used to complement the Continuous Plankton Recorder (CPR) survey that continuously sampled a route between Norfolk (Virginia, USA; 39° N, 71° W) and Argentia (Newfoundland; 47° N, 54° W) over the period 1995?1998. Over this period, we examined physical structures (i.e. SST and SSH) and climatic forcing associated with space-time phytoplankton structure. Along this route, the phytoplankton structures were mainly impacted by the changes in surface flow along the Scotian Shelf rather than significantly influenced by the mesoscale features of the Gulf Stream. These changes in water mass circulation caused a drop in temperature and salinity along the Scotian Shelf that induced changes in phytoplankton and zooplankton abundance

From CNNs to Shift-Invariant Twin Models Based on Complex Wavelets

We propose a novel antialiasing method to increase shift invariance and prediction accuracy in convolutional neural networks. Specifically, we replace the first-layer combination "real-valued convolutions + max pooling" ($\mathbb{R}$Max) by "complex-valued convolutions + modulus" ($\mathbb{C}$Mod), which is stable to translations. To justify our approach, we claim that $\mathbb{C}$Mod and $\mathbb{R}$Max produce comparable outputs when the convolution kernel is band-pass and oriented (Gabor-like filter). In this context, $\mathbb{C}$Mod can be considered as a stable alternative to $\mathbb{R}$Max. Thus, prior to antialiasing, we force the convolution kernels to adopt such a Gabor-like structure. The corresponding architecture is called mathematical twin, because it employs a well-defined mathematical operator to mimic the behavior of the original, freely-trained model. Our antialiasing approach achieves superior accuracy on ImageNet and CIFAR-10 classification tasks, compared to prior methods based on low-pass filtering. Arguably, our approach's emphasis on retaining high-frequency details contributes to a better balance between shift invariance and information preservation, resulting in improved performance. Furthermore, it has a lower computational cost and memory footprint than concurrent work, making it a promising solution for practical implementation

Révolution bolivarienne et ordre mondial

Depuis une dizaine d’années, on s’était habitué à rechercher les sources de résistances à la globalisation néolibérale en-dehors des structures étatiques traditionnelles, jugées dépassées par un processus qui en remettait en question à la fois la nature et la portée. Toutefois, dans un contexte également marqué par l’annonce - certes un peu précipitée - de la « fin des idéologies », voici qu’un pays jusque-là plus ou moins sans histoire se lance dans une « révolution protagoniste et populaire » qui vient remettre en cause la plupart des préjugés contemporains consacrant l’inévitabilité de la globalisation ou encore l’indépassabilité de la démocratie représentative libérale. C’est pourquoi il nous a semblé pertinent de nous pencher sur ce processus bolivarien, à bien des égards inédit, et de chercher à en comprendre les dynamiques particulières au-delà de l’exubérance de son bouillant comandante, qui a tendance, volontairement ou malgré lui, à détourner l’attention des véritables enjeux sociaux, économiques et politiques qui sont soulevés à l’heure actuelle au Venezuela et, plus largement, dans l’ensemble de la région sud-américaine. Ce mémoire porte donc non pas sur des personnes, mais sur un processus, celui de la révolution bolivarienne, et de ses incidences sur un ordre mondial plus que jamais en proie à des bouleversements aux conséquences imprévisibles

The impact of turbulence and phytoplankton dynamics on foam formation, seawater viscosity and chlorophyll concentration in the eastern English Channel

The space-time dynamics of chlorophyll a concentration and seawater excess viscosity has been investigated in the hydrographically contrasting inshore and offshore water masses of the eastern English Channel. This was done during the phytoplankton spring bloom dominated by Phaeocystis globosa before and after the very large-scale formation of foam induced by an increase in wind-driven turbulence and the related wave breakings. The results suggest that the dynamics of chlorophyll a concentration and seawater excess viscosity are differentially controlled by the formation of foam through the intensity of the spring bloom and wind-generated turbulence

Modeling of MMCs With Controlled DC-Side Fault Blocking Capability for DC Protection Studies

The fault current characteristics in dc systems depend largely on the response, and hence also the topology, of the ac-dc converters. The presently used ac-dc converter topologies may be categorized into those with controlled or uncontrolled fault blocking capability and those lacking such capability. For the topologies of the former category, generic models of the dc-side fault response have not yet been developed and a characterization of the influence of control and sensor delays is a notable omission. Therefore, to support accurate and comprehensive dc system protection studies, this paper presents three reduced converter models and analyzes the impact of key parameters on the dc-side fault response. The models retain accurate representation of the dc-side current control, but differ in representation of the ac-side and internal current control dynamics, and arm voltage limits. The models were verified against a detailed (full-switched) simulation model for the cases of a full-bridge and a hybrid modular multilevel converter, and validated against experimental data from a lab-scale prototype. The models behave similarly in the absence of arm voltage limits, but only the most detailed of the three retains a high degree of accuracy when these limits are reached