Stratification d'un lac | EPFL Graph Search

La stratification des lacs est la tendance des lacs à former des couches thermiques séparées et distinctes par temps chaud. Les lacs stratifiés présentent généralement trois couches distinctes : l'épilimnion comprenant la couche chaude supérieure, la thermocline (ou métalimnion), couche intermédiaire qui peut changer de profondeur tout au long de la journée, et l'hypolimnion, plus froid, s'étendant jusqu'au fond du lac. Définition La stratification thermique des lacs fait référence à un changement de température à différentes profondeurs dans ceux-ci, et est due au changement de densité de l'eau avec la température. L'eau froide est plus dense que l'eau chaude et l'épilimnion est généralement constitué d'eau qui n'est pas aussi dense que l'eau de l'hypolimnion. Cependant, la température de densité maximale pour l'eau douce est de . Dans les régions tempérées, où l'eau du lac se réchauffe et se refroidit au fil des saisons, un modèle cyclique de renversement se produit qui

Thin layers of phytoplankton and harmful algae events in a coastal upwelling system

Bieito Fernandez Castro

We combined time-series from a monitoring program and specific field observations in order to: 1) describe the characteristics of thin layers of phytoplankton (TLP) in the Galician Rías Baixas (NW Iberian Peninsula), 2) investigate the relationship between spatially-extended TLP (se-TLP) events and environmental variables, and 3) analyze the relationship between TLP occurrence and cell densities of toxin-producing harmful algal bloom (THAB) species. Between 2012 and 2015, a total of 118 TLP were detected in any of the 39 stations weekly sampled in the study area in the frame of the monitoring program, which represents a frequency of occurrence of 2%. Most TLP (84%) were detected between May-August and located slightly below shallow pycnoclines. Eight se-TLP events, i.e., when at least five stations were simultaneously influenced by the occurrence of a TLP, were identified during the period 2012–2015. Six out eight of these events happened during summer-upwelling conditions, and according to a Principal Component Analysis (PCA) they were positively correlated with thermal stratification. The other two events, detected during spring-downwelling, were positively correlated with haline stratification. A logistic regression model including six variables (surface temperature, relative humidity, bottom salinity, a proxy for the intrusion of the Miño River plume into Ría de Vigo, Miño River flow, and bottom temperature) explained 42% of the variance in the occurrence of se-TLP events. The temporal persistence of TLP events was confirmed by observations carried out during specific cruises, which showed how TLP were formed and disappeared over short periods of time in response to changes in vertical mixing. TLP were more frequently observed in Ría de Pontevedra, which was characterized by longer harvesting closures due to detection of Dinophysis toxins (DSP: diarrhetic shellfish poisoning toxins and pectenotoxins) above regulatory levels. About 25% of the TLP detected in the Rías Baixas were associated with increased cell densities of potentially toxic Pseudo-nitzschia species (ASP: amnesic shellfish poisoning) and of D. acuminata. These results suggest that a relationship may be established between the occurrence of TLP and the growth and/or accumulation of the main THAB species in the Galician Rías Baixas.

2020

Towards patient-tailored interventions for stroke recovery: graph theoretical approaches and modeling of non-invasive brain stimulation

Giorgia Giulia Evangelista

Stroke is a disease that causes the death of precious brain cells that will never grow back and which will affect one in four humans at some point in their life. While therapies and treatments for this debilitating condition are fairly standard, their outcomes are highly heterogeneous, with deficits all too often leading to permanent disability. These unsatisfactory outcomes point to the need for a paradigm shift towards precision medicine, where therapies and treatments are tailored to individual patients. This in turn requires a better understanding of the mechanisms underlying post-stroke impairment and recovery. Recent technological innovations provide an unprecedented opportunity to acquire a detailed picture of the anatomy and electrophysiology of the human brain non-invasively. Therefore, we apply multimodal methodologies to investigate the inner workings of the stroke-affected brain with a view to establishing biomarkers and creating stratifications that maximize the effectiveness of stroke therapy by making it personalized.In the first part of this thesis, network-theoretic models of the brain known as connectomes are created, under the assumption that these provide sufficiently detailed information about the individual patient to inform personalized recommendations. In particular, we leverage existing knowledge about how brain networks are organized into specialized modules connected by a "rich-club" of hubs. In a novel contribution to the field of connectomics, we combine these connectomes with detailed images of the lesion to obtain a network-theoretic lesion profile and a map of the non-lesioned part of the network, which we expect to be a better reflection of true brain connectivity. Using this methodology, we establish that the rich-club, in addition to being a source of network robustness, is also a source of vulnerability in that attacks against it lead to poor behavioral outcomes disproportionate to the size of the attacks.The creation of in-silico simulation of individual patients presents a great opportunity to quickly experiment with treatments, making it a potential game-changer in medical research. However, current technology is not able to faithfully capture the complexity of a whole human brain. The complex brain abnormalities inherent to stroke only add to the difficulty of the challenge. Any models one might wish to create must strike a balance between technological feasibility and sensitivity to inter-patient variability across a large parameter space. In the second part of this thesis, we investigate whether one major source of inter-patient variability, namely the size and location of the lesion, induces substantial variability in the effects of non-invasive brain stimulation to be included as factors in the design of stimulation protocols. Based on our modeling of the stimulation, personalizing stimulation protocols by taking the lesion into account produces little added value, except possibly when one is targeting brain areas close to the lesion and the lesion is significantly large.The proposed studies represent a step of the march towards precision medicine, by examining sources of variability in stroke patients and by associating them with variability in some outcomes of interest. Each step of this march brings stroke therapy farther from the status quo of homogeneous treatments and heterogeneous outcomes and closer to the ideal of heterogeneous treatments and homogeneously positive outcomes.

EPFL2022

Effects of biogenic surfactants on the spatial variability of surface water temperature under low-wind conditions

David Andrew Barry, Mehrshad Foroughan

Natural slicks are manifested by changing reflective properties of water surfaces through inhibition of wave-growth and damping short gravity-capillary waves. Such influence on near-surface hydrodynamics can in turn affects the air-water exchange of momentum, heat, and gas. Here, we present observations from a field campaign carried out in Lake Geneva in Spring 2019, which show the response of Lake Surface Water Temperature (LSWT) to transient light winds. For the purpose of this research, we used an autonomous catamaran equipped with near-surface water temperature sensors, a weather station, and an RGB camera taking slanted view images of the water surface. The second imagery package includes a LWIR camera carried by a balloon, providing thermal images with resolution of O(1 m) from altitude of about 500 m. As our result demonstrates, following the onset of low winds (U10 < 3 m/s) roughness patterns appeared on the surface that resembles slick/non-slick regions commonly observed on the water bodies. Such patterns have an exact correspondence with surface temperature contrasts of up to 2℃. To associate smooth and choppy surface conditions with the effects of biogenic surfactants, enrichment factor of Fluorescence Dissolved Organic Matter is measured as a proxy from samples collected at different field campaigns. The results point to the spatial variability of LSWT at the sub-pixel satellite scale. It is shown that such warm and cool patches can persist on the water surface after the surface roughness patterns disappeared following a drop in wind speed (U_10 < 2 m/s). Hence, such interaction between transient wind and random-like distribution of natural slicks leads to heterogeneous surface mixing. This can in turn play a role in the generation of intermittency of air-water fluxes, particularly during strong lake stratification and low-wind conditions.

2020

Thin layers of phytoplankton and harmful algae events in a coastal upwelling system

Bieito Fernandez Castro

2020

Towards patient-tailored interventions for stroke recovery: graph theoretical approaches and modeling of non-invasive brain stimulation

Giorgia Giulia Evangelista

EPFL2022

Effects of biogenic surfactants on the spatial variability of surface water temperature under low-wind conditions

David Andrew Barry, Mehrshad Foroughan

2020