Análisis de medidas de salinidad cerca de islas de la plataforma continental francesa del Golfo de Vizcaya

The Bay of Biscay is characterised by large fresh water inputs from two major rivers, the Loire and the Gironde, with a mean discharge of about 900 m3/s. To describe the spatial and temporal variability of the Loire and Gironde plumes, continuous salinity measurements were made at 4 locations on French coastal islands over periods lasting at least two years. These islands are located at a distance from the large local gradients linked to local inputs. Whereas sea surface salinity in the centre of the Bay of Biscay is 35.6, salinities at the coast vary between about 35 in summer to occasionally less than 30 in late winter or in spring. Minimum salinities do not occur at the same time along the coast, indicating a strong spatial gradient. Near the Loire and Gironde estuaries, a strong temporal correlation between discharges and salinity variation appears. Far from the estuaries, some drops in salinity seem to be correlated with a change of wind regime. In summer, a general decrease from north to south is found along the coast and may be linked to northwesterly wind conditions, which drive surface water to SW of the bay. These data constitute the first set of time series of salinity. They will be pursued to assess the interannual variability which is presumed to be high because of the large variability in river discharges and wind regimes.El golfo de Vizcaya se caracteriza por los importantes aportes de agua dulce que recibe de dos grandes ríos, Loira y Garona, de un caudal medio de 900m3/s. Para medir la variabilidad espacial y temporal de estas plumas, se han realizado medidas continuas de salinidad en 4 puntos situados en sendas islas de la costa francesa durante períodos mínimos de dos años. Estas islas están alejadas de los fuertes gradientes relacionados con aportes locales. Mientras que la salinidad en la superficie del mar en el centro del golfo es de 35.6, en la costa oscila entre 35 en verano y menos de 30 en ciertos momentos al final del invierno o durante la primavera. Los mínimos de salinidad no se dan simultáneamente a lo largo de toda la costa, lo que indica un fuerte gradiente espacial. Cerca de los estuarios, ciertas disminuciones de salinidad parecen estar correlacionadas con cambios en los regímenes de viento. En verano se observa una disminución general desde el norte hacia el sur a lo largo de la costa y que puede estar relacionada con unas condiciones de viento del noroeste que conduce el agua de la superficie hacia el sudoeste del golfo. Estos datos constituyen el primer conjunto de series temporales de salinidad cuyo estudio se continuará para evaluar la variabilidad interanual que se supone elevada debido a la fuerte variabilidad de las descargas de los ríos y de los regímenes de viento

Tipología de las estructuras hidrológicas modeladas y observadas en el Golfo de Vizcaya

Collecting hydrological observations over the whole Bay of Biscay continental shelf can require several weeks. As a result, the observations are not truly synoptic and the interpretation of hydrological structures is corrupted by the time-lags between observations at distinct locations. We evaluated the effects of non-synoptic sampling during a spring cruise (17 April to 13 May 2000), using outputs from 3D hydrodynamic models as a substitute for true synoptic sampling. We developed a method for clustering hydrological regions based on the vertical structure of the water column and temporal changes in hydrography. In parallel, outputs from the 3D model were compared with field observations. The results show that in spring 2000 the Bay of Biscay continental shelf could be separated into six principal hydrological regions, one of them being characterised by vertical stability and low temporal variability. This region corresponds to an area where pelagic fish spawning activity is known to be persistently low.La recogida de datos hidrológicos en el conjunto de la plataforma continental del Golfo de Vizcaya puede requerir varias semanas. Como resultado, estas observaciones no son verdaderamente sinópticas y la interpretación de las estructuras hidrológicas está perturbada por los desfases temporales entre observaciones de distintas localidades. En este estudio, se evalúan los efectos del muestreo no sinóptico durante una campaña de primavera (17 de abril al 13 de mayo de 2000), usando los resultados de modelos hidrodinámicos tridimensionales como sustituto de un muestreo verdaderamente sinóptico. A través del método cluster se realizó una agrupación de las regiones hidrológicas, sobre la base de la estructura vertical de la columna de agua y los cambios temporales en la hidrografía. En paralelo, las simulaciones del modelo tridimensional son comparadas con las observaciones in situ. Los resultados muestran que en la primavera de 2000 la plataforma continental del Golfo de Vizcaya puede separarse en 6 regiones hidrológicas principales, una de las cuales se caracteriza por una estabilidad vertical y una baja variabilidad temporal. Esta región corresponde a un área donde se conoce que de manera persistente la freza de peces pelágicos es limitada

Variabilidad hidrológica de mesoescala inducida por los vientos del noroeste sobre la plataforma continental del golfo de Vizcaya

On the French continental shelf of the Bay of Biscay the variability of the surface salinity distribution has been mainly investigated at the seasonal and inter-annual scales. Here, new mesoscale features such as lower-salinity lenses observed in model results are investigated by hydrological measurements acquired during 8 cruises (1997-2000). These lenses are 50-80 km wide and ~30 m thick and occur during westerly to northerly wind events that push offshore the less saline water of river plumes. These water masses detached from the coast are replaced with upwelled saltier water at the coast, so coastal upwelling is often observed at the same time along Landes and southern Brittany coasts. We show that in addition to the influence of seasonal and inter-annual variability of the wind and river outflows, short term meteorological variability may drive mesoscale structures on this continental shelf.La salinidad de la plataforma continental francesa del golfo de Vizcaya fue estudiada a escalas anuales e interanuales principalmente. A partir de medidas efectuadas durante ocho campañas marinas (1997- 2000) y del empleo de modelos matemáticos han podido estudiarse nuevas estructuras hidrológicas aisladas de menor salinidad en forma de lentejas. Estas lentejas han medido entre 50 y 80 km de diámetro y 30 m de espesor. Este fenómeno ha sido observado durante, o después, de fuertes vientos del cuadrante oeste o noroeste que favorecen el desplazamiento del agua desalada de las plumas fluviales hacia el mar abierto. Al separarse de la pluma, estas masas de agua son remplazadas por otra, mas salada, que resurge en la zona costera. De esta manera, puedan observarse simultáneamente afloramientos costeros en las regiónes francesas de Landes y del sur de Bretaña. Se muestra que, además de la variabilidad anual e interanual de los vientos y del caudal de los ríos, los fenómenos meteorológicos a corto plazo pueden condicionar las estructuras hidrológicas de la meso escala de la plataforma continental

Statistical properties and time-frequency analysis of temperature, salinity and turbidity measured by the MAREL Carnot station in the coastal waters of Boulogne-sur-Mer (France)

In marine sciences, many fields display high variability over a large range of spatial and temporal scales, from seconds to thousands of years. The longer recorded time series, with an increasing sampling frequency, in this field are often nonlinear, nonstationary, multiscale and noisy. Their analysis faces new challenges and thus requires the implementation of adequate and specific methods. The objective of this paper is to highlight time series analysis methods already applied in econometrics, signal processing, health, etc. to the environmental marine domain, assess advantages and inconvenients and compare classical techniques with more recent ones. Temperature, turbidity and salinity are important quantities for ecosystem studies. The authors here consider the fluctuations of sea level, salinity, turbidity and temperature recorded from the MAREL Carnot system of Boulogne-sur-Mer (France), which is a moored buoy equipped with physico-chemical measuring devices, working in continuous and autonomous conditions. In order to perform adequate statistical and spectral analyses, it is necessary to know the nature of the considered time series. For this purpose, the stationarity of the series and the occurrence of unit-root are addressed with the Augmented–Dickey Fuller tests. As an example, the harmonic analysis is not relevant for temperature, turbidity and salinity due to the nonstationary condition, except for the nearly stationary sea level datasets. In order to consider the dominant frequencies associated to the dynamics, the large number of data provided by the sensors should enable the estimation of Fourier spectral analysis. Different power spectra show a complex variability and reveal an influence of environmental factors such as tides. However, the previous classical spectral analysis, namely the Blackman–Tukey method, requires not only linear and stationary data but also evenly-spaced data. Interpolating the time series introduces numerous artifacts to the data. The Lomb–Scargle algorithm is adapted to unevenly-spaced data and is used as an alternative. The limits of the method are also set out. It was found that beyond 50% of missing measures, few significant frequencies are detected, several seasonalities are no more visible, and even a whole range of high frequency disappears progressively. Furthermore, two time-frequency decomposition methods, namely wavelets and Hilbert–Huang Transformation (HHT), are applied for the analysis of the entire dataset. Using the Continuous Wavelet Transform (CWT), some properties of the time series are determined. Then, the inertial wave and several low-frequency tidal waves are identified by the application of the Empirical Mode Decomposition (EMD). Finally, EMD based Time Dependent Intrinsic Correlation (TDIC) analysis is applied to consider the correlation between two nonstationary time series

Variation in Size and Growth of the Great Scallop Pecten maximus along a Latitudinal Gradient

Understanding the relationship between growth and temperature will aid in the evaluation of thermal stress and threats to ectotherms in the context of anticipated climate changes. Most Pecten maximus scallops living at high latitudes in the northern hemisphere have a larger maximum body size than individuals further south, a common pattern among many ectotherms. We investigated differences in daily shell growth among scallop populations along the Northeast Atlantic coast from Spain to Norway. This study design allowed us to address precisely whether the asymptotic size observed along a latitudinal gradient, mainly defined by a temperature gradient, results from differences in annual or daily growth rates, or a difference in the length of the growing season. We found that low annual growth rates in northern populations are not due to low daily growth values, but to the smaller number of days available each year to achieve growth compared to the south. We documented a decrease in the annual number of growth days with age regardless of latitude. However, despite initially lower annual growth performances in terms of growing season length and growth rate, differences in asymptotic size as a function of latitude resulted from persistent annual growth performances in the north and sharp declines in the south. Our measurements of daily growth rates throughout life in a long-lived ectothermic species provide new insight into spatio-temporal variations in growth dynamics and growing season length that cannot be accounted for by classical growth models that only address asymptotic size and annual growth rate

Exploration multi-échelle de la circulation côtière dans le golfe de Gascogne

The Bay of Biscay continental shelf is the site of a variety of physical processes, none of which are permanently dominant. The variability of the circulation from daily to interannual scales has been explored through both modelling and observation. Emphasis will be placed on one of the peculiarities of the Atlantic Plateau: the importance of river flows whose catchment areas represent half of the metropolitan territory. The mixing of these fresh waters with marine waters forms plumes that oscillate according to winds and general circulation. Increasingly refined modelling and a growing number of observations now make it possible to describe their fate and their extension as far as the English Channel. The general circulation on the shelf is weak and mainly under the influence of winds, as shown by two recent studies based on float trajectories and current measurements. However, some very energetic current structures have been described recently. These are the fall current, which occurs almost every year and carries water northward, and the even stronger Landes current, which occurs for a few days in the southern Gulf following a westerly gale. While most of my research focused on subtidal scales (time scale greater than one day), my perspectives are oriented towards the study of high-frequency hydrodynamics. These have been inspired by recent coastal observations made in different settings. Indeed, off the Croisic, internal oscillations of several metres in the thermocline have been measured at frequencies ranging from a few minutes to half an hour; along the Landes coast, an astonishing seasonality of tidal currents has been highlighted, since they decrease by a factor of three as soon as the summer stratification disappears. Moreover, in the bay of Miquelon, the bottom temperatures (between 30 and 60m deep) oscillate in summer by several degrees (up to 12°C) at an almost diurnal frequency (26h) while the tide is semi-diurnal. All these observations reveal the presence of internal waves down to very shallow depths and have, to date, never been the subject of specific studies along the French coasts, although their role on the mixing of water masses and coast-wide transport can be potentially very important. This theme is developed and the avenues for exploration through observation and modelling exposed.  Le plateau continental golfe de Gascogne est le siège de processus physiques variés dont aucun ne domine en permanence. La variabilité de la circulation de l'échelle journalière à l échelle interannuelle a été explorée à la fois par la modélisation et l'observation. L'accent sera mis sur l'une des particularités du plateau atlantique : l'importance des apports fluviaux dont les bassins versants représentent la moitié du territoire métropolitain. Le mélange de ces eaux douces avec les eaux marines forme des panaches qui oscillent selon les vents et la circulation générale. La modélisation de plus en plus fine et les observations en nombre croissant permettent désormais de décrire leur devenir et leur extension jusqu'à la Manche. La circulation générale sur le plateau est faible et principalement sous l'influence des vents comme le montrent deux études récentes basées sur des trajectoires de flotteurs et des mesures de courant. Cependant, quelques structures courantologiques très énergétiques ont été décrites récemment. Il s'agit du courant automnal qui se manifeste presque chaque année et transporte les eaux vers le Nord et du courant des Landes, plus fort encore, qui se manifeste durant quelques jours dans le Sud du golfe à la suite d'un coup de vent d'Ouest. Alors que l'essentiel de mes recherches se focalisait sur des échelles subtidales (échelle de temps supérieure à la journée), mes perspectives s'orientent vers l'étude de l'hydrodynamique à haute fréquence. Celles-ci ont été inspirées par des observations côtières récentes effectuées dans différents cadres. En Effet, au large du Croisic, des oscillations internes de plusieurs mètres de la thermocline ont été mesurées à des fréquences variant de quelques minutes à la demi-heure; le long de la côte de Landes, une saisonnalité étonnante des courants de marée a été mise en évidence puisque ceux-ci diminuent d'un facteur trois dès que la stratification estivale disparaît. Par ailleurs, dans la rade de Miquelon, les températures de fond (entre 30 et 60m de profondeur) oscillent en été de plusieurs degrés (jusqu'à 12°C) à une fréquence quasi diurne (26h) alors que la marée est semi-diurne. Toutes ces observations révèlent la présence d'ondes internes jusqu'à de très faibles profondeurs et n'ont, à ce jour, jamais fait l'objet d'études spécifiques le long des côtes françaises alors que leur rôle sur le mélange des masses d'eau et les transports côte-large peut être potentiellement très important. Cette thématique est développée et les pistes d'exploration par l'observation et la modélisation exposées