Pascal Lazure

3D modelling of seasonal evolution of Loire and Gironde plumes on Biscay Bay continental shelf

Abstract A 3D model of the Atlantic shelf has been developed and applied to study shelf dynamics and evolution of hydrology. The model takes the combined effects of tide, wind, river discharges and surface heat flux into account. Time scales vary from 1 day to several years. Following a brief description of the model and forcing variables, the behaviour of the Loire and Gironde plumes is described, first for winter and spring, then over a period of several years, under realistic forcing. The results show that plume evolution depends on the high variability of river runoff and winds. Model simulations performed over 7 years (1990 to 1996) have highlighted several features of these plumes: 1) In early winter and periods of high river runoff, plumes usually spread northwards and along shore. 2) During winter, vertical stratification is weak on the shelf. 3) Near the bottom, low-salinity water spreading hardly varies, reaching about the same extent each year. The low-salinity waters are located in the north of each estuary, often not reaching the 50 m isobath. 4) When river discharges are reduced and prevailing winds are from the north-west, the northward spreading of plumes may be stopped. In that case, plumes may be driven offshore or southwards. This path change usually occurs in spring. Salinity gradients become weaker under mixing and spreading effects. The low-salinity strip along the shore seldom builds up again, and the shelf circulation of water masses becomes mainly wind-driven.

Quantitative description of habitat suitability for the juvenile common sole (Solea solea, L.) in the Bay of Biscay (France) and the contribution of different habitats to the adult population

Abstract This study describes the spatial distribution of young-of-the-year sole based on autumnal beam trawl surveys conducted in the Bay of Biscay (France) during a 15-y period. Previous studies showed that habitat suitability for juvenile sole varies according to physical factors such as bathymetry, sediment structure and river plume influence. These factors, which are known exhaustively for the entire Bay of Biscay from static maps (bathymetry and granulometry) or temporal maps based on a hydrodynamic model (the river plume), were used as descriptors in a generalised linear model of habitat suitability in order to characterise the distribution of juvenile 0-group sole according to delta distribution. This model was used to identify the habitats in which juvenile 0-group sole are concentrated. The respective areas of these habitats were determined from a Geographic Information System (GIS), and their respective contribution to the sole population in the Bay of Biscay was calculated in terms of the estimated number of young fish (GIS area×density derived from the model). Despite the great variability of survey data, this quantitative approach emphasises the highly important role of restricted shallow, muddy estuarine areas as nursery grounds of sole in the Bay of Biscay and demonstrates the relation between interannual variations of nursery habitat capacity (with respect to estuarine extent) and sole recruitment.

The influence of local and non-local forcing effects on the subtidal circulation of Patos Lagoon

Some basic features concerning the subtidal circulation of Patos Lagoon were studied through time series analysis of wind, freshwater discharge, and water level records, as well as by means of experiments carried out with a 3D numerical model. The results indicate that during low to moderate river discharge the wind is the main forcing mechanism in time scales associated with meteorological fronts. The two types of wind action, local and non-local effects, are distinguished and their relative importance is evaluated. Salt water enters the system due to a combination of both remote and local wind effects that favors the development of a pressure gradient towards the lagoon during southwesterly winds. This situation is reversed when northeasterly winds dominate. In the inner parts of the lagoon, local wind plays the major role by inducing set up/set down oscillations. An upwind return flow is then developed under these conditions. During high flood periods, normally observed in late winter, the circulation is driven by freshwater discharge.

Hydrology and circulation in the Strait of Hormuz and the Gulf of Oman—Results from the GOGP99 Experiment: 1. Strait of Hormuz

In October and early November 1999, the GOGP99 experiment collected hydrological, currentmeter, tide recorder, thermistor and drifting buoy data near the Strait of Hormuz. Data analysis provides the water mass structure in the Strait: Persian Gulf Water (PGW) core is banked against the Omani coast, while Indian Ocean Surface Water (IOSW) lies near the Iranian coast. These water masses are most often covered by a homogeneous surface layer. Thermohaline characteristics of the PGW core decrease substantially downstream, from the Persian/Arabian Gulf to the Gulf of Oman. PGW and IOSW thermohaline characteristics and distribution also exhibit notable changes at periods shorter than a month as shown by repeated hydrological sections. The tidal signal measured south of the Strait by moored ADCP and thermistor chains has predominant semi-diurnal M2 and S2 and diurnal K1 components and possesses a complex vertical structure. Tidal intensification near the surface pycnocline is associated with noticeable internal waves. At subtidal timescale, mooring recordings confirm the water mass variability observed in the repeated hydrological sections. The mixed layer also deepens substantially during the 1-month period. Finally, trajectories of surface buoys drogued at 15 m exhibit reversals over periods characteristic of changes in wind direction.

Modelling eutrophication events in a coastal ecosystem. Sensitivity analysis

To study the eutrophication of Vilaine bay, we developed a three-dimensional hydrodynamic model and a biological model simulating phosphorus and nitrogen cycles and dissolved oxygen. To couple these models, advective currents, calculated by the hydrodynamic model, were averaged over time and space. The resulting ecological box model simulates seasonal evolution of nutrients, phytoplankton and oxygen reasonably well and reproduces the bays spatial heterogeneity. Following the calibration step, the models behaviour was analysed by means of various sensitivity studies, which led to several conclusions about eutrophication of the bay. In winter and spring, the Vilaine bay behaves like an open system enriched by the Loire plume. During the rest of the year, the bay can be considered as a closed system where the river Vilaine is the primary source of nutrients. Oxygen depletion is mostly due to consumption in the water column (plankton community respiration and remineralization). Reduction of nitrogen inputs from the river are more effective in reducing phytoplankton biomass and hypoxia than reduction in phosphorus loadings. Better knowledge of the benthic stocks of nutrients and processes controlling them is required to confirm the previous results. Spatial and temporal scales used in the model allow the simulation of seasonal trends but cannot reproduce episodic events.

Motility and autotoxicity in Karenia mikimotoi (Dinophyceae)

Karenia mikimotoi is one of the most common red-tide dinoflagellates proliferating in the eastern North Atlantic and around Japan. Kills of marine fauna are associated with its blooms. In mixed water columns it migrates vertically, while in stratified water columns, the population remains confined within pycnocline layers. Wind events, increasing mixing and agitation initiate declines in its populations. This paper is focused on the formulation of mortality rate relative to shear rate. Autotoxicity is demonstrated by the use of a synthetic toxin. Bioconvection observed in cultures allows the establishment of a trade-off between phototropism, which leads to the local accumulation of cells, and their autotoxicity, which would prevent cell concentration. The combination of these processes allows diffusion of the toxin into the underlying water, where it subsequently degrades. Confinement of the population in the pycnocline layer results also from another trade-off between growth conditions and shear-rate-modulated mortality. A simplified encounter kernel was introduced into the population dynamics equation to account for a mortality factor. Under realistic forcing conditions with a small number of parameters, this model reproduced the confinement of the population in the pycnocline layer, the proper timing and the duration of the recurrent K. mikimotoi bloom on the Ushant front (France).

Influence of the Gironde estuary outputs on cadmium concentrations in the waters: consequences on the Marennes-Oléron bay (France)

The Gironde, an estuary in Southwestern France, has been highly contaminated by cadmium for many years, and the results of the French mussel watch suggest that this contamination is not restricted to the estuary, but could be responsible for relatively high levels of Cd in the oysters from the Marennes-Oleron bay located nearby. This hypothesis has been evaluated through three cruises carried out in the Gironde estuary and plume, the Marennes-Oleron bay, and the Charente estuary, a river of minor importance that influences directly this bay. The field results from the Pertuis cruise (May 1991) and two others conducted in January and December of the same year show that the Gironde is an important source of dissolved cadmium for the bordering continental shelf (11 t year–1), while the Charente inputs are far lower (60 kg year–1). Dissolved Cd concentrations on the shelf vary according to the salinity, ranging from 0.76 nmol L–1 close to the Gironde to 0.15 nmol L–1 far from the shore. The field data as well as simulations from a 3D hydrodynamic model show that the Gironde plume reaches the Antioche Straits north of the Marennes-Oleron bay, and that the Charente plume is diluted in the bay by marine waters strongly influenced by the Gironde. This study confirms that the Gironde estuary is an important source of cadmium for the Marennes-Oleron bay.

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.

Issues About Retrieving Sea Surface Salinity in Coastal Areas From SMOS Data

This paper aims at studying the quality of the sea surface salinity (SSS) retrieved from soil moisture and ocean salinity (SMOS) data in coastal areas. These areas are characterized by strong and variable SSS gradients [several practical salinity units (psu)] on relatively small scales: the extent of river plumes is highly variable, typically at kilometric and daily scales. Monitoring this variability from SMOS measurements is particularly challenging because of their resolution (typically 30-100 km) and because of the contamination by the nearby land. A set of academic tests was conducted with a linear coastline and constant geophysical parameters, and more realistic tests were conducted over the Bay of Biscay. The bias of the retrieved SSS has been analyzed, as well as the root mean square (rms) of the bias, and the retrieved SSS compared to a numerical hydrodynamic model in the semirealistic case. The academic study showed that the Blackman apodization window provides the best compromise in terms of magnitude and fluctuations of the bias of the retrieved SSS. Whatever the type of vegetation cover, a strong negative bias, greater than 1 psu, was found when nearer than 36 km from the coast. Between 44 and 80 km, the type of vegetation cover has an impact of less than a factor 2 on the bias, and no influence further than 80 km from the coast. The semirealistic study conducted in the Bay of Biscay showed a bias over ten days lower than 0.2 psu for distances greater than 47 km, due to an averaging over various geometries (coastline orientation, swath orientation, etc.). The bias showed a weak dependence on the location of the grid point within the swath. Despite the noise on the retrieved SSS, contrasts due to the plume of the Loire River and the Gironde estuary remained detectable on ten-day averaged maps with an rms of 0.57 psu. Finally, imposing thresholds on the major axis of the measurements brought little improvement to the bias, whereas it increased the rms and could lead to strong swath restriction: a 49-km threshold on the major axis resulted in an effective swath of 800-900 km instead of 1200 km.

Modélisation numérique des crises anoxiques (malaïgues) dans la lagune de Thau (France)

Abstract In summer, the Thau lagoon is often affected by anoxic outbreaks, which induce shellfish mortality. A 3D hydrodynamic model has been set up to simulate currents driven by the wind and the sea temperature. It has been coupled to a biochemical model including oxygen, organic matter and sulphides to reproduce the ‘malaigue’ event. Firstly the model has been applied to the 1997 anoxic crisis that broke out in the Meze area. It raises the importance of wind data and the impact of shellfish farming structures that lower velocity currents. The model has also been applied to the 1982 and 1987 anoxic events, using historical data. The results point out that the ‘malaigue’ depends on the localization of the original flare up spots and the trophic status of the lagoon. Secondly, the model has been used to assess different modifications on locations of shellfish farming areas in order to find the best way of reducing the anoxic crisis. The oxygen level could be improved by extending the distance between the shellfish areas and the coast or by reducing shellfish density.