Hyperspectral sensors provide informative data in many fields related to Earth observation. On the coastal zone, the inversion of radiative transfer models of the light has shown the ability to estimate parameters characterizing the water column. Particularly the water column depth, its concentration of non-algal particles, in phytoplankton, and the bottom reflectance can be retrieved. In this paper, the ability of hyper-spectral data to estimate the parameters of the coastal zone thanks to the semi-analytical Lee model is studied according to the way the measurement is made: number of bands, impact of spectral response at each band and without any additional concepts such as probabilistic concepts.

We consider the study of performing likelihood-based inference in Lee's radiative transfer semi-analytical model. This model is widely used for the inversion of bathymetric products in coastal areas. We perform assessment of uncertainty on retrieved bathymetry in terms of hypothesis testing. Uncertainty results are compared to Monte Carlo simulations showing that inference based on the likelihood ratio statistic is much more reliable than the Wald statistic. Our results suggest that the Wald statistic based on Fisher observed information outperforms the Cramer-Rao lower bound in evaluating the estimation uncertainty. Confidence intervals derived from likelihood ratio statistic reveal the asymmetry of the maximum likelihood uncertainty arguing that variance-based uncertainties may be irrelevant for inferring uncertainty on bathymetric products.

A method is proposed to analytically propagate the measurement uncertainties of multispectral or hyperspectral satellite sensors from the top of the atmosphere to the surface as well as through the correction of specular reflections from the sun, in a maritime application context. The method is applied to the Sentinel2-MSI sensor and validated by a Monte Carlo technique. The resulting surface reflectance uncertainties can again be propagated to subsequent treatment processes such as the inversion of the radiative transfer model in the water column.

A pair of self-contained acoustic Doppler current profilers (SC-ADCPs) operating with different frequencies were moored on a muddy sea bottom at about 20 m depth in the Bay of Vilaine off the French Atlantic coast. With their acoustic beams oriented upwards, the SC-ADCPs ensonified most of the water column. The results of several months of in situ recorded echo intensity data spanning 2 years (2003 to 2004) from the dual-frequency ADCPs are presented in this paper. The aim was to estimate suspended particle mass concentration and mean size. A concentration index CI is proposed for the estimation of particle concentration. Based on theory the CI-unlike the volume backscatter strength-does not depend on particle size. Compared with in situ optical data, the CI shows reasonable precision but not increased with respect to that of the highest-frequency backscatter strength. Concerning the mean particle size, despite a lack of quantitative validation with optical particle-size measurements, the method yielded a qualitative discrimination of mineral (small) and organic (large) particles. This supports the potential of dual-frequency ADCPs to quantitatively determine particle size. A cross-calibration of the transducers of each ADCP shows that a specific component of the precision of the backscatter strength measured by ADCP depends on the acoustic frequency, the cell thickness and the ensemble integration time. Based on these results, the use of two ADCPs operating with distinctly different frequencies (two octaves apart) or a single dual-frequency ADCP is recommended.

The French Atlantic shelf is subjected to strong anthropic influences (urban, industrial and agricultural discharges) of two main rivers (Loire and Gironde).The extension and consequences of these continental loadings for the nutrient and chlorophyll distribution have never been studied before on the Bay of Biscay continental shelf as a whole.We present the first synoptic view of the nutrient distribution and evolution on the French Atlantic shelf.Nutrient concentrations of the surface layer were studied during four cruises in April, June, September 1999 and March 2000.Until June, the freshwater inputs induce a nitrate gradient from river mouths to offshore waters in the vicinity of the 100 m isobath.The Redfield's ratio study highlights the nitrate excess in river loadings.The early spring situation is characterised by high N:P ratios in front of the two estuaries and by a potential Si-limitation in the northern part.Nitrate removal continues in spite of the Plimitation and the increase in silicate concentrations during summer supposes high regeneration processes.At the end of summer, the water column is thermally stratified and the surface mixed layer is totally depleted in nitrate.

The general trend in ecosystem modelling is to improve the spatial resolution by shifting from rough box-models to fine 3D models. Despite the continuous speeding-up of computing, 3D models involving numerous state variables may remain intractable, especially for parameter calibration, when processes with long half-life periods (i.e, from years to decades) are introduced, such as the behaviour of organic matter in sediment and population dynamics of benthic species. In these cases, a first approach can be provided by fast-running box-models, if they take into account the most crucial hydrodynamic properties of the system. In a macrotidal shelf sea such as the English Channel, the long-term horizontal transport can be summarized by the tidal residual circulation, and the vertical stratification can be sketched by a two- or three-layered integral model.This paper compares the results obtained in the English Channel area by the same biogeochemical equations of pelagic primary production, coupled to 1) a two-layered box-model 2) a three-layered box-model (i.e., with an intermediate cline layer between surface and bottom ones) and 3) a fine-gridded 3D model. Comparison is focused firstly on thermal stratification and summer dinoflagellate blooms in the north-western Channel and secondly on the haline stratification and the sequence of blooms obtained in the eutrophicated Seine river plume. Comparison shows that box-models act as low-pass filters which reproduce correctly the weekly mean time-course, but greatly reduce the variance locally observed in a tide-oscillating plume region. As far as global characteristics are concerned, such as the annual primary production, or the percentage of variation in annual production after reducing the nutrient loadings, the box and 3D models gave very similar results. This conclusion reinforces the usefulness of using box-models as a first approach in long-term processes, for which a long transient phase is expected before reaching the annual periodic solution.

Among the various missions assigned to the SHOM and responding to the French Navy and public administration requests (involved in environmental affairs), one of the recent objectives is to ensure better knowledge of the continental maritime area. Among coastal projects, the one we deal in this study consists in evaluating the optical properties of shelf waters. This project called "Turbidity" focuses more particularly on turbidity processes having an impact on submarine visibility. This project is structured around three topics: (1) spatial and in-situ observations of the turbidity, (2) acoustic measurements for detection of particles loads and (3) a modelling approach. The modelling theme, presented here, involves development of simulation tools coupling hydrodynamics (the physical frame is provided by the MARS3D model developed by Ifremer), biological and sedimentary dynamics (provided by the model SiAM3D developed by Ifremer). An optical model completes this tool, in order to convert the turbidity parameters in terms of submarine visibility. The actual configuration of the model simulates the turbid waters above the French Atlantic continental shelf. The project includes also an adaptation of this framework to the Persian gulf area. The model takes into account the mineral and organic (living and non-living) particles which influence the optical properties of water. The biological production follows specific conditions of light, nutrients inputs and ocean dynamics. And the mineral particles are coming from river discharges and exchanges fluxes with the sediment (erosion and deposit). The complexity of the processes requires a modelling approach. At this complexity of particles origin, adds the complexity of the circulation of waters masses and particulate transport in coastal area (dynamic of freshwater plumes, gradients of water's density, influence of mixing by winds, tides and waves, and interactions of these processes with coastline and bathymetry). Consistent with the observations, the model reproduces reasonably well the main algal and mineral seasonal structures. The two mains axes planned to improve quantitative assessment are (1) the modelling of particulate structures and (2) the optical model: (1) modelling presents encouraging results but is a- ctually limited by a coarse spatial resolution of the model, by simplified forcing fluxes (wave, wind and sunshine) and by simplified hydro-sedimentary and biological schemes; (2) the optical module is based on empirical laws coming from literature and is limited to a monochromatic approach; however, in-situ measurements cruises are planned in order to be suited to the specific characteristics of the study areas. The difficulty of this feasibility study is based on the fact that neither in-situ measurements, nor model estimations are directly linked to the visibility parameters. In-situ, visibility is assessed with the measurement of inherent optical properties. With the model, it is deduced from the particles concentrations. Moreover, the biological and sedimentary transport models are not able to reproduce the whole nature and multitude of particles and molecules influencing the optical properties. All this makes the visibility distances difficult to assess.

Different scales of hydrological and biological patterns of the Bay of Biscay are assessed using space‐borne and airborne optical remote sensing data, field measurements and a 3‐dimensional biophysical model. If field measurements provide accurate values on the vertical dimension, ocean colour data offer frequent observations of surface biological patterns at various scales of major importance for the validation of ecosystem modelling. Although the hydro‐biological model of the continental margin reproduces the main seasonal variability of surface biomass, the optical remote sensing data have helped to identify low grid resolution, input inaccuracies and neglect of swell‐induced erosion mechanism as model limitations in shallow waters. Airborne remote sensing is used to show that satellite data and field measurements are unsuitable for comparison in the extreme case of phytoplankton blooms in patches of a few hundred metres. Vertically, the satellite observation is consistent with near surface in situ measurements as the sub‐surface chlorophyll maximum usually encountered in summer is not detected by optical remote sensing. A mean error (δC/C) of 50.5% of the chlorophyll‐a estimate in turbid waters using the SeaWiFS‐OC5 algorithm allows the quantitative use of ocean colour data by the coastal oceanographic community.

In biological modelling of the coastal phytoplankton dynamics, the light attenuation coefficient is often expressed as a function of the concentrations of chlorophyll and mineral suspended particulate matter (SPM). In order to estimate the relationship between these parameters over the continental shelf of the northern Bay of Biscay, a set of in situ data has been gathered for the period 1998–2003 when SeaWiFS imagery is available. These data comprise surface measurements of the concentrations of total SPM, chlorophyll, and irradiance profiles from which is derived the attenuation coefficient of the photosynthetically available radiation, KPAR. The performance of the IFREMER look-up table used to retrieve the chlorophyll concentration from the SeaWiFS radiance is evaluated on this new set of data. The quality of the estimated chlorophyll concentration is assessed from a comparison of the variograms of the in situ and satellite-derived chlorophyll concentrations. Once the chlorophyll concentration is determined, the non living SPM, which is defined as the SPM not related to the dead or alive endogenous phytoplankton, is estimated from the radiance at 555 nm by inverting a semi-analytic model. This method provides realistic estimations of concentrations of chlorophyll and SPM over the continental shelf all over the year. Finally, a relationship, based on non living SPM and chlorophyll, is proposed to estimate KPAR on the continental shelf of the Bay of Biscay. The same formula is applied to non living SPM and chlorophyll concentrations, observed in situ or derived from SeaWiFS radiance.

In order to study the occurrence of Gymnodinium mikimotoï bloom on the French Atlantic coast, a three-dimensional model of primary production was set up. The biological model was coupled to a hydrodynamic model previously developed at Ifremer, by the intermediary of transport, diffusion and heat fluxes. The cycles of three limiting elements for the phytoplankton growth were modelised: nitrogen, phosphorus and silicon. The second phase of the study aimed at reproducing the summer dinoflagellate blooms. In this new approach a model of the species Gymnodinium mikimotoï was coupled with the previous biomass model which provides the physical and chemical environments. The study focuses on the results obtained by this specific sub-model. The parametrisation was based on some physiological studies reported from laboratory cultures and on parameters existing in the literature. Consistent with the observations, the model reproduces the sub-surface cell concentrations, in the zone of minimal turbulence. The formulation retained for the temperature effect on the growth rate leads to a strong influence of temperature on the onset of the bloom and the mortality rate controls the vertical distribution. Nevertheless, as formulated here, the model underestimates the cell density.