Sylvain Ouillon

Coupling satellite data with in situ measurements and numerical modeling to study fine suspended-sediment transport: a study for the lagoon of New Caledonia

This paper investigates the potential of remotely sensed data to map turbidity in a coral reef lagoon and to calibrate a numerical model of fine suspended-sediment transport. Simultaneous measurements of turbidity depth-profile and above-water spectral reflectance integrated according Landsat 7 ETM+ band 2 spectral sensitivity provide a linear regression relationship for the southwest lagoon of New Caledonia (r2=0.95, n=40). This relationship is applied to an empirically atmospherically corrected Landsat ETM+ image of the lagoon acquired on October 23, 2002. A comparison between Landsat estimates of turbidity and concurrent measurements at 14 stations indicates that the mean standard error in the satellite-estimated turbidity is 17.5%. The numerical model introduced in Douillet et al. (2001) is used to simulate the transport of fine suspended sediments in the lagoon in October 2002. A calibration of the erosion rate coefficient required by the model is proposed using in situ turbidity profiles and the remotely sensed turbidity field. In situ data are used to tune locally the erosion rate coefficient, while satellite data are used to determine its spatial zonation. We discuss necessary improvements in coupled studies of fine-sediment transport in coastal zones, namely relationships between turbidity and sediment concentration, integration of wave influence in the model, and correction of bottom reflection in satellite data processing.

Optical algorithms at satellite wavelengths for Total Suspended Matter in tropical coastal waters

Is it possible to derive accurately Total Suspended Matter concentration or its proxy, turbidity, from remote sensing data in tropical coastal lagoon waters? To investigate this question, hyperspectral remote sensing reflectance, turbidity and chlorophyll pigment concentration were measured in three coral reef lagoons. The three sites enabled us to get data over very diverse environments: oligotrophic and sediment-poor waters in the southwest lagoon of New Caledonia, eutrophic waters in the Cienfuegos Bay (Cuba), and sediment-rich waters in the Laucala Bay (Fiji). In this paper, optical algorithms for turbidity are presented per site based on 113 stations in New Caledonia, 24 stations in Cuba and 56 stations in Fiji. Empirical algorithms are tested at satellite wavebands useful to coastal applications. Global algorithms are also derived for the merged data set (193 stations). The performances of global and local regression algorithms are compared. The best one-band algorithms on all the measurements are obtained at 681 nm using either a polynomial or a power model. The best two-band algorithms are obtained with R412/R620, R443/R670 and R510/R681. Two three-band algorithms based on Rrs620.Rrs681/Rrs412 and Rrs620.Rrs681/Rrs510 also give fair regression statistics. Finally, we propose a global algorithm based on one or three bands: turbidity is first calculated from Rrs681 and then, if < 1 FTU, it is recalculated using an algorithm based on Rrs620.Rrs681/Rrs412. On our data set, this algorithm is suitable for the 0.2-25 FTU turbidity range and for the three sites sampled (mean bias: 3.6 %, rms: 35%, mean quadratic error: 1.4 FTU). This shows that defining global empirical turbidity algorithms in tropical coastal waters is at reach.

Inversion of reflectance spectra of nonchlorophyllous turbid coastal waters

Abstract This article deals with the inversion of reflectance spectra measured in the Rhone River mouth area during a flood event in terms of parameters characterizing the turbid water components. The optically active constituents considered are the total suspended matter, mainly composed of sediment, of known concentration ranging from 3.7 mg L −1 to 234 mg L −1 , and yellow substance. A simple reflectance model is presented. Scattering is treated by Mie theory applied to a collection of particles of real refractive index m p and obeying a Junge size distribution law. A sensitivity study performed on the model shows that both concentration and refractive index and both cutoff particle diameters of the Junge size distribution law cannot be accurately determined from a given reflectance spectrum. Two model parameters, among a total of six, are estimated from model inversion: m p and a parameter characterizing the yellow substance absorption spectrum, a y0 =a y (440 nm). The derived values are realistic as compared to the values that can be found in literature. Other combinations of parameters could be determined by inversion but measurement of m p and a y0 from reflectance spectra already has interesting applications in a satellite remote sensing perspective. Increasing the number of parameters of the model is feasable, but much work is needed to envisage the inverse problem.

Circulation and suspended sediment transport in a coral reef lagoon: The south-west lagoon of New Caledonia

The south-west lagoon of New Caledonia is a wide semi-open coral reef lagoon bounded by an intertidal barrier reef and bisected by numerous deep inlets. This paper synthesizes findings from the 2000-2008 French National Program EC2CO-PNEC relative to the circulation and the transport of suspended particles in this lagoon. Numerical model development (hydrodynamic, fine suspended sediment transport, wind-wave, small-scale atmospheric circulation) allowed the determination of circulation patterns in the lagoon and the charting of residence time, the later of which has been recently used in a series of ecological studies. Topical studies based on field measurements permitted the parameterisation of wave set-up induced by the swell breaking on the reef barrier and the validation of a wind-wave model in a fetch-limited environment. The analysis of spatial and temporal variability of suspended matter concentration over short and long time-scales, the measurement of grain size distribution and the density of suspended matter (1.27 kg l(-1)), and the estimation of erodibility of heterogeneous (sand/mud, terrigenous/biogenic) soft bottoms was also conducted. Aggregates were shown to be more abundant near or around reefs and a possible biological influence on this aggregation is discussed. Optical measurements enabled the quantification of suspended matter either in situ (monochromatic measurements) or remotely (surface spectral measurements and satellite observations) and provided indirect calibration and validation of a suspended sediment transport model. The processes that warrant further investigation in order to improve our knowledge of circulation and suspended sediment transport in the New Caledonia lagoon as well as in other coral reef areas are discussed, as are the relevance and reliability of the numerical models for this endeavour.

Surface suspended matter off the Rhone river mouth from visible satellite imagery

Abstract The quantification of surface Total Suspended Matter (TSM) off the Rhone river mouth is investigated using SPOT-XS and Landsat-TM images collected in November 1994. The method of satellite data calibration from in situ measurements is discussed. It is based on TSM concentration measurements through filtration of surface water samples and on radiometric data collected with a 256-channel spectroradiometer in the range 400–1100 nm. Empirical relationships between reflectance and TSM concentration are established by linear regression analysis of the data. These relationships are then applied to the image data corrected from atmospheric and surface effects. A good agreement, within a factor of less than two, was found on derived TSM concentration values as compared with sea truth data. The results for three different river flows are presented: a quasi-normal river discharge, a flood situation and an intermediate situation. The TSM corresponding concentration maps are presented and discussed.

Suspended matter dispersion in the Ebro ROFI: an integrated approach

Abstract River discharges represent the main source of telluric freshwater and suspended matter in coastal areas. Spatio-temporal complex mechanisms of exchanges in the river plume are investigated for successively highly stratified and mixed situations, using in situ measurements, satellite images and numerical modelling. The study, as part of the “fluxes across narrow shelves” research project, is focused on the Ebro river plume (Spain). Three specific situations encountered in the Ebro site, and for which in situ data, satellite images and numerical simulations are available, have been studied. Depending on the river flow and wind conditions, three-dimensional salinity, temperature and suspended matter fields exhibit weak to strong horizontal and vertical diffusion patterns leading to a challenging case for modelling. Occurrence of fronts at the plume boundaries is shown to inhibit exchanges near the river mouth but to enhance the region of mixing. Association of this latter process with wind effect influences the space scale of mixing and determines the occurrence of fluxes across the shelf break.

Phytoplankton distribution and productivity in a highly turbid, tropical coastal system (Bach Dang Estuary, Vietnam)

Phytoplankton diversity, primary and bacterial production, nutrients and metallic contaminants were measured during the wet season (July) and dry season (March) in the Bach Dang Estuary, a sub-estuary of the Red River system, Northern Vietnam. Using canonical correspondence analysis we show that phytoplankton community structure is potentially influenced by both organometallic species (Hg and Sn) and inorganic metal (Hg) concentrations. During March, dissolved methylmercury and inorganic mercury were important factors for determining phytoplankton community composition at most of the stations. In contrast, during July, low salinity phytoplankton community composition was associated with particulate methylmercury concentrations, whereas phytoplankton community composition in the higher salinity stations was more related to dissolved inorganic mercury and dissolved mono and tributyltin concentrations. These results highlight the importance of taking into account factors other than light and nutrients, such as eco-toxic heavy metals, in understanding phytoplankton diversity and activity in estuarine ecosystems.

Inherent optical properties and satellite retrieval of chlorophyll concentration in the lagoon and open ocean waters of New Caledonia

The retrieval of chlorophyll-a concentration from remote sensing reflectance (Rrs) data was tested with the NASA OC4v4 algorithm on the inner New Caledonian lagoon (Case 2) and adjacent open ocean (Case 1) waters. The input to OC4v4 was Rrs measured in situ or modeled from waters inherent optical properties (2001-2007). At open ocean stations, backscattering and absorption coefficients were correlated with chlorophyll (R(2)=0.31-0.51, respectively), in agreement with models for Case 1 waters. Taking spectrofluorometric measurement as reference, the OC4v4 model leads to an average underestimation of 33% of the chlorophyll concentration. For the lagoon waters, OC4v4 performed inadequately because the backscattering coefficient, highly correlated with turbidity and suspended matter (R(2)=0.98), was poorly correlated to chlorophyll (R(2)=0.42). The OC4v4 performance was better in deep lagoon waters for stations with a TDT index (Tchla x depth/turbidity) higher than 19 mg m(-2) NTU(-1) (R(2)=0.974, bias=10.2%). Global Imager Rrs provided a good estimate of Tchla (R(2)=0.79, N=28) in the deeper part of the lagoon.

Bio-optical properties of the marine cyanobacteria Trichodesmium spp.

Bio-optical spectral properties were determined on fresh suspensions of Trichodesmium spp. collected in a tropical lagoon and put in seawater tanks (total chlorophyll concentrations range between 0.1 and 3.8 mg m -3). The spectrum of the backscattering coefficient was a hyperbolic function with a slope of 1.2, often showing troughs at 440, 550 and 676 nm, due to absorption peaks of chlorophyll and phycoerythrin. The absorption spectrum computed with a specific beta correction for Trichodesmium, showed a blue to red ratio (B/R) equivalent to the one of a single colony (B/R=2), and also showed the double peak of mycosporine-like amino acids (MAAs, 330 and 360 nm). The CDOM absorption spectrum showed minor MAA peaks when cyanobacterial concentrations were above 1 mg Chl a m -3. The chlorophyll a-specific backscattering and absorption coefficients at 442 nm were respectively 0.0126 m 2 (mg.chl a) -1 and 0.027 m 2 (mg chl a) -1. Suspensions in tanks exhibited a high backscattering ratio at 660 nm (b˜ b=b bp/b p). The above-water reflectance spectrum clearly showed troughs at the wavelength of the pigment absorption peaks. Datasets of Trichodesmium normalized absorption, backscattering and reflectance spectra will allow its detection with future hyperspectral ocean colour sensors.

A three-component model of ocean color and its application in the Ebro River Mouth Area

Abstract A three-component reflectance model that takes into account the contributions of dissolved organic matter, phytoplankton, and mineral particles is described. The principle of inversion of the model, in terms of chlorophyll a plus phaeopigments concentration, chl , sediment refractive index, m r , and absorption coefficient of yellow substance at 440 nm, a y (440), is exposed. The inversion procedure is applied to experimental data collected in the Ebro river plume area. The obtained estimates of chl are accurate only in order of magnitude. However, we demonstrate that m r and a y (440) determinations are robust against chl variations within −50%, +100% of experimental values. Investigation of possible variations with wavelength of m r , using measured concentrations and a y (440) estimates, evidences a slight dependency as was previously observed. We support in conclusion that the inversion procedure could be exploited in a two-step process. First a y (440) and the mean value of m r are estimated from measured water quality parameters. Then m r is calculated at each wavelength, considering a y (440) value determined in the first step.