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Mudflats and mud suspension observed from satellite data in French Guiana
Marine Geology 208 (2004) 153 – 168 www.elsevier.com/locate/margeo
Mudflats and mud suspension observed from satellite data in French Guiana J.M. Froidefond a,*, F. Lahet b, C. Hu c, D. Doxaran a, D. Guiral b, M.T. Prost d, J.-F. Ternon b a
De´partement de Ge´ologie et Oce´anographie (UMR EPOC), Universite´ Bordeaux-1, Avenue des Facultes 33405, Talence Cedex, France b Institut de Recherche pour le De´veloppement (IRD) BP 165-97323, Cayenne Ce´dex, France c Institute for Marine Remote Sensing (IMaRS), College of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701, USA d Museu Paraense Emı´lio Goeldi, Campus de Pesquisa, Av. Perimetral, Terra Firme, Belem, Para, Brazil Received 8 July 2002; accepted 14 April 2004
Abstract The littoral of French Guiana is characterized by the north-westward migration of large mud banks along shore and by high suspended particulate matter concentrations (SPMC) coming from the Amazon River. A correspondence function is established between in situ optical data and SPMC measurements obtained from three coastal surveys on the Kaw River, the Mahury estuary, and the adjacent continental shelf, respectively. This function is applied to SPOT satellite images to estimate SPMC distributions near the sea surface on five dates between 1998 and 2001. Two typical situations have been observed: the mud suspensions are generally confined near the coast by a strong north-westward coastal current, while in one case (SPOT scene of 2 July 2001) the turbid plumes, probably resulted from the wave action on the mud banks, are directed seaward. SPMC varies from more than 300 mg l 1 adjacent to the intertidal zone to a few mg l 1 offshore. Turbid plumes are observed downstream, with regard to the Guyana Current, of small islands along the coast. Mud undulations are observed on the emergent surface of the mudflats, parallel to the crest waves and characterized by a wavelength of 200 – 1000 m. These structures are interpreted to form by wave action on the soft mudflat surface. Mangrove colonization of the mudflat follows these primary topographic structures. D 2004 Elsevier B.V. All rights reserved. Keywords: mud bank; suspended particulate matter; remote sensing; Amazon River; French Guiana
1. Introduction
* Corresponding author. Tel.: +33-5-56-84-60-00; fax: +33-556-84-08-48. E-mail addresses: [email protected] (J.M. Froidefond), [email protected] (F. Lahet), [email protected] (C. Hu), [email protected] (D. Guiral), [email protected] (M.T. Prost), [email protected] (J.-F. Ternon). 0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2004.04.025
The French Guiana is strongly influenced by fresh and turbid Amazonian outflows. Immense argillaceous suspensions (1.1 – 1.3 10 9 tons year 1 , Meade et al., 1985) are discharged at the mouth of the Amazon River. Of this total, 6.3 F 2 108 tons year 1 are deposited on the continental shelf close to the Amazon River mouth (Kuehl et al., 1986, Nit-
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trouer and DeMaster, 1986), while about 1.5 108 tons year 1 are transported in suspension, and about 1 108 tons year 1 move along Guyanas in the form of mud banks (Eisma et al., 1991). The Guyana Current, an extension to the North Equatorial current off Brazil (Muller-Karger et al., 1988), goes along the coast from southeast to northwest. In 1961, during an aerial survey, the Delft Hydraulics Laboratory (DHL, 1962) observed 21 mudflats along the coast between the Waini River in Guyana and Cayenne in French Guiana. Initial mud bank development took place at Cabo Cassipore (Allison et al., 2000). The average ‘‘wavelength’’ was approximately 45 km and varied between 30 and 60 km. The extensive mudflats are overgrown with mangroves at the highest levels. These mudflats are the emerged parts of mud banks (NEDECO, 1968, Augustinus, 1978). A vertical sequence of massive and laminated mud with discontinuity features is present in almost all cores taken and analysed by Rine and Ginsburg (1985) from the coastal mud banks of Surinam. Comparison of successive sounding maps shows that mud banks slowly move from east to west. The mean celerity was calculated to be 1.5 km year 1 (NEDECO, 1968). The mechanism of propagation of mud banks is a topic of debate. Wells and Coleman (1981) proposed two hypotheses. The first, which they prefer, is the action of the swells that suspends the sediments and transports them along the coast. The second hypothesis is based on observations showing the ‘‘transport en masse’’ of mud by flow sliding (Wells et al., 1980). But the existence of the Guyana Current can also be a good candidate to explain the mud bank migration. The objective of this study is to quantify and to map the suspended particulate matter concentrations (SPMC) from satellite images and to help interpret how the Guyana coastal mud banks are formed and how they migrate along the coast. In fact, these mud suspensions are marker of surface currents because of the very weak settling velocity of sediments (LCHF, 1978). Quantification of SPMC is also useful to design dredging strategies on navigational channels and to estimate the fluvial solid discharges to the ocean. Moreover, high SPMC has an inhibitive effect on primary production (due to light limitation) in coastal waters and, its estimation is essential in understanding the biological mechanisms.
Below, we first describe the area by visualization of a satellite image. Then, empirical regression functions are derived based on field measurements to estimate SPMC from satellite data. These functions are then used to estimate SPMC distributions and to estimate transport rates.
2. Study area The study area is located in French Guiana between Cayenne and the mouth of the Approuague River. Fig. 1 shows the area from a SPOT-4 image recorded on 2 July 2001. From west to east, the area includes the peninsula of Cayenne, the mouth of the Mahury River, the littoral swamp of Kaw, crossed by the Kaw River, and the mouth of the Approuague River. Along this coast, the displacement of mud banks was known before 1875, mentioned in technical reports of marine officers (Lemiere, 1953) and preserved in the archives of the French Hydrographic Survey. These mud banks were described in these reports in the following way: they are constituted by very soft mud on which the sea is always quiet; the western part of these banks is generally steep while the oriental part is sloping gently. These banks move slowly westward on a bottom constituted by hard mud. The commander Yayer (1948), during his hydrographic survey in 1936 –1937, also indicated the passage of soft mud banks. During this mission, the hard bottom was sounded by means of a 15-kg lead, and the thickness of the soft mud was measured with a perch. He noted that the thickness of the layer of soft mud was most frequently 1 m. Intertidal mudflats are quickly covered with mangroves. Then, studies were carried out between 1955 and 1978 (LCHF, 1978) for the access to the harbour of Cayenne (on the Mahury River) and for a project of exploitation of bauxite on the mountain of Kaw. Between 1978 and 1984, the coast of French Guiana was bordered by six major mudflats ranging from 20 to 40 km in length and migrating along the coast to the west-northwest towards Surinam, at rates ranging from 0.32 to 1.22 km year 1 with a mean rate of 0.9 km year 1 (Froidefond et al., 1988). A comparable study of the location of the mudflats between 1955 and 1984 reveals a more rapid displacement. Between 1984 and 1988, a mud bank reached the coast of
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Fig. 1. The study area composed of the southeast part of the French Guiana, Cayenne, the main rivers, the emergent parts (mudflats) of the mud banks, and their turbid plumes (SPOT scene, 02 July 2002).
Cayenne. Sandy beaches separated by rocky caps characterizing this coast were gradually silted up. Then, the mangrove swamp developed in front of the coast masking completely the sight on the ocean. Generally, these beaches are associated with cheniers (coastal ridges) on the west part of the French Guiana coast (Prost, 1989). At present, a new mud bank is located in front of Cayenne, and another bank is located to the east of the swamp of Kaw. The morphology of this coast and the turbid plumes are well visible on satellite images (Fig. 1). Typically, the emerged part of the mud bank, called ‘‘vasie`re’’ (mudflat), is attached to the coast. The Kaw River, at the center of the study zone, is located between two mud banks of Approuague on the east and of Cayenne on the west.
The average angle between the crests of the mud banks like ridges and the coast varies between 20j and 30j, with an average of 24j. Generally, the mudflats and the intermediate troughs have a gentle slope which might be less than 1:3000 with the exception of the northwest flank of the mudflats, which has a relatively steep slope, 1:500 (NEDECO, 1968; Allison et al., 2000). Fluid mud occurs most commonly on the lee side or western flank of the mud shoals (NEDECO, 1968). Soft tidal flat deposits are typically 1 to 2 m thick and overlie a firm clay base. Sedimentation of mud leads to the formation of slingmud, a very dense suspension of sediments. The sedimentologic measurements made on suspended mud in the Mahury mouth indicate a median grain size of 1– 2 Am, originated from the Amazon (Parra
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and Pujos, 1998). In the soft mud, the water content can reach from 150% to 200%, with concentrations of 400 to 500 g l 1 (LCHF, 1978). The clay minerals are typically composed of Illite, 37%; kaolinite, 27%; smectite, 20%; and chlorite, 16% (Jouanneau and Pujos, 1987). Average current speed is reported to be 0.3– 0.6 m s 1 (NEDECO, 1968). The tide currents increase the current in flow (0.8 – 0.9 m s 1) and decrease during the ebb (0.25 m s 1) with a low deviation toward the coast during the flow and offshore during the ebb. On the Cayenne mud bank (Pujos and Froidefond, 1995), in October 1988, the currents at the surface and at the bottom were towards the west at flood (260 –270j) and towards the northwest at ebb (300 – 320j). Their surface velocities (0.15 – 0.25 m s 1) increase due to the sea breeze in the morning (0.4 – 0.6 m s 1). SPMC at sea surface was 100 –300 mg l 1 in the morning and greater than 500 mg l 1 in the afternoon, when the sea is wavy. Tides are semidiurnal and also synchronous along the entire Guiana Coast (Allersma, 1971). Two high tides and two low tides of different height occur each day in 25 h 50 min along the coast of French Guiana. The tidal range, during spring tide, is about 2.5 m (SHOM, 1975). Waters are especially muddy at low tide. Samples reveal that SPMC begins to increase near midtide, reach a maximum at low tide, and then decrease to the following high tide (Wells and Coleman, 1981). Waves on the shelf are typically 1 –2 m high but sometimes (< 1% of the year) can exceed 4 m (NEDECO, 1968). The highest waves coincide with the periods of the strongest winds (February – May), whereas the lowest waves coincide with periods of weakest winds (June –November). Average period is 6– 8 s, with swell from North Atlantic storms occasionally exceeding 15 s. Historical data show that sea and swell arrive from the northeast quadrant during all months of the year (with 50% arriving from N50jE to N70jE, NEDECO, 1968). Examination of time series records (Wells and Coleman, 1981) indicates that waves are deformed and solitary-like wave profiles result. Solitary waves are characterized descriptively as isolated crests separated by flat ‘‘troughs’’ lying at still-water level. Suspended concentrations are highest in fluid mud regions, where solitary waves occur. This suggests that a tremendous potential exists for sedi-
ment transport by wave acting alone (Wells and Coleman, 1981).
3. Methods Different studies show the possibility of quantifying SPMC from water color (Whitlock et al., 1981; Curran and Novo, 1988). Reflectance spectra vary in terms of two inherent optical properties: total absorption coefficient, a(k), and total back-scattering coefficient, bb(k) (Prieur and Sathyendranath, 1981, Gordon et al., 1988). The two coefficients, a(k) and bb(k), vary as a function of the concentration of mineral particles, phytoplankton (chlorophyll-a), and dissolved organic matter (yellow substances). Remote sensing reflectance, Rrs(k) (sr 1), is defined as the ratio of water-leaving radiance (Lw) to downwelling irradiance (Ed): RrsðkÞ ¼ Lw ðkÞ=Ed ðkÞ
ð1Þ
(Mobley, 1999). For coastal turbid waters, Rrs(k) can be expressed as (Doxaran et al., 2002) RrsðkÞc0:529
f ½bb ðkÞ=ðaðkÞ þ bb ðkÞÞ Q
ð2Þ
where f/Q (sr 1) is a factor related to the solarviewing geometry and particle-scattering phase function. It is very difficult to measure the f/Q ratio directly, particularly because the sky light reflection on the sea surface. In situ measurements are often used to obtain empirical relationships between particle concentrations and reflectance at selected wavelength (Froidefond et al., 1999; Doxaran et al., 2002). These relationships may then be used to quantify phytoplankton and mineral particles from satellite data after the atmospheric effects are corrected (Sathyendranath et al., 2000). In coastal waters and estuaries, characterized by high mineral particles concentrations, such as in this study, the phytoplankton and yellow substance contributions can be ignored in a first approximation. SPOT satellite images are used in this study, because the spatial coverage (60 60 km) and pixel resolution (20 20 m) are suitable for biologic and
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sedimentologic studies. The SPOT data cover three to four spectral channels (http://spot4.cnes.fr/): Band XS1, 500 –590 nm; Band XS2, 610 – 680 nm; Band XS3, 780 – 890 nm (near infrared, NIR); Band XS4 1580 –1750 nm (middle infra red, MIR). The method to describe SPMC of surface waters from SPOT images was based on a four-step empirical approach: (1) field sampling of the study area to obtain Rrs spectra and SPMC; (2) regression between SPMC and Rrs; (3) atmospheric correction of the satellite data to obtain Rrs at sea level for the area; and (4) application of the regression relationship to the satellite Rrs data to obtain SPMC distribution maps. Two assumptions are inherent in the process. First, field and satellite measurements were conducted under similar solar illumination and viewing conditions, although they were collected on different dates. Second, the mineralogical composition and the particle size of suspended sediments did not vary significantly between the time of field and satellite measurements. 3.1. Field measurements In situ measurements were collected in the coastal waters of French Guiana during three field campaigns in November 1998 (Mahury River), April 1999 (con-
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tinental shelf between Cayenne and the Oyapock River) and November – December 2000 (Kaw River and Mahury River). The procedures used to collect the Rrs data for these field campaigns have been described in detail (Froidefond et al., 2002). Briefly, a Spectron SE-590 spectroradiometer was used to measure the water-leaving radiance (Lw) and the irradiance (Ed) from a standard reflectance plate. Rrs is then obtained as Eq. (1): Rrs(k) (sr 1) = Lw(k)/Ed(k). Measurement protocols are regularly updated. That of Mobley (1999) is now recommended. Concurrent with the optical measurements (within 1 – 2 min), water samples were collected at approximately 30 cm beneath the sea surface. Total SPMC was measured from sample filtration through membranes of 0.47 Am porosity. The measured reflectance spectra are shown in Fig. 2. At a given wavelength, spectral amplitudes clearly increase with SPMC. The reflectance spectra exhibit the same overall shape and, in particular, maxima and shoulders in the vicinity of 580, 650, 690, and 810 nm, respectively. These spectra are similar to those measured at the Rhoˆne River mouth during a flood event (Forget and Ouillon, 1998) or in the Gironde estuary (Doxaran et al., 2002). The emergent mudflat presents similar spectra excepted in the NIR wavelength where the Rrs values are clearly higher (Fig. 2).
Fig. 2. Reflectance spectra measured on the coastal waters of French Guiana in November 1998 (Mahury River), April 1999 (continental shelf), and November – December 2000 (Kaw River and Mahury River). The spectrum of the emerged mudflat is different, particularly in the near infrared part.
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3.2. Relationship between satellite-equivalent reflectance and SPMC Rrs spectra from field measurements were used to compute the Rrs in the SPOT channels with the following equation: Z
kmax
Lw ðkÞ SXSi ðkÞ
RrsXSi ¼ Zkmin kmax
dk
November 2000, respectively. With the channel XS1, the fit equation (SPMC = 2.78[exp(131.24 RrsXS1)]) 2 has a regression coefficient R = 0.83. With XS3, the fit equation (SPMC = 7211 RrsXS3 + 14.25) has a regression coefficient equal to 0.61, and we found that channel XS2 is the most sensitive to changes in SPMC, with a regression coefficient R2 = 0.87 (Fig. 4) where SPMC ranges between 3.5 and 500 mg l 1.
ð3Þ Ed ðkÞ SXSi ðkÞ
dk
SPMC ¼ 3:4707½expð109:31
RrsXS2Þ
R2 ¼ 0:87
ð4Þ
kmin
where i is equal to 1, 2, or 3 and stands for the SPOTHRV channels, RrsXSi(k) is for channel XSi, and SXSi(k) the spectral sensitivity (Fig. 3). Spectral sensitivity data can be obtained from either the header file of each SPOT-HRV image or SPOT-HRV technical manual (CNES, 1986). A test of the November 2000 Rrs data for the two sensitivity data sets shows that the difference is < 5%. In this study, we use the typical sensitivity data (CNES, 1986) for data processing. The Rrs-SPMC regression analysis for each SPOT channel was carried out on 32 data pairs, of which, 10 were obtained on the continental shelf in April 1999, 10 were obtained on the Kaw River, and 12 on the Mahury River and the Cayenne River in November 1998 and in
This function (Eq. (4)) is used to calculate the SPMC distributions from the SPOT images. 3.3. Satellite data processing and atmospheric corrections Five SPOT scenes Level-2A (radiometrically and geometrically corrected) were acquired for the period between June 1998 and December 2001. The characteristics of these scenes are listed in Table 1 (the incident angle is the angle between the vertical at the centre of the image and the vector to the satellite). Four spectral channels are available except for the June 1998 SPOT scene. Because of the atmospheric attenuation (absorption and scattering), the total signal
Fig. 3. Spectral sensitivities of XS1, XS2, and XS3 channels of SPOT-HRV sensor. Values deduced from the leader file of the 5 December 2000 image are represented with continuous lines. Typical values given by SPOT-HRV technical manual (CNES, 1986) are represented by crosses, triangles, and rounds.
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Fig. 4. Correspondence function used to quantify SPMC from SPOT-XS2 data.
(radiance) received by the SPOT sensor, Lt, is (Gordon and Wang, 1994): Lt ¼ Lr þ La þ tLw ;
ð5Þ
-where Lr is due to molecular (Rayleigh) scattering, La is from aerosol scattering (including aerosol– Rayleigh interactions), Lw is the water-leaving radiance to be derived, and t is the diffuse transmittance to propagate Lw to the sensor. For simplicity, the wavelength dependence of each term is omitted. Several other factors that affect the total signal, including the sun glint, whitecaps, ozone, and other gaseous absorption, are not considered because they can be either avoided or corrected. The goal of the atmospheric correction is to retrieve Lw(k) from Lt(k). Because of the complexity of the atmosphere, the limited number of spectral channels on SPOT, and the large signal from the highly turbid environment, it is neither realistic nor necessary to use a sophisticated real-time atmospher-
ic correction scheme with the precision of SeaWiFS images (Hu et al., 2001). Numerical models (MODTRAN, LOWTRAN, or 6S) are often used to estimate the atmospheric effects. However, the aerosol properties must be known a priori to use the models. Unfortunately, these meteorological data are not available in French Guiana. Hence, an empirical method similar to that of Smith and Milton (1999) was used. It is based on in situ reflectances measured from a number of calibration targets. The targets are several times the pixel size with uniform colors (reflectances). For each SPOT channels (XS1, XS2, and XS3), the corresponding digital numbers (DN) from the satellite sensor are compared with the target reflectances, and a regression relationship is derived. Five types of targets were measured in the field and selected for the regression analysis (Table 2): sand, laterite, fine gravels, mud (mudflat), and offshore water. Fig. 5 shows the regression between in situ Rrs and SPOT DN for image XS2 (2 July 2001) used in SPMC calculations. The regression
Table 1 Characteristics of SPOT scenes and corresponding time of low tides Date
Time (UTC)
1998/06/20 1999/08/15 2000/12/05 2001/12/14 2001/07/02
13 14 14 14 13
h h h h h
57 00 09 16 49
min min min min min
41 12 08 02 38
s s s s s
Number of bands
Incidence angle
Sun elevation
Low tide (UTC)
3 4 4 4 4
9.7j 9.0j 15.8j 28.9j 24.8j
61.0j 65.0j 57.9j 57.5j 59.2j
11 h 12 min 17 h 07 min 8 h 58 min 12 h 14 min 13 h 57 min
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Table 2 In situ reflectances of the targets Reference areas (targets)
Dry sands, beach of Montjoly Sea surface (offshore) Mudflat (Pointe Mahury) Laterite (Crique Fouille´e) Fine gravels (Degrad harbour)
4. Results and discussion In situ reflectances Rrs (sr 1) Rrs XS1 0.0668 0.0190 0.0318 0.0445 0.0764
Rrs XS2 0.0986 0.0095 0.0372 0.0859 0.0828
4.1. Distribution of SPMC on the eastern French Guiana Coast Rrs XS3 0.1305 0.0006 0.0531 0.0954 0.0891
coefficients (R2) are 0.98, 0.95, 0.99, 0.90, and 0.97 for 20 June 1998, 15 August 1999, 5 December 2000, 2 June 2001, and 14 December 2001, respectively. These regression equations were then applied to the SPOT-XS2 image to derive Rrs and SPMC distributions. 3.4. Uncertainties in the SPMC estimations The SPMC estimates from the satellite images have uncertainties associated with (1) errors in the optical measurements, which may reach up to 20%; (2) 1- to 2-min difference between in situ SPMC and optical measurements; (3) limited number of samples used in the regression between Rrs and SPMC; (4) limited number of data points in the regression between SPOT DN and Rrs; and (5) the assumption that the atmosphere is homogenous across the whole image, which may also lead to additional errors. Considering all these factors, SPMC estimates are possibly with uncertainties of 25% to 35%.
The SPMC map obtained from the XS2 image of 2 July 2001 is shown in Fig. 6A. The image was taken just at low tide at the beginning of the flood phase (low tide: 12:10 UT; tide range: 2 m). Two large turbid plumes are associated with the Approuague mud bank near the Approuague River mouth and with the Cayenne mud bank. SPMC varies between >450 mg l 1 near the emergent mudflat and 50 mg l 1 offshore. These turbid plumes are weakly diverted north-westward in the direction of the Guyana Current. Mud banks seem to be the source of these turbid plumes that extend 15 – 20 km offshore. At the Mahury River mouth, a turbidity maximum appears approximately 5 km inland from the coast. Upriver, turbidities are reduced, likely due to the sedimentpoor nature of these rivers that drain the humid forest (Jouanneau and Pujos, 1987). However, the low turbidity river waters are generally dark-brown because of the high concentrations of coloured dissolved organic carbon (yellow substance), potentially altering the SPMC calibration for these inland areas. Similar to the 2 July image, the 14 December 2001 image (Fig. 6B) was recorded at the beginning of the flood tide (low tide: 13:58 UT; tidal range: 2.4 m). SPMC near the coast is higher than those found on 2 July. A wide band (5 –10 km) of turbidities (SPMC
Fig. 5. Example of regression function used with the target method to obtain a SPOT-XS2 atmospherically corrected image.
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Fig. 6. (A) Distribution of SPMC at sea surface from the SPOT-XS2 image of 02 July 2001. (B) Distribution of SPMC at sea surface from the SPOT-XS2 image of 14 December 2001.
between 350 and 450 mg l 1) extends along the coast. Turbid plumes in the form of draperies, stretched out in the orientation of the Guyana current, are visible with SPMC ranging from 50 to 100 mg l 1. In the north, at 35 km from the coast, the turbidity is equal or
lower than 20 mg l 1. Turbid coastal waters penetrate into the mouths of the Mahury and Kaw rivers, because river discharges are low in this season. The contrast between 2 July and 14 December images is probably due to the stronger Guyana current.
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Fig. 7. Turbid plumes behind small islands in front of Cayenne (SPOT-XS2, 20 June 1998). SPMC varies between 1 and 60 mg l 1.
This current seasonality is also synchronous with trade wind-driven wave seasonality. Along the Suriname coast, NEDECO (1968) also observed a seasonal reduction in SPMC towards the open sea; in the trade winds season (e.g., December – February, average current f 0.5 m s 1), the concentrations were 400 mg l 1 near the coast, 50 mg l 1 at 20 km offshore, and 10 mg l 1 at 30 km offshore, whereas during the calm season (e.g., May – September, average current ~0.2 m.s 1), they were no more than 150, 12, and, 1– 2 mg l 1, respectively. Unfortunately, geophysical data (currents, waves, winds) during these satellite acquisitions were inexistent to confirm these hydrodynamic processes. Wind data are available from the Rochambeau airport (Cayenne) located a few kilometres away from the coast. Hence, wind speed and direction cannot be considered representative of wind at sea. Moreover, due to the lack of wave recorder, no in situ measurements were available to explain wave structures observed on the SPOT images. SPMC map derived from the 20 June 1998 SPOT data is shown in Fig. 7, where turbid plumes were observed at the back of small islands (rocky outcrops) in front of Cayenne. SPMC is relatively low, ranging between 10 and 60 mg l 1. The low concentrations near the mud banks show that the current is probably not at the origin of the mud resuspension. This observation is in agreement with those of Wells and Coleman (1981), who think that waves are at the
origin of the mud resuspension on the mud banks. Other turbid plumes were observed at the back of two small islands (Grand Connetable and Petit Connetable) at 15 km from the Approuague coast on 5 December 2000 SPOT image (Fig. 8). These plumes
Fig. 8. Turbid plumes behind small islands off the Approuague River mouth (SPOT-XS2, 5 December 2000).
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indicate the presence of a relatively strong Guyana current, steered westward along the coast. 4.2. Emergent mudflat morphology The SPOT image (channel XS2) of 14 December 2001 shows the mangrove swamps (dark tint) near the Kaw River mouth and the Approuague intertidal mudflat (Fig. 9). SPMC varies between >500 mg l 1, near the Kaw River mouth, and 8 mg l 1, at 40 km off the coast. Wave crests are also visible offshore. Close to the coast, the distance between crests decreases, and evidence of wave breaking (white spots) start to appear where SPMC is overestimated. The Approuague bank is characterized by the absence of wave breaking due to the frictional damping effect of the fluid mud, as mentioned by Wells and Coleman (1981) and Allison et al. (2000). The mangrove swamps appear dark in the image due to the fact that vegetation does not reflect enough red lights (channel XS2). The emergent mudflat appears darker than surrounding turbid waters. This darker tint
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may originate from a film of microphytobenthos, which engenders a high chlorophyll absorption in the red (absorbance of chlorophyll-a at 670 nm). Light is selectively absorbed by the water depending on the wavelength. Generally speaking, the absorption increases with wavelength. For the XS3 (NIR) and XS4 (MIR) channels, the water absorption is so high that the sea surface appears black on the images. In contrast, the mangrove swamps appear white because of their high reflectivity at these wavelengths (Fig. 10A). Structures in parallel bands divert the mudflat drainage channels towards the east are visible along the lending (western) edge of the mudflat associated with the Approuague mud bank. The XS4 image (Fig. 10B) shows black lines between bands observed on the XS3 image, due to higher water absorption. These black zones correspond to surface water of only 1 or 2 cm thick. They separate clearer bands corresponding to pads of mud. These surface structures, characterized by wavelengths between 200 and 1000 m, could be formed by layers of mud successively deposited on the mudflat.
Fig. 9. The mouth of the Kaw River surrounded by mangrove swamps (dark tint), and to the right, the Approuague emergent mudflat (SPOTXS2, 14 December 2001). Tints are all the clearer as SPOT radiances are high. At the top of the image (in the north), the crests of waves are visible. Close to the coast, the distance between the crests decreases and marks of foams (white spots) are visible.
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Fig. 10. (A) Emergent mudflat of the Approuague mud bank observed from the SPOT image in the near infrared band (XS3, 2 July 2001). (B) Emergent mudflat of the Approuague mud bank (SPOT-XS4, 2 July 2001). On the mudflat, the surfaces weakly covered by the water appear in black and reveal big ridges oblique to the coast.
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Fig. 11. Cross-section used to measure the surface transport rates.
4.3. Littoral transport rates The alongshore-suspended sediment transport rate along the Guyana coast has been reported to have large seasonal variability ‘‘from less than 2 106 tons month 1 from August to September, to 25 106 tons month 1 from April to May’’ (NEDECO, 1968). With SPMC maps derived from SPOT satellite images, an attempt was made to test this concept by estimating instantaneous sediment transport rate (i.e., the quantity of suspended sediments transported north-westward by considering a theoretical speed of the Guyana current). Three realistic speed values, derived from ground-based observations of flow along the French Guiana coast (LCHF, 1978; Pujos and Froidefond, 1995), were used to calculate this transport: a strong current, 0.4 m s 1; a mean current, 0.3 m s 1; and a weak current, 0.2 m s 1. The surface transport rates were calculated through a 25-km north – south transect (Fig. 11) to the east of the Kaw River (4j46V25UN and 52j05V11UW). The MIR image was used to locate the shoreline to avoid integrating the intertidal mudflat. The surface mass of suspended sediments within a 25 km (length) 1 m (width) 1 m (thickness) box was estimated as: 1
Mkðkg m Þ ¼ R½Ci LZi
ð6Þ
where Ci is the concentrations of the ith pixel, L is the length (L = 20 m) and Zi = 1 m. The instantaneous transport rate (R) at the sea surface, through this N –S transect was estimated as: Rðkg s1 Þ ¼ MkV sina
ð7Þ
where a is the angle between the N – S transect and the Guiana current direction and assumed to be 65j (Fig. 10A). The transport rate was calculated with the three current speed values: V1 = 0.4 m s 1, V2 = 0.3 m s 1, and V3 = 0.2 m s 1. Results are listed in Table 3. On 20 June 1998, sedimentary load in suspension along the 25-km transect was the smallest among the study dates, and the transport rate was estimated to range from 20 to 41 kg/s. Turbid plumes appeared to extend behind the small islands in front of Cayenne (Fig. 7). The length of these plumes depends on the turbulences induced by the strong Guyana current directed north-westward (concentrations < 100 mg l 1 at 300 m offshore). Waves are not visible. On 15 August 1999, the sedimentary load and the transport rate (Table 3) increased. Waves and tide currents are thought to be responsible for the increase. For example, turbid plumes are visible at the entrance of the Approuague River. Their fronts are underlined with darker lines that may be associated with natural organic slicks (Fig. 12). Waves are visible but with short wavelengths. On 5 December 2000, sedimentary load and transport rate further increased. High turbidity was located
Table 3 Transport rates (1 m thickness) across the 25-km transect Date (SPOT XS2)
Mk (kg m 1) Tr N – S
R (kg s 1) V1
R (kg s 1) V2
R (kg s 1) V3
20-06-1998 15-08-1999 05-12-2000 02-07-2001 14-12-2001
114 379 9185 1047 3315
41 138 3329 379 1201
31 103 2496 285 901
20 69 1664 190 601
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However, they are certainly crude estimates due to the several assumptions used in the calculation.
5. Conclusion
Fig. 12. Example of sun-glitter effect on the Approuague mudflat, (SPOT-XS2, 15 August 1999). SPMC was overestimated due to this phenomenon. The fronts of turbid plumes are well visible on this image.
in a 10-km coastal band, where SPMC exceeded 500 mg l 1. Off the coast, SPMC was approximately 300 mg l 1. Further offshore, SPMC decreased abruptly to 25 mg l 1. Dark water (SPMC < 100 mg l 1), originated from the Mahury River, was carried away along the Cayenne coast. Long-wavelength waves are visible. Turbid plumes appeared behind the small islands (Connetable Islands) offshore the Approuague River (Fig. 8), showing a strong Guyana current similar to those recorded on 20 June 1998. On 2 July 2001, the suspended mass and transport rate (Table 3) were relatively low. High turbidity was located in front of the intertidal mudflats and moved north-westward (Fig. 6A). There were no visible current marks or wave crests. On 14 December 2001, high SPMC was located along a 10-km-wide coastal band (Fig. 6B), where the distribution of SPMC was similar to those recorded on 5 December 2000. The above estimates are in gross agreement with those reported by NEDECO (1968).
The study of SPOT satellite images over the French Guiana coastal areas provides new results concerning the distribution of suspended sediments. The method used to quantify and to map the SPMC was based on in situ reflectance measurements. The atmospheric correction of SPOT images was also based on in situ reflectance measurements from various homogeneous targets. Despite the limited number of points used to establish a correspondence function between concentration and reflectance, and despite the simplified atmospheric correction and some degree of wavebreaking (foam) contamination on the image, the derived turbidity gradients as well as the SPMC values appear to be agree with surface turbidities measured in previous ground-based studies of the Guyana coast (NEDECO, 1968; LCHF, 1978). There is substantial spatial and temporal variability in the derived SPMC values. While values generally decreased offshore from 100 to 500 mg l 1 at the shoreline to 20 to 1 mg l 1 at 30 km offshore, smallscale spatial variations are observed and can be linked to the physical forcing mechanisms that act upon this coastal zone. The high coastal turbidities are always associated with waves that are visible on the 20 m resolution SPOT images. These observations are consistent with the results of previous studies (Wells and Coleman, 1981), suggesting that waves are a primary resuspension mechanism, and the Guyana coastal current transports this material along the coast. Turbid plumes appeared behind the small coastal islands, formed by the turbulence stimulated by the Guyana Current. These marks of current were observed on the images of 26 June 1998, 5 December 2000, and 14 December 2001. The mouths of estuaries show a large range of turbidity associated with frontal processes and seasonal variations in shore-facing coastal current. In the upstream of the rivers, the turbidity is generally small compared with the increased turbidity in the estuaries due to coastal water intrusion. Otherwise, turbid coastal waters may intrude into estuaries to create a turbidity increase.
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These results show that the quantification and the periodic synoptic SPMC mapping of the Amazonian suspensions can be envisaged with satellite imagery. Further improvements are necessary, particularly to take into account more realistic current speed values and to reduce the uncertainties in the satellite SPMC estimates. At the current stage, the number of SPOT images is not enough to estimate the surface sedimentary flux with sufficient accuracy. The Moderate Resolution Imaging Spectraradiometer (MODIS) satellite sensors (http://modis.gsfc.nasa.gov), which provide daily observations of the ocean’s surface SPMC distribution, will allow better understanding of the coastal environments particularly to trace the origin and fate of suspended sediments over great distances, and therefore provide relevant, timely information to coastal resource managers and scientists. Acknowledgements This work was supported by the PNEC-Guyane project (Programme National d’Environnement Cotier) and the U.S. National Aeronautics and Space Administration (NASA grant NAG5-10738). We thank the CNES for providing SPOT images at low cost (ISIS program). We are grateful to the staff of IRD for their logistic support and scientific discussions. References Allersma, E., 1971. Mud on the oceanic shelf off Guiana. Symp. Invest. Resour. Caribb. Sea and adjacent Reg. UNESCO, Paris, pp. 193 – 203. Allison, M.A., Lee, M.T., Ogston, A.S., Aller, R., 2000. Origin of Amazon mudbanks along the northeastern coast of South America. Mar. Geol. 163, 241 – 256. Augustinus, P.G., 1978. The changing shoreline of Surinam (South America). Thesis University of Utrecht: Soc. Invest. Hist. Natur. Stud. Utrecht, 95. CNES, 1986. Guide des utilisateurs de donnees SPOT. Manuel de reference, vol. 1. Centre National d’Etude Spatiale. Curran, P.J., Novo, E.M.M., 1988. The relationship between suspended sediment concentration and remotely sensed spectral radiance: a review. J. Coast. Res. 4, 351 – 368. DHL, 1962. Demerara coastal investigation. Report on Siltation of Demerara Bar Channel and Coastal Erosion in British Guiana. Delft Hydraulics Laboratory, The Netherlands. Doxaran, D., Froidefond, J.M., Lavender, S.J., Castaing, P., 2002. Spectral signature of highly turbid waters. Application with
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SPOT data to quantify suspended particulate matter. Remote Sens. Environ. 81, 149 – 161. Eisma, D., Augustinus, P.G., Alexander, C., 1991. Recent and subrecent changes in the dispersal of Amazon mud. Neth. J. Sea Res. 28, 181 – 192. Forget, P., Ouillon, S., 1998. Surface suspended matter off the Rhone river mouth from visible satellite imagery. Oceanol. Acta 21, 739 – 749. Froidefond, J.M., Pujos, M., Andre´, X., 1988. Migration of mud banks and changing shoreline in French Guiana. Mar. Geol. 84, 19 – 30. Froidefond, J.M., Castaing, P., Prud’homme, R., 1999. Monitoring suspended particulate matter fluxes and patterns with the AVHRR/NOAA-11 satellite. Application to the Bay of Biscay. Deep-Sea Res. II. Spec. Issue Ecofer 46, 2029 – 2055. Froidefond, J.M., Gardel, L., Guiral, D., Parra, M., Ternon, J.F., 2002. Spectral remote sensing reflectances of coastal waters in French Guiana under the Amazon influence. Rem. Sens. Environ. 80, 225 – 232. Gordon, H.R., Wang, M., 1994. Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm. Appl. Opt. 33, 443 – 455. Gordon, H.R., Brown, O.B., Evans, R.H., Brown, J.W., Smith, R.C., Baker, K.S., Clark, D.K., 1988. A semi-analytical radiance model of ocean color. J. Geophys. Res. 93, 10909 – 10924. Hu, C., Carder, K.L, Muller-Karger, F.E., 2001. How precise are SeaWiFS ocean color estimates? Implications of digitizationnoise errors. Remote Sens. Environ. 76, 239 – 249. Jouanneau, J.M., Pujos, M., 1987. Suspended matter and bottom deposits in the Mahuryestuarine system (French Guiana): environmental consequences. Neth. J. Sea res. 21, 191 – 202. Kuehl, S.A., DeMaster, D.J., Nittrouer, C.A., 1986. Nature of sediment accumulation on the Amazon continental shelf. Cont. Shelf Res. 6, 209 – 226. LCHF, 1978. Etude portuaire de la Guyane lie´e a` l’implantation de l’industrie papetie`re. Rapport d’e´tude en nature. 4e`me partie: synthe`se ge´ne´rale des e´tudes. Laboratoire Central d’Hydraulique de France, No. 94700, 74pp. Lemiere, G., 1953. Mission hydrographique des coˆtes de la Guyane Francß aise (1947 – 1948). Recherche hydrographique sur le re´gime des coˆtes. Imprimerie Nationale, 28e`me cahier, 628, 168 – 196. Meade, R.H., Dunne, T., Richey, J.E., Santos, U.M., Salati, E., 1985. Storage and remobilization of suspended sediment in the lower Amazone River of Brazil. Science 228, 488 – 490. Mobley, C.D., 1999. Estimation of the remote-sensing reflectance from above surface measurements. Appl. Opt. 38, 7442 – 7455. Muller-Karger, F.E., McClain, C.R., Richardson, P.L., 1988. The dispersal of the Amazon’s water. Nature 333, 56 – 59. NEDECO, 1968. Transportation study. Report on hydraulic investigation. Nedeco Eng. Consult. The Hague, The Netherlands. Nittrouer, C.A., DeMaster, D.J., 1986. Sedimentary processes on the Amazon continental shelf: past, present and future research. Cont. Shelf Res. 6, 5 – 30. Parra, M., Pujos, M., 1998. Origin of late Holocene fine grained sediments on the French Guiana shelf. Cont. Shelf Res 18, 1613 – 1629.
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Prieur, L., Sathyendranath, S., 1981. An optical classification of coastal and oceanic waters based on the specific spectral absorption curves of phytoplancton pigments, dissolved organic matter and other particulate materials. Limnol. Oceanogr. 26, 671 – 689. Prost, M.T., 1989. Coastal dynamics and chenier sands in French Guiana. In: Augustinus, P.G.E.F. (Ed.), Cheniers and Chenier Plains. Mar. Geol. 90, 259 – 267. Pujos, M., Froidefond, .J.M., 1995. Water masses and suspended matter circulation on the French Guiana continental shelf. Cont. Shelf Res. 15, 1157 – 1171. Rine, J.M., Ginsburg, R.N., 1985. Depositional facies of a mud shoreface in Suriname, South America—a mud analogue to sandy, shallow-marine deposits. J. Sediment. Petrol. 55, 633 – 652. Sathyendranath, S., Bukata, R.P., Arnone, R., Dowell, M.D., Davis, C.O., Babin, M., Berthon, J.F., Kopelevich, O.V., Campbell, J.W., 2000. Colour of case 2 waters. Remote Sensing of Ocean Colour in Coastal, and other Optically-Complex, Waters. Report, vol. 3. IOCCG, Dartmouth, Canada, pp. 23 – 46.
SHOM, 1975. Instructions nautiques: Ame´rique du Sud (Coˆte Est). Serv. Hydrogr. Oce´anogr. Mar., Paris, Se´r. H. 4, 11 – 97. Smith, G., Milton, E.J., 1999. The use of the empirical line method to calibrate remotely sensed data to reflectance. Int. J. Remote Sens. 20 (13), 2653 – 2662. Wells, J.T., Coleman, J.M, 1981. Physical processes and finegrained sediment dynamics, coast of Surinam, South America. J. Sediment. Petrol. 51, 1053 – 1068. Wells, J.T., Prior, D.B., Coleman, J.M, 1980. Flowslides in muds on extremely low angle tidal flats, northeastern South America. Geology 8, 272 – 275. Whitlock, C.H., Poole, L.R., Usry, J.W., Houghton, W.M., Witte, W.G., Morris, W.D., Gurganus, E.A., 1981. Comparison of reflectance with backscatter and absorption parameters for turbid waters. Appl. Opt. 20, 517 – 527. Yayer, M.J., 1948. Missions hydrographiques de la Guyane Francßaise (1936 – 1937). Annales Hydrographiques, vol. 19. Service Hydrographique de la Marine, Paris, pp. 31 – 120.
Mudflats and mud suspension observed from satellite data in French Guiana J.M. Froidefond a,*, F. Lahet b, C. Hu c, D. Doxaran a, D. Guiral b, M.T. Prost d, J.-F. Ternon b a
De´partement de Ge´ologie et Oce´anographie (UMR EPOC), Universite´ Bordeaux-1, Avenue des Facultes 33405, Talence Cedex, France b Institut de Recherche pour le De´veloppement (IRD) BP 165-97323, Cayenne Ce´dex, France c Institute for Marine Remote Sensing (IMaRS), College of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701, USA d Museu Paraense Emı´lio Goeldi, Campus de Pesquisa, Av. Perimetral, Terra Firme, Belem, Para, Brazil Received 8 July 2002; accepted 14 April 2004
Abstract The littoral of French Guiana is characterized by the north-westward migration of large mud banks along shore and by high suspended particulate matter concentrations (SPMC) coming from the Amazon River. A correspondence function is established between in situ optical data and SPMC measurements obtained from three coastal surveys on the Kaw River, the Mahury estuary, and the adjacent continental shelf, respectively. This function is applied to SPOT satellite images to estimate SPMC distributions near the sea surface on five dates between 1998 and 2001. Two typical situations have been observed: the mud suspensions are generally confined near the coast by a strong north-westward coastal current, while in one case (SPOT scene of 2 July 2001) the turbid plumes, probably resulted from the wave action on the mud banks, are directed seaward. SPMC varies from more than 300 mg l 1 adjacent to the intertidal zone to a few mg l 1 offshore. Turbid plumes are observed downstream, with regard to the Guyana Current, of small islands along the coast. Mud undulations are observed on the emergent surface of the mudflats, parallel to the crest waves and characterized by a wavelength of 200 – 1000 m. These structures are interpreted to form by wave action on the soft mudflat surface. Mangrove colonization of the mudflat follows these primary topographic structures. D 2004 Elsevier B.V. All rights reserved. Keywords: mud bank; suspended particulate matter; remote sensing; Amazon River; French Guiana
1. Introduction
* Corresponding author. Tel.: +33-5-56-84-60-00; fax: +33-556-84-08-48. E-mail addresses: [email protected] (J.M. Froidefond), [email protected] (F. Lahet), [email protected] (C. Hu), [email protected] (D. Guiral), [email protected] (M.T. Prost), [email protected] (J.-F. Ternon). 0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2004.04.025
The French Guiana is strongly influenced by fresh and turbid Amazonian outflows. Immense argillaceous suspensions (1.1 – 1.3 10 9 tons year 1 , Meade et al., 1985) are discharged at the mouth of the Amazon River. Of this total, 6.3 F 2 108 tons year 1 are deposited on the continental shelf close to the Amazon River mouth (Kuehl et al., 1986, Nit-
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trouer and DeMaster, 1986), while about 1.5 108 tons year 1 are transported in suspension, and about 1 108 tons year 1 move along Guyanas in the form of mud banks (Eisma et al., 1991). The Guyana Current, an extension to the North Equatorial current off Brazil (Muller-Karger et al., 1988), goes along the coast from southeast to northwest. In 1961, during an aerial survey, the Delft Hydraulics Laboratory (DHL, 1962) observed 21 mudflats along the coast between the Waini River in Guyana and Cayenne in French Guiana. Initial mud bank development took place at Cabo Cassipore (Allison et al., 2000). The average ‘‘wavelength’’ was approximately 45 km and varied between 30 and 60 km. The extensive mudflats are overgrown with mangroves at the highest levels. These mudflats are the emerged parts of mud banks (NEDECO, 1968, Augustinus, 1978). A vertical sequence of massive and laminated mud with discontinuity features is present in almost all cores taken and analysed by Rine and Ginsburg (1985) from the coastal mud banks of Surinam. Comparison of successive sounding maps shows that mud banks slowly move from east to west. The mean celerity was calculated to be 1.5 km year 1 (NEDECO, 1968). The mechanism of propagation of mud banks is a topic of debate. Wells and Coleman (1981) proposed two hypotheses. The first, which they prefer, is the action of the swells that suspends the sediments and transports them along the coast. The second hypothesis is based on observations showing the ‘‘transport en masse’’ of mud by flow sliding (Wells et al., 1980). But the existence of the Guyana Current can also be a good candidate to explain the mud bank migration. The objective of this study is to quantify and to map the suspended particulate matter concentrations (SPMC) from satellite images and to help interpret how the Guyana coastal mud banks are formed and how they migrate along the coast. In fact, these mud suspensions are marker of surface currents because of the very weak settling velocity of sediments (LCHF, 1978). Quantification of SPMC is also useful to design dredging strategies on navigational channels and to estimate the fluvial solid discharges to the ocean. Moreover, high SPMC has an inhibitive effect on primary production (due to light limitation) in coastal waters and, its estimation is essential in understanding the biological mechanisms.
Below, we first describe the area by visualization of a satellite image. Then, empirical regression functions are derived based on field measurements to estimate SPMC from satellite data. These functions are then used to estimate SPMC distributions and to estimate transport rates.
2. Study area The study area is located in French Guiana between Cayenne and the mouth of the Approuague River. Fig. 1 shows the area from a SPOT-4 image recorded on 2 July 2001. From west to east, the area includes the peninsula of Cayenne, the mouth of the Mahury River, the littoral swamp of Kaw, crossed by the Kaw River, and the mouth of the Approuague River. Along this coast, the displacement of mud banks was known before 1875, mentioned in technical reports of marine officers (Lemiere, 1953) and preserved in the archives of the French Hydrographic Survey. These mud banks were described in these reports in the following way: they are constituted by very soft mud on which the sea is always quiet; the western part of these banks is generally steep while the oriental part is sloping gently. These banks move slowly westward on a bottom constituted by hard mud. The commander Yayer (1948), during his hydrographic survey in 1936 –1937, also indicated the passage of soft mud banks. During this mission, the hard bottom was sounded by means of a 15-kg lead, and the thickness of the soft mud was measured with a perch. He noted that the thickness of the layer of soft mud was most frequently 1 m. Intertidal mudflats are quickly covered with mangroves. Then, studies were carried out between 1955 and 1978 (LCHF, 1978) for the access to the harbour of Cayenne (on the Mahury River) and for a project of exploitation of bauxite on the mountain of Kaw. Between 1978 and 1984, the coast of French Guiana was bordered by six major mudflats ranging from 20 to 40 km in length and migrating along the coast to the west-northwest towards Surinam, at rates ranging from 0.32 to 1.22 km year 1 with a mean rate of 0.9 km year 1 (Froidefond et al., 1988). A comparable study of the location of the mudflats between 1955 and 1984 reveals a more rapid displacement. Between 1984 and 1988, a mud bank reached the coast of
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Fig. 1. The study area composed of the southeast part of the French Guiana, Cayenne, the main rivers, the emergent parts (mudflats) of the mud banks, and their turbid plumes (SPOT scene, 02 July 2002).
Cayenne. Sandy beaches separated by rocky caps characterizing this coast were gradually silted up. Then, the mangrove swamp developed in front of the coast masking completely the sight on the ocean. Generally, these beaches are associated with cheniers (coastal ridges) on the west part of the French Guiana coast (Prost, 1989). At present, a new mud bank is located in front of Cayenne, and another bank is located to the east of the swamp of Kaw. The morphology of this coast and the turbid plumes are well visible on satellite images (Fig. 1). Typically, the emerged part of the mud bank, called ‘‘vasie`re’’ (mudflat), is attached to the coast. The Kaw River, at the center of the study zone, is located between two mud banks of Approuague on the east and of Cayenne on the west.
The average angle between the crests of the mud banks like ridges and the coast varies between 20j and 30j, with an average of 24j. Generally, the mudflats and the intermediate troughs have a gentle slope which might be less than 1:3000 with the exception of the northwest flank of the mudflats, which has a relatively steep slope, 1:500 (NEDECO, 1968; Allison et al., 2000). Fluid mud occurs most commonly on the lee side or western flank of the mud shoals (NEDECO, 1968). Soft tidal flat deposits are typically 1 to 2 m thick and overlie a firm clay base. Sedimentation of mud leads to the formation of slingmud, a very dense suspension of sediments. The sedimentologic measurements made on suspended mud in the Mahury mouth indicate a median grain size of 1– 2 Am, originated from the Amazon (Parra
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and Pujos, 1998). In the soft mud, the water content can reach from 150% to 200%, with concentrations of 400 to 500 g l 1 (LCHF, 1978). The clay minerals are typically composed of Illite, 37%; kaolinite, 27%; smectite, 20%; and chlorite, 16% (Jouanneau and Pujos, 1987). Average current speed is reported to be 0.3– 0.6 m s 1 (NEDECO, 1968). The tide currents increase the current in flow (0.8 – 0.9 m s 1) and decrease during the ebb (0.25 m s 1) with a low deviation toward the coast during the flow and offshore during the ebb. On the Cayenne mud bank (Pujos and Froidefond, 1995), in October 1988, the currents at the surface and at the bottom were towards the west at flood (260 –270j) and towards the northwest at ebb (300 – 320j). Their surface velocities (0.15 – 0.25 m s 1) increase due to the sea breeze in the morning (0.4 – 0.6 m s 1). SPMC at sea surface was 100 –300 mg l 1 in the morning and greater than 500 mg l 1 in the afternoon, when the sea is wavy. Tides are semidiurnal and also synchronous along the entire Guiana Coast (Allersma, 1971). Two high tides and two low tides of different height occur each day in 25 h 50 min along the coast of French Guiana. The tidal range, during spring tide, is about 2.5 m (SHOM, 1975). Waters are especially muddy at low tide. Samples reveal that SPMC begins to increase near midtide, reach a maximum at low tide, and then decrease to the following high tide (Wells and Coleman, 1981). Waves on the shelf are typically 1 –2 m high but sometimes (< 1% of the year) can exceed 4 m (NEDECO, 1968). The highest waves coincide with the periods of the strongest winds (February – May), whereas the lowest waves coincide with periods of weakest winds (June –November). Average period is 6– 8 s, with swell from North Atlantic storms occasionally exceeding 15 s. Historical data show that sea and swell arrive from the northeast quadrant during all months of the year (with 50% arriving from N50jE to N70jE, NEDECO, 1968). Examination of time series records (Wells and Coleman, 1981) indicates that waves are deformed and solitary-like wave profiles result. Solitary waves are characterized descriptively as isolated crests separated by flat ‘‘troughs’’ lying at still-water level. Suspended concentrations are highest in fluid mud regions, where solitary waves occur. This suggests that a tremendous potential exists for sedi-
ment transport by wave acting alone (Wells and Coleman, 1981).
3. Methods Different studies show the possibility of quantifying SPMC from water color (Whitlock et al., 1981; Curran and Novo, 1988). Reflectance spectra vary in terms of two inherent optical properties: total absorption coefficient, a(k), and total back-scattering coefficient, bb(k) (Prieur and Sathyendranath, 1981, Gordon et al., 1988). The two coefficients, a(k) and bb(k), vary as a function of the concentration of mineral particles, phytoplankton (chlorophyll-a), and dissolved organic matter (yellow substances). Remote sensing reflectance, Rrs(k) (sr 1), is defined as the ratio of water-leaving radiance (Lw) to downwelling irradiance (Ed): RrsðkÞ ¼ Lw ðkÞ=Ed ðkÞ
ð1Þ
(Mobley, 1999). For coastal turbid waters, Rrs(k) can be expressed as (Doxaran et al., 2002) RrsðkÞc0:529
f ½bb ðkÞ=ðaðkÞ þ bb ðkÞÞ Q
ð2Þ
where f/Q (sr 1) is a factor related to the solarviewing geometry and particle-scattering phase function. It is very difficult to measure the f/Q ratio directly, particularly because the sky light reflection on the sea surface. In situ measurements are often used to obtain empirical relationships between particle concentrations and reflectance at selected wavelength (Froidefond et al., 1999; Doxaran et al., 2002). These relationships may then be used to quantify phytoplankton and mineral particles from satellite data after the atmospheric effects are corrected (Sathyendranath et al., 2000). In coastal waters and estuaries, characterized by high mineral particles concentrations, such as in this study, the phytoplankton and yellow substance contributions can be ignored in a first approximation. SPOT satellite images are used in this study, because the spatial coverage (60 60 km) and pixel resolution (20 20 m) are suitable for biologic and
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sedimentologic studies. The SPOT data cover three to four spectral channels (http://spot4.cnes.fr/): Band XS1, 500 –590 nm; Band XS2, 610 – 680 nm; Band XS3, 780 – 890 nm (near infrared, NIR); Band XS4 1580 –1750 nm (middle infra red, MIR). The method to describe SPMC of surface waters from SPOT images was based on a four-step empirical approach: (1) field sampling of the study area to obtain Rrs spectra and SPMC; (2) regression between SPMC and Rrs; (3) atmospheric correction of the satellite data to obtain Rrs at sea level for the area; and (4) application of the regression relationship to the satellite Rrs data to obtain SPMC distribution maps. Two assumptions are inherent in the process. First, field and satellite measurements were conducted under similar solar illumination and viewing conditions, although they were collected on different dates. Second, the mineralogical composition and the particle size of suspended sediments did not vary significantly between the time of field and satellite measurements. 3.1. Field measurements In situ measurements were collected in the coastal waters of French Guiana during three field campaigns in November 1998 (Mahury River), April 1999 (con-
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tinental shelf between Cayenne and the Oyapock River) and November – December 2000 (Kaw River and Mahury River). The procedures used to collect the Rrs data for these field campaigns have been described in detail (Froidefond et al., 2002). Briefly, a Spectron SE-590 spectroradiometer was used to measure the water-leaving radiance (Lw) and the irradiance (Ed) from a standard reflectance plate. Rrs is then obtained as Eq. (1): Rrs(k) (sr 1) = Lw(k)/Ed(k). Measurement protocols are regularly updated. That of Mobley (1999) is now recommended. Concurrent with the optical measurements (within 1 – 2 min), water samples were collected at approximately 30 cm beneath the sea surface. Total SPMC was measured from sample filtration through membranes of 0.47 Am porosity. The measured reflectance spectra are shown in Fig. 2. At a given wavelength, spectral amplitudes clearly increase with SPMC. The reflectance spectra exhibit the same overall shape and, in particular, maxima and shoulders in the vicinity of 580, 650, 690, and 810 nm, respectively. These spectra are similar to those measured at the Rhoˆne River mouth during a flood event (Forget and Ouillon, 1998) or in the Gironde estuary (Doxaran et al., 2002). The emergent mudflat presents similar spectra excepted in the NIR wavelength where the Rrs values are clearly higher (Fig. 2).
Fig. 2. Reflectance spectra measured on the coastal waters of French Guiana in November 1998 (Mahury River), April 1999 (continental shelf), and November – December 2000 (Kaw River and Mahury River). The spectrum of the emerged mudflat is different, particularly in the near infrared part.
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3.2. Relationship between satellite-equivalent reflectance and SPMC Rrs spectra from field measurements were used to compute the Rrs in the SPOT channels with the following equation: Z
kmax
Lw ðkÞ SXSi ðkÞ
RrsXSi ¼ Zkmin kmax
dk
November 2000, respectively. With the channel XS1, the fit equation (SPMC = 2.78[exp(131.24 RrsXS1)]) 2 has a regression coefficient R = 0.83. With XS3, the fit equation (SPMC = 7211 RrsXS3 + 14.25) has a regression coefficient equal to 0.61, and we found that channel XS2 is the most sensitive to changes in SPMC, with a regression coefficient R2 = 0.87 (Fig. 4) where SPMC ranges between 3.5 and 500 mg l 1.
ð3Þ Ed ðkÞ SXSi ðkÞ
dk
SPMC ¼ 3:4707½expð109:31
RrsXS2Þ
R2 ¼ 0:87
ð4Þ
kmin
where i is equal to 1, 2, or 3 and stands for the SPOTHRV channels, RrsXSi(k) is for channel XSi, and SXSi(k) the spectral sensitivity (Fig. 3). Spectral sensitivity data can be obtained from either the header file of each SPOT-HRV image or SPOT-HRV technical manual (CNES, 1986). A test of the November 2000 Rrs data for the two sensitivity data sets shows that the difference is < 5%. In this study, we use the typical sensitivity data (CNES, 1986) for data processing. The Rrs-SPMC regression analysis for each SPOT channel was carried out on 32 data pairs, of which, 10 were obtained on the continental shelf in April 1999, 10 were obtained on the Kaw River, and 12 on the Mahury River and the Cayenne River in November 1998 and in
This function (Eq. (4)) is used to calculate the SPMC distributions from the SPOT images. 3.3. Satellite data processing and atmospheric corrections Five SPOT scenes Level-2A (radiometrically and geometrically corrected) were acquired for the period between June 1998 and December 2001. The characteristics of these scenes are listed in Table 1 (the incident angle is the angle between the vertical at the centre of the image and the vector to the satellite). Four spectral channels are available except for the June 1998 SPOT scene. Because of the atmospheric attenuation (absorption and scattering), the total signal
Fig. 3. Spectral sensitivities of XS1, XS2, and XS3 channels of SPOT-HRV sensor. Values deduced from the leader file of the 5 December 2000 image are represented with continuous lines. Typical values given by SPOT-HRV technical manual (CNES, 1986) are represented by crosses, triangles, and rounds.
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159
Fig. 4. Correspondence function used to quantify SPMC from SPOT-XS2 data.
(radiance) received by the SPOT sensor, Lt, is (Gordon and Wang, 1994): Lt ¼ Lr þ La þ tLw ;
ð5Þ
-where Lr is due to molecular (Rayleigh) scattering, La is from aerosol scattering (including aerosol– Rayleigh interactions), Lw is the water-leaving radiance to be derived, and t is the diffuse transmittance to propagate Lw to the sensor. For simplicity, the wavelength dependence of each term is omitted. Several other factors that affect the total signal, including the sun glint, whitecaps, ozone, and other gaseous absorption, are not considered because they can be either avoided or corrected. The goal of the atmospheric correction is to retrieve Lw(k) from Lt(k). Because of the complexity of the atmosphere, the limited number of spectral channels on SPOT, and the large signal from the highly turbid environment, it is neither realistic nor necessary to use a sophisticated real-time atmospher-
ic correction scheme with the precision of SeaWiFS images (Hu et al., 2001). Numerical models (MODTRAN, LOWTRAN, or 6S) are often used to estimate the atmospheric effects. However, the aerosol properties must be known a priori to use the models. Unfortunately, these meteorological data are not available in French Guiana. Hence, an empirical method similar to that of Smith and Milton (1999) was used. It is based on in situ reflectances measured from a number of calibration targets. The targets are several times the pixel size with uniform colors (reflectances). For each SPOT channels (XS1, XS2, and XS3), the corresponding digital numbers (DN) from the satellite sensor are compared with the target reflectances, and a regression relationship is derived. Five types of targets were measured in the field and selected for the regression analysis (Table 2): sand, laterite, fine gravels, mud (mudflat), and offshore water. Fig. 5 shows the regression between in situ Rrs and SPOT DN for image XS2 (2 July 2001) used in SPMC calculations. The regression
Table 1 Characteristics of SPOT scenes and corresponding time of low tides Date
Time (UTC)
1998/06/20 1999/08/15 2000/12/05 2001/12/14 2001/07/02
13 14 14 14 13
h h h h h
57 00 09 16 49
min min min min min
41 12 08 02 38
s s s s s
Number of bands
Incidence angle
Sun elevation
Low tide (UTC)
3 4 4 4 4
9.7j 9.0j 15.8j 28.9j 24.8j
61.0j 65.0j 57.9j 57.5j 59.2j
11 h 12 min 17 h 07 min 8 h 58 min 12 h 14 min 13 h 57 min
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Table 2 In situ reflectances of the targets Reference areas (targets)
Dry sands, beach of Montjoly Sea surface (offshore) Mudflat (Pointe Mahury) Laterite (Crique Fouille´e) Fine gravels (Degrad harbour)
4. Results and discussion In situ reflectances Rrs (sr 1) Rrs XS1 0.0668 0.0190 0.0318 0.0445 0.0764
Rrs XS2 0.0986 0.0095 0.0372 0.0859 0.0828
4.1. Distribution of SPMC on the eastern French Guiana Coast Rrs XS3 0.1305 0.0006 0.0531 0.0954 0.0891
coefficients (R2) are 0.98, 0.95, 0.99, 0.90, and 0.97 for 20 June 1998, 15 August 1999, 5 December 2000, 2 June 2001, and 14 December 2001, respectively. These regression equations were then applied to the SPOT-XS2 image to derive Rrs and SPMC distributions. 3.4. Uncertainties in the SPMC estimations The SPMC estimates from the satellite images have uncertainties associated with (1) errors in the optical measurements, which may reach up to 20%; (2) 1- to 2-min difference between in situ SPMC and optical measurements; (3) limited number of samples used in the regression between Rrs and SPMC; (4) limited number of data points in the regression between SPOT DN and Rrs; and (5) the assumption that the atmosphere is homogenous across the whole image, which may also lead to additional errors. Considering all these factors, SPMC estimates are possibly with uncertainties of 25% to 35%.
The SPMC map obtained from the XS2 image of 2 July 2001 is shown in Fig. 6A. The image was taken just at low tide at the beginning of the flood phase (low tide: 12:10 UT; tide range: 2 m). Two large turbid plumes are associated with the Approuague mud bank near the Approuague River mouth and with the Cayenne mud bank. SPMC varies between >450 mg l 1 near the emergent mudflat and 50 mg l 1 offshore. These turbid plumes are weakly diverted north-westward in the direction of the Guyana Current. Mud banks seem to be the source of these turbid plumes that extend 15 – 20 km offshore. At the Mahury River mouth, a turbidity maximum appears approximately 5 km inland from the coast. Upriver, turbidities are reduced, likely due to the sedimentpoor nature of these rivers that drain the humid forest (Jouanneau and Pujos, 1987). However, the low turbidity river waters are generally dark-brown because of the high concentrations of coloured dissolved organic carbon (yellow substance), potentially altering the SPMC calibration for these inland areas. Similar to the 2 July image, the 14 December 2001 image (Fig. 6B) was recorded at the beginning of the flood tide (low tide: 13:58 UT; tidal range: 2.4 m). SPMC near the coast is higher than those found on 2 July. A wide band (5 –10 km) of turbidities (SPMC
Fig. 5. Example of regression function used with the target method to obtain a SPOT-XS2 atmospherically corrected image.
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Fig. 6. (A) Distribution of SPMC at sea surface from the SPOT-XS2 image of 02 July 2001. (B) Distribution of SPMC at sea surface from the SPOT-XS2 image of 14 December 2001.
between 350 and 450 mg l 1) extends along the coast. Turbid plumes in the form of draperies, stretched out in the orientation of the Guyana current, are visible with SPMC ranging from 50 to 100 mg l 1. In the north, at 35 km from the coast, the turbidity is equal or
lower than 20 mg l 1. Turbid coastal waters penetrate into the mouths of the Mahury and Kaw rivers, because river discharges are low in this season. The contrast between 2 July and 14 December images is probably due to the stronger Guyana current.
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Fig. 7. Turbid plumes behind small islands in front of Cayenne (SPOT-XS2, 20 June 1998). SPMC varies between 1 and 60 mg l 1.
This current seasonality is also synchronous with trade wind-driven wave seasonality. Along the Suriname coast, NEDECO (1968) also observed a seasonal reduction in SPMC towards the open sea; in the trade winds season (e.g., December – February, average current f 0.5 m s 1), the concentrations were 400 mg l 1 near the coast, 50 mg l 1 at 20 km offshore, and 10 mg l 1 at 30 km offshore, whereas during the calm season (e.g., May – September, average current ~0.2 m.s 1), they were no more than 150, 12, and, 1– 2 mg l 1, respectively. Unfortunately, geophysical data (currents, waves, winds) during these satellite acquisitions were inexistent to confirm these hydrodynamic processes. Wind data are available from the Rochambeau airport (Cayenne) located a few kilometres away from the coast. Hence, wind speed and direction cannot be considered representative of wind at sea. Moreover, due to the lack of wave recorder, no in situ measurements were available to explain wave structures observed on the SPOT images. SPMC map derived from the 20 June 1998 SPOT data is shown in Fig. 7, where turbid plumes were observed at the back of small islands (rocky outcrops) in front of Cayenne. SPMC is relatively low, ranging between 10 and 60 mg l 1. The low concentrations near the mud banks show that the current is probably not at the origin of the mud resuspension. This observation is in agreement with those of Wells and Coleman (1981), who think that waves are at the
origin of the mud resuspension on the mud banks. Other turbid plumes were observed at the back of two small islands (Grand Connetable and Petit Connetable) at 15 km from the Approuague coast on 5 December 2000 SPOT image (Fig. 8). These plumes
Fig. 8. Turbid plumes behind small islands off the Approuague River mouth (SPOT-XS2, 5 December 2000).
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indicate the presence of a relatively strong Guyana current, steered westward along the coast. 4.2. Emergent mudflat morphology The SPOT image (channel XS2) of 14 December 2001 shows the mangrove swamps (dark tint) near the Kaw River mouth and the Approuague intertidal mudflat (Fig. 9). SPMC varies between >500 mg l 1, near the Kaw River mouth, and 8 mg l 1, at 40 km off the coast. Wave crests are also visible offshore. Close to the coast, the distance between crests decreases, and evidence of wave breaking (white spots) start to appear where SPMC is overestimated. The Approuague bank is characterized by the absence of wave breaking due to the frictional damping effect of the fluid mud, as mentioned by Wells and Coleman (1981) and Allison et al. (2000). The mangrove swamps appear dark in the image due to the fact that vegetation does not reflect enough red lights (channel XS2). The emergent mudflat appears darker than surrounding turbid waters. This darker tint
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may originate from a film of microphytobenthos, which engenders a high chlorophyll absorption in the red (absorbance of chlorophyll-a at 670 nm). Light is selectively absorbed by the water depending on the wavelength. Generally speaking, the absorption increases with wavelength. For the XS3 (NIR) and XS4 (MIR) channels, the water absorption is so high that the sea surface appears black on the images. In contrast, the mangrove swamps appear white because of their high reflectivity at these wavelengths (Fig. 10A). Structures in parallel bands divert the mudflat drainage channels towards the east are visible along the lending (western) edge of the mudflat associated with the Approuague mud bank. The XS4 image (Fig. 10B) shows black lines between bands observed on the XS3 image, due to higher water absorption. These black zones correspond to surface water of only 1 or 2 cm thick. They separate clearer bands corresponding to pads of mud. These surface structures, characterized by wavelengths between 200 and 1000 m, could be formed by layers of mud successively deposited on the mudflat.
Fig. 9. The mouth of the Kaw River surrounded by mangrove swamps (dark tint), and to the right, the Approuague emergent mudflat (SPOTXS2, 14 December 2001). Tints are all the clearer as SPOT radiances are high. At the top of the image (in the north), the crests of waves are visible. Close to the coast, the distance between the crests decreases and marks of foams (white spots) are visible.
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Fig. 10. (A) Emergent mudflat of the Approuague mud bank observed from the SPOT image in the near infrared band (XS3, 2 July 2001). (B) Emergent mudflat of the Approuague mud bank (SPOT-XS4, 2 July 2001). On the mudflat, the surfaces weakly covered by the water appear in black and reveal big ridges oblique to the coast.
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Fig. 11. Cross-section used to measure the surface transport rates.
4.3. Littoral transport rates The alongshore-suspended sediment transport rate along the Guyana coast has been reported to have large seasonal variability ‘‘from less than 2 106 tons month 1 from August to September, to 25 106 tons month 1 from April to May’’ (NEDECO, 1968). With SPMC maps derived from SPOT satellite images, an attempt was made to test this concept by estimating instantaneous sediment transport rate (i.e., the quantity of suspended sediments transported north-westward by considering a theoretical speed of the Guyana current). Three realistic speed values, derived from ground-based observations of flow along the French Guiana coast (LCHF, 1978; Pujos and Froidefond, 1995), were used to calculate this transport: a strong current, 0.4 m s 1; a mean current, 0.3 m s 1; and a weak current, 0.2 m s 1. The surface transport rates were calculated through a 25-km north – south transect (Fig. 11) to the east of the Kaw River (4j46V25UN and 52j05V11UW). The MIR image was used to locate the shoreline to avoid integrating the intertidal mudflat. The surface mass of suspended sediments within a 25 km (length) 1 m (width) 1 m (thickness) box was estimated as: 1
Mkðkg m Þ ¼ R½Ci LZi
ð6Þ
where Ci is the concentrations of the ith pixel, L is the length (L = 20 m) and Zi = 1 m. The instantaneous transport rate (R) at the sea surface, through this N –S transect was estimated as: Rðkg s1 Þ ¼ MkV sina
ð7Þ
where a is the angle between the N – S transect and the Guiana current direction and assumed to be 65j (Fig. 10A). The transport rate was calculated with the three current speed values: V1 = 0.4 m s 1, V2 = 0.3 m s 1, and V3 = 0.2 m s 1. Results are listed in Table 3. On 20 June 1998, sedimentary load in suspension along the 25-km transect was the smallest among the study dates, and the transport rate was estimated to range from 20 to 41 kg/s. Turbid plumes appeared to extend behind the small islands in front of Cayenne (Fig. 7). The length of these plumes depends on the turbulences induced by the strong Guyana current directed north-westward (concentrations < 100 mg l 1 at 300 m offshore). Waves are not visible. On 15 August 1999, the sedimentary load and the transport rate (Table 3) increased. Waves and tide currents are thought to be responsible for the increase. For example, turbid plumes are visible at the entrance of the Approuague River. Their fronts are underlined with darker lines that may be associated with natural organic slicks (Fig. 12). Waves are visible but with short wavelengths. On 5 December 2000, sedimentary load and transport rate further increased. High turbidity was located
Table 3 Transport rates (1 m thickness) across the 25-km transect Date (SPOT XS2)
Mk (kg m 1) Tr N – S
R (kg s 1) V1
R (kg s 1) V2
R (kg s 1) V3
20-06-1998 15-08-1999 05-12-2000 02-07-2001 14-12-2001
114 379 9185 1047 3315
41 138 3329 379 1201
31 103 2496 285 901
20 69 1664 190 601
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However, they are certainly crude estimates due to the several assumptions used in the calculation.
5. Conclusion
Fig. 12. Example of sun-glitter effect on the Approuague mudflat, (SPOT-XS2, 15 August 1999). SPMC was overestimated due to this phenomenon. The fronts of turbid plumes are well visible on this image.
in a 10-km coastal band, where SPMC exceeded 500 mg l 1. Off the coast, SPMC was approximately 300 mg l 1. Further offshore, SPMC decreased abruptly to 25 mg l 1. Dark water (SPMC < 100 mg l 1), originated from the Mahury River, was carried away along the Cayenne coast. Long-wavelength waves are visible. Turbid plumes appeared behind the small islands (Connetable Islands) offshore the Approuague River (Fig. 8), showing a strong Guyana current similar to those recorded on 20 June 1998. On 2 July 2001, the suspended mass and transport rate (Table 3) were relatively low. High turbidity was located in front of the intertidal mudflats and moved north-westward (Fig. 6A). There were no visible current marks or wave crests. On 14 December 2001, high SPMC was located along a 10-km-wide coastal band (Fig. 6B), where the distribution of SPMC was similar to those recorded on 5 December 2000. The above estimates are in gross agreement with those reported by NEDECO (1968).
The study of SPOT satellite images over the French Guiana coastal areas provides new results concerning the distribution of suspended sediments. The method used to quantify and to map the SPMC was based on in situ reflectance measurements. The atmospheric correction of SPOT images was also based on in situ reflectance measurements from various homogeneous targets. Despite the limited number of points used to establish a correspondence function between concentration and reflectance, and despite the simplified atmospheric correction and some degree of wavebreaking (foam) contamination on the image, the derived turbidity gradients as well as the SPMC values appear to be agree with surface turbidities measured in previous ground-based studies of the Guyana coast (NEDECO, 1968; LCHF, 1978). There is substantial spatial and temporal variability in the derived SPMC values. While values generally decreased offshore from 100 to 500 mg l 1 at the shoreline to 20 to 1 mg l 1 at 30 km offshore, smallscale spatial variations are observed and can be linked to the physical forcing mechanisms that act upon this coastal zone. The high coastal turbidities are always associated with waves that are visible on the 20 m resolution SPOT images. These observations are consistent with the results of previous studies (Wells and Coleman, 1981), suggesting that waves are a primary resuspension mechanism, and the Guyana coastal current transports this material along the coast. Turbid plumes appeared behind the small coastal islands, formed by the turbulence stimulated by the Guyana Current. These marks of current were observed on the images of 26 June 1998, 5 December 2000, and 14 December 2001. The mouths of estuaries show a large range of turbidity associated with frontal processes and seasonal variations in shore-facing coastal current. In the upstream of the rivers, the turbidity is generally small compared with the increased turbidity in the estuaries due to coastal water intrusion. Otherwise, turbid coastal waters may intrude into estuaries to create a turbidity increase.
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These results show that the quantification and the periodic synoptic SPMC mapping of the Amazonian suspensions can be envisaged with satellite imagery. Further improvements are necessary, particularly to take into account more realistic current speed values and to reduce the uncertainties in the satellite SPMC estimates. At the current stage, the number of SPOT images is not enough to estimate the surface sedimentary flux with sufficient accuracy. The Moderate Resolution Imaging Spectraradiometer (MODIS) satellite sensors (http://modis.gsfc.nasa.gov), which provide daily observations of the ocean’s surface SPMC distribution, will allow better understanding of the coastal environments particularly to trace the origin and fate of suspended sediments over great distances, and therefore provide relevant, timely information to coastal resource managers and scientists. Acknowledgements This work was supported by the PNEC-Guyane project (Programme National d’Environnement Cotier) and the U.S. National Aeronautics and Space Administration (NASA grant NAG5-10738). We thank the CNES for providing SPOT images at low cost (ISIS program). We are grateful to the staff of IRD for their logistic support and scientific discussions. References Allersma, E., 1971. Mud on the oceanic shelf off Guiana. Symp. Invest. Resour. Caribb. Sea and adjacent Reg. UNESCO, Paris, pp. 193 – 203. Allison, M.A., Lee, M.T., Ogston, A.S., Aller, R., 2000. Origin of Amazon mudbanks along the northeastern coast of South America. Mar. Geol. 163, 241 – 256. Augustinus, P.G., 1978. The changing shoreline of Surinam (South America). Thesis University of Utrecht: Soc. Invest. Hist. Natur. Stud. Utrecht, 95. CNES, 1986. Guide des utilisateurs de donnees SPOT. Manuel de reference, vol. 1. Centre National d’Etude Spatiale. Curran, P.J., Novo, E.M.M., 1988. The relationship between suspended sediment concentration and remotely sensed spectral radiance: a review. J. Coast. Res. 4, 351 – 368. DHL, 1962. Demerara coastal investigation. Report on Siltation of Demerara Bar Channel and Coastal Erosion in British Guiana. Delft Hydraulics Laboratory, The Netherlands. Doxaran, D., Froidefond, J.M., Lavender, S.J., Castaing, P., 2002. Spectral signature of highly turbid waters. Application with
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