Satellite assessment of the coupling between in water suspended particulate matter and mud banks dynamics over the French Guiana coastal domain

Journal of South American Earth Sciences 44 (2013) 25e34

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Satellite assessment of the coupling between in water suspended particulate matter and mud banks dynamics over the French Guiana coastal domain V. Vantrepotte*, E. Gensac, H. Loisel, A. Gardel, D. Dessailly, X. Mériaux INSU-CNRS, UMR 8187, LOG, Laboratoire d’Océanologie et des Géosciences, Université Lille Nord de France, ULCO, 32 avenue Foch, 62930 Wimereux, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 December 2011 Accepted 15 November 2012

Particulate suspended matter concentration (SPM) were estimated over a 8 year time period (2002e2010) in the coastal waters of French Guiana from a regional algorithm applied to the MODIS monthly reflectance measurements. Comparison between SPOT images and MODIS-SPM maps revealed the strong spatiotemporal coupling between in water SPM and the dynamics of local (i.e. Kourou and Cayenne) mud banks. Highest MODIS SPM values (>13 g m
3 approximately) can be significantly associated with the subtidal part of the banks as well as to the related turbid plume. The migration of these mud banks induces strong interannual changes in SPM reaching up to 6% year
1 within increasing and decreasing patchy areas distributed alternatively along the coastline of French Guiana. Mud banks migration rates derived from MODIS SPM data reach in average about 2 km year
1 in agreement with previous studies. The MODIS time series have allowed for a detailed description of the seasonality and interannual variations in the in-water SPM loads. Seasonal changes in SPM are related to the onset of the trade wind season. Marked non-linear patterns including a sharp evolution in the SPM values around 2005 as well as additional high frequency modulations have been emphasized within the upward and downward SPM trend regions. Concurrent temporal variations in the frequency of northward swells (favoring mud banks migration and reworking) as well as interannual changes in the amount of sediment delivered by the Amazon River have been shown to play a major role in the SPM temporal patterns observed in the French Guiana coastal waters. Our results clearly demonstrate the advantage for ocean color data to describe mud banks dynamics through the assessment of in water SPM temporal variability. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Suspended matter Mud banks Ocean color MODIS French Guiana

1. Introduction The coastal waters stretching between the mouths of Amazon (Brazil) and Orinoco Rivers (Venezuela) represent the world longest muddy coastal domain. This seashore hosts huge mud banks which are formed on the Amapá coast (North of Brazil) and migrate along the Guiana’s coastline under the influence of swells and currents. The mud banks dynamics over this area is strongly conditioned by the marine particulate matter originating from the Amazon River outputs (Gardel and Gratiot, 2005). The dynamics of the sedimentary discharge from the Amazon River has been widely documented (Gibbs, 1967; Meade et al., 1979, 1985; Bordas et al., 1988; Filizola, 1999). The most recent study (Martinez et al., 2009) highlights a suspended sediment discharge reaching about * Corresponding author. Tel.: þ33 21996414; fax: þ33 21996401. E-mail address: [email protected] (V. Vantrepotte). 0895-9811/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsames.2012.11.008

8.108 t year
1. Each year, around 15e20% of this sediment supply migrates along the Guiana coasts (Augustinus, 1978; Wells and Coleman, 1978; Eisma et al., 1991; Allison et al., 2000). A significant amount of this suspended sediment is trapped near Cabo Cassipore (North of Brazil) and participate to the mud banks formation (Allison et al., 2000). The remaining part of the Amazon suspended sediment is transported along the Guiana coast thanks to the North Brazilian and Guiana currents (Wells and Coleman, 1978). The mud banks migration processes strongly affect the morphology of French Guiana coastal domain. Mud banks are spaced of about 15e25 km (interbank areas), and are estimated to be of 10e60 km long, 20e30 km wide and 5 m thick (Allison et al., 2000; Froidefond et al., 1988). Mud banks protect the coasts from erosion which occurs in interbank phase while the massive and fast accumulation of mud along the coast can create significant issues for navigation (implying daily dredging in the concerned harbors) and tourism activities. Further, consolidated mud flats are rapidly

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colonized by mangroves which have a relevant impact on regional biogeochemical processes (Fromard et al., 2004; Proisy et al., 2009; Gensac et al., 2011). Numerous uncertainties still remain on the underlying mechanisms driving the mud banks formation and migration processes (Anthony et al., 2010). Recent studies based on high resolution satellite imagery have allowed a better description of the morphology and dynamics of the French Guiana mud banks (e.g. Froidefond et al., 2004; Gardel and Gratiot, 2005). The latter authors have emphasized the potential of LANDSAT and SPOT images to be used for estimating mud banks migration rates which can reach up to 3 km year
1. These previous studies, despite their high spatial precision, were however limited by the temporal resolution of the data used, preventing a detailed and continuous monitoring of the mud banks migration processes. In that context, the estimation of total suspended particulate matter concentration (SPM) from ocean color remote sensing can provide relevant information on the mud banks dynamics through the assessment of its impact on the in-water sediment loads. The spatial resolution of the optical remote sensing sensors is obviously lower (1  1 km for MOderate Resolution Imaging Spectroradiometer, MODIS) than the one used in the previously reported studies (between 10 and 30 m for Landsat and SPOT images). However, the latter potential limitation is largely compensated by the spatial (synoptic view) and temporal (long lasting time series, 9 year for MODIS, daily data) coverage offered by the optical satellite observations. Algorithms initially developed in the open ocean for estimating SPM were based on the covariation of the phytoplankton Chlorophyll a and suspended matter concentration. This assumption is not valid in the coastal ocean where the sources of particulate matter are heterogeneous, and usually independent from the phytoplankton dynamics (IOCCG, 2000). The assessment of SPM loads in coastal waters is therefore based on the strong positive correlation existing between the marine reflectance in the red part of the visible spectrum and the amount of particulate matter present in the water body. Accordingly, various single band general (Nechad et al., 2010) or regional (Froidefond et al., 2002, 2004; Vantrepotte et al., 2011a) inversion algorithms have been developed in recent years providing SPM estimates with a satisfying accuracy. The objective of this work is to illustrate the co-variation between surface SPM concentration and the mud banks resuspension and migration processes and therefore demonstrate the potential for ocean color SPM estimates for investigating the dynamics of French Guiana mud banks and its impacts on these coastal waters. In the frame of this study, SPM spatio-temporal variability in the French Guiana coastal waters is analyzed from MODIS monthly time series covering the time period June 2002 to November 2010. First, the presence of significant interannual changes in water masses turbidity over the MODIS 8-yr period is evaluated using non-parametric statistical analyses. In addition, a detailed description of the temporal patterns in SPM loads over the French Guiana coast is provided from the decomposition of the MODIS monthly time series into a seasonal, an irregular and a trend-cycle term using the Census X-11 procedure which advantages for describing the seasonality and non-linear long term changes in biogeochemical products have been previously emphasized from various applications on satellite time series (Vantrepotte and Mélin, 2009; Vantrepotte et al., 2011b; Mélin et al., 2011). Further, the identified temporal patterns are discussed and related to the influence of the local and larger scale environmental forcings.

2. Material and methods 2.1. Ocean color data MODIS-derived remote sensing reflectance, Rrs, at the 1  1 km2 spatial resolution, were acquired from the NASA Goddard Distributed Archive Center (reprocessing 2009.1) for the time period June 2002 to November 2010, using the standard algorithm for cloud detection. In practice, monthly time series (N ¼ 102) were considered in order to optimize the spatial coverage of the investigated region. These monthly data have been computed by averaging, for a given month and for each satellite pixel, the daily SPM maps derived from the MODIS reflectance measurements. 2.2. Suspended matter concentration estimates A recent study stressed that SPM can reliably be assessed in coastal turbid waters from the water marine reflectance in the red part of the visible spectrum which variability is mostly driven by the water particulate matter content (Nechad et al., 2010). The assessment of the suspended particulate matter concentration (SPM, in g m
3) from the radiometric measurement of Rrs was performed accordingly using this generic single band algorithm expressed as follows:

TSM ¼

A$rw þB 1
rw =C

(1)

where rw corresponds to the water reflectance (with rw ¼ Rrs$ p) and A, B and C the coefficients of the nonlinear regression. In the context of the present study we use rw (678) which corresponds to the MODIS band in the red. This formulation has been regionally adapted to the coastal waters of French Guiana allowing better estimates of the SPM concentration than the general relationship by Nechad et al. (2010) for the studied region. Equation parameters A, B and C were equal to 260, 1.092 and 0.142, respectively (Vantrepotte et al., 2011a). This algorithm was based on in situ SPM values collected during various cruises in the corresponding region gathering SPM loads ranging from 3 to 325 g m
3 (average of 34 g m
3) and has been validated against in situ data (SPM estimated/SPM measured ¼ 1.11, variation coefficient ¼ 30%, mean average relative difference ¼ 23%; Vantrepotte et al., 2011a). 2.3. Environmental forcings Various descriptors have been considered in order to describe the temporal variation in the environmental conditions over the MODIS time period analyzed. Information on the liquid discharges and surface particulate matter concentration delivered by the Amazon River to the marine domain were assessed from the OREHYBAM at the Obidos (Brazil) station (http://www.ore-hybam.org/ ). Wind stress and wind speed as well as swell parameters (height, period and direction) were computed from the NOAA-WavewatchIII model (http://polar.ncep.noaa.gov/waves/index2.shtml) and then averaged over an area encompassing the coastal waters of French Guiana (3e7 N; 50e55 W). 2.4. Statistical analyses The analysis of time series of SPM concentration and environmental forcing descriptors have been analyzed in order to identify the temporal patterns, including long term changes and seasonal variation over the considered MODIS time period. The temporal series analysis was conducted at the level of each grid point, taking

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into account the spatial variations in the temporal coverage of the data record. In practice, if valid data values for a given month m (for example January) and grid point are present less than 50% of the years (over a potential maximum of 8), all values for m are removed, in effect creating for that grid point time series with an annual cycle of varying length, or period, p < 12 (for the example, from February to December). Among these series, those with more than 25% of missing values are excluded. Occasional gaps remaining in the series have been conversely filled using the eigen vector filtering method (Ibanez and Conversi, 2002; see the detailed methods in Vantrepotte and Mélin, 2009, 2011; Vantrepotte et al., 2011b; Mélin et al., 2011). A time series X(t) (here the monthly series of SPM) can be decomposed as X(t) ¼ S(t) þ T(t) þ I(t), where S, T and I represent, respectively, the seasonal, the trend-cycle, and the irregular (or residual) component. In practice, this decomposition has been performed by using the Census X-11 method which is based on an iterative bandpass filter algorithm that explicitly allows the consideration of inter-annual variations in the seasonal cycle shape (Pezzulli et al., 2005; Vantrepotte and Mélin, 2011). The relative part of variance of the initial series associated with the component S(t), I(t) and T(t) can be then estimated in order to identify the main spatial patterns of temporal variability. The computation of the trend-cycle term derived from the X-11 decomposition procedure has been shown to be particularly adapted for describing nonlinear patterns from various applications performed on diverse ocean color products (Vantrepotte and Mélin, 2011; Vantrepotte et al., 2011a,b; Mélin et al., 2011). In addition, the presence of significant monotonic change in the data over the period investigated has been assessed using the seasonal Kendall test applied on X(t). The amplitude of the observed changes (in % yr
1) has been quantified using the Sen’s slope estimator (Gilbert, 1987). The Census X-11 procedure has been applied also applied on the monthly averaged wind, river discharges and swell data in order to be consistent with the MODIS temporal resolution. 3. Results and discussion 3.1. SPM distribution over the French Guiana coastal domain The 8-yr average SPM concentration derived from the MODIS marine reflectance reveals the presence of high SPM values within a continuous coastal band covering the whole coastal domain of French Guiana (Fig. 1). SPM concentration varies from few g m
3 (<3 g m
3) in the offshore regions (w30 km from the coast) to more than 70 g m
3 for the most nearshore pixels (>10 mg m
3, 10 km from the coast) in agreement with in situ measurements (Loisel et al., 2009). Note however that the maximal SPM loads observed from the current MODIS data set remain at the lower end of estimates previously documented in the same region and computed from high resolution SPOT images (reaching up to 400 g m
3, Froidefond et al., 2002, 2004). The latter feature is explained by the fact that reflectance measurement provided by the MODIS sensor does not allow to capture the most ultra nearshore marine domain due to the presence of clouds, bright pixels and adjacency effects (Santer and Schmechtig, 2000). Further, note that the consideration of monthly data for the present study tends to smooth the water SPM content especially when compared with high resolution daily images which enable, for instance, to account for varying tidal conditions that induces significant spatial modulation in the water SPM content (Froidefond et al., 2004). Some peculiar patterns however mark the presence of mud banks in the latter area with typical offshore extension particularly visible in front of Kourou (5 90 4100 N; 52 370 3300 W) and northern Cayenne (4 560 1100 N; 52180 3600 W). The spatial correspondence

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Fig. 1. MODIS 8-year average SPM concentration (g m
3) over the Guiana coastal waters.

between these mud banks and the MODIS SPM concentration has been checked from the concomitant SPOT images (Figs. 2 and 3) acquired at the beginning (September 2003 for the Kourou and Cayenne mud banks, respectively) and at the end (August 2010 for both sites) of the considered time series. The comparison between MODIS SPM maps and SPOT images indicates that the SPM spatial patterns depicted correspond more likely to the subtidal part of the leading edge of the mud banks particularly affected by resuspension processes (Gratiot et al., 2007). Further, high SPM concentrations also encompass the turbid plume associated with this part of the bank which is oriented northeastward following the direction of the Guyana current system. Conversely, the very nearshore inter and subtidal part of the bank often corresponds to masked (i.e. “flagged”) areas although it can occasionally and partially be captured from the MODIS measurements. From the latter comparisons, it can be assumed that a threshold of about 13 g m
3 can be used to significantly delineate areas associated with the subtidal part of the Guianese mud banks. 3.2. Temporal variability in SPM loads The trend analysis of the SPM concentration over the 8 years MODIS monthly time period within the French Guiana coastal waters reveals the presence of strong positive or negative long term changes in particulate matter loads reaching up to 6% per year (Fig. 4). Interestingly, these areas present patchy distributions showing with a clear alternation between increasing and decreasing SPM regions (Fig. 4). Note that these strong SPM trend regions are separated by thinner areas where the SPM concentration remained unchanged over the MODIS time period considered (Fig. 4). Two couples of decreasing and increasing SPM areas can be therefore identified over the coastal waters of French Guiana: one in front of the Kourou region and the other one in the Cayenne region (Fig. 4). These long term changes in the marine turbidity, which can be captured from changes in the optical characteristics of these waters, clearly reflect the impact of the migration of the corresponding Guianese mud banks. The morphodynamical processes driving the mud banks migration have been extensively documented in previous works (Augustinus, 1978; Froidefond et al., 1988; Eisma et al., 1991; Lefebvre et al., 2004; Gardel and Gratiot,

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Fig. 2. Comparison between SPM spatial distributions (g m
3) derived from MODIS monthly data and concurrent SPOT images in August 2003 (a,b) and September 2010 (c,d) in the region corresponding to the Kourou mud bank and showing the location of the mud flat and associated turbid plume. Grey areas correspond to flagged data.

2005; Gratiot et al., 2007; Winterwerp et al., 2007; Anthony et al., 2008). Basically, it can be described as follows. The back of the bank is composed by consolidated mud. This part of the mud bank, not protected by subtidal soft mud, is eroded and fluidized essentially through the action of waves as well as in a lesser extent through the influence of tidal and wind-induced geostrophic currents (Anthony et al., 2008). The particulate matter so mobilized is then transported to the leading edge of the bank resulting in a displacement of the mud bank along the coastline following to the direction of the residual coastal currents system here oriented from the southeast to northwest (e.g. Anthony and Dolique, 2004; Allison and Lee, 2004). Considering this conceptual scheme, it can be assumed that the position of the mud banks at the beginning of the MODIS time series includes the SPM decreasing area and the intertrend area (representing the initial front of the bank). Conversely, the position of the Kourou and Cayenne mud banks at the end of the MODIS series correspond to the regions encompassing intertrend area and the SPM increasing pixels. The previous assumption is confirmed from the corresponding SPOT images which provide a detailed description of the mud banks position at different time steps (Fig. 4). The previous framework has been considered for deriving, from the previously depicted trend SPM patterns (i.e. upward,

downward and inter-trend areas), various characteristics of the coastal waters significantly affected by the particulate matter originating from the mud banks. These characteristics, including for instance information on the alongshore and offshore extension of these turbid areas, deliver indirect information on the mud banks morphology and dynamics. The marine domain significantly impacted by the mud banks resuspension processes located in front of Kourou and Cayenne can be estimated to be on average of 20 km long and 13 km wide. The corresponding surface (w260 km2), although not being a straightforward estimate of the mud banks surface, falls within the range of previous computations performed in the same region from higher resolution imagery (Gardel and Gratiot, 2005). In addition to these morphological aspects, the following of the spatio-temporal evolution of the SPM concentration within the coastal regions showing significant trends in SPM can also be used for assessing mud banks dynamics. This is illustrated from the timeespace diagrams computed for two alongshore transects crossing the SPM trends regions (Fig. 5). The temporal evolution of the transect barycentre (Gardel and Gratiot, 2005) corresponding to the highest SPM values (>13 m g
3) allows for an indirect estimate of the mud bank migration rates. Average approximates derived from the MODIS SPM data for the Kourou and Cayenne mud banks range between 1.6 and 2 km year
1. This

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Fig. 3. Comparison between SPM spatial distributions (g m
3) derived from MODIS monthly data and concurrent SPOT images in August 2003 (a,b) and September 2010 (c,d) in the region corresponding to the Cayenne mud bank and showing the location of the mud flat and associated turbid plume. Grey areas correspond to flagged data.

falls into the order of magnitude of previous computations performed from SPOT and Landsat imagery (Froidefond et al., 1988; Gardel and Gratiot, 2005). Further, our results which are closer to those by Gardel and Gratiot (2005) tend to confirm the probable

acceleration during the last decade of the mud banks migration over the French Guiana coast. It is worth underlining that the latter estimates of the mud banks morphology and migration velocities derived from ocean

Fig. 4. Significant trends in SPM detected over the French Guiana coast from the 8 year MODIS time series (a) (in % year
1) and comparison with the mud banks estimated position and morphology (b). SPOT estimates of the location of the mud flat and subtidal extension of the mud bank at the beginning and end of the MODIS time series considered are represented by white and dark grey surfaces and dashed lines, respectively. The subtidal mud bank extension has been determined from swell damping characteristics (SPOT images).

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Fig. 5. Time space diagrams computed over two along shore transect crossing the areas presenting significant 2002e2010 trends in SPM in the Kourou (a) and Cayenne (b) coastal regions (from points A to B and C to D, respectively, see Fig. 4a). The red line shows the position of the transect barycentre associated with the highest SPM (see text). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

color imagery are conditioned by the spatial resolution of the optical sensor used in the frame of this study (i.e. MODIS, 1  1 km). However, the general agreement between MODIS derived estimates and those inferred from high resolution aerial photographs and satellite images (e.g. SPOT imager) confirm the potential for SPM ocean color data to be used as a proxy for monitoring mud banks impacts on the adjacent coastal domain and consequently for monitoring mud banks dynamics. Further, although the limitation represented by the optical sensor spatial resolution, using quantitative estimates of SPM concentration derived from ocean color remote sensing allow for a detailed description of the temporal evolution of the SPM loads associated with the mud banks dynamics. This information delivered by these continuous long lasting measurements of high temporal resolution are hardly available from other approaches and can help to elucidate the actual impact of the hydrodynamic forcing on mud banks migration processes and therefore deliver relevant insight on the underlying processes. 3.3. Seasonal and non-linear patterns in SPM loads The outputs of the Census X-11 procedure provide a detailed description of the SPM temporal patterns corresponding to the different areas of interest detected from the trend analysis performed over the French Guiana coastal domain. In practice, zonal average of X-11 trend-cycle, seasonal and irregular time series were computed within each increasing or decreasing SPM areas associated with the Cayenne and Kourou mud banks (Fig. 6). The seasonality explain a moderate part of the SPM temporal variability in the previous areas with a relative contribution of the component S(t) to the total variance of the series being lower than 25%. The major part of the temporal changes in SPM is explained by interannual patterns (>60%) while sub-annual evolution accounts for the remaining part of variability in the SPM data (w15%). The seasonal cycle in the SPM loads associated with the Kourou and Cayenne mud banks are shown in the Fig. 6. Basically, maximal SPM concentrations usually occur from September to November even though a strong year-to-year variation in the SPM seasonal cycle amplitude and shape. The seasonality observed in the SPM concentration can be related to that of the hydrodynamic conditions. The high SPM data between September and November coincide with the beginning of high swell (and high winds) time period in the French Guiana coastal waters. Similar observations have been reported by Gratiot et al. (2007) who showed that the effect of waves on mud re-suspension processes is particularly

marked at the onset of the high wave energy period (typically from October to May) which corresponds to that of the trade winds season. The previous authors have related this feature to a favored mobilization of the bottom particulate matter after a long lasting low energy waves time period when important quantities of mud are readily available. The mobilized particulate matter is then preferentially transported toward the coast due to the action of swell and currents (Allison and Lee, 2004). However, the corresponding ultra nearshore areas are often not or only partially visible from the current MODIS observations (see Section 3.1). This feature might explain the decrease of particulate loads in the sub-tidal part of the mud banks and associated turbid plumes regions after the beginning of the highest wave season. The temporal evolution of the X-11 trend-cycle term reveals that the long term increasing or decreasing changes detected in the SPM are not monotonic but correspond more likely to strong non-linear features. A strong spatial consistency in the SPM interannual variations is found within the different areas considered as emphasized by the low dispersion around the different zonal average signals (Fig. 6). Further, the long term evolution in the SPM loads are almost identical between the upward and downward SPM areas corresponding to the Kourou and Cayenne mud banks system, as emphasized by the highly significant correlation found between the corresponding time series (r2 > 0.9, p < 0.001). This underlines the parallel dynamics of the Cayenne and Kourou mud banks and their similar sensitivity to environmental forcing. First, sharp downward or upward shifts in SPM loads are observed from 2005 to 2006 and from 2008 to 2010, respectively, consistently for the two mud banks areas considered (Fig. 6). The fast decrease in SPM from 2005 to 2006 might translate a concomitant sudden change in the regional environmental conditions. The general increasing or decreasing SPM patterns can indeed be associated with the successive mud banks migration phases. Basically, the highly turbid waters observed both at the beginning of the decreasing time series and at the end of the increasing time series (Fig. 6) correspond preferentially to the front part of the mud banks where the mud can be easily mobilized through resuspension processes therefore explaining the high turbidity of these marine waters. Conversely, the weak and stable values of SPM until 2006 for the upward SPM time series correspond to an inter-bank situation where the presence of consolidated mud tends to lessen the impact of re-suspension processes on the marine surface water characteristics (Fig. 6). The low SPM loads from 2006 to 2010 associated with the decreasing SPM trend average time series mark the impact of the remaining and less concentrated mud located at the back of the bank (Fig. 6).

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Fig. 6. X-11 terms regional statistics associated with the significant SPM trends areas associated with the Kourou (a: decreasing and b: increasing area) and Cayenne (c: decreasing and d: increasing area) mud bank regions (see definition of the regions in Fig. 4). From the left to the right, panels show the average normalized time series for S(t) and T(t) (blackline), respectively, with the envelope figuring the dispersion of the data around the regional average signal (mean þ 1 standard deviation).

Additionally to these long term general trends, higher frequency modulations are also observed from the SPM X-11 trend-cycle time series. Indeed, various sharp peak events of varying amplitude lasting one year or less are visible on the SPM trend series are identified both on the SPM increasing and decreasing time series. These secondary oscillations are especially found at the beginning of 2003, 2005, 2007 and 2010. The Census X-11 analysis was also similarly computed for various variables describing potential concurrent interannual

changes in the hydrological and hydro-dynamical conditions over the French Guiana coast (see Section 2.3). Long term variations in hydrodynamic conditions have been first investigated. The latter can induce evolution in the mud banks re-suspension, erosion and therefore in sediments transport and mud banks migration processes and can therefore potentially explain the nonlinear trend patterns in SPM found within the French Guiana coast. The action of swells reaching the nearshore domain is known to be a major driving factor for the mud banks

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dynamics (Anthony et al., 2008; Gardel et al., 2011). Swells are characterized by their direction, period and amplitude. The two latter parameters (wave period and significant swell height, Fig. 7a) do not present any significant interannual feature allowing an explanation of the general long term pattern or the different peak events reported for the SPM. Conversely, significant co-variation is found between the SPM (p < 0.05 and p < 0.001 for downward and upward time series, respectively) and swell direction interannual evolutions. In particularly, our results suggest that more frequent episodes of northern swell might favor re-suspension processes and mud banks reworking in the turbid areas associated with the Kourou and Cayenne mud banks (Fig. 7b). As a matter of fact, the presence of a long lasting period of northward swell is for instance observed before the sharp decrease in SPM after 2005 within the SPM downward regions as well as during the abrupt increase in the SPM upward areas from 2008. This result is in agreement with recent works by Gardel et al. (2011) who showed that the punctual arrival of distant swell generated by storms and cyclones in the northern Atlantic might be responsible for relevant episodes of mud-bank reworking and bar formation in the Kourou area. Note that no direct co-variation exists between wind data for the period

2002e2010 (i.e. regional average wind speed and wind stress) and the long term changes in SPM (not shown). The wind parameters considered in the frame of this study correspond to regional average over a large coastal zone (3e7 N; 50e55 W) that can assumed to be representative of the long term evolution of this forcing. However, detailed data on local wind changes might be useful for assessing the exact role of this environmental forcing. Besides variations in hydrodynamic forcings, the impact of long term variation in terrestrial inputs has been also characterized. No direct link between interannual changes in French Guiana local river discharges (Kourou and Cayenne rivers), which remains stable over the MODIS time period, and SPM concentration can be established (not shown). This confirms the restricted impact of these local forcings on the inputs of mud present over the Guianese coastal waters as already underlined by previous works (e.g. Gardel and Gratiot, 2005; Anthony et al., 2008). Conversely, a significant co-variation is found between the long term evolution in the Amazon River particulate matter concentration over the 2002e 2007 time period and the SPM trend patterns depicted for Kourou and Cayenne mud banks regions (p < 0.001 for decreasing areas and p < 0.05 for increasing areas). Especially, the high

Fig. 7. Time series and corresponding X-11 trend-cycle terms associated with the regional average (a,b) swell significant height and (c,d) direction (derived from the NOAA Wavewatch-III, Model) as well as with (e,f) SPM concentration (g m
3) in the Amazon river surface waters measured at the Obidos station (Brazil, ORE-HYBAM measurement network). The solid and dashed red lines in the panels, b, d and f show the normalized SPM X-11 trend-cycle series averaged within the downward and upward SPM areas identified in the region of Kourou (see Fig. 4a). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

V. Vantrepotte et al. / Journal of South American Earth Sciences 44 (2013) 25e34

frequency peak events increasing episodically the SPM loads at the beginning of 2003, 2005 and 2007 are also clearly visible for the trend patterns associated with SPM loads transported by the Amazon River (Fig. 7). The fate of the particulate matter originating from the Amazon River is diverse. A part of the SPM delivered to the marine waters is catch northern to the Amazon River mouth due to regional currents (Anthony et al., 2010). Although the underlying complex processes still remain poorly understood (Anthony et al., 2010), this trapped mud leads to the formation of huge banks representing the source of the mud banks migrating over the French Guiana coast. Another part of the Amazon particulate matter reaches the coastal waters of the French Guiana on short timescale (Eisma et al., 1991) and might therefore be responsible of sudden increases in SPM concentration observed in the MODIS time series. Interestingly, the long term decrease in SPM loads over the Kourou and Cayenne areas between 2002 and 2006 is also associated with a concurrent long term diminution of about 35% of the inputs in particulate matter from the Amazon River (Fig. 7c). This long term changes in the Amazon River solid discharge has been previously documented by Martinez et al. (2009) and is not related to a parallel evolution in the corresponding liquid discharge. The impact of this interannual decrease in the Amazon sediments inputs is presumably masked by the predominant impact of mud banks migration processes on the local changes in SPM documented in the present study. However, it can be assumed that the long term decrease in the Amazon solid discharges between 2002 and 2006 might have played an additional role on the observed downward SPM evolution over this time period. This assumption can unfortunately not be confirmed from the current data set due to the lack of continuous measurements of Amazon sediment loads from 2007 to 2010. These results confirm however the predominant role of Amazon River inputs on the SPM dynamics, including long term and high frequency variations, within the French Guiana coastal domain. 4. Conclusions This study illustrates for the first time the potential for SPM estimates derived from ocean color reflectance measurements for characterizing and monitoring the impact of mud banks resuspension and migration processes in the French Guiana coastal waters. The comparison between MODIS derived SPM maps and corresponding high resolution SPOT images indicates that high SPM loads correspond essentially to the frontal subtidal part of the mud banks as well as to the associated turbid plume. The analysis of 8-year MODIS time series reveals the presence of strong trends in SPM data that can be significantly associated with the mud banks migration processes affecting the French Guiana coastal domain. Ocean color SPM derived estimates of the surface marine area affected by the mud banks re-suspension as well as of the mud banks migration rates are in agreement to those obtained from previous studies performed on SPOT or Landsat imagery. Despite a lower spatial resolution when compared to the latter measurements, the advantage of ocean color data stands mainly in its ability to describe precisely seasonal and interannual patterns in surface SPM loads providing an indirect description of the mud banks dynamics. Both seasonal and interannual patterns in SPM concentrations present a high consistency within and between the Kourou and Cayenne mud bank areas. The SPM seasonality confirms that the onset of the trade wind season corresponds to the most important phase of mud mobilization (Gratiot et al., 2007). The detailed description of the long term evolution in the MODIS-SPM data suggests that the mud banks migration processes more likely correspond to sharp nonlinear interannual patterns rather than to gradual monotonic features. In addition, higher frequency peak events have been shown to affect the amount of particulate matter

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present within the coastal waters of French Guiana. These interannual changes in SPM have been significantly associated with concurrent changes in the intensity of the terrestrial inputs from the Amazon River. This emphasizes the importance of a sustainable monitoring of the amount of particulate matter delivered by this riverine system to the marine domain as it is currently performed by the ORE-HYBAM measurements network. Additionally, interannual changes in SPM have been also related to the orientation of the swell reaching the coastal waters of French Guiana. Indeed, a greater occurrence of northward swells seems to favor mud resuspension and mud banks reworking confirming results from previous works (Gardel et al., 2011). Apart the latter environmental descriptors, no clear evidence of concurrent changes has been emphasized between SPM long term changes and the other ancillary parameters considered in this study for describing changes in the hydrodynamic conditions (i.e. wind stress and swell height and period). A consideration of the local behavior of these parameters as well as the inclusion of additional descriptors such as those derived from altimetry might help to further elucidate our observations. Further, the observed interannual changes in the banks migration velocities might be related to morphology of the coastline (including the length of the shoreline linear and its orientation, the vicinity from local rivers, variations in the seabed nature.) which impacts on the mud banks dynamics have been clearly demonstrated (Gardel and Gratiot, 2005). In that context, our results have clearly emphasized the interest of using SPM ocean color data for modeling purpose aiming to integrate the impact of these different factors which effects are intimately interrelated through complex relationships. The use of ocean color data at higher spatial resolution (MERIS, 500 m) as well as the integration of a cloud mask allowing a better distinction between cloudy and highly turbid areas would provide a better description of the SPM nearshore patterns. The method presented in this work and illustrated from an application on the French Guiana coastal waters will be applied in future works on ocean color SPM data computed over a larger spatial window encompassing coastal areas from the Amazon to the Orinoco river mouths. The analysis of ocean color time series in the latter coastal band would provide new insights on the fate of the terrestrial particulate matter delivered by the Amazon and its actual impact on the Guianese coast, which currently remain to be better characterized. Acknowledgments This research has been funded by CNES through the TOSCA/ Coulcot project as well as by the GlobCoast project supported by the Agence Nationale pour la Recherche (ANR, Paris). We are grateful to the “Papi Jo” crew. The authors also would like to thank the NASA MODIS Project and the NASA/GSFC/DAAC for the production and distribution of MODIS data. The ORE-HYBAM program is acknowledged for providing Amazon River discharges data. The NOAA Wavewatch group is acknowledged for providing swell and wind data. References Allison, M.A., Lee, M.T., 2004. Sediment exchange between Amazon mudbanks and fringing mangroves in French Guiana. Marine Geology 208, 169e190. Allison, M.A., Lee, M.T., Ogston, A.S., Aller, R.C., 2000. Origin of Amazon mudbanks along the northeastern coast of South America. Marine Geology 163, 241e256. Anthony, E.J., Dolique, F., 2004. The influence of Amazon-derived mud banks on the morphology of sandy headland-bound beaches in Cayenne, French Guiana: a short to long term perspective. Marine Geology 208, 249e264. Anthony, E.J., Gardel, A., Gratiot, N., Proisy, C., Allison, M.A., Dolique, F., Fromard, F., 2010. The Amazon-influenced muddy coast of South America: a review of mudbank e shoreline interactions. Earth-science Reviews 103, 99e121.

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