Gabriel ; Reygondeau
email: gabriel.reygondeau@ird.fr

Gregory Beaugrand
email: gregory.beaugrand@univ-lille1.fr

Grégory Beaugrand

Water column stability and Calanus finmarchicus

Keywords: thermocline, Calanus finmarchicus , North Atlantic Ocean , Macroecology, Exponentially Weighted Moving Average Exponentially Weighted Moving Average, Calanus finmarchicus, thermocline, North Atlantic Ocean, Macroecology

Introduction

The thermocline is often defined as a vertical zone of rapid temperature change in the water column located below the surface layer of rapid mixing [START_REF] Kaiser | Marine ecology: processes, systems, and impacts[END_REF]. It can be seasonal or permanent. Seasonal thermoclines are located in extratropical regions, being shallow in spring and summer, deep at the beginning of the autumn, and disappearing in winter [START_REF] Tomczak | Regional Oceanography: an Introduction 2[END_REF]. In the tropics, winter cooling is not strong enough to destroy the seasonal thermocline, leading to a permanent thermocline called the tropical thermocline [START_REF] Tomczak | Regional Oceanography: an Introduction 2[END_REF]. In high latitudes, temperature profiles vary seasonally as a function of wind stress and ice cover. From spring to autumn, a dicothermal layer separates the upper from the deeper zones of the water column and in winter, the water column is homogenous [START_REF] Pickard | Descriptive Physical Oceanography: An Introduction 5[END_REF]. [START_REF] Pickard | Descriptive Physical Oceanography: An Introduction 5[END_REF] described a depth range of seasonal and tropical thermoclines from 0 to 500 m. Below 500 m and up to 1000 m, the seasonal thermocline is known as the permanent or oceanic thermocline (i.e. transition zone from warm to cold waters of great oceanic depth). This type of thermocline is not considered in this paper [START_REF] Tomczak | Regional Oceanography: an Introduction 2[END_REF]. In the North Atlantic Ocean, both the depth and the intensity of the thermocline vary with the latitude and the local hydrodynamics [START_REF] Longhurst | Ecological Geography of the Sea[END_REF].

Methods for estimating the depth of the thermocline from profile data fall into two broad categories of threshold methods [START_REF] Thomson | Estimating mixed layer depth from oceanic profile data[END_REF] (Table 1): 1) Methods using a depth-to-depth temperature or density difference [START_REF] Defant | Physical Oceanography[END_REF][START_REF] Kara | An optimal definition for ocean mixed layer depth[END_REF][START_REF] Kara | Naval Research Laboratory Mixed Layer Depth (NMLD) Climatologies[END_REF][START_REF] Wijffels | Direct observations of the Ekman balance at 10°N in the Pacific[END_REF], 2) Methods using a temperature or density difference between a reference depth and other depths in the profile [START_REF] Lamb | On the mixed-layer climatology of the north and topical Atlantic[END_REF][START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF]Monterey and Levitus, 1997;[START_REF] Sprintall | Salinity considerations in the oceanic surface mixed layer[END_REF][START_REF] Weller | Observations of the vertical structure of the oceanic boundary layer[END_REF]. For a long time, thermocline depth was considered as the Mixed Layer Depth (MLD) until Sprintall and F o r P e e r R e v i e w [START_REF] Sprintall | Salinity considerations in the oceanic surface mixed layer[END_REF] showed that there was a difference between the two hydrodynamical features [START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF][START_REF] Sprintall | Salinity considerations in the oceanic surface mixed layer[END_REF]. However, due to a lack of salinity data, or because salinity data tend to be noisier than temperature data, MLD is sometimes linked to a steplike change in water temperature, with specified steps in the range 0.018-0.58°C [START_REF] Thomson | Estimating mixed layer depth from oceanic profile data[END_REF].

The structure of the water column influences the biodiversity of pelagic ecosystems and related biogeochemical cycles [START_REF] Bopp | Response of diatoms distribution to global warming and potential implications: A global model study[END_REF][START_REF] Kara | An optimal definition for ocean mixed layer depth[END_REF][START_REF] Rutherford | Environmental controls on the geographic distribution of zooplankton diversity[END_REF][START_REF] Sarmiento | Ocean biogeochemical dynamics[END_REF]. The stratification of the water column has an effect on the spatial distribution of plankton and on the life cycle of many pelagic organisms [START_REF] Beare | Temporal patterns in the surface abundance of C. finmarchicus and C. helgolandicus in the northern North Sea (1958-1996) inferred from the Continuous Plankton Recorder data[END_REF]. For example, [START_REF] Rutherford | Environmental controls on the geographic distribution of zooplankton diversity[END_REF] showed a strong relationship between the diversity of foraminifera and the properties of the water column at a global scale.

Calanus finmarchicus, a subarctic oceanic species found north of the Oceanic Polar Front (Dietrich, 1964), is among the most studied species of copepods in the world [START_REF] Mauchline | The biology of calanoid copepods[END_REF]. The species plays a central role in transferring the primary production to high trophic levels [START_REF] Dickson | Effects of a changing windfield on cod stocks of the North Atlantic[END_REF]. It is the prey of many young stages of fish and the fluctuations of Calanus population have been linked to the variation in the stock of some species of fish [START_REF] Beaugrand | Plankton effect on cod recruitment in the North Sea[END_REF]. Some authors have suggested that the structure of the water column could influence the vertical and horizontal distribution of the species [START_REF] Helaouët | Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas[END_REF]Williams and Lindley, 1980a;Williams and Lindley, 1980b;[START_REF] Williams | Vertical distribution of Calanus finmarchicus and C. helgolandicus (Crustacea: Copepoda)[END_REF][START_REF] Williams | Vertical distribution of Calanus finmarchicus and C. helgolandicus in relation to the development of the seasonal thermocline in the Celtic Sea[END_REF]. [START_REF] Helaouët | Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas[END_REF] used a proxy of water turbulence induced by atmospheric forcing (i.e. the cube of the wind speed) and data from the Continuous Plankton Recorder (CPR) survey (sampling between 0 and 10 m) to suggest a possible influence of turbulence or of the the mixed layer depth on the abundance of C. finmarchicus [START_REF] Helaouët | Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas[END_REF]. Williams (1980[START_REF] Williams | Vertical distribution of Calanus finmarchicus and C. helgolandicus in relation to the development of the seasonal thermocline in the Celtic Sea[END_REF] supposed that at the southern boundary of the species' spatial distribution, the stratification could stop the vertical flux of organic particule and consequently increase the inter-specific competition for food with C. helgolandicus. [START_REF] Williams | Vertical distribution of Calanus finmarchicus and C. helgolandicus in relation to the development of the seasonal thermocline in the Celtic Sea[END_REF] concluded that it was likely that the survival rate of C.finmarchicus located beyond the thermocline diminished due to food limitation.

The objectives of this paper were to investigate at a macroecological scale the influence of the structure of the water column (i.e. depth and intensity of the thermocline) on Calanus finmarchicus. First, we implemented a new numerical procedure, based on a technique used in statistical quality control and called Exponentially Weighted Moving Average (EWMA) to identify and quantify the depth and intensity of the thermocline. Second, the procedure was tested in neritic and oceanic regions. Third, this procedure was used on more than 1 million temperature profiles, allowing us to map the seasonal variability of the thermocline depth and intensity over the North Atlantic Ocean. Finally, the relationships between the structure of the water column and the relative frequency of C. finmarchicus were investigated from copepodite stage 1 to adults. Using a new database, monthly and interannual changes were investigated to indicate the different ways in which thermocline descriptors could affect C. finmarchicus population dynamics.

Method

Biological data

Biological data used in this study were located in the northern part of the North Atlantic Ocean and extended from 80 °W to 19.5 °E of longitude and 25.5 to 73 °N of latitude (Fig. 1a). Biological data came from different sources: the TASC programme ( from 1977to 2006;Labrador data in 1997Labrador data in , 2001Labrador data in and 2002. . Spatial distribution of the dataset is presented in Figure 1a and details of sampling methods are shown in Supplementary Table 1.

Chlorophyll-a data were gathered from the same surveys as the C. finmarchicus database and completed by downloaded data from the World Ocean Database (www.nodc.noaa.gov). Chlorophyll-a from the upper five meters were retained and integrated.

Physical data

Physical data utilised in this work were located in the North Atlantic Ocean, extending from 81.5 °W to 23.5 °E of longitude and 18.5 °N to 75 °N of latitude. Temperature and bathymetric data came from different high resolution profile sources. Some were provided by Conductivity-Temperature-Depth (CTD) data samples collected during the above biological cruises. Others came from the National Oceanic and Atmospheric Administration (NOAA, data provided by Maureen Taylor). Lastly, some data were downloaded from the World Ocean Database (www.nodc.noaa.gov) including Mechanical BathyThermograph (MBT), eXpendable BathyThermograph (XBT) and Profiling Floats (PFL) data. Data encompassed the period 1876-2008 (Fig. 1b). All profiles were taken from the surface to 500 m to focus on seasonal thermocline. The main surface currents and frontal structures of the region covered in this study are indicated in Fig. 1c. The techniques used to identify the depth and intensity of the thermocline have generally been empirical in the past. In this study, we designed a new procedure, which is summarised in Fig. 2. The procedure was divided into 4 steps:

Analyses

Identification of intensity and depth of the thermocline

Step 1: for each profile, two tests were performed before estimating the thermocline. The first test measured the number of points in each profile. When the number of measures was strictly below 5 (threshold defined empirically in this paper), no estimation was made. The second test measured the resolution of the data. If the profile had a difference of more than 50 m and 20 m between successive depths (threshold defined empirically) in oceanic and neritic regions respectively, the information was considered too scarce and the estimation of depth and intensity of the thermocline was not calculated.

Step 2: the temperature of the profile was linearly interpolated for each interval of 5 m from the original profile (in this study) [START_REF] Lam | Spatial interpolation methods: a review[END_REF]. This interpolation was used to regulate the gap from one sample to another. No extrapolation was carried out here. Interpolation started at 2 meters to minimise the influence of the diurnal fluctuation of the water temperature and the noise related to the instrument in the first few meters [START_REF] De Boyer Montégut | Mixed layer depth over the global ocean: An examination of profile data and a profile-based climatology[END_REF]. The procedure ended at the maximal depth of the profile (maximum 500 m). The interpolation technique was applied because the use of the weighted moving average in Step 3 needed equally spaced data.

Step 3: a filtering technique, named Exponentially Weighted Moving Average (EWMA) [START_REF] Montgomery | Introduction to statistical quality control[END_REF], was applied on each temperature profile. This technique was chosen because it enables low or high persistent values to be better identified (Beaugrand and Ibanez, unpublished data). The technique is similar to the "cumulative sums", another technique frequently used in statistical quality control [START_REF] Ibañez | Application de la méthode des sommes cumulées à l'analyse des séries chronologiques en océanographie[END_REF]. The technique was calculated by an iterative procedure and as follows:

1 0 ). 1 ( . - - + = z z E A E λ λ (1) F o r P e e r R e v i e

w

Where z E is the value of the moving average at depth z, 1 z E is the value of the moving average at depth z-1, 0 A is the value of the initial dataset at depth z 0 and λ is the filtering factor which is typically comprised between 0 and 1 and often fixed at 0.3 (Beaugrand and Ibanez, in preparation). The technique also enables a reduction of the noise inherent to this kind of data. The type of filtering is asymmetric contrary to most of the filtering methods (simple or weighted moving average, see Legendre & Legendre, 1998). This makes the method more sensitive to local variations.

Step 4: depth-to-depth absolute temperature difference d(T) was calculated every 5 meters as follows:

Za Zb T T T d - = ) ( (2)
Where T Zb is the temperature at depth Z b and T Za is the temperature at depth Z a ., a and b ranging from 2 to 500m. The depth-to-depth temperature difference amplitude (i.e. the maximal minus the minimal difference in temperature calculated for each profile) was chosen to be the variable of the threshold method. This absolute difference amplitude marks a standardised increase or decrease of water temperature. In some cases, this variable can give the depth of the temperature inversions which occur at the base of barrier layers and in Polar

Regions. When the depth-to-depth temperature difference amplitude was lower than a threshold (see Analysis 2), the thermocline intensity was fixed at the maximal difference of temperature and the thermocline depth was made equal to the maximal depth of the profile. In other cases, the thermocline intensity was fixed at the maximal value of d(T) and the depth of the thermocline was the depth at which the maximal value of d(T) was detected.

Before applying the procedure of identification of the depth and intensity of the thermocline (see previous analysis, step 4), it was important to select a threshold that allows the numerical procedure described in this paper to distinguish between a homogenous and a stratified water column. The selection of the threshold was done by performing 3 analyses.

First, we examined monthly changes in the maximum depth-to-depth temperature difference (see Fig. 3) to evaluate the possible range of the different thresholds which could separate a homogenous from a stratified water column during a year, following descrptions in the literature [START_REF] Longhurst | Ecological Geography of the Sea[END_REF][START_REF] Pedlosky | Ventilation, potential-vorticity homogenization and the structure of the ocean circulation[END_REF]. This analysis was conducted at a large-scale, covering the whole supratropical region of the North Atlantic and adjacent seas to consider the full range of conditions experienced by C. finmarchicus. We examined the interval of maximum annual variation (see Fig. 3) to temporarily select some thresholds for the next analysis in this interval.

Second, we tested the new procedure against two types of averaged thermal profiles.

Two regions were chosen to observe bi-monthly changes in the depth and intensity of the thermocline (see Fig. 1 and4). The first station (see Fig 4a) was located between 42°N and 44°N and 66°W and 71°W and represented a neritic area of 132m mean depth. A total of 17,846 profiles were considered in this area. The second area (see Fig. 4b) was located between 44°N and 46°N and 25°W and 30°W. The region represented an oceanic area of 2620 meters mean depth. A total of 1,004 profiles were considered in this region.

Temperature data were linearly interpolated every 5 meters for each profile from 2 meters to the maximal depth. A mean temperature was calculated for each depth and each two-week period. Mean bi-monthly depth and intensity of the thermocline were also calculated using the techniques of [START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF] and [START_REF] Defant | Physical Oceanography[END_REF]. The [START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF] method assesses the depth of the thermocline by calculating the differences between a reference point (here 0 m) and subsequent depths. A threshold of 0.5°C was selected by the author [START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF]. The [START_REF] Defant | Physical Oceanography[END_REF] technique computes differences between successive depths with no prior transformation of the data. The threshold selected by the author was 0.015°C m because of the resolution of the profiles after interpolation, which corresponded to 0.03, 0.05, 0.06, 0.08°C.m -1 . For both regions, the analysis of the intensity of thermocline was used to quantify the period of restratification and destratification. Restratification was considered as the first significant increase (i.e. increase of 5% from one point to the next) in the intensity of the thermocline. Destratification corresponds to the first significative decrease in the intensity of the thermocline (i.e. decrease of 5% from one point to the next). The Thresholds and methods were compared to the thermal profiles of the two regions.

Third, both depth and intensity of the thermocline were mapped and the different thresholds examined at a macro-scale (Fig. 5). For each geographical cell of 1° of longitude and 1° of latitude, median depth and intensity of the thermocline were mapped after application of the techniques on each profile between 17.5 °E and 80.5 °W and between 30.5 and 72.5 °N. Spatial variation was computed using an isotropic variogram [START_REF] Matheron | Traité de géostatistique appliquée[END_REF] so as to compensate for gaps in the database, a kriging method based on the mgstat package of matlab (www.mgstat.sourceforge.net) was used. The spatial scale at which these parameters were mapped was selected to compare characteristic features of the whole supratropical region detected by other procedures (e.g. the transition zone between permanently and seasonally stratified regions at 40°N). Mapping was realised in spring (from April to June), summer (from July to September), autumn (from October to December) and winter (from January to March), using the method of [START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF] and the technique proposed in this paper with different thresholds (see Analysis 1; see Fig. 5). Our results were then compared to existing knowledge on North Atlantic circulation (Fig 1c .). The maps of the selected thresholds for the depth of the thermocline are represented in Figure 6 and the intensity of the thermocline in Figure 7.

Relationships between the structure of the water column and Calanus finmarchicus

The objectives of these analyses were (1) to characterize the tolerance of each copepodite stage of C. finmarchicus to both the depth and the intensity of the thermocline and

(2) to understand how the thermocline may affect the spatial distribution of the different copepodite stages.

Due to the variability in the sampling techniques among biological datasets (see Supplementary Table 1), a direct comparison based on abundance was likely to give biased results [START_REF] Nichols | Mesh selection of copepodite and nauplius stages of four calanoid copepod species[END_REF][START_REF] Ohman | On birth and death in the sea[END_REF]. Therefore, we worked on relative frequency (Legendre and Legendre, 1998). To compute this index, the abundance data for each sampling sites and copepodite stages were integrated (BONGO and MOCNESS stratified dataset) on the whole water column sampled and converted into presence or absence. Then, for each geographical cell (spatial grid of 1° of longitude and latitude), month and year, the sum of the number of presences was divided by the total number of samples for each copepodite stage. Due to the likely overestimation or underestimation (mesh of 35-96 µm for early stages) of some specific copepodites by the different sampling techniques [START_REF] Nichols | Mesh selection of copepodite and nauplius stages of four calanoid copepod species[END_REF][START_REF] Ohman | On birth and death in the sea[END_REF], their potential influences were tested to estimate potential biases in the dataset. Relative presence was computed with and without all the different sampling techniques. No significant difference was observed between datasets, therefore the analyses presented in this article were performed with the whole database. different stages of the species as a function of the intensity of the thermocline was assessed from 0 °C.5m -1 to 2 °C.5m -1 using a regular interval of 0.05°C.5m -1 . A tolerance diagram was also built to examine the relative presence of the different stages of C. finmarchicus as a function of the depth of the thermocline from 0 m to 200 m by using a regular interval of 25 m. This procedure was applied for each every month and each copepodite stage (from stage 1 to adult) (see Fig. 8).

Secondly, correlations between the relative presence of C. finmarchicus and both the intensity and depth of the thermocline were calculated at the scale of the North Atlantic Ocean for the months between April and June (see Fig. 9). Prior to performing this analysis, the relative presence of the species was averaged for categories of 0.05°C.m -1 of thermocline intensity ranging from 0 to 1.5°C.5m -1 and for categories of 10m of depth between 0 and 200 m. Only intervals with an observation were considered when performing the linear regression.

Third, an average of thermocline intensity and depth was calculated for the whole region where C. finmarchicus occurs at both monthly and year-to-year scales. The relative presence of C. finmarchicus was reported on the graphs to examine potential relationships between the species (copepodite stage 1, 5 and adults) and the mean characteristics of the thermocline (see Fig. 10). The objective of this analysis was therefore to study the relationships between the species and the thermocline by averaging the space and focussing on time at both seasonal and year-to-year scales.

Relationships between C. finmarchicus and environmental parameters

To understand how stratification affects the relative frequency of the species, the relation between thermocline (depth and intensity), surface chlorophyll-a and C. finmarchicus was investigated. Data on relative presence of C. finmarchicus were averaged for categories of chlorophyll-a of 0.075µg.l -1 between 0 and 4.2µg.l -1 . Then, correlations were calculated between the relative presence of C. finmarchicus and the concentration of chlorophyll-a for all copepodite stages (see Fig. 11). Data of chlorophyll a were averaged for categories of 0.05°C.m -1 of thermocline intensity ranging from 0 to 1.5°C.5m -1 and for categories of 10m

for the depth of the thermocline from 0 to 200 m in continental shelf regions and from 0 to 500 m in oceanic regions. Then, correlations were calculated between both the intensity and the depth of the thermocline and the chlorophyll-a concentration (see Fig. 12).

Results

Selection of a threshold to discriminate a homogeneous from a stratified water column

A fundamental step before running the technique for each thermal profile was to choose a relevant threshold to discriminate between a homogeneous and a stratified water column. This was done by performing three analyses. The first analysis used frequency histograms of the depth-to-depth temperature difference amplitude (i.e. maximum depth-todepth temperature difference) found in each profile and each month (Fig. 3). Between December and April when the water column was mainly homogeneous and June and September when the water column was mostly stratified, pronounced changes (maximal annual variation) were observed between 0.1°C.5m -1 and 0.4°C.5m -1 (Fig. 3). This analysis enabled the selection of potential thresholds between 0.1°C.5m -1 and 0.4°C.5m -1 to discriminate between a homogeneous and a stratified water column. Such a range of potential thresholds coincided with values commonly found in the literature [START_REF] Thomson | Estimating mixed layer depth from oceanic profile data[END_REF].

The second analysis tested our procedure with different thresholds in the range determined previously (see Fig. 3) against two other already published methods [START_REF] Defant | Physical Oceanography[END_REF][START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF] in a neritic (Fig. 4a) and an oceanic region (Fig. 4b). In the neritic region during the winter mixing period from December to April, the method of Defant and Levitus and our procedure based on a threshold of 0.15°C.5m -1 localised the depth of the thermocline shallower than with other thresholds (Fig. 4a). The restratification period was detected in April in the neritic region and at the end of May in the oceanic region with a significant increase in the slope of the depth of the thermocline (Fig 4). This period was well identified with all the techniques and thresholds except for the threshold of 0.4°C.5m -1 in the neritic region (Fig 4a). In the oceanic region (Fig. 4b), the methods of Levitus and Defant and our technique based on thresholds of 0.15°C.5m -1 localised the thermocline between April to May. When thresholds of 0.3°C.5m -1 and 0.4°C.5m -1 were used, the technique identified the establishment of the thermocline at the beginning of June. During the summer stratification (strong stratification between June and September), the methods of Levitus and Defant represented the depth of the thermocline shallower than our method. The destratification period detected in September for both regions was well detected with all the procedures with the notable exception of the technique of Levitus that localised the destratification in October.

Among all the techniques and thresholds, the results indicated that our procedure based on a threshold of 0.25°C.5m -1 gave the closest match in term of depth. Furthermore, the intensity assessed from our technique with a threshold of 0.25°C.5m -1 gave a satisfactory summary of the structure of the water column (Fig. 4). Similar results were observed in the oceanic region (Fig. 4b). Here also, the threshold of 0.25°C.5m -1 gave the best compromise between a too conservative result (threshold of 0.4°C.5m -1 ) and too much sensitivity (Defant, followed by Levitus). In all cases, the procedure of Defant was too sensitive and was therefore excluded from the next analysis.

At a basin scale, the comparison of the Levitus technique with our method based on the thresholds 0.25, 0.30 and 0.40°C.5m -1 showed a good correspondence in the spatial changes in the depth of the thermocline (Fig. 5). However, some differences were observed at the boundary between permanently and seasonally stratified regions. The Levitus method did not clearly detect this well-known boundary. The new technique gave the best contrast between both permanently and seasonally stratified regions (PSWW: permanently stratified water in winter, Fig. 1c) when the smallest threshold was used. We therefore selected the threshold of 0.25°C.5m -1 in the next analyses.

Seasonal changes in the spatial distribution of the depth and the intensity of the thermocline

Seasonal variability in the spatial distribution of the depth and the intensity of the thermocline were examined in the North Atlantic sector using our procedure with a threshold of 0.25°C.5m -1 (Fig. 6 and Fig. 7). Mapping of seasonal changes in the depth of the thermocline clearly located the Subtropical Convergence (PSWW and PSWS, Fig 1c). This front separates the permanently stratified regions of the North Atlantic Subtropical Province from the seasonally stratified extratropical provinces of the Atlantic Westerly Winds Biome as it was termed by [START_REF] Longhurst | Ecological Geography of the Sea[END_REF]. In the north of the Subtropical Convergence, deep mixing occurred and in most regions (e.g. the Subarctic Gyre) winter mixing was greater than 500m in winter (Fig. 6). The meso-scale variability associated with the path of the Gulf Stream was particularly well detected in winter and the singularity of the South West European Basin clearly identified (Fig. 1b). A very interesting feature was the detection of the influence of the circulation around the Subarctic Gyre that increases the depth of the mixing in summer and autumn (Fig. 6c-d). The knowledge about the spatial distribution of C.

finmarchicus [START_REF] Heath | Spatial demography of Calanus finmarchicus in the Irminger Sea[END_REF][START_REF] Helaouët | Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas[END_REF][START_REF] Planque | Calanus and environment in the eastern North Atlantic. I. Spatial and temporal patterns of C. finmarchicus and C. helgolandicus[END_REF] and the present results suggest that the species is predominantly present in those oceanic regions characterised by deep winter mixing. This condition is important but sea surface temperature is another essential controlling factor [START_REF] Helaouët | Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas[END_REF].

Mapping of the seasonal changes in the intensity of the thermocline showed the contrast between the winter and summer on the one hand and coastal and oceanic regions on the other hand (Fig. 7). In summer, the strongest thermoclines were detected over continental shelves and in the Mediterranean Sea where maximal intensity values were greater than 1°C.5m -1 . In oceanic regions, intensity values oscillated between 0 and 0.3°C.5m -1 in temperate and in subpolar regions and 0.3 and 0.9°C.5m -1 in subtropical regions north of the Subtropical Convergence. Local hydro-dynamic features such as the Ligurian Front and the Flamborough front were also detected (Fig. 7 and Fig. 1b). The impact of surface currents on the intensity of the thermocline was clear. For example, weaker values were observed in the path of the Gulf Stream and around the Subarctic Gyre (Fig. 7).

Relationships between the structure of the water column and Calanus finmarchicus

To examine potential relationships between C. finmarchicus and the structure of the water column, we represented the relative frequency of the species as a function of the depth and the intensity of the thermocline (Fig. 8) for all the months from the copepodite stage 1 to the adult. The mapping of the relative frequency of the species as a function of the depth and intensity of the thermocline and the months revealed that young copepodite stages and especially copepodite stage 1 occurred in regions where the water column was not stratified (Fig. 8). Copepodite stage 1 individuals were less frequent in July. However, it was observed that a greater number of these stages were found in regions where the intensity of the thermocline was lower than 1 and the thermocline deeper than 50 m. Therefore, it is likely that copepodite stage 1 is less frequent during this month because it is not within its environmental niche (Fig. 8) [START_REF] Helaouët | Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas[END_REF]. When the individuals become older, stratification had much less influence and the species occurred in regions with both shallow and stronger thermocline. Adult stages seemed however to have a less wide range of habitats, than stage 5. The increase of tolerance to stratification occurring during the life cycle of the species is not consistent with the seasonal course of the thermocline.

To investigate more deeply the potential link between C. finmarchicus and the structure of the water column at a macroscale, we represented the mean relative frequency of the species as a function of the depth and the intensity of the thermocline (Fig. 9) for the months of maximum abundance. The results confirmed previous observations, showing a negative relationship between the species and the intensity of the thermocline. This relationship was especially high for copepodite stage 1. The negative relationship was not detected in May for Stage 5 (Fig. 9a). The examination of the sensitivity of the relationship, measured here by the strength of the correlation, indicated that adult stages were more sensitive to thermocline intensity than stage 5 but less than stage 1. The same sensitivity was found for all the stages when correlations were calculated between the relative presence of C.

finmarchicus and the thermocline depth (Fig. 9b). The positive relationship between the relative presence of the species and the thermocline depth was however substantially lower for stage 5 in June.

An analysis was performed to investigate the temporal relationships between both thermocline depth and intensity and the relative presence of C. finmarchicus on both seasonal and year-to-year scales (Fig. 10). The analysis revealed that the youngest stages had their maximum relative presence just before the establishment of the thermocline (Fig. 10a). Stage 5 did not exhibit this relationship. On a year-to-year scale (period 1977-2005; n=28), the

Relationships between C. finmarchicus and chlorophyll a concentration

Positive and significant relationships were detected between chlorophyll a and the relative presence of C. finmarchicus from stages 1 to 3. Then, the correlation became null for stage 4 and significantly negative for C5 and adults.

Relationships between the structure of the water column and chlorophyll a concentration

The relationship between water column stratification and chlorophyll-a was examined in both neritic (Fig. 12a) and oceanic (Fig. 12b) regions of the North Atlantic. Negative and significant relationships between chlorophyll-a and both intensity and depth of the thermocline were detected. Mean chlorophyll-a concentration was low in homogenous water (intensity of the thermocline < 0.25 °C.5m -1 and deep thermocline depth) and slightly increased when the intensity of the thermocline increased. Maximum chlorophyll-a concentration was located in the interval between 0.2 to 0.4 °C.5m -1 of intensity of the thermocline and between 40 and 60m of depth of the thermocline in the neritic area and between 100 and 230m in the oceanic area.

Discussion

The technique proposed to determine the depth and intensity of the thermocline is a gradient threshold method. This technique is usually considered to be a less consistent estimator of the thermocline than the "difference approach" [START_REF] Thomson | Estimating mixed layer depth from oceanic profile data[END_REF]. However, because of problems related to the instruments used to sample or the presence of rapid environmental variation over a short depth range, temperature profiles are often piecewiselinear rather than smoothing varying [START_REF] Brainerd | Surface mixed and mixing layer depths[END_REF]. The use of EWMA [START_REF] Montgomery | Introduction to statistical quality control[END_REF]Beaugrand and Ibanez, in preparation) gave us the possibility to obtain sharp gradient-resolved temperature profiles. EWMA was chosen because (1) the technique reduces the noise associated with temperature profiles and (2) in contrast to classical moving average methods, it is more sensitive to local variations (rapid temperature change), which is an important property for an accurate detection of the depth of the thermocline.

In this article, a thermocline is defined as the depth of maximal temperature change in the water column (maximal temperature gradient) and the concept is used as an indicator of water column stability. A prerequisite to the use of the technique was to define a threshold to separate a stratified from a homogenous water column. Three analyses were performed on different time and space scales and compared to two already existing methods [START_REF] Defant | Physical Oceanography[END_REF][START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF] with different possible thresholds. The technique, applied to both neritic and oceanic regions (Fig. 4), revealed that the Defant method and our technique based on a threshold of 0.15°C.5m -1 were too sensitive because they overrated the depth of the thermocline during winter and identified restratification too early. De Boyer Montégut et al.

(2004) noticed that the method of [START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF] did not accurately localise the initiation of stratification (Fig. 4). The analysis performed at a macroscale (Fig. 5) and the comparison with profiles (see Fig. 4) showed that the general pattern of thermocline depth was respected in comparison to the Levitus technique. The comparisons with the profiles (see Fig. 4 and5)

showed that the threshold of 0.25°C.m -1 gave the best results (see also the location of hydrodynamic features in Fig 1c). The spatial distribution and seasonal variation of the thermocline appear to be in agreement with the knowledge on water column characteristics across the North Atlantic Ocean [START_REF] Beaugrand | Geographical distribution and seasonal and diel changes of the diversity of calanoid copepods in the North Atlantic and North Sea[END_REF][START_REF] Sarmiento | Ocean biogeochemical dynamics[END_REF][START_REF] Sprintall | Salinity considerations in the oceanic surface mixed layer[END_REF][START_REF] Tomczak | Regional Oceanography: an Introduction 2[END_REF]. In Atlantic central gyres, the thermocline is deep and rapidly increases across frontal structures or main surface currents. Major currents such as the Gulf Stream, the Labrador Current or the European Continental Shelf Current are identified in our results and seasonal frontal structures (Oceanic Polar Front or Flamborough Front, Iceland-Faroe Front) are well represented (Fig. 6 and7). The general pattern proposed in this study respects the location and the seasonality of these currents, and the quantification of the intensity of the thermocline provides additional information. A comparison between the monthly changes in these two descriptors (Fig. 4) and the description of the change in the characteristics of the thermocline by [START_REF] Longhurst | Ecological Geography of the Sea[END_REF] are in agreement. Nevertheless, examination of seasonal changes in the depth of the thermocline shows that our technique described a deeper thermocline than previous studies [START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF][START_REF] Sprintall | Salinity considerations in the oceanic surface mixed layer[END_REF]. Such a result can be explained either by the absence of consideration of the diurnal variation in the depth of thermocline in our analyses [START_REF] De Boyer Montégut | Mixed layer depth over the global ocean: An examination of profile data and a profile-based climatology[END_REF] or by the choice of the threshold of 0.25°C.m -1 . Indeed, the greater the threshold, the less sensitive the method is regarding temperature change.

The present study used a new database on C. finmarchicus that can be complementary to the data collected by the Continuous Plankton Recorder (CPR) survey [START_REF] Beaugrand | Plankton effect on cod recruitment in the North Sea[END_REF].

The present database is a compilation of several studies and surveys on the different copepodite stages of C. finmarchicus (supplementary Table 1.). This dataset has samples at different levels of the water column, information that cannot be obtained with CPR data (sampling at a constant depth of 7 m). Many authors have suggested an effect of stratification on the life cycle or the spatial distribution of different types of planktonic organisms [START_REF] Planque | Calanus and environment in the eastern North Atlantic. I. Spatial and temporal patterns of C. finmarchicus and C. helgolandicus[END_REF][START_REF] Rutherford | Environmental controls on the geographic distribution of zooplankton diversity[END_REF]. While [START_REF] Planque | Calanus and environment in the eastern North Atlantic. I. Spatial and temporal patterns of C. finmarchicus and C. helgolandicus[END_REF] suspected an effect of stratification on the spatial distribution of Calanus finmarchicus and C. helgolandicus, [START_REF] Rutherford | Environmental controls on the geographic distribution of zooplankton diversity[END_REF] provide evidence of a positive influence of this parameter on foraminifera diversity at a global scale. At a higher trophic level, mechanisms by which the column stability affects fish (tuna, anchovy, sardines) have been widely studied and are relatively well understood [START_REF] Beare | An increase in the abundance of anchovies and sardines in the north-western North Sea since 1995[END_REF][START_REF] Bertignac | A spatial population dynamics simulation model of tropical tunas using a habitat index based on environmental parameters[END_REF][START_REF] Block | Environmental preferences of yellowfin tuna (Thunnus albacares) at the northern extent of its range[END_REF][START_REF] Brill | Horizontal movements and depth distribution of large adult yellowfin tuna (Thunnus albacares) near the Hawaiian Islands, recorded using ultrasonic telemetry: implications for the physiological ecology of pelagic fishes[END_REF][START_REF] Lehodey | El Niño Southern Oscillation and tuna in the western Pacific[END_REF]. Beare and colleagues (2004) have revealed that water column stability could affect nutrition of small pelagic fish and thereby alter their growth and mortality or delay their biological development. For many Thunnidae, the thermocline affects their vertical and horizontal distributions [START_REF] Bertignac | A spatial population dynamics simulation model of tropical tunas using a habitat index based on environmental parameters[END_REF][START_REF] Block | Environmental preferences of yellowfin tuna (Thunnus albacares) at the northern extent of its range[END_REF][START_REF] Brill | Horizontal movements and depth distribution of large adult yellowfin tuna (Thunnus albacares) near the Hawaiian Islands, recorded using ultrasonic telemetry: implications for the physiological ecology of pelagic fishes[END_REF] and many reports suggest that a change in the depth of the thermocline (for example due to the Niño effect) might change the catchability of these species [START_REF] Lehodey | El Niño Southern Oscillation and tuna in the western Pacific[END_REF]. Williams (1980aWilliams ( , 1980Williams ( b, 1985) ) speculated that, at the southern edge of the spatial distribution of C. finmarchicus, the vertical position of the thermocline may affect the abundance and the location of the species. Williams (1980b) found that the vertical position of C. finmarchicus was altered when the thermocline was stronger.

This author suggested that this phenomenon allows the species to be separated from its congeneric species C. helgolandicus located shallower in the water column. The author speculated that this difference was related to the warmer thermal preference of C.

helgolandicus. Subsequently, [START_REF] Williams | Vertical distribution of Calanus finmarchicus and C. helgolandicus in relation to the development of the seasonal thermocline in the Celtic Sea[END_REF] supposed that this mechanism may also affect the nutrition strategy of the two species which compete for phytoplankton.

The link between stratification and C. finmarchicus was examined in different ways to detect and better understand how the species might be influenced by the characteristics of the water column stability. The macroecological study revealed a change in the environmental tolerance to stratification parameters occurring during the life cycle (Fig. 8). The results show that even if early life stages are found in different stratification conditions, they occurred more frequently (relative presence >0.5) in weakly stratified waters (0.1 and 0.3 C.5m -1 of intensity of thermocline and >50 m of depth of the thermocline) (Fig. 8 and9). Older developmental stages were less sensitive to stratification (Fig. 8 and9). Weak conditions of stratification were found before the establishment of the thermocline (March to May) when, in parallel, an increase in the relative presence of all copepodite stages was detected (Fig 10). The restratification period was characterised by an increase of the intensity of the thermocline (ranging between 0.2 and 0.5°C.m -) combined with a shallowing of the depth of the thermocline and exhibited an annual maximum of chlorophyll-a (Fig 12). Given the positive relationship between early stage and chlorophyll-a concentration (Fig 11) and the negative relationship between chlorophyll-a and the intensity of the thermocline (Fig. 12), it appears that the restratification period offers optimal environmental conditions for the species. Indeed, the results were in accordance with the observations made by [START_REF] Saumweber | Estimating potential diapause duration in Calanus finmarchicus[END_REF], who suggested that from March to May copepodite stages 5 emerged from diapause and became adults ready to spawn.

In the Arctic and Subarctic pelagic domain, sea surface temperature and chlorophyll-a concentration were driven by the incoming solar radiation and water stratification [START_REF] Longhurst | Ecological Geography of the Sea[END_REF][START_REF] Sverdrup | On conditions for the vernal blooming of phytoplankton[END_REF]. These two environmental parameters influence growth, nutrition, survival rate and therefore both temporal and spatial changes in the abundance of C.

finmarchicus [START_REF] Heath | Spatial demography of Calanus finmarchicus in the Irminger Sea[END_REF][START_REF] Helaouët | Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas[END_REF]. Furthermore, Cook et al.

(2007) showed that early life stages were highly sensitive to both parameters suggesting that with low food concentration and high temperature conditions, young copepodites tended to disappear. Having said that, in winter and summer, the water column offers unsuitable environmental conditions for early life stages of C. finmarchicus. During these periods, survival rates of young copepodite stages may decrease due to food limitation in winter or too higher temperature in summer. It is only during the restratification period when the thermocline shallows, that epipelagic water contains both high food concentration (phytoplankton bloom) and an optimal temperature range for the reproduction and development of naupliar and young copepodite stages [START_REF] Helaouët | Macroecology of Calanus finmarchicus and C. helgolandicus in the North Atlantic Ocean and adjacent seas[END_REF]Cook et al., 2007). Based on our results and on previous findings [START_REF] Hirst | Mortality of marine planktonic copepods: global rates and patterns[END_REF]Ohman et al., 2004;[START_REF] Plourde | Long-term seasonal and spatial patterns in mortality and survival of Calanus finmarchicus across the Atlantic Zone Monitoring Programme region, Northwest Atlantic[END_REF], we suggest that stratification affects the occurrence of this subarctic species via both food concentration and temperature.

Change in global air/sea temperature in the Northern Hemisphere are assumed to move the boundaries between permanently and seasonally stratified waters polewards but also to reinforce the stratification in high latitudes according to Atmosphere-Ocean General

Circulation models (Intergovernmental Panel on Climate Change, 2007;[START_REF] Sarmiento | Response of ocean ecosystems to climate warming[END_REF]. In addition, the seasonal timing in the establishment of the thermocline is also expected to shift [START_REF] Walther | Ecological responses to recent climate change[END_REF]. These alterations could affect phytoplankton blooms and all associated trophic webs [START_REF] Edwards | Impact of climate change on marine pelagic phenology and trophic mismatch[END_REF][START_REF] Gowen | Regional differences in stratification and its effect on phytoplankton production and biomass in the northwestern Irish Sea[END_REF]. The ecological-niche and ecophysiological models of C. finmarchicus suggest changes in the spatial distribution of the species in the next decades (scenario A2 and B2) without including explicitly the influence of stratification. Our results suggest that the intensification of stratification or changes in its seasonal progression could alter the survival rate of early life stages of C. finmarchicus. As a consequence, this effect might limit the expected northward movement if the temperature continues to warm in the North Atlantic sector. The contraction of the species range could have a significant impact for ecosystem functioning by bottom-up or top-down propagation through the food web [START_REF] Beaugrand | Plankton effect on cod recruitment in the North Sea[END_REF] and have a potential impact on some biogeochemical cycles [START_REF] Beaugrand | Decadal changes in climate and ecosystems in the North Atlantic Ocean and adjacent seas[END_REF]. Z=0m ∆T= 0.2°C [START_REF] Levitus | Climatological Atlas of the World Ocean[END_REF] Z= 0 m ∆T= 0.5°C [START_REF] Lamb | On the mixed-layer climatology of the north and topical Atlantic[END_REF] Z=0m ∆T= 1.0°C [START_REF] Martin | Simulation of the mixed layer at OWS November and Papa with several models[END_REF] Z=0m ∆T= 0.1°C [START_REF] Price | Diurnal cycling: Observations and models of the upper ocean response to diurnal heating, cooling, and wind mixing[END_REF] Z=0m ∆T= 0.5°C Sprintall and Tomczack (1990) Z= 0 m ∆T= 0.5°C [START_REF] Wijffels | Direct observations of the Ekman balance at 10°N in the Pacific[END_REF] ∆Z= 2.5 m ∂σ θ /∂z = 0.01 kg.m -4 [START_REF] Weller | Observations of the vertical structure of the oceanic boundary layer[END_REF] Z= 10 m ∆σ θ = 0.15 kg m -3 [START_REF] Wagner | Decadal-scale trends in mechanisms controlling meridional sea surface temperature gradients in the tropical Atlantic[END_REF] Z=0m ∆T= 1.0°C [START_REF] Obata | Global verification of critical depth theory for phytoplankton bloom with climatological in situ temperature and satellite ocean color data[END_REF] Z=0m ∆T= 0.5°C Monterey and Levitus (1997) Z=0m ∆T= 0.5°C

F

  Physical observations (i.e. mean depth and intensity of the thermocline) were then integrated for each biological sample. Physical data were attributed to biological samples with the same month and year (at a temporal resolution of 15 days) and being at a maximum spatial distance of 0.2° of latitude or longitude. The first analysis characterised the tolerance of the species to the thermocline intensity and depth. The mean relative frequency of the

  thermocline (intensity and depth) was significantly highly negative for stage 1. No relationship was detected between thermocline intensity and the other stages (C5 and adult). Positive relationships between the relative presence of C. finmarchicus and the depth of the thermocline were also detected for all stages.

Figure Captions Figure 1 .Figure 2 .

 Captions12 Figure Captions

Figure 3 .

 3 Figure 3. Frequency histograms of the maximal depth-to-depth temperature (in °C) difference

Figure 4 .Figure 5 .

 45 Figure 4. (a) Upper figure: Monthly changes in the thermal profile of the water column in the

Figure 6 .

 6 Figure 6. Seasonal spatial changes in the mean depth of the thermocline for the period (a)

Figure 7 .

 7 Figure 7. Seasonal spatial changes in the mean intensity of the thermocline for the period (a)

Figure 8 .

 8 Figure 8. Tolerance diagram of C. finmarchicus against the characteristics of the thermocline.

Figure 9 .

 9 Figure 9. Relationships between both the intensity (a) and the depth (b) of the thermocline

Figure 10 .

 10 Figure 10. (a) Monthly changes in the intensity of the thermocline (in grey) and the relative

Figure 11 .

 11 Figure 11. Macroecological relationships between the relative frequency of copepodite stages

Figure 12 .Figure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .Figure 7 .Figure 8 .Figure 9 .Figure 11 .Figure 12 .

 121234567891112 Figure 12. Macroecological relationships between both the intensity and the depth of the

  al. 2007) in 1963 ; the UK Marine Productivity project data held at BODC (www.bodc.ac.uk, Heath et al. 2007) from 2001 to 2002 ; the U. S Northeast continental shelf bongo survey (data provided by Jack Jossi)

	F o r
	P
	e
	e r
	R
	e
	v i e
	w
	TransAtlantic Study
	on Calanus; http://tasc.imr.no) from 1995 to 1998 ; the U. S Georges Bank programme

(http://globec.whoi.edu) from 1995 to 2000; the India survey (data provided by X. Irigoien) from 1971 to 1975; the Norwestlant programme (www.st.nmfs.noaa.gov/plankton;

Heath et

  Ohman, M. D., Eiane, K., Durbin, E. G., Runge, J. A., and Hirche, H. J. (2004) A comparative study of Calanus finmarchicus mortality patterns at five localities in the North Atlantic. ICES J.Mar. Sci., 61,[687][688][689][690][691][692][693][694][695][696][697]

	o r F o r F o r F o r F o r F o r F o r F o r
	P P P P P P P P
	e e e e e e e e
	e r e r e r e r e r e r e r e r
	R R R R R R R R
	e e e e e e e e
	v i e v i e v i e v i e v i e v i e v i e v i e
	w w w w w w w w

Table Legends Table 1 .

 Legends1 Selected equations for detecting the depth of the thermocline or mixed layer depth.Methods are sorted chronologically. T is temperature and σ θ is potential density. Z denotes the depth and ∆Z, ∆T and ∆σ θ denote respectively the difference in depth and temperature and density.

	Supplementary

Table 1 .

 1 Characteristics of all databases used in this study.

http://mc.manuscriptcentral.com/jplankt Journal of Plankton Research

Acknowledgements

The authors are grateful to all the people who helped to find or shared data: the programme TASC (Trans-Atlantic Study on Calanus; http://tasc.imr.no), the U. S Georges Bank programme (http://globec.whoi.edu); M. Heath for providing data from the UK Marine Productivity programme held at BODC (www.bodc.ac.uk), M. Taylor and J. Jossi for providing the U. S Northeast continental shelf bongo survey data, T. O'Brien and the web site Copepoda (www.st.nmfs.noaa.gov/plankton ), X. Irigoien and E. Head. Discussions with C. de Boyer-Montégut and F. Ibanez have improved our reflexion on the methodology used in this paper and the interpretation of the results. We are grateful to A.C Gandrillon for her help in the editing of the paper. Finally, the authors are grateful to the referees who helped to improve the study.


Florent Jangal
email: florent.jangal@onera.fr

Marie Anne

Anne Littaye

Michel Morel

Jean-Pierre Mano

Extraction of Suspicious Behavior of Vessels in the Exclusive Economic Zone #

Introduction

Constant growth of world maritime transport and significant economic stakes of territorial water management have prompted the international community to invest in maritime global security research [START_REF]Review of maritime transport[END_REF]. In this context, the ScanMaris project, which is founded by the French National Research Agency (ANR), aims to continuously monitor activities in the Exclusive Economic Zone (EEZ) activities and detect abnormal behavior using both observation systems and external data sources.

The ScanMaris software workshop is based on the assumption that global surveillance of the EEZ cannot only use target detection systems. Indeed, whatever the performances of tracking algorithms are, they cannot estimate how suspicious the vessel tracks are. Some external information, like insurance database or transported goods, can improve behavior analysis.

We propose to collect numerous information on the observed area (i.e. vessel locations and tracks, weather condition, previous committed offenses, destination, etc.) in order to create an enriched map of the scene. Then, the map is analyzed and abnormal behaviors are picked out.

ScanMaris consists in four complementary layers: observation of maritime activities with several kind of sensor, fusion of sensed data, enrichment of information with heterogeneous sources (e.g. EQUASIS safety-related base, weather condition, LLOYDS insurance base, history of vessels, delimitation of maritime zones, etc.) and classification of the overall behavior.

The issues of such an approach are target detection up to the EEZ limits, management of a large amount of data, analysis of each vessel activity and classification of behaviors.

Sensors

We have chosen the High Frequency Surface Wave Radar (HFSWR) to detect targets up to the EEZ limits. Other sensors like Automatic Identification System (AIS) or VHF radars will complete or overlay surface wave measurements.

HFSWR

High Frequency Surface Wave Radar is a promising low-cost surveillance system of the EEZ. In the high frequency band (from 3 MHz to 30 MHz) waves can be propagated along the interface between the earth and the air. HFSWR takes advantage of this property to detect targets over the horizon [START_REF] Menelle | Full digital high frequency surface wave radar: French trials in the Biscay bay[END_REF].

ScanMaris will use vessel tracks acquired by a full digital HFSWR demonstrator which has been recently set up in the southwest of France. As shown in Fig. 1 the demonstrator aerials are located very close to the shoreline in order to maximize the excitation of surface waves.

Data combination

HFSWR cannot be the sole data source of the project: vessels are tracked but not identified by the radar. AIS, which is a cooperative system, make it possible to get the vessel identification. Information from regional safety a security centers also contain identification numbers.

In the common coverage range, we will associate tracks and identifiers. Then, this information is integrated to the enriched map.

Enriched Map and Inference Engine

All available information on the observed scene are kept in a space-time map: each set down element on the enriched map includes an object location (e.g. vessel position) or a validity area (e.g. fishing zones), an event date and a remanence duration.

In the midst of this large amount of information some details point to uncommon behaviors. Nevertheless, uncommon behaviors are not necessarily fraudulent (e.g. fishing vessel sailing slowly outside a stormy fishing zone). Find a classification criterion which remains valid despite law evolutions, vessel locations or weather conditions is not possible.

We are considering the possibility to use several interconnected Inference Engines (IEs) to state either the normality or the abnormality of events [START_REF] Iris | Evaluation of Spatial Multidimensional Approach for the Analysis of Disaster Data[END_REF]. The challenge is to deal with multidisciplinary information (i.e. bathymetry, radar tracks, insurance database, weather, etc.) and to use variable criterion.

IEs will not analyze all the enriched map: this would be needlessly time consuming since fraud is a marginal activity. Thus, interesting details will be extracted beforehand by Adaptive Multi-Agent System

Adaptive Multi-Agent Systems

Adaptive Multi-Agent Systems seems to be well-suited to extract uncommon behaviors [START_REF] Di Marzo Serugendo | Selforganisation and emergence in multi-agent systems: an overview[END_REF]. There are numerous similarities between ScanMaris issues and some AMAS issues. That is, extract some distinctive behaviors or features from a global scene (i.e. the enriched map) where each element is autonomous (i.e. vessels).

Vessels, which can be thought as cooperative agents, affect the environment by leaving traces. Conversely, each agent is influenced by the environment, in particular, by the traces of other agents. Agents get space-time information from the enriched map and put down traces on it.

When an uncommon event is detected, a request is sent to the IEs. Then, IEs reply the normality/abnormality status of the event. If an accumulation of abnormal event occurs an alert is sent to the user.

As mentioned before abnormality do not signify outlaw. AMAS must have the opportunity to continuously learn which behavior should be pointed out or not. So, the workshop contains two AMAS: learning AMAS and an operative AMAS Fig. 2. As shown bellow, the learning AMAS receives alert acknowledgments from the user. Acknowledgment specifies the relevance of each alert. Moreover, the user can inform the learning AMAS of undetected illicit activity.

Regularly the content of learning AMAS is transferred into the operative AMAS. In order to adapt the system, each agent of both AMAS will continuously redefine themselves the remanence of leaved traces as well as limits of the space-time vicinity area.

Obviously, data access strategy of AMAS can dramatically impacts the workshop efficiency. High access rate will overload the system while low access rate will reduce the accuracy. This last element highlight, if necessary, the important role plays by data management in the ScanMaris project.

Conclusion

Global security of the Exclusive Economic Zone is a present challenge since over 90% of world trade travels by sea. We think that the transversal approach of ScanMaris will improve the EEZ management.

The key points of the projects are: the fusion of several sensor information, in particular data provided by High Frequency Surface Wave Radar; the continuous update of an enriched map using heterogeneous sources; the extraction of uncommon events performed by Adaptive Multi-Agent Systems and the interconnected Inference Engines which state the normality/abnormality of each pointed out uncommon behaviors.

The project has begun since January 2008 and the first results will be presented at the conference.

Figure 1 :

 1 Figure 1: French Biscay Bay HFSWR

Figure 2 :

 2 Figure 2: AMAS architecture

Acknowledgments

We would like to thank the French National Research Agency for its financial contribution to the project, regional safety and security centers (DAM and CROSS) for their support and all members of the project's steering committee for their support and advices.


A C Vieira

L A Rocha

S Mischler

Influence of SiC reinforcement particles on the tribocorrosion behaviour of Al-SiC p FGM's in 0.05M NaCl solution

Keywords: Al alloy/SiC p composites, FGM, tribocorrosion 1. Introduction

The main aim of this work was to study and understand the influence of SiC particles on the corrosion and tribocorrosion of Al matrix composite materials. For that, Al-SiC p functionally graded composites were produced by centrifugal casting and different SiC p contents were achieved. Their mechanical properties were improved by age-hardening heat-treatments. The tribocorrosion behaviour was studied in 0.05M NaCl solutions using a reciprocating motion tribometer involving an alumina ball sliding against the Al based samples. Above critical SiC particles content the matrix alloy surface was found protected against wear by SiC particles protruding from the surface. Below this threshold content, the SiC reinforcement was inefficient and the wear rate of the composite was the same as the non reinforced alloy.

improvement of mechanical, thermal, electromagnetic, biochemical or biomechanical properties [START_REF] Gasik | Industrial applications of FGM solutions[END_REF][START_REF] Ogawa | Theoretical study on fabrication of functionally graded material with density gradient by a centrifugal solid-particle method[END_REF][START_REF] Duque | Functionally graded aluminum matrix composites produced by centrifugal casting[END_REF][START_REF] El-Hadad | Fabrication of Al-Al 3 Ti/Ti 3 Al functionally graded materials under a centrifugal force -Review[END_REF][START_REF] Lin | Design optimization of functionally graded dental implant for bone remodeling[END_REF][START_REF] Watari | Biocompatibility of materials and development to functionally graded implant for bio-medical application[END_REF][START_REF] Hassanin | Functionally graded microceramic components[END_REF]. For instances, by using centrifugal casting, SiC particles can be dispersed in an Al-based matrix with a graded distribution originating an Al-SiC p FGM [START_REF] Watanabe | Particle size distributions in functionally graded materials fabricated by the centrifugal solid-particle method[END_REF][START_REF] Watanabe | Control of composition gradient in a metal ceramic functionally graded material manufactured by the centrifugal method[END_REF]. Therefore, it is possible to combine surface hardness and high wear resistance with high bulk toughness [START_REF] Vieira | Dry sliding wear of Al alloy/SiC p functionally graded composites: Influence of processing conditions[END_REF].

In fact, Al-SiC p FGM's, have shown a great potential for several industries [START_REF] Vieira | Dry sliding wear of Al alloy/SiC p functionally graded composites: Influence of processing conditions[END_REF][START_REF] Velhinho | Tribocorrosion Studies in Centrifugally Cast Al-matrix SiC p -reinforced Functionally Graded Composites[END_REF], being their major applications in electronic packaging industry, brake rotor assemblies or pistons rings in automotive industries and turbine components or rocket nozzles (relatively hightemperature environments) for advanced aircrafts and aerospace vehicles [START_REF] Gasik | Industrial applications of FGM solutions[END_REF][START_REF] Gao | Modeling the solidification of functionally graded materials by centrifugal casting[END_REF][START_REF] Uemura | The activities of FGM on new applications[END_REF][START_REF] Mishnaevsky | Functionally gradient metal matrix composites: Numerical analysis of the microstructure-strength relationships[END_REF][START_REF] Miyamoto | Functionally graded materials: Design, processing and applications[END_REF].

Because of the relatively good corrosion properties of aluminium, Al-SiC p FGM's are candidate materials for tribocorrosion applications were contact is exposed to the combined degradation by wear and corrosion. Examples of such tribocorrosion applications are chemical pumps, piston rings, cylinder walls and components in marine structures [START_REF] Fang | Synergistic effects of wear and corrosion for Al 2 O 3 particulate-reinforced 6061 aluminium matrix composites[END_REF][START_REF] Mischler | Triboelectrochemical techniques and interpretation methods in tribocorrosion: A comparative evaluation[END_REF][START_REF] Landolt | Electrochemical methods in tribocorrosion: a critical appraisal[END_REF]. However, relatively little is known about the corrosion and tribocorrosion behaviour of aluminium based FGM's.

The reinforcement by SiC particles can influence the corrosion behaviour of the Al matrix.

Preferential attack at the reinforcement/matrix interface (due to Al 4 C 3 formation at the interface), creation of local cathodes on conductive SiC catalysing the reduction of oxygen and/or modifications of the matrix alloy microstructure (alteration of the size and distribution of intermetallic phases) during composite production were phenomena reported in the literature as having influence on the corrosion resistance of Al-SiC p composites [START_REF] Fang | Synergistic effects of wear and corrosion for Al 2 O 3 particulate-reinforced 6061 aluminium matrix composites[END_REF][START_REF] Montoya-Dávila | Effect of SiC p multimodal distribution on pitting behaviour of Al/SiC p composites prepared by reactive infiltration[END_REF][START_REF] Pardo | Influence of reinforcement proportion and matrix composition on pitting corrosion behaviour of cast aluminium matrix composites (A3xx.x/SiC p )[END_REF]. Clearly, these effects are highly dependent on the nature of the matrix Al alloy, on the SiC particles and on the heat treatments. Also, the tribocorrosion behaviour of the material appears to be determined by the area fraction of reinforcement, particle mean size and tribological testing parameters. A. Velhinho et al [START_REF] Velhinho | Tribocorrosion Studies in Centrifugally Cast Al-matrix SiC p -reinforced Functionally Graded Composites[END_REF] investigated the tribocorrosion behaviour of Al-SiC p functionally graded composites against cast iron pins in water using a unidirectional pin on disk tribometer. Two volume fractions (12.6 and 35.7%) of SiC particles with median size of 118 µm were considered.

Those authors reported that the presence of water facilitated catastrophic SiC particle pull-out, and therefore significantly increased wear. Indeed, higher volume fractions of SiC particles were found to cause larger wear. On the other hand, J.R. Gomes et al [START_REF] Gomes | Wear mechanisms in functionally graded aluminium matrix composites: Effect of the presence of an aqueous solution[END_REF] also studied Al-SiC p functionally graded composites but using different experimental conditions: reciprocating sliding and a normal load of 10N. Tests were conducted by using cast iron pins counterfaces in a 3% NaCl solution and SiC particles area fraction varying between 25.8% and 33.4%. In this case, the wear rate of the composites was not significantly affected by the presence of the aqueous solution. In contrast to Velhinho et al [START_REF] Velhinho | Tribocorrosion Studies in Centrifugally Cast Al-matrix SiC p -reinforced Functionally Graded Composites[END_REF] results, higher amounts of reinforcement lead to lower wear rates. This behaviour was attributed to the capability of the SiC particles to anchor protective tribolayers and to their load-supporting effect allowing the protection of the Al matrix, contributing to a lower total wear of the composite material.

C.-K. Fang et al [START_REF] Fang | Synergistic effects of wear and corrosion for Al 2 O 3 particulate-reinforced 6061 aluminium matrix composites[END_REF] studied the synergistic effects of wear and corrosion for Al 2 O 3 particulate-reinforced 6061 Al matrix composites and postulated that, although the incorporation of reinforcement was detrimental to the corrosion resistance of the material, its influence on wear-corrosion was favourable. Therefore there is still a need for a mechanistic understanding of the involved phenomena.

This study aim was to gain a better understanding of the tribocorrosion mechanisms of Al-SiC p functionally graded composites. For this a model system consisting in Al-SiC p samples sliding against inert alumina balls in 0.05M NaCl was investigated. Aluminium matrix composites materials with a gradient in SiC content were fabricated using centrifugal casting. Samples of different SiC contents were obtained by machining the composite ingots in different locations. Heat treatments were carried out in order to improve the mechanical properties of the matrix. The corrosion mechanisms were evaluated using immersion and electrochemical techniques. During tribocorrosion the OCP was recorded in order to gain information on the prevailing mechanisms.

Experimental details

Materials

A non-commercial Al-10Si-4.5Cu-2Mg (wt. %) was selected as matrix of the composite.

The alloy was home-developed in order to present specific properties, such as good castability (by adding Si), heat-treatment capability (by adding Cu) and good wettability (by adding Mg).

As reinforcement, SiC particles with 37.6 µm as grain size distribution (10% of volumetric fraction) were selected. The functionally graded materials (FGM) were processed by centrifugal casting, using radial geometry, with 1500 rpm centrifugal speed.

Two different post-processing age hardening heat-treatments were made: solution treatment at 500 ⁰C (2h and 8h), followed by quenching, plus an artificial aging at 160 ⁰C during 512 min. The solution treatments were made in air, in tubular furnace, while the artificial aging was made in a thermostatic silicone bath (Model Haake F6).

In this paper the samples solution treated during 2h were identified as FGM-S2h while the samples solution treated during 8h were identified as FGM-S8h. To use as reference, a non heat-treated sample was also studied and was identified as FGM-NHT.

The FGM microstructure was characterised using XRD (Cu K radiation, continuous Bragg Brentano mode, scan step size of 0.02⁰, Bruker D8 Discover equipment), SEM/EDS (Nano-SEM FEI Nova 200), and optical microscopy.

The particle area fraction was estimated by image analysis. For that, 25 to 30 images were acquired in an optical microscope and subsequently analysed with computer aided image analysis (Image tool 3.0). Macro Vickers hardness measurements (30 Kg, 20s dwell time) were performed on all samples.

Corrosion tests

The samples were machined from the centrifugal cast-ring, in order to test the most exterior zone of the ring. Previous to each corrosion test, the samples were wet-polished up to 1200 mesh in SiC abrasive paper.

The electrochemical solution used was 0.05M NaCl (pH = 6.2). The pH was measured before and after the corrosion tests. No variation in the pH was detected during the tests.

Two types of tests were carried out: immersion tests (to characterize localized corrosion mechanisms) and potentiodynamic polarization measurements (polarization curves). In the first tests, the samples were immersed in the solution (open circuit conditions) during 30 min and afterwards the corroded samples were analysed by SEM. Regarding the polarization curves, a potential sweep rate of 0.5 mV/s in the noble direction (starting from cathodic values) was used. The samples were stabilized in the solutions during 1h before these measurements. A three-electrode electrochemical cell configuration integrating a standard double wall glass cell was used: a standard calomel reference electrode (SCE), a Pt counter electrode and the FGM samples connected as working electrode (WE) to the potentiostat (Autolab PGSTAT 30 under GPES software). In this paper, all potentials are given with respect to SCE.

Tribocorrosion tests

Disks were machined from the most exterior part of the centrifugal cast-ring, with ∅ = 20 mm (+/-0.1 mm) and 5 mm (+/-0.1) thick. Previous to each test, the samples were polished down to 3 µm diamond spray (from Struers). The surface roughness (R a ) was measured after polishing. R a was of 0.21 ± 0.07 µm in the FGM samples. R a values of the unreinforced centrifuged Al matrix was of 0.04 ± 0.01 µm.

The tribocorrosion experiments were performed using the solution described above. The pH was evaluated before and after tribocorrosion tests to verify that no changes occurred during rubbing. The tests were done at OCP conditions in a reciprocating ball-on-plate tribometer (ball sliding against a stationary working electrode), with 1Hz frequency, 4 mm stroke length, 11.4 mm/s as sliding velocity and 4N as normal applied load (at approximately 22 ºC and relative humidity of 40%). The counterbody was an alumina ball (∅ = 6 mm). Details on the tribometer used in the present study can be found elsewhere [START_REF] Vieira | Mechanical and electrochemical deterioration mechanisms in the tribocorrosion of Al alloys in NaCl and in NaNO 3 solutions[END_REF][START_REF] Stojadinovic | Effect of electrode potential on the tribocorrosion of tungsten[END_REF].

An electrochemical cell was mounted on the tribometer with a three electrodes configuration. The FGM sample was connected to a Wenking LB 95 L potentiostat as working electrode. A Pt counter electrode and a standard calomel reference electrode completed the electrochemical set-up.

The profiles of the wear tracks were quantified using non-contact scanning laser profilometry (UBM Telefokus instrument). Three profiles across the wear track for each sample were measured. The wear volume was calculated by multiplying the depth mean values by the track's length (4 mm) and by the width.

The worn surfaces were analysed by SEM/EDS. EDS spectra were obtained under an acceleration voltage of 15 KeV. The SEM/EDS equipment used were a Nano-SEM model -FEI Nova 200.

Results

Materials microstructural characterization

The FGM-NHT, FGM-S2h and FGM-S8h microstructural characterization was made by XRD and SEM, previous to corrosion and tribocorrosion tests. From the XRD pattern, presented in Figure 1, FGM-NHT, FGM-S2h and FGM-S8h samples presented the same phases: α-Al, Si, θ (Al 2 Cu), Q (Al 4 Cu 2 Mg 8 Si 7 ), (Al 8 Si 6 Mg 3 Fe) and SiC. The first five phases were from the Al-matrix [START_REF] Vieira | Effect of Al 2 Cu precipitates size and mass transport on the polarization behavior of age-hardened Al-Si-Cu-Mg alloys in 0.05 M NaCl[END_REF], being SiC the reinforcement phase.

The microstructures of FGM-NHT, FGM-S2h and FGM-S8h are presented in Figure 2. No significant differences can be observed between the samples, being the only dissimilarity related with the whitest phase identified by EDS as the Cu-rich phase θ-Al 2 Cu [START_REF] Vieira | Effect of Al 2 Cu precipitates size and mass transport on the polarization behavior of age-hardened Al-Si-Cu-Mg alloys in 0.05 M NaCl[END_REF]. In Figure 2d) an EDS spectra obtained on the θ-Al 2 Cu phases from the FGM-NHT sample is presented, confirming the presence of Al and Cu. Similar spectra were obtained in FGM-S2h and FGM-S8h samples. The heat treatment on the FGM samples leads to more fine and dispersed Cu-rich phases. From our previous work [START_REF] Vieira | Effect of Al 2 Cu precipitates size and mass transport on the polarization behavior of age-hardened Al-Si-Cu-Mg alloys in 0.05 M NaCl[END_REF], the distribution and size of the Q-Al 4 Cu 2 Mg 8 Si 7 and the -Al 8 Si 6 Mg 3 Fe phases on Al matrix alloys was shown to be not affected by the heattreatments. On the other hand, the θ-Al 2 Cu and Si phases becomes more fine and dispersed after the heat-treatment, being the decrease in size of the θ-Al 2 Cu phase confirmed by image analysis.

Table 1 lists the SiC particle area fraction as determined by image analysis. The hardness values of each sample are also presented in Table 1. Hardness increases with increasing SiC particle area fraction. This correlation was already reported in previous works [START_REF] Vieira | Dry sliding wear of Al alloy/SiC p functionally graded composites: Influence of processing conditions[END_REF][START_REF] Mondal | High stress abrasive wear behavior of aluminum hard particle composites: Effect of experimental parameters, particle size and volume fraction[END_REF].

Regarding the hardness mean values presented in Table 1, the lower hardness values were obtained in FGM-NHT (samples without heat-treatment), while the highest values were obtained with the FGM-S2h and FGM-S8h heat-treated samples.

Corrosion behaviour

The surface morphology of FGM-NHT sample, after immersion tests, is presented in Figure 3. There is preferential dissolution of the Al matrix around the θ-Al 2 Cu phases (Figure 3a), by the formation of a galvanic couple between the Cu-rich phases (preferential cathodes) and the surrounding Al matrix (preferential anodes). This mechanism was confirmed as the main corrosion mechanism presented in the Al alloy matrix [START_REF] Vieira | Effect of Al 2 Cu precipitates size and mass transport on the polarization behavior of age-hardened Al-Si-Cu-Mg alloys in 0.05 M NaCl[END_REF]. From Figure 3b), no preferential attack at the reinforcement/matrix interface as well as preferential dissolution of Al matrix around the SiC reinforcement particle can be noticed. Therefore, no effect of SiC reinforcement particles on the corrosion mechanisms can be suggested. This shows that SiC reinforcement does not alter, in the present conditions, the corrosion mechanisms of these Al alloys. Similar behaviour was observed in FGM-S2h and FGM-S8h samples when immersed in NaCl solution.

Tribocorrosion behaviour

The evolution of the open circuit potential (OCP) with time during tribocorrosion testing of FGM-NHT, FGM-S2h and FGM-S8h samples of different SiC particle contents are presented in Figure 5a), Figure 5b) and Figure 5c), respectively. Each test was identified in accordance with the SiC particle area fraction value presented on the tested sample surface. For comparison, the results obtained with unreinforced Al matrix samples (without SiC p and identified in the figures with 0%) are also presented. Before rubbing, the corrosion potential values are similar in all the samples (approximately -0.6V). The OCP decreases when rubbing This is where the paper number, author and title should go 8 starts. This behaviour is usually attributed to the breaking of the passive film on the wear track area by the abrading alumina ball [START_REF] Mischler | Triboelectrochemical techniques and interpretation methods in tribocorrosion: A comparative evaluation[END_REF][START_REF] Ponthiaux | Electrochemical techniques for studying tribocorrosion processes[END_REF][START_REF] Taylor | An electrochemical investigation on fretting corrosion of a number of pure metals in 0.5M sodium chloride[END_REF][START_REF] Bethune | Electrochemical studies of fretting corrosion[END_REF]. Once rubbing stops depassivation ceases and the potential recovers its initial value.

During rubbing, between 1200s and 1800s, samples without SiC and samples with lower SiC particles area fraction values, have OCP values in the range of -0.9V to -1.0V. This trend indicates that during rubbing, the material surface in being constantly depassivated. However, for higher SiC particles area fraction values (for instances, 23 ± 4 % in Figure 5a), after the initial cathodic shift, the OCP recovers during rubbing reaching the value observed before sliding, suggesting that repassivation of the sample surface occurred during rubbing.

In case of repassivation occurring during rubbing, a correlation between the evolution of OCP and friction coefficient with time was observed (Figure 6a), where negative peaks in OCP corresponds to positive peaks in friction coefficient. When repassivation did not occur during rubbing, the coefficient of friction attained more stable values within short time (Figure 6b). This behaviour was observed systematically in heat-treated samples as well on the non heattreated material.

The wear volume estimated after the tribocorrosion samples are presented as a function of the SiC particles area fraction in Figure 7. For comparison data, the unreinforced alloys [START_REF] Vieira | Mechanical and electrochemical deterioration mechanisms in the tribocorrosion of Al alloys in NaCl and in NaNO 3 solutions[END_REF] are also plotted. The SiC reinforcement does not affect significantly wear volume values up to a volume fraction of approximately 15% above which a steady decrease in wear is observed.

The minimum wear volume values were achieved to volume fraction similar or higher than 18%. The low wear volumes exhibited by FGM-S2h and FGM-S8h samples are likely due to the increase in hardness induced by the heat treatment.

Two typical wear patterns were observed using SEM. The first one corresponds to heat treated and non heat treated samples with SiC contents lower or equal to 15% where large plastic flow and material smearing characterise the worn surface (Figure 8a). The two large pits (approximately 30 µm) observed in Figure 8a) are likely due to the pull out of SiC particles.

The second characteristic pattern is observed on samples with SiC contents higher than 18%.

These samples present a more corrugated topography with visible SiC particles (Figure 8b)

protruding from the surface as shown in the magnified micrograph shown in Figure 8c) and Figure 8d). Between 15% and 18% of SiC particle area fraction, intermediate mechanisms were observed.

Rubbing induced surface transformations in the contacting area of the alumina balls as shown in Figure 9. The transformation consisted in crystallographic etching of the surface and an area of a layer partially covering the ball. This layer was material transferred from the composite counter body. EDS analysis revealed the presence of Al, Cu, Si and Mg but not of SiC (Z1 and Z2 in Figure 9b). Although the detected Al could be attributed both to the alumina ball and to the Al matrix alloy, the presence of Si, Cu and Mg strongly suggested the presence of the metallic matrix in the alumina surface. Regarding Z3 zone from Figure 9b), only Al and O elements were identified, indicating that, in this zone, only alumina is present.

Discussion

Two main mechanisms usually contribute to material degradation in the tribocorrosion of passive metals: wear-accelerated corrosion and mechanical wear [START_REF] Mischler | Triboelectrochemical techniques and interpretation methods in tribocorrosion: A comparative evaluation[END_REF][START_REF] Landolt | Corrosion and Surfaces Chemistry of Metals[END_REF]. The latter mechanism involves the mechanical removal of metallic particles by counter-body asperities or trapped third body particles digging below the metal surface. Wear-accelerated corrosion arises from the fact that an asperity rubbing on a metal surface produces a track of clean metal which is usually more sensitive to corrosion than the original metal protected by a thin oxide film (passive film). In the present case, the cathodic shift of the potential observed at the onset of rubbing (Figure 5) reveals that depassivation and thus wear-accelerated corrosion occur. When rubbing stops the potential recovers its initial (before rubbing) value. For samples with the highest SiC particle area fraction values, the potential recovery occurs already during rubbing indicating that depassivation ceased. This particular effect can be explained by the appearance of reinforcement SiC particles standing proud of the aluminium matrix surface (Figure 8c).

These protruding SiC particles limit or even suppress direct contact between the metal and the alumina ball as schematically illustrated in Figure 10a). In such situation, depassivation of the aluminium matrix is not expected to occur and thus the wear track recovers the passive state of the surrounding areas and the galvanic coupling effect disappears. For the lowest SiC particle area fraction contents the direct metal/ball contact (Figure 10b) is maintained during the entire rubbing period as indicated by the observed stable cathodic potential shift and the absence on protruding SiC particles. Furthermore, strong wear action on the matrix can lead to pull-out of This is where the paper number, author and title should go 10

SiC particles that loose the mechanical support of the surrounding matrix, as shown in Figure 8a).

Figure 11 summarizes the observations concerning the passivity recovery during rubbing as a function of heat treatment and SiC content. All samples with a SiC content of 18% or higher exhibit passivity recovery during rubbing. This threshold concentration corresponds well with the SiC content above which protruding reinforcement particles are observed in the wear tracks. It also corresponds to the SiC content above which wear significantly decreases (Figure 7). Clearly, protruding particles not only limits plastic deformation (Figure 8) and thus depassivation but they also protect against wear. Interestingly, below 18% the wear intensity is not affected by the presence of SiC reinforcement, the composite materials exhibiting the same wear rate as the un-reinforced alloys.

The data obtained here do not permit drawing conclusions about the possible influence of heat treatment on the threshold SiC content for wear transition. More experiments with reinforcement contents around 18% are needed to obtain statistically relevant results.

Nevertheless the heat treatment has a clear effect on the overall wear rate. In particular, before the transition, wear is more severe on the non heat-treated alloy probably due to the lower hardness of the metal matrix.

The two situations depicted in Figure 10 may also explain the correlation found between the coefficient of friction and the corrosion potential (Figure 6). Indeed, when plastic flow occurs, depassivation is large and thus the potential attains very negative values. This situation corresponds to a direct contact of the alumina counter ball (covered with transferred material) and the composite metal and thus to a specific frictional interaction. When SiC particles protrudes from the surface the contact is established between these particles and the counter resulting in a different frictional force and the reduction in plastic deformation of the metal matrix and thus of the cathodic shift of the corrosion potential. The SiC-counter ball contact (Figure 10a) produces less friction than the metal matrix one (Figure 10b). This can be explained by the smooth appearance of the protruding SiC particles and by the suppression of large plastic deformation they induce.

Conclusions

The corrosion and tribocorrosion behavior of Al-SiC p FGM's in 0.05M NaCl was investigated using samples obtained by centrifugal casting and therefore, presenting different SiC contents in the most exterior part of the tested surfaces. The conclusions of this study are:

-The introduction of SiC particles and heat-treatments do not affect the corrosion behavior of the composite materials. The polarization curves follow similar trends with or without reinforcement particles.

-Two tribocorrosion mechanisms observed were depending on SiC p content: above 18% the SiC particles were found protruding from the surface and thus protecting the surrounding metal matrix against wear and wear accelerated corrosion. Below 18% the SiC particles had no effect on wear and generalized plastic flow, larger wear and wear accelerated corrosion characterized the FGM's wearing surfaces. This is where the paper number, author and title should go 16

List of Figures

List of Tables: Table 1. Characterization of the samples tested in the tribocorrosion tests in 0.05M NaCl solution, regarding the SiC particle area fraction on the surface sample and macro Vickers hardness (HV 30 ). This is where the paper number, author and title should go This is where the paper number, author and title should go 30

FGM-NHT FGM-S2h FGM-S8h

Figure 4

 4 Figure 4 presents the potentiodynamic polarisation curves of the FGM samples in NaCl solution. For comparison, the polarization curve obtained with unreinforced Al matrix (without SiC reinforcement particles) is also presented. The reproducibility of the tests being very good,only one curve per condition was plotted. No significant differences can be detected between the different samples. E corr is similar to all the samples (≈ -0.60V). Therefore, in NaCl solution, apparently, neither the presence of SiC particles nor the heat-treatment does significantly influence the corrosion behaviour of the material.

Figure 1 .

 1 Figure 1. XRD patterns obtained in the Al-SiC p FGM-NHT, FGM-S2h and FGM-S8h samples.

Figure 2 .

 2 Figure 2. SEM micrographs obtained in backscattering electron mode (BE mode) of: a) FGM-NHT; b) FGM-S2h; c) FGM-S8h; d) EDS spectra obtained on the θ phases presented in FGM-NHT sample.

Figure 3 .

 3 Figure 3. SEM micrographs obtained in FGM-NHT sample after immersion tests of 30 min in 0.05M NaCl solution (backscattering electron mode): a) Identification of θ-Al 2 Cu phases; b) Identification of SiC particles. Similar micrographs were identified in FGM-S2h and in FGM-S8h samples.

Figure 4 .

 4 Figure 4. Polarisation curves of FGM-NHT, FGM-S2h, FGM-S8h and unreinforced Al matrix samples, in 0.05M NaCl solution.

Figure 5 .

 5 Figure 5. Corrosion potential evolution with time during the tribocorrosion tests in: a) FGM-NHT; b) FGM-S2h; c) FGM-S8h samples tested in 0.05M NaCl solution. Each test is identified in accordance with the SiC particle area fraction value presented on the sample surface.

Figure 6 .

 6 Figure 6. Friction coefficient and corrosion potential evolution during the tribocorrosion tests in 0.05M NaCl, for: a) FGM-NHT with 23 ± 4% SiC; b) FGM-S2h with 14 ± 6% SiC;

Figure 7 :

 7 Figure 7: Relation between the wear volume values measured after tribocorrosion tests and the SiC particle area fraction values of FGM-NHT, FGM-S2h and FGM-S8h samples in 0.05M NaCl.

Figure 8 .

 8 Figure 8. SEM micrographs obtained after the tribocorrosion tests in 0.05M NaCl solution in the wear track of FGM-NHT sample with: a) SiC p particle area fraction of 15 ± 3% obtained in secondary electron mode (SE mode); b) SiC p particle area fraction of 23 ± 4%, (SE mode); c) and d) SiC p particle area fraction of 23 ± 4%, (BE mode; 40º tilted).

Figure 9 :

 9 Figure 9: SEM micrographs obtained in the alumina counterbody after the tribocorrosion tests in 0.05M NaCl tested with the FGM-NHT sample (SE mode).

Figure 10 :

 10 Figure 10: Schematic representation of two wear mechanisms suggested: a) Mechanical contact between the protruded SiC reinforcement particles and the Al 2 O 3 counterbody; b) Mechanical contact between the Al matrix and the Al 2 O 3 counterbody.

Figure 11 :

 11 Figure 11: Relation between the potential values measured 100s before stopping rubbing (at 1700s) and the SiC particle area fraction values of FGM-NHT, FGM-S2h and FGM-S8h samples tested in 0.05M NaCl. Black symbols represent the situation where potential recover during rubbing was observed.

  Figure 3

25

 25

  Figure 7

26

 26

  Figure 8

28

 28

  Figure 9

29

 29

Figure 10 Al

 10 Figure 10

rtic le a re a fra c tio n No potential recovering With potential recoveringTable 1 1 2

 12

	FGM -NHT FGM-S2h	1 0 fraction (%) 1 2 15 ± 3 17 ± 5 23 ± 4 14 ± 6 S iC p a SiC particle area 1 4 16 ± 4	1 6	1 8 HV 30 135 ± 6 141 ± 3 159 ± 13 176 ± 4 180 ± 6	2 0	2 2	2 4 HV 30 (mean values) 140 ± 18 179 ± 10
		18 ± 3		189 ± 11
		11 ± 3		180 ± 0
	FGM-S8h	18 ± 3		181 ± 6			177 ± 9

This is where the paper number, author and title should go

Acknowledgements

The research team was financially supported by the Portuguese Foundation for Science and Technology (FCT -Portugal) under a PhD scholarship (SFRH / BD / 27911 / 2006). The authors thank also to Dr.ª Edith Ariza (University of Minho) and Pierre Mettraux (EPFL) for SEM analysis.