Jordan Curt
email: jordan.curt@edf.fr

Matteo Capaldo

François Hild

Stéphane Roux

Modal analysis of a wind turbine tower by digital image correlation

Operational Modal Analysis is a method widely used in the wind industry to characterize structures and perform structural health monitoring or numerical model calibration.

To reach this goal, optical techniques are becoming appealing because of their non-intrusiveness and their full-field feature in contrast to sensor-based methods. An innovative method is proposed herein for modal measurement with a single camera via Digital Image Correlation. The first resonance frequency of the tower is well captured from images of a shut down turbine.

Introduction

Operative Modal Analysis (OMA) aims at identifying the modal properties of a structure based on vibration data collected when it is in service, without any knowledge about the excitation. Such analysis can, for instance, help monitoring the structure health [START_REF] Farrar | An introduction to structural health monitoring[END_REF]. In the wind turbine industry, such analysis is required because of the loading complexity undergone by the structure, originating from aeroelastic phenomena. Several methods have been proposed in the literature, which can be classified into two categories, namely, intrusive and non-intrusive approaches. Intrusive methods consist in placing sensors on the structure, such as accelerometers [START_REF] Rytter | Vibrational based inspection of civil engineering structures[END_REF][START_REF] Devriendt | Monitoring resonant frequencies and damping values of an offshore wind turbine in parked conditions[END_REF], strain gauges [START_REF] Benedetti | Structural health monitoring of wind towers: remote damage detection using strain sensors[END_REF] or optical fibres [START_REF] Hj Bang | Shape estimation and health monitoring of wind turbine tower using a fbg sensor array[END_REF]. These methods present as main drawback their intrusiveness (i.e., they are demanding for installation and maintenance, which imply additional costs) and their restricted scope (i.e., point measurements).

Non-intrusive methods are much less common. Micro-wave radar systems [START_REF] Aj Chen | Wind-induced vibration analysis and remote monitoring test of wind turbine power tower[END_REF] and Light Detection And Ranging (LIDAR) [START_REF] Mitchell | Development of a high-speed wideband wavelength tunable infrared laser source for real time wind turbine array sensing applications[END_REF] techniques have been proposed but their cost remains prohibitive for a larger implementation. To tackle the above-mentioned drawbacks, photogrammetric techniques appear as promising for operative modal analysis. Very few works have been proposed until now. One notable exception is Ref. [START_REF] Özbek | Optical monitoring and operational modal analysis of large wind turbines[END_REF], where the author proposed photogrammetry for such a purpose. The principle was to carry out an OMA of a wind turbine tower using two cameras. Reflective patches were placed on the top of the tower and on the blades. The rigid body motions of these patches were tracked over time. The difficulties of this method are the installation of patches, the required camera calibration for stereo-imaging, and finally the acquisition of images which are to be performed at night to exploit patch reflectiveness, namely, the turbine was illuminated with an LED based flash system.

A simpler way of performing a modal analysis is presented herein with a single camera. More precisely, global digital image correlation [START_REF] Hild | Digital image correlation. Optical Methods for Solid Mechanics. A Full-Field Approach[END_REF] is used with an integrated approach for the full-field displacement measurement of a wind turbine tower in its fore-aft direction. Modal analysis is then performed based on displacement measurements.

Methodology

Digital Image Correlation

Digital Image Correlation (DIC) is a displacement field measurement technique that allows the image of a surface in its reference state to be registered with that of the deformed configuration. These two images are respectively described by matrices f and g representing the gray levels associated with the pixel at position x. By noting u the displacement field between the two images, the following equation

f (x) = g(x + u(x)) (1)
accounts for gray level conservation between the two images. The objective is to find the displacement field that minimizes the sum of quadratic gray level differences. Integrated DIC [START_REF] Hild | Digital image correlation. Optical Methods for Solid Mechanics. A Full-Field Approach[END_REF] allows for the addition of kinematic hypotheses to help solving the inverse (ill-posed) problem by adapting to the conditions of the experiment. It is convenient to discretize the displacement field via finite element shape functions n i (x) with unknown nodal displacements a i

u(x) = i a i n i (x) (2)
Based on the minimization of the quadratic differences between the two members of Equation ( 1), displacements are determined via a modified Gauss-Newton algorithm where incremental corrections to the displacement is obtained from the tangent (linear) problem

[M]{δa} = {b} (3)
with

M ij = x (n i (x) • ∇f (x))(n i (x) • ∇f (x)) (4)
and

b i = x (n i (x) • ∇f (x))(f (x) -g{a} (x)) (5)
where [M] is the DIC matrix and {b} the second-member. In the above equation, g{a} is the deformed image g, corrected by the current determination of the displacement field

g{a} (x) = g(x + a i n i (x)) (6)

Integrated Approach

The uncertainty of displacement measurement depends on several parameters: the quality of the camera (and more specifically the "noise" of the acquired images), the contrast of the observed surface, which is generally marked by a random speckle pattern with high contrast on a small scale, and the "complexity" of the displacement field to be measured (the more numerous the degrees of freedoms, the higher the displacement uncertainty). Under classical conditions, typically in the laboratory, the uncertainty of displacement measured by DIC is of the order of 10 -2 pixel. For the present case, due to very low contrast and very small motion amplitudes, the number of degrees of freedom must be drastically lowered. One possible way is to integrate a priori knowledge from a numerical model of the tower and lower the dimension of the subspace of sought solutions. This approach is called integrated DIC [START_REF] Leclerc | Integrated digital image correlation for the identification of mechanical properties[END_REF].

The choice of modes is made according to the considered problem. In the present case, the displacement of a wind turbine tower in the fore-aft direction depends on several mechanical parameters, such as for instance, its moment of inertia as a function of the height, mass and inertia of the rotor, and the unknown external loading. The final objective is to capture resonance frequencies of a wind turbine tower. Thus, an appropriate basis appears to be the finite element modal basis. The model of the chosen tower is based on beam elements with variable crosssections (i.e., 100 beam elements are used in the present situation). The Rotor-Nacelle Assembly (RNA) is modeled as an added mass with inertia and stiffness. The approximate values of these parameters are obtained with a 3D model from the DIEGO tool [START_REF] Milano | Impact of high order wave loads on a 10 mw tension-leg platform floating wind turbine at different tendon inclination angles[END_REF], which is the aeroelastic code for wind turbines developed at EDF R&D. Thus, the geometry of the tower is a mandatory input for this approach. The whole modelling is done with a multiple beams finite element code in Matlab [START_REF] Desmond | [END_REF]. The first three modes, determined by a modal analysis with a Matlab code, are shown in Figure 1. The frequency calculation will be carried out from the mode weighting of the temporal signal. With this method the mode shapes are inputs of the problem, not outputs like other classical methods. This knowledge integration in the calculation is of paramount importance to lower measurement uncertainty.

Displacement measurement and experimental set up

Pictures of the wind turbine tower are acquired with a camera positioned 170 m away from the structure (Figure 2(a)). The camera is positioned along the perpendicular axis to the RNA orientation. One of the resulting pictures (cropped here to the region of interest) acquired by the camera is displayed in Figure 2(b). The hardware parameters of the optical setup are reported in Table 1.

And the turbine parameters of the wind turbine are reported in Table 2. Generally, the structures of interest for DIC are speckled in order to provide high contrast at a fine scale and thus reduce the uncertainty of displacement measurements. However, this is not feasible here. It is proposed to exploit the only accessible information, namely, the boundary between the tower and the sky, where gray level gradients are the highest and well oriented to measure transverse displacements.

Acquisitions are carried out under in-service condition. Thus rigid body translations of the camera may occur during the test. A mode corresponding to horizontal translation is considered to take into account this phenomenon. The Supervisory Control And Data Acquisition (SCADA) system enables the RNA to always be in the wind direction thanks to data collected from anemometers. Since it is proposed to measure the displacements of a wind turbine tower from a single point of view in order to access vibration frequencies, it is necessary that RNA remain fixed during the acquisition in the camera field of view. A change in orientation of the rotor would alter the modal analysis due to inertia changes with respect to the field of view plan. It is therefore proposed, for the sake of robustness, to perform the measurements on a shut down wind turbine with blocked rotor, pitch and yaw. Keeping the yaw fixed for a wind turbine in operation is not allowed by park operators. In this situation, the operational excitation acting on the wind turbine is likely to be white. With a rotating rotor, it can be expected that the harmonics would dominate the response and alter the frequency analysis. In [START_REF] Özbek | Optical monitoring and operational modal analysis of large wind turbines[END_REF] it has been shown with multiple OMAs that the first turbine frequency is independent from wind velocity, and if the turbine is on operation or not. Indeed, the gyroscopic effects are very small and their impact on the first frequency is negligible. This assertion is not true for high order modes. Even if it were possible, difficulties are likely to occur if the turbine was functioning. The brightness conservation must be ensured at best when using DIC. Depending on the sun orientation, the blade shadow could appear on the tower and may alter the DIC results. Although brightness and contrast corrections could be designed to mitigate these effects, they would increase the number of unknowns and in turn the uncertainties. Last, the expected vibration amplitude is of the order of 1 mm for an 80 m high tower. The size of one pixel is about 3 cm. Thus the motion amplitude to be captured (at most 3 centipixel) is very challenging.

In the present study, only two degrees of freedom are used: one related to rigid body translations and one associated with the first vibration mode obtained from the numerical model. In DIC, it is possible to determine directly from the DIC matrix [M] the covariance matrix [C] associated with the measurement uncertainty of all considered degrees of freedom of DIC

[C] = 2σ 2 [M] -1 ( 7
)
were σ is the standard deviation of acquisition noise, which is assumed to be white and Gaussian. Thus, the a priori uncertainty estimates are the following. For the first mode, it corresponds to a maximum displacement of 0.15 mm at the tower top. In comparison, the method proposed in Ref. [START_REF] Özbek | Optical monitoring and operational modal analysis of large wind turbines[END_REF] reported a measurement uncertainty of 5 mm. The developed technique enables very low measurement uncertainty levels to be reached as compared to common techniques. Four sets of acquisitions are performed with 1150 pictures acquired for each sequence.

Results

This section discusses the results obtained with the previously described methodology. First, the time evolution of the amplitude of each considered degree of freedom is studied, then the frequency analysis is performed.

Displacement measurement

The amplitudes of both degrees of freedoms are plotted as functions of time in Figure 3(a) for a single series of pictures. The results obtained with two other series of pictures are shown for comparison purposes in Figure 3(b). A significant slow drift of the camera is seen, thereby confirming the need for including such rigid body translations as relevant degree of freedom.

For the first vibration mode, oscillating amplitudes are clearly visible. Depending on the image series, the amplitudes of oscillations differ. This change is attributed to the fluctuating wind excitation during the 20 s acquisitions. However, the vibration frequencies look similar. To further investigate this point, the displacement is band-pass filtered in the range 0.15-3 Hz in order to suppress very low and very high frequency components that are irrelevant (reflecting respectively unsteady wind conditions and measurement uncertainties).

Frequency analysis

For the four series, the power spectra of the vibration mode amplitude are plotted in Figure 4(a). The peak frequency (at about 0.35 Hz) is in the expected range. Indeed it has not been possible to perform a measurement with accelerometers. The turbine provider estimates the first frequency at 0.4 Hz. Differences between design and real on-site frequencies are common, the cause can be foundation particularities. Figure 4(b) shows the four peak frequencies as measured independently for each series. Despite the variable in-service conditions, and the small duration of each series (no more than 7 periods are captured) the standard deviation of the peak frequency is 0.006 Hz, which is remarkably low. Such scatter is greater than what would be expected from an accelerometer (i.e., typically of the order of 0.001 Hz), but this level is regarded as satisfactory given the equipment used herein. It is expected that the scatter would be lowered if acquisitions were performed for a longer duration. Such a tool can thus be used for rapid estimations of a turbine first resonance frequency with very minimal instrumentation.

Model uncertainty

The considered vibration mode was directly extracted from a simple finite element model. Several parameters are not known with high accuracy. In this subsection, it is proposed to study the influence of the most uncertain parameter, namely, the RNA stiffness. Five values are tested around the initial guess.

Large variations of the RNA stiffness are tested and the impact on the first mode shape (as shown in Figure 5) is visible but modest. In turn, the resonance frequencies measured with DIC display very minute variations (i.e., less than 1 mHz over the entire range of variation for the RNA stiffness). Because the chosen displacement basis has been restricted to two modes only, even an approximate mode shape is enough to capture its contribution in the displacement field over time. Moreover, even if the amplitude of variation of the vibration mode is affected by the spatial shape of the mode, it does not necessarily impact the estimate of the peak frequency. These results show that the proposed methodology for measuring the frequency of the first vibration mode is extremely robust.

Conclusion and Perspectives

In this work, an innovative approach was developed to identify the first resonance frequency of a wind turbine tower with a single camera when the turbine was shut down. Integrated digital image correlation was conducted with the help of a numerical model of the tower to estimate the spatial shape of the fundamental vibration mode. It enables very low measurement uncertainty levels to be reached (i.e., less than one millimeter). From 4 sets of images, a very small scatter (i.e., less than 0.01 Hz) was found on the estimate of the fundamental frequency, which is considered as very low given the extreme simplicity of the optical set-up. This method relies on prior knowledge of geometrical structure characteristics and approximate mechanical parameters such as rotor-nacelle assembly inertia, mass and stiffness. Yet, it was shown that the measurement was very robust to even important parameter changes of the models since only the spatial shape of the fundamental vibration mode matters.

In this work, only the first resonance frequency of the tower was measured. Calculations were also performed including the second vibration mode. They resulted in increased uncertainties associated with the different degrees of freedom. The first mode was still identifiable. However, the amplitude of the second mode was significantly lower, and revealed to be of the order of magnitude of noise. A longer time series, under presumably stronger wind excitation, could possibly make this second vibration frequency accessible. Temporal regularization of the DIC scheme [START_REF] Berny | Mode-enhanced space-time dic: applications to ultrahigh-speed imaging[END_REF][START_REF] Jailin | Full field modal measurement with a single standard camera[END_REF] could also be a perspective to enhance the sensitivity and robustness of the method.

Figure 1 .

 1 Figure 1. Three first modes of the wind turbine tower model calculated with a multiple beams FE code. The y-axis is dimensionless with respect to the tower height.

Figure 2 .

 2 Figure 2. (a) Set-up for image acquisition of the wind turbine tower. (b) Picture of the wind turbine tower taken by the camera.

Figure 3 .

 3 Figure 3. (a) Time history of the amplitudes of the two degrees of freedom (translation in blue, and first vibration mode in orange) considered in the DIC analysis for series # 1. (b) Time amplitude of the first vibration mode for series # 1 and # 3 showing similar oscillations although with different amplitudes depending on series.

Figure 4 .

 4 Figure 4. Power spectrum (a) and estimated peak frequency values with corresponding error bars (b) for the four pictures sets. The power spectrum is the squared Fast Fourier Transform.

Figure 5 .

 5 Figure 5. Influence of the RNA stiffness on the first mode shape. The y-axis is dimensionless with respect to the tower height.

Table 1 .

 1 DIC hardware parameters.

	Parameter	Value
	Camera	PCO-edge CMOS 5.5
	Stand-off distance	180 m
	Frame rate	56 fps
	Number of pictures 1150
	Lens	50 mm

Table 2 .

 2 Turbine parameters.

	Parameter	Value
	Height	76 m
	Diameter	2.96-4.3 m
	Thickness	1.8-2.3 cm
	Rated power 3 MW

Acknowledgments

The authors would like to thank ANRT, EDF Research and Development for their financial and operational support of this work.


Galen Strawson's Free

Jun-Ichi Yano

Determinism, Morality, Free Will An essay

Keywords: Determinism, Morality, Free Will

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Summary:

The present short essay argues that a traditionally accepted order between the determinism, the free will, and the moral responsibility must be reversed. The determinism provides a basis for the moral responsibility. Existence of the moral responsibility, in turn, ensures the free will.

The question of relations between the human moral responsibility and free will is often constructed in such a manner that the free will is a basis for our moral responsibility.

Here, I propose that this question is better formulated in the other way round: we first define a basis of the moral responsibility, and once our capacity for moral responsibility is established, the notion of the free will naturally follows.

To establish the notion of the moral responsibility, I first point out that it comes from a construction of Western society: We have to take a moral responsibility because our society asks for it. To live in such a society, the parents bring up their children to bear moral responsibilities. That is why we have it. This is a major detour of this essay, and the claim is rather subjective, and I am even ready to accept a criticism as a racist. However, I believe, this detour is important to make the point stated in the next single-sentence paragraph clear: Looking at far-oriental societies, people are not asked to bear such a responsibility. For example, in my own personal observations, in China, people are just asked to follow certain rules, or follow an order of someone superior, such as a parent, a work supervisor. Or simply give their favors to those who they like or who are considered to be important for them. Either the case, what follows as their action does not involve any sense of "moral" in Western sense. [Look at the bahavior of the people at any major cross sections in Beijing : it just reveals that people do not know how to behave with total strangers : in short, lack of sense of moral.] Thus, we must first recognize that the moral responsibility is a culturally based notion.

1 * Ironically, the determinism is consistent with the notion of moral responsibility 1) . To be responsible, one must make every decision in a logically consistent manner, so to say, in such a manner that nothing is arbitrary 2) . In other words, a moral decision must follow a well-defined set of logic: an analogy with a computer program may be useful here. Every time we face a need for a moral decision, we run this system of logic for a moral decision, like running a computer program. If a logic system is well defined, a process of reaching a decision must be totally determinisitc. Otherwise, we are inserting something irrational and random into a process of moral decision. Of course, an arbitrary decision would not be responsible, because we cannot be accountable with an arbitrary decision in a literal sense that we cannot explain.

One may say that, however, if our decision process is totally deterministic, it is just mechanical (though it is more to do with a software rather than a hardware). To this, I would point out that our decision logic is hardly a fixed one, but evolves with us as getting older, and thus as achieving more maturity. We can imagine that there is a hyper-logic (or upper logic) within ourselves to revise our set of moral logic principles. In short, we learn from our moral erros (our moral decisions are evaluated by ourselves afterwards, that would be a part of the moral responsibility), and grow throughout our life, so that we can make (in general) better moral decisions as we are getting older. We may interpret that this process is handled by a type of upper-level software system that supervises those basic moral programs. Of course, such an upper-level software (i.e., , upper-level of moral logic) must also be deterministic. In principle, we can also imagine our hyper-hyper moral logic to check our hyper moral logic, and this hierarchy may extend to infinity. A numer of levels that one has for moral decisions may be considered a sign of a degree of maturity of a givern person.

Though I do not disccuss any further here, we may note here that a further expansion of the software analogy can justify to argue that we have mind, self, soul, etc as types of macros in this complex software system. * The issue of the free will is vast, and it is beyond the scope of this short essay to even define it carefully 3) . However, once a self autonomy of our moral system is established as 1) A similar argument is also developed by Susan Wolf in Asymmetriucal freedom, Journal of Philosophy, LXXVII, 3 (March 1980): 151-166.

2) Looking at differently, here I am defining the moral responsibility as a capacity of making a moral decision in a logically defendable manner.

3) For example, Thomas Aquinas defines the free will as "a free judgment arising from a type of logical and deterministic software system as already argued above, we can say that the existence of a self autonomy in ourselves can also be considered a mark of a free will within ourselves 4) .

It is often argued that the existence of free will within us is a basis of our moral responhsibility. However, realize: when we face a serious moral decision, we are not quite free to choose, in a literal sense, as we wish. Rather, under a given circmustance, by following our own moral principles, we are no longre free to choose: our moral principle simply urges us to take a partiocular choice in a rather inevitable manner.

My favorite cases to consider are decisions of intellectuals under the Nazis regime. Many chose to leave Germany (or Hungary), including Albert Einstein (physicist) as well as Erich Kleiber (conductor), Vera Bartók (composer), and Karl Barth (theologian). On the other hand, some intellectuals chose to collaborate with the Nazis, those include Werner Heisenberg (physicist), Wilhelm Furtwängler (conductor), Richard Strauss (composer). One of the most unique cases would be the decision of Dietrch Bonhoeffefer (theologian) to remain in Germany as a resistance : did he have a choice to leave Germany or to collaborate with the Nazies? For him, clearly, this was an inevitable choice.

If one has a strong sense of moral responsibility, like those people I just mentioned, under a critical situation, one is no longre free to choose, but a decision becomes an inevitable one. This fact is supported by an analogy of our moral responsibility with a deterministic software system, as suggested above. Under such a stark situation, the moral choice is deterministic. However, we should not say that those decisions were not from their own free wills.

They were not forced to make these decision by anyone else, though there were certainly pressures on them. It would be best interpreted that they made those inevitable decisions out of their free wills. In this manner, a determinisim associated with a moral decision does not contradict with the free will, but rather establishes it.

*

Looking at other way round, we have to realize that cases easily demonstrating our ability of choosing "alternative possibilities" by free will, without any inevitablenss, are reason" in Summa Theologica, First Part of the Second Part, Q17, Article 1 by following earlier philosophers.

4) In this respect, I do not interpret the free will in terms of the "principle of alternative possibilities" (cf., Harry Frankfurt, Alternative possibilities and moral responsibility, Journal of Philosophy, LXVI, 23 (December 4, 1969): 829-839), as elaborated further below.

rather benain and inconsequential, like choosing a type of cheese for your burger or a dressing for your salad at a restaurent 5) . Unless you have a strong preference, the choice is inconsequential, hurting no one by whatever you choose. Under such a circmustance, we literally demonstrate our freedom to choose, by chossing a choice out of whim. It appears to us that we are using a kind of random-number generator in our moral-decision software.

However, if we believe in determinisim of any physical processes, even such a virtually random choice must be deterministic, in the same sense as the so called random-number generators in computers are based on deterministic algorithms.

Probably, an important point is that we usually do not choose in a totally random manner, but based on a certain logic: like since you took chedder last time, you take something else this time. However, such a logic is hardly a serious one: it just demonstrates that our human brain is not quite well adapted to generate a pure random number so we need a certain justification to choose. Again, it can all be well understood in analogy with a computer software.

If wished, we can still continue to discuss about the free will as a matter of choice from "alternative possibilities". However, as I already pointed out, the question of whether we have such a capacity does not suggest any serious moral implications. Lack of such a choice will would rather be considered an indication of a strong sense of moral responsibility.

The following example would appear rather redicurious, but make this point clear in a very simple setting. Imagine a person who finds a strong moral responsibility to take chedder cheese every time when she orders a burger. So every time she orders a burger, she asks for chedder as well. This is her completely fixed behavior, almost mechanical. It would look like that she has absolutely no free will in choosing cheese for burgers. But does it actually prove a lack of her free will?

We should interprete it in the other way round: the fact that she always order chedder with a burger is a sign of her sense of strong moral responsibility, and she does it out of her free will. We may even argue that those common people who choose a type of cheese out of whim every time ordering a burger rather lack a real free will. *

The question of our free will to choose should be taken more seriously when a situation is more serious. For example, when we face lot of political pressures, like under the Nazis regime. For me, the question of the free will is whether one has a capacity of making one's 5) We must also reaize that availability of such choices is, again, a cultural matter: we encounter this situation more frequently in US. Under other cultures, once we choose a menu to order, it is left as chef's responsibility to prepare the food in the best manner. own decision regardless how much external pressures one faces. To be able to make such a chose, one must hold a very strong sense of moral responsibility. Such a strong sense of responsibility is established only by a robust logical system of decision making, that makes the decision deterministic under a given full circmustance. A strong free will is rather supported by a very robust deterministic decision system that one holds within oneself.


Ben Fida

Salem

Olfa Ben Said

Ezzeddine Mahmoudi

Robert Duran

Mathilde Monperrus

Distribution of organic contamination of sediments from Ichkeul Lake and Bizerte Lagoon, Tunisia

Analyses of organochlorine pesticides (OCPs), polycyclic aromatic hydrocarbons (PAHs), polychlorinated bi-phenyls (PCBs), and butyl tins (BuSn) were conducted on sediments from Ichkeul Lake-Bizerte Lagoon watershed (Tunisia). A total of 59 compounds (16 PAHs, 12 PCBs, 22 OCPs and 9 BuSn) were measured in 40 surface sediment samples collected during two campaigns. High concentrations of total PAHs were identified in the lagoon ranging from 122 to 19600 ng•g -1 . Several OCPs, including endrin, dieldrin, and lindane (Hexachlorocyclohexane or HCH or BHC) were found in high concentrations in Ichkeul Lake, ranging from 28 to 2012 ng g -1 . PAHs and OCPs varied seasonally, in response to the complex hydrology of the watershed. The concentrations of total PCBs ranged between 0.04 and 10.653 ng g -1 and suggests low total PCBs sediment contamination, when compared to most international criteria. Total BuSn concentrations range between 67 and 526 ng•g -1 , which are relatively low when compared to most international criteria and ecological risk assess-ments. This is the first study of organic contamination in Ichkeul Lake (RAMSAR and UNESCO World Heritage site).

Polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs) are classes of persistent organic pollutants (POPs). These compounds are ubiquitous environmental contaminants of great concern in coastal marine sediments these contaminants enter into the environment from numerous sources, including river run-off, atmospheric precipitation, industry, households and all types of transportation [START_REF] Wen | Linked sediment/contaminant transport model for rivers with application to the Buffalo River, New York[END_REF][START_REF] Chang | Atmospheric polycyclic aromatic hydrocarbons (PAHs) in Asia: a review from 1999 to 2004[END_REF]. POPs have been detected in various environmental matrices and biota of aquatic ecosystems which receive these pollutants from agricultural and industrial activities [START_REF] Kukucka | Source identification, spatio-temporal distribution and ecological risk of persistent organic pollutants in sediments from the upper Danube catchment[END_REF][START_REF] Sharma | Environment and human exposure to persistent organic pollutants (POPs) in India: a systematic review of recent and historical data[END_REF]. Several studies have shed light on the presence of industrial pollutants or priority substances [START_REF] Wenzel | Chelate-assisted phytoextraction using canola (Brassica napus L.) in outdoors pot and lysimeter experiments[END_REF][START_REF] Deycard | Contributions and potential impacts of seven priority substances (As, Cd, Cu, Cr, Ni, Pb, and Zn) to a major European Estuary (Gironde Estuary, France) from urban wastewater[END_REF] in different environmental matrices [START_REF] Shu | Analysis of polycyclic aromatic hydrocarbons in sediment reference materials by microwave-assisted extraction[END_REF][START_REF] Vane | Assessment of polyaromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) in surface sediments of the Inner Clyde Estuary, UK[END_REF][START_REF] Ben Said | Bacterial community structure of sediments of the Bizerte Lagoon (Tunisia), a Southern Mediterranean Coastal Anthropized Lagoon[END_REF][START_REF] Opel | Inside-sediment partitioning of PAH, PCB and organochlorine compounds and inferences on sampling and normalization methods[END_REF]. Sediments are thought to be the final compartment in which POPs accumulate. The rapid sorption of POPs to sediment particles and their long persistence in aquatic sediments make them a practical media to assess contamination of an aquatic ecosystem. Although environmental concentrations are highest near the sources, the presence of contaminants in places distant from primary sources indicates that POPs are persistent in the environment and can be transported at long distances [START_REF] Pozo | Levels of persistent organic pollutants (POPs) in sediments from Lenga estuary, central Chile[END_REF]. Pollutants from different origins that accumulate in sediments suggest that this matrix is the principal reservoir of environmental contaminants [START_REF] Birch | Accumulation of metallic contaminants in surficial sediments on a high energy continental shelf (Sydney, Australia)[END_REF].Many of these compounds are toxic to aquatic life and potentially harmful to humans [START_REF] Tolosa | Aliphatic and aromatic hydrocarbons in marine biota and coastal sediments from the Gulf and the Gulf of Oman[END_REF][START_REF] Sharma | Environment and human exposure to persistent organic pollutants (POPs) in India: a systematic review of recent and historical data[END_REF]. Coastal lagoons and lakes are generally ecosystems stressed by the impact of large cities, industrial activities and intensive agriculture. Rivers and estuaries can serve as important sources of metal and organic contaminants to coastal marine environments [START_REF] Pozo | Levels of persistent organic pollutants (POPs) in sediments from Lenga estuary, central Chile[END_REF][START_REF] Yang | Farmer and retailer knowledge and awareness of the risks from pesticide use: a case study in the Wei River catchment[END_REF]. Bottom sediments are complicated by interfering factors (such as binding of organic pollutants) that require complex separation chemistry to produce accurate measurements. In Tunisia, during the past decades researchers have taken the determination of PAHs [START_REF] Ben Said | Bacterial community structure of sediments of the Bizerte Lagoon (Tunisia), a Southern Mediterranean Coastal Anthropized Lagoon[END_REF][START_REF] Mzoughi | Méthodologie de l'extraction des hydrocarbures aromatiques polycycliques. Application à des sédiments de la lagune de Bizerte (Tunisie)[END_REF][START_REF] Trabelsi | Polycyclic aromatic hydrocarbons in superficial coastal sediments from Bizerte Lagoon, Tunisia[END_REF] and organotin [START_REF] Mzoughi | Organotin speciation in Bizerte lagoon (Tunisia)[END_REF] in the sediment of Bizerte Lagoon, but only one study was performed for the determination of PCB contamination in surface sediments at Monastir Bay (Tunisia) [START_REF] Nouira | Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) in surface sediments from Monastir Bay (Tunisia, Central Mediterranean): occurrence, distribution and seasonal variations[END_REF]. Even throughout Tunisia, there is no study reported for the determination of pesticides in sediments but pesticides were investigated in other biological matrices such as human milk, from Bizerte and various Tunisia places [START_REF] Ben Hassine | Determination of chlorinated pesticides, polychlorinated biphenyls, and polybrominated diphenyl ethers in human milk from Bizerte (Tunisia) in 2010[END_REF][START_REF] Ennaceur | Distribution of polychlorinated biphenyls and organochlorine pesticides in human breast milk from various locations in Tunisia: levels of contamination, influencing factors, and infant risk assessment[END_REF], in mullet and sea bass [START_REF] Ben Ameur | Concentration of polychlorinated biphenyls and organochlorine pesticides in mullet (Mugil cephalus) and sea bass (Dicentrarchus labrax) from Bizerte Lagoon (Northern Tunisia)[END_REF]. Pollutants from agricultural activities may impact the health of aquatic organisms and subsequently local and migratory birds. Even though Ichkeul Lake is an important migratory area for thousands of birds annually [START_REF] Stevenson | Palaeoecological and documentary records of recent environmental change in Garaet el Ichkeul: a seasonally saline lake in NW Tunisia[END_REF], it has never been evaluated for POPs contamination.

The current study aimed to investigate the occurrence and distribution patterns of OCPs, PAHs, PCBs and BuSn compounds in sediment of the Ichkeul Lake-Bizerte Lagoon complex (Tunisia). The QuEChERS method validated by [START_REF] Yang | Rapid multiresidue determination for currently used pesticides in agricultural drainage waters and soils using gas chromatography-mass spectrometry[END_REF] for the analysis of pesticides in soil was adapted for the analysis of OCPs, PAHs and PCBs in sediment samples and was applied with Gas chromatography-mass spectrometry (GC-MS) for the determination of these pollutants. Sediment composition and contaminant profiles from different stations of each ecosystem and the entire complex were compared in order to evaluate the organic contamination and the potential sources.

Organic solvents (hexane, acetone, acetonitrile, and dichloromethane) were of analytical grade and were supplied by Sigma Aldrich. Buffer salt mixture was obtained from Agilent. Deionized water used in the experiment was obtained from a Milli-Q water Ultrapure Water Purification Systems (France).

PCB standard solutions, either as compound mix, containing 28,31,70,52,101,138,118,153,149,180,194 pérylène, Dibenzo[a,h]anthracène), known as the 16 priority polycyclic aromatic hydrocarbons of US EPA list, as well as two deuterated internal standards phenanthrene-D10 and perylene-D12 were supplied as pure reference materials (purity: 99.5%) also from Sigma Aldrich. OCPs standard solutions of HCB, aldrin, dieldrin, heptachlor, heptachlor epoxide isomer B, endrin, endrinaldehyde, α and β endosulfan, endosulfan sulfate, 4,4′-DDE, 4,4′-DDD, 4,4′-DDT, α and β chlordane, α and β nonachlor and an internal standards atrazine-D5 were purchased from Sigma Aldrich. Standard solutions were prepared in acetonitrile. Tributyltin (TBT) chloride (96%), dibutyltin (DBT) dichloride (97%), monobutyltin (MBT) trichloride (95%) were obtained from Sigma-Aldrich, whereas trimethyltin (TMeT) chloride (98%), dimetyltin (DMeT) dichloride (95%), monometyltin (MMeT) trichloride (98 + %) were obtained from Strem Chemicals (Newburyport, MA, USA). The isotopically enriched butyltin species used were purchased from ISC Science (Oviedo, Spain): a mix of MBT, DBT and TBT enriched in 119 Sn (82.4%) at 0.110, 0.691 and 1.046 μg•g -1 , respectively.

All stock solutions (1000 μg•g -1 as Sn) were prepared by dissolving the corresponding salt in a 3:1 mixture of acetic acid/methanol and were kept in the dark at 4 °C until use. Working solutions of the organotin compounds were prepared daily before analysis by dilution of the stock solutions with 1% HCl in ultrapure water.

Ichkeul Lake -Bizerte Lagoon complex is located at the Northern region of Tunisia. The Tinja River connects the Lake to the Lagoon, and several rivers feed the hydrosystem complex (Fig. 1). The complex has been exploited for fishing activities for several centuries and the lagoon for mussel farming since 1964. Bizerte Lagoon is open to the Mediterranean Sea by a channel. The area of Bizerte Lagoon is approximately 128 km 2 , and it is subjected to contamination from two industrial zones, one with oil refinery one with cement plant… [START_REF] Ben Said | Bacterial community structure of sediments of the Bizerte Lagoon (Tunisia), a Southern Mediterranean Coastal Anthropized Lagoon[END_REF] and agricultural lands located on the Bizerte Lagoons' edge. Covering 90 km 2 , Ichkeul Lake is a Biosphere reserve, a Ramsar site, and a UNESCO World Heritage Site [START_REF] Stevenson | Palaeoecological and documentary records of recent environmental change in Garaet el Ichkeul: a seasonally saline lake in NW Tunisia[END_REF][START_REF] Kraïem | Sites description, water chemistry and vegetation transect of Tunisian lakes. Fish populations study of the nine CASSARINA lakes[END_REF][START_REF] Ramdani | North African wetland lakes: characterization of nine sites included in the CASSARINA project[END_REF][START_REF] Ben | Evolution des parmètres physico-chimiques des eaux du lac Ichkeul après la réalisation des aménagements hydrauliques (Tunisie)[END_REF]. The hydrology of the complex is seasonal. During winter, Ichkeul Lake is fed by fresh water from the rivers increasing the water level. At that time, large volumes of water are spilled into Bizerte Lagoon through the Tinja River. During the summer, water levels in Ichkeul Lake are low due to high evaporation rates, coupled with lower river inflows. As a result, saline water from Bizerte Lagoon enters Ichkeul Lake through the Tinja River. This results in a saline water connection between Ichkeul Lake and the Mediterranean Sea and formerly had a seasonal salinity regime, freshwater increasing with winters' rainfall and decreasing in summer. Ichkeul Lake receives principally agricultural inputs as it is surrounded by a vast agricultural area [START_REF] Stevenson | Palaeoecological and documentary records of recent environmental change in Garaet el Ichkeul: a seasonally saline lake in NW Tunisia[END_REF].

A total of 40 surface sediment samples were collected by Van Veen grab from 20 stations of the Ichkeul Lake-Bizerte Lagoon complex, during two sampling periods in September 2011 and April 2012.Selected stations included mouths of rivers tributary to the lake and lagoon, stations adjacent to industrial areas, and two stations at the centers of the lake and lagoon. The locations of the sample stations are identified in Fig. 1. The sediments were placed in glass bottles and were kept frozen at -20 °C. Before analysis samples were freeze dried in a ZirBus Vaco2 lyophiliser and sieved.

Water column temperature, pH, dissolved oxygen, salinity and conductivity were determined in the field with a handheld multiparameter system WTW Multi-197i (Table 1). Nitrite and nitrate were analyzed following the procedure of [START_REF] Wood | Determination of nitrate in sea water by cadmium-copper reduction to nitrite[END_REF]. Ammonia was determined according to [START_REF] Aminot | Manuel des analyses chimiques en milieu marin[END_REF]. Phosphate levels were determined as described in [START_REF] Murphy | A modified single solution method for the determination of phosphate in natural waters[END_REF]. For chlorophyll a (Chla), seawater samples were filtered through Whatman GF/F filters. Pigment concentrations were determined using the standard spectrophotometric method [START_REF] Parsons | Biological Oceanographic Processes[END_REF], following extraction with 10 mL 90% acetone overnight at 4 °C in the dark. Suspended material was analyzed according to [START_REF] Baretta-Becker | Testing the microbial loop concept by comparing mesocosm data with results from a dynamical simulation model[END_REF]. Total organic carbon (TOC) of sediment samples was analyzed with the TOC Analyzer (SHIMADZU H544051).

The samples were analyzed according to an adapted QuEChERS method for the simultaneous analysis of OCPs, PAHs and PCBs in sediment by gas chromatography-mass spectrometry [START_REF] Salem | Pesticides in Ichkeul Lake-Bizerte Lagoon Watershed in Tunisia: use, occurrence, and effects on bacteria and free-living marine nematodes[END_REF]. BuSn were analyzed according to methods validated by [START_REF] Moreno | Simultaneous determination of monomethylmercury, monobutyltin, dibutyltin and tributyltin in environmental samples by multi-elementalspecies-specific isotope dilution analysis using electron ionisation GC-MS[END_REF] The samples were analyzed according to an adapted QuEChERS method for the analysis of OCPs, PAHs and PCBs in sediment by gas chromatography-mass spectrometry. The QuEChERS method validated by [START_REF] Yang | Rapid multiresidue determination for currently used pesticides in agricultural drainage waters and soils using gas chromatography-mass spectrometry[END_REF] for the analysis of pesticides in soil was adapted for the analysis of OCPs, PAHs and PCBs in sediment samples. Briefly, an aliquot of 5 g of sediment was weighed into a polypropylene tube (50 mL capacity). Then, 4 mL ultrapure water was added, the tube was manually shaken and the internals standards (atrazine d5, PCB 30, phenanthrene-D10 and perylene-D12) were introduced. Then, 20 mL of extraction solvent (dichloromethane-acetone (50:50 v:v)) were added and the tube was shaken vigorously by hand for 1 min. Then, a citrate buffer salt mixture (4 g MgSO 4 , 1 g NaCl, 0.5 g disodium citrate sesquihydrate and 1 g of trisodium citrate dehydrate) was added and the tube was shaken vigorously manually (5 min). Finally, the tube was centrifuged for 3 min at 2500 rpm and 10 mL of the supernatant were transferred into another polypropylene tube (15 mL capacity) already containing 900 mg of MgSO 4 and 150 mg primary secondary amine (PSA). The tube was shaken vigorously by hand for 30 s and centrifuged for 3 min at 2500 rpm. The extracts were then dried under a gentle argon stream using a Turbovap LV Concentration Evaporator system. The dried residue was re-dissolved with 1 mL acetonitrile and kept at -20 °C until analysis.

The analyses were performed by using a gas chromatograph (Agilent 7890A) coupled to a mass spectrometer (Agilent 5975C) with an electron impact ionization source (EI). The GC-MS system was equipped with an Agilent DB5-MS UI column. The carrier gas was helium used at 1 mL min -1 flow rate. The ion source temperature and the quadrupole temperatures were kept at 230 °C and 150 °C, respectively. A sample volume of 1 μL of the extracts was injected in pulsed splitless mode at an inlet temperature of 280 °C. The column temperature was programmed as follows: the initial oven temperature was set at 80 °C for 1 min, increased to 160 °C at 10 °C/min (hold 5 min), then ramped at 3 °C/min to 300 °C (hold 2 min). The MS interface temperature was maintained at 300 °C. The quantification was carried out in the selected ion monitoring mode (SIM) selecting two characteristics fragments ions for each compound. Two replicates were performed for each sample with two injections. An internal standard correction was used for the quantification of the analytes. The accuracy of the method was evaluated by the recoveries obtained for the certified reference sediment. The obtained data show acceptable recoveries ranging from 60% to 103% for PAHs, from 76 to 131% for PCBs and from 81 to 137% for pesticides.

Data were detected and their concentrations were analyzed using mean ± SD.

The physical and chemical parameters observed in the sampling stations are summarized in Table 1.Water temperature at all sampled stations exhibited comparable temporal fluctuation from 21.8 to 24.6 °C in September and from 22.5 to 23.7 °C in April (Table 1). In dry season, salinity varied from 34.9 to 37.7 PSU in Bizerte Lagoon and from 34.4 to 39.7 PSU in Ichkeul Lake. In rainy season, salinity varied from 31.2 to 34.4 PSU in Bizerte Lagoon and from 2.3 to 2.6 PSU in Ichkeul Lake. The reduction in salinity between seasons at Ichkeul Lake from 35 to 2.5 PSU is due to the contribution of continental water from dams and rivers. In Bizerte Lagoon, the channel and marine stations 9, 10 are submitted to the marine influence with higher salinities.

All PAH target compounds were detectable in sediment samples collected from the complex Ichkeul Lake-Bizerte Lagoon. Total PAHs ranged from 122 to 19,600 ng•g -1 dry wt with mean of 1576 ng•g -1 dry wt. Bizerte Lagoon was much more contaminated with PAHs than Ichkeul Lake for two campaigns September and April. In September 2011, stations 11, 18 and 19 were the most contaminated at Bizerte Lagoon. Stations 9 and 12 were the most affected in April (Fig. 2). The station 11 located in the way of boats between Mediterranean Sea and Menzel Bourguiba harbor, and stations 18, near the industrial area, were contaminated by PAHs in April 2011, and previously reported by [START_REF] Ben Said | Bacterial community structure of sediments of the Bizerte Lagoon (Tunisia), a Southern Mediterranean Coastal Anthropized Lagoon[END_REF]. It is not surprising to found high PAHs levels in the stations 9 and 12 since they are located at the channel of Bizerte Lagoon, which is probably impacted by maritime traffic of Menzel Bourguiba Harbor. The results are comparable to those reported previously [START_REF] Mzoughi | Organotin speciation in Bizerte lagoon (Tunisia)[END_REF][START_REF] Ben Said | Bacterial community structure of sediments of the Bizerte Lagoon (Tunisia), a Southern Mediterranean Coastal Anthropized Lagoon[END_REF]. Station 17 at Bizerte Lagoon was the less contaminated in April than in September. This station is always subjected to water flow with permanent water renewal eliminating all type of contaminant [START_REF] Harzallah | Transport des polluants dans la lagune de Bizerte simulé par un modèle de circulation de l'eau. 30[END_REF].

For the Ichkeul Lake, stations 1, 5, and 6 were most PAHs contaminated in September 2011. In April, stations 1, 2 and 5 were most affected by PAHs. Whatever season, the least PAHs contamination in Ichkeul Lake was at station 7. These results indicate that contamination by PAHs in the lagoon and lake mainly due to accumulation of PAH in sediment from the maritime traffic. Seasonal variation showed that contamination by PAHs was stronger in rainy season (April) than in dry season (September) for the two ecosystems studied. These results were consistent with those obtained by [START_REF] Mzoughi | Organotin speciation in Bizerte lagoon (Tunisia)[END_REF][START_REF] Mzoughi | Méthodologie de l'extraction des hydrocarbures aromatiques polycycliques. Application à des sédiments de la lagune de Bizerte (Tunisie)[END_REF]. Individual PAHs with the highest concentrations are naphthalene, indeno [1,2,3 cd]pyrene, phenanthrene, benzo[b]fluoranthene, and fluoranthene, in descending order (Fig. 3a andb). Comparison with threshold published by the National Oceanic and Atmospheric Administration (NOAA): the Threshold Effects Level (TEL) and Probable Effects Level (PEL), showed that the PAHs contents in stations 6, 9, 10, 12, 18 exceeded the TEL (Table 2), station 12 exceeding the PEL.

Pesticides were detectable in sediment from all prospected stations of the complex. Total OCPs ranged from 34 to 2021 ng•g -1 dry wt with mean of 1576 ng•g -1 dry wt. Lake Ichkeul was more contaminated by OCPs than Bizerte lagoon regardless of the season. Stations 2, 8 and 1 were the most contaminated (Fig. 4). The maximum concentration of endrin was recorded at station 2 and the minimal at station 6 (Fig. 5a). Regarding Dieldrin, the highest concentrations were observed at stations 8, 2 and 1(Fig. 5). The lowest concentration was detected at the station 6. Lindane (HCH) was detected in all stations with comparable concentrations. The maximum dose was detected in station 8 and the minimum at station 6 (Fig. 5a). Endosulfan has been detected in the sediments of different stations with maximum at stations 8 and 2 (Fig. 5a). Heptachlor, Aldrin, Hexachlorobenzene (HCB), dichlorodiphenyltrichloroethane (DDTs), chlordane and nonachlor were detected in the different stations, but at very low concentrations (Fig. 5a andb). A maximum total concentration of up to 2021 ng•g -1 dry wt OCPs was detected in the sediments from stations 8 (Fig. 4). Interestingly station 2 is located near agricultural areas (4102 ha) of the office of public lands. Station 8 was the only station chosen because it is not located in the mouth of a river in order to estimate the effect of direct runoff from agricultural areas. According to the results, it is likely that direct runoff had a greater effect in the transport of pesticides that rivers flow. This observation may be explained by a dilution effect in rivers. These concentrations were in the same range of those detected by [START_REF] Ben Ameur | Concentration of polychlorinated biphenyls and organochlorine pesticides in mullet (Mugil cephalus) and sea bass (Dicentrarchus labrax) from Bizerte Lagoon (Northern Tunisia)[END_REF] in the tissues of fishes Mugilcephalus and Dicentrarchus labrax in Bizerte Lagoon from 1.25 to 227 ng•g -1 dry wt. Lindane (HCH), Dieldrin and Endrin are the most represented in the sediments of Ichkeul Lake. The OCPs were detected at low levels in all stations of Bizerte Lagoon. The OCPs contents exceeded the threshold of the standards published by NOAA (Table 2) in almost all stations in the lake and lagoon. The effect of the OCPs detected on bacteria and free-living marine nematodes in station 6, 8, 13, 17 was investigated in [START_REF] Salem | Pesticides in Ichkeul Lake-Bizerte Lagoon Watershed in Tunisia: use, occurrence, and effects on bacteria and free-living marine nematodes[END_REF]. Concentrations of total PCBs detected in the sediment of Ichkeul Lake-Bizerte Lagoon complex were ranged between 0.004 and 10.653 ng•g -1 dry wt. The levels were relatively low. This result is comparable to the range of concentrations of PCBs detected in sediment from Monastir Bay in Tunisia (1.1 and 9.3 ng•g -1 ) [START_REF] Nouira | Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) in surface sediments from Monastir Bay (Tunisia, Central Mediterranean): occurrence, distribution and seasonal variations[END_REF]. However, the contents detected in the tissues of fish Mugilcephalus and Dicentrarchus labrax from Bizerte Lagoon were higher in the range of 1.43-156 ng•g -1 wt [START_REF] Ben Ameur | Concentration of polychlorinated biphenyls and organochlorine pesticides in mullet (Mugil cephalus) and sea bass (Dicentrarchus labrax) from Bizerte Lagoon (Northern Tunisia)[END_REF], probably due to biomagnification. There was not a clear seasonal and spatial variation between the Ichkeul Lake and the Bizerte Lagoon (Fig. 6) as reported previously by [START_REF] Nouira | Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) in surface sediments from Monastir Bay (Tunisia, Central Mediterranean): occurrence, distribution and seasonal variations[END_REF], except for station 9 at Bizerte Lagoon which was more contaminated than the other stations in April, and for station 5 at Ichkeul Lake in September (Fig. 6). The complex Ichkeul Lake-Bizerte Lagoon was not affected by PCBs. PCBs contents exceeding neither the threshold set by NOAA nor the Interim sediment quality guidelines (ISQGs) for total PCBs, which is determined at 21.5 μg•kg -1 dry wt for marine sediment and at 34.1 μg•kg -1 dry wt for freshwater sediment. The probable effect levels (PELs) for total PCBs being 189 μg•kg -1 for marine sediment and 277 μg•kg -1 for freshwater sediment.

Concentrations of total BuSn detected in the sediment of Ichkeul Lake-Bizerte Lagoon complex were ranged between 67.5 and 526.1 ng•g -1 dry wt. Generally there is no difference between two stations (lake and lagoon) and seasons (Fig. 7), except the station 20 from the lagoon indicates a high level of BuSn (526.1 ng•g -1 dry wt). The lagoon stations close to the ship repair industry are the most contaminated. As regards the Lake, station 1 where load the fishing boats is the most contaminated and the stations submitted to maritime traffic are also contaminated. Concentrations of BuSn detected in the sediments of the lake/lagoon complex were relatively low compared to norms. The spatial distribution of BuSn in the sediment of complex shows that stations subjected to the action of maritime traffic are most contaminated but the contents are substandard.

The Ichkeul Lake was found with high levels of OCPs contents. The OCPs input is probably coming from the agricultural areas located at the edge of this aquatic ecosystem. Bizerte Lagoon was found with high levels of PAHs contents, which are probably coming from the maritime traffic. The PCBs and BuSn content levels were found low indicating that there was not a real contamination by these compounds. The hydrological characteristics of the complex Ichkeul Lake-Bizerte Lagoon, such as seasonal variation of flow rates, may influence the spatial and temporal distribution of persistent organic pollutants in sediments. It is thus important to survey the impact of human activities on the Ichkeul Lake-Bizerte Lagoon hydrologic complex in order to protect this area with high ecosystemic value.

  and singles compounds solutions of PCB 209 and PCB 30 were purchased from Sigma Aldrich. The 16 priority PAHs (Naphtalène, Acénaphtène, Acénaphtylène, Fluorène, Phénanthrène, Anthracène, Fluoranthène, Pyrène, Benzo[a]anthracène, Chrysène, Benzo[b]fluoranthène, Benzo [k]fluoranthène, Benzo[a]pyrène, Indéno[1,2,3-cd]pyrène, Benzo[ghi]

Fig. 1 .

 1 Fig. 1. Map of the Ichkeul Lake-Bizerte Lagoon complex indicating the location of the sampling sites (numbers).

Fig. 2 .

 2 Fig. 2. Spatial distribution of total PAHs in sediments of Ichkeul Lake-Bizerte Lagoon complex. Gray rectangle: concentrations of total PAHs in April; black rectangle: concentrations of total PAHs in September.

Fig. 4 .

 4 Fig. 4. Spatial distribution of total OCPs in sediments of Ichkeul Lake-Bizerte Lagoon complex. Gray rectangle: concentrations of total OCPs in April; black rectangle: concentrations of total OCPs in September.

Fig. 5 .

 5 Fig. 5. a. Concentrations (ng•g -1 dry wt) of OCPs in sediment of different sites from Lake Ichkeul in April. b. Concentrations (ng•g -1 dry wt) of OCPs in sediment of different sites from Bizerte lagoon in April.

Fig. 6 .

 6 Fig. 6. Spatial distribution of totals PCBs (ng•g -1 ) in sediments of complex Ichkeul Lake-Bizerte Lagoon. Gray rectangle: concentrations of totals PCBs in April; black rectangle: concentrations of totals PCBs in September.

Fig. 7 .

 7 Fig.7. Spatial distribution of total BuSn (ng•g -1 ) in sediments of complex Ichkeul Lake-Bizerte Lagoon. Gray rectangle: concentrations of totals BuSn in April; black rectangle: concentrations of totals BuSn in September.

Table 1

 1 Physicochemical parameters of water from the complex Ichkeul Lake-Bizerte Lagoon.

	ST/HAPs T°C	pH	Salinity	Conductivity	Dissolved	Nitrite	Nitrate	Phosphate	Ammonium	Chla	Suspended	Organic
				(PSU)	(ms cm -1 )	oxygen	(μM)	(μM)	(μM)	(μM)	(μg L -1 )	material (mg)	carbon
						(mg l -1 )							(mg g -1 )
	September
	ST1 Sep	21.8 7.52 39.1	58.9	7.4	0.07	11.30	0.05	5.47	0.27	0.01	3.15
	ST2 Sep	22.6 7.71 39.7	59.5	10	0.64	6.19	0.30	14.88	0.40	0.04	3.02
	ST3 Sep	23.2 7.89 37.0	55.9	8.4	0.19	5.34	5.73	12.52	0.13	0.02	3.02
	ST4 Sep	23.3 8.06 37.3	56.2	9.6	0.00	18.26	0.12	6.05	0.27	0.02	2.90
	ST5 Sep	23.4 8.05 35.4	53.8	8.2	0.82	1.91	2.69	16.05	0.94	0.02	2.94
	ST6 Sep	23.5 8.04 36.4	55.1	8.8	0.03	6.07	0.37	4.29	0.67	0.01	3.76
	ST7 Sep	23.2 8.15 35.4	53.7	8.7	0.06	21.47	0.69	1.94	0.80	0.01	3.06
	ST8 Sep	23.6 8.08 36.1	54.6	9.5	0.13	18.22	0.44	11.94	1.47	0.02	3.14
	ST10 Sep 24.1 7.33 37	56.1	6.3	0.51	0.59	0.51	19.00	0.16	0.03	3.55
	ST11 Sep 24.6 7.72 36.4	55.4	5.4	0.84	9.52	0.05	15.47	0.27	0.03	3.51
	ST12 Sep 23.9 7.77 36.5	55.3	5.5	0.34	17.22	0.37	11.35	0.14	0.03	4.34
	ST13 Sep 24.3 7.66 36.4	55.4	5.9	0.69	7.02	0.62	18.41	0.35	0.03	2.12
	ST14 Sep 24.5 7.70 37.4	56.5	6	0.03	17.82	0.65	6.64	0.95	0.04	4.12
	ST15 Sep 24.3 7.67 36.7	55.5	5.6	0.52	7.83	0.62	13.70	0.73	0.06	2.90
	ST16 Sep 24.4 7.76 36.8	55.9	5.9	0.88	13.48	1.51	36.64	0.49	0.04	3.41
	ST17 Sep 24.5 7.79 34.9	53.4	6.1	0.64	3.37	1.08	16.64	0.67	0.04	5.08
	ST18 Sep 24.6 7.74 37.4	56.6	6.1	0.49	5.20	0.98	21.94	0.53	0.03	5.72
	ST19 Sep 24.4 7.84 37.7	57.0	6.2	0.56	15.88	0.15	13.11	0.84	0.03	7.24
	April
	ST1 Apr	22.8 8.24 2.4	4.6	9.6	0.09	1.73	0.02	7.32	0.45	0.04
	ST2 Apr	22.5 8.24 2.5	4.9	11	0.53	6.09	0.09	11.09	0.68	0.06
	ST3 Apr	23.8 8.32 2.6	5.0	9.2	0.09	2.41	0.74	13.98	0.41	0.05
	ST4 Apr	23.6 8.32 2.6	5.0	10.8	0.07	1.76	0.16	9.83	0.52	0.04
	ST5 Apr	23.2 8.32 2.6	5.01	9	0.57	7.22	1.26	14.54	2.06	0.03
	ST6 Apr	23.4 8.32 2.4	4.7	9.7	0.03	1.28	0.17	6.87	1.50	0.04
	ST7 Apr	23.1 8.31 2.3	4.4	9.3	0.09	3.65	0.41	10.85	1.75	0.04
	ST8 Apr	22.9 8.27 2.6	5.0	10.7	0.15	1.97	0.27	3.94	3.98	0.06
	ST9 Apr	23.4 8.18 32.2	49.3	8.5	0.41	3.71	0.36	11.64	1.73	0.02
	ST10 Apr 23.2 8.21 32.4	45.6	8.5	0.46	2.91	0.32	13.91	0.39	0.02
	ST11 Apr 23.5 8.34 24.5	40.2	8.4	0.32	4.78	0.63	18.79	0.57	0.02
	ST12 Apr 22.9 8.33 30	46.3	8.7	0.50	5.99	0.54	10.19	0.36	0.03
	ST13 Apr 23.1 8.34 27	42.6	8.6	0.57	8.21	1.46	8.37	0.49	0.02
	ST14 Apr 23.2 8.34 28.8	45.1	8.4	0.71	11.43	1.22	16.22	0.74	0.04
	ST15 Apr 23.7 8.37 28.5	43.5	8.6	0.01	0.97	1.64	11.07	1.44	0.03
	ST16 Apr 23.6 8.39 28.6	45.1	8.5	0.60	5.81	0.32	7.88	0.96	0.05
	ST17 Apr 23.7 8.42 29	45.3	8.7	0.74	6.91	1.72	11.54	0.91	0.03
	ST18 Apr 23.2 8.46 28.4	45.1	8.5	0.50	3.72	0.50	16.02	0.65	0.02
	ST19 Apr 23.6 8.45 29.1	45.6	8.6	0.53	4.23	0.36	8.74	1.97	0.04
	ST20 Apr 23.5 8.40 29.2	45.7	8.2	0.46	5.98	0.16	8.34	0.76	0.01

Table 2

 2 Threshold effects level (TEL) (ng•g -1 dry wt) of OCPs from NOAA screening quick references table.

	Compounds	Marine sediment	Freshwater sediment
	Lindane	0.32	0.94
	Chlordane	2.26	4.5
	p,p DDD	1.22	3.54
	p,p DDE	2.07	1.42
	p,p DDT	1.19	-
	DDT total	3.89	6.98
	Dieldrin	0.715	2.85
	Endrin	-	2.67
	Heptachlor epoxide	-	0.6

Acknowledgements

The authors would like to thank Mme Nabiha M'barek (National Agency of Environment Protection Tunisia) and Mr. Taher Kaabi (Commissariat Regional Commission for Agricultural Development of Bizerte) for their logistic contribution. We acknowledge the Regional Platform for Environmental Microbiology PREMICE supported by the Aquitaine Regional Government Council (France) and the urban community of Pau-Pyrénées (France).

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.marpolbul.2017.09.024.