Noël Groga
email: groga7@yahoo.fr

Allassane Ouattara

Sébastino Da Costa

Alain Dauta

Olivier Beauchard

Jacques Moreau

Germain Gourene

Pascal Laffaille
email: pascal.laffaille@ensat.fr

Water quality and water-use conflicts in lake taabo (ivory coast)

Keywords: Conflicts over Water Use, Pollution, Tropical Lake, Lake Structure

The Lake Taabo (Ivory Coast, Africa) results of the construction of the Taabo dam on the Bandama River. The changes in the water level of the 69-km² lake depend on 1) the rainfall linked to alternating dry/wet seasons; 2) the extraction of water for human uses; 3) the discharge of water from the upstream dam and the volumes tur-bined by the Kossou dam; 4) the various anthropic effects (discharge of untreated waste water from towns and industries, and leaching from agricultural land). The average concentrations of nutrients (NH 4 -N: 1.1 mg/L, NO 3 -N: 1.62 mg/L, PO 4 -P: 10 mg/L, SiO 2 : 15 mg/L) and chlorophyll a (from 4.8 to 16.5 µg/L, average 11.4 µg/L) indicates some degree of eutrophication. The cumulated effects that threaten the ecosystem (degradation of water quality and eutrophication) are such that they are likely to interfere with various water uses. In a context of growing health and environmental concerns in Africa, this study demonstrates conflicts between different uses of this water resource and the urgent need for an appropriate policy including specific monitoring of lake water quality, wastewater control, and a programme to reduce agricultural fertilizers.

INTRODUCTION

The conflicts around water are not new. Water is a valuable and essential natural resource, the quantity and quality of which are decisive factors for human and animal health [START_REF] Kazi | Assessment of water quality of polluted lake using multivariate statistical techniques: A case study[END_REF]. Lake water is used for human and animal consumption, for fishing, irrigation, transportation, the generation of hydroelectricity, and for a whole range of other domestic, industrial and agricultural uses. Lake ecosystems are not only reserves of fresh water, they can also act as foci around which human activities that can stimulate and support regional economies [START_REF] Burton | Le plus grand lac du mexique peut-il être sauvé?[END_REF] tend to cluster. However, these activities may not all be mutually compatible. The quality of the surface water is influenced both by natural processes (soil erosion, rainfall, evaporation…) and by human activities (agricultural, urban and industrial waste water) [START_REF] Strobl | Network design for water quality monitoring of surface freshwaters: A review[END_REF]. The exponential growth of human populations, the development of intensive agriculture and increasing industrialisation are the main causes of the deterioration of water quality at all levels. West Africa is currently experiencing a very real problem of the overexploitation of hydrosystems, because urbanisation is concentrating populations beside rivers and lakes. This kind of problem has occurred in the basin of Komaduju river and of Lake N'Guru (Nigeria), Lake Diama (Senegal), and River Lumani (Burkina Faso), where the waste water produced by the major cities and the increasing use of agricultural fertilizers has contributed to a dangerous degradation of water quality, and the spread of waterborne diseases such as onchocerciasis and schistosomiasis [START_REF] Niasse | Réduire la vulnérabilité de l'afrique de l'ouest aux impacts du climat sur les ressources en eau, les zones humides et la désertification[END_REF][START_REF] Horeau | L'or bleu des barrages se ternit... colloque eau et santé, impacts sanitaires et nutritionnels des hydro-aménagements en Afrique[END_REF].

The Lake Taabo region (Ivory Coast) is an example of this. This hydro-electric dam located downstream from the Kossou dam is exposed to the impacts of various different anthropic actions, such as the generation of hydroelectricity, the discharge of unprocessed effluents from the surrounding zones, the use of traditional fishing methods, the leaching of agrochemicals… This lake is therefore exposed to severe anthropic pressure leading to weakening of the ecosystem, and disruption of the balance between the various uses of water. In conflicts over water use (Figure 1) it appears that socio-economic drivers, the services expected, and the problems directly linked to water use are both interdependent and contradictory. This study, carried out on Lake Taabo reflects these conflicts over water use in a region currently undergoing rapid development. On the basis of the water quality data measured, this study draws up an inventory of the topics that are leading to problems and the indicator parameters, which constitutes a first step towards finding solutions in the medium term, with regard to development, sustainable management and monitoring of water resources.

MATERIAL AND METHODS

Study Site

Lake Taabo (06Û40' latitude Nord and 5Û longitude West; Figure 2) is the regulating lake of the Kossou dam. Located downstream from the confluence of the White Bandama and the Marahoué (or Red Bandama), Lake Taabo has a catchment area of 58,700 km 2 . The lake itself covers an area of ±80 km 2 , and is generally subjected to a daily fluctuation of the water level of ±3 meters. The population living in the vicinity of the lake is estimated to be about 140,000 inhabitants. With nearly 240 year-round commercial fishermen, the average total annual fish catch is 220 metric tons. Traditional subsistence crops occupy huge areas, notably cocoa (7,741 ha) and coffee (173 ha) beans, with annual productions of 3,000 and 121 metric tonnes respectively (data from the Agriculture is the main activity in the region. It occupies 85% of the land in the catchment area of the reservoir and uses large amounts of agrochemicals (10,400 metric tonnes per year). Furthermore, during the last ten years, the invasion of water hyacinth has resulted in the progressive eutrophication of the lake, leading to major problems for water use [START_REF] Véi | Suivi et évaluation de l'impact socio-temporel d'un projet d'aménagement du territoire en afrique de l'ouest. l'exemple du barrage de taabo en côte d'ivoire[END_REF][START_REF] Kouamé | Macroinvertebrate communities associated with macrophyte habitats in a tropical man-made lake (Lake Taabo, Côte d'Ivoire)[END_REF].

Sampling

The samples were taken during five campaigns between July 2006 and June 2007, using an opaque PVC Van Dorn-type bottle with a capacity of 2.5 litres, mounted horizontally in order to maintain the sampling depth. The sampling depths in the lake were as follows: surface (-40 cm); -1 m; -2 m; -3 m. -4 m. The stations chosen took into account the succession of aquatic habitats and human activities from upstream to downstream. There were six stations (Figure 2): furthest upstream, the reference station Sahoua (S1), in the upper flow at Ahondo (S2), in the deep segment of the left bank of the lake at Courandjourou (S3), in an agricultural zone at Taabo-village wharf (S4), in an urban zone on the right bank at Taabo-City (S6), and finally the furthest downstream at the Taabo-village dam (S5). A GPS MLR SP 12X satellite navigator was used to assist in determining the location of the stations. The water transparency was determined in situ, from the depth (Zs) at which the Secchi disk disappeared. Since 15% of the incident light reached this depth, the depth of the euphotic layer (Zeu) could then then evaluated using the following equation: Zeu = 2.43 × Zs [START_REF] Wetzel | Limnological Analysis[END_REF]. Determinations of the temperature (in ÛC), the conductivity (in µS/cm), the total dissolved solids (TDS; in mg/l), the dissolved oxygen (DO; in mg/l), the pH and the turbidity (in NTU) of the water were carried out in the field, using a multiparametric WTW 340i instrument.

Analysis of Nutrient

In the laboratory, the water samples were analysed in order to determine the concentrations of ammoniacal nitrogen (mg/L of NH 4 + ), nitrate (mg/L of N-NO 3 ), nitrites (mg/L of N-NO 2 ), phosphate (mg/L of PO 4

3-) and silica (mg/L of SiO 2 -). The samples were filtered through Whatman GF/C fibreglass filters. Then, in accordance with standard operating procedures [START_REF]Standard Methods for the Examination of Water and Wastewater, 17th Edition[END_REF], the concentrations of these parameters were determined using a HACH DR 2000 model spectrophotometer at a specified wavelength.

Analysis of Chlorophill a

To measure the chlorophyll a (chl-a) concentration, 250 ml of river water was passed through 0.7 µm glass fibre filters (Whatman GF/C, diameter of 25 mm) during the 10 hours after sampling. Because sampling took place over 2 days, filters were stored at -80ÛC for 48 h, following which chl-a was extracted in 97% methanol at 75ÛC. The extinction of extracts was measured at 750 nm and 665 nm before and after adding hydrochloric acid. The chorophyll-a content was calculated according to Herbland et al. [START_REF] Herbland | Size structure of phytoplankton in the equatorial Atlantic Ocean[END_REF].

Data Analysis

Wilcoxon and Kruskal-Wallis tests were used to test the effects of season, station and depth on each of the 13 physicochemical descriptors. The spatio-temporal analysis was then analysed by means of a normalised principal component analysis (PCA) and processed by intergroup analyses [START_REF] Doledec | Rythmes saisonniers et composantes stationnelles en milieu aquatique-I. description d'un plan d'observations complet par projection de variables[END_REF] to test the extent of the effects of the three factors (season, station and depth) on the multidimensional model. The test procedure was based on 9999 random permutations of the lines in the data table [START_REF] Manly | Randomization and Monte Carlo Methods in Biology[END_REF].

RESULTS

Physicochemical Characteristics

The water temperature in Lake Taabo (Table 1) re-mained fairly high all year round (on average 28.8ÛC), with little variability (interval of confidence: 1.7ÛC).

The dynamics of the values of the turbidity and transparency of the water showed the same trends as those of the Total Dissolved Solids (TDS) and the conductivity (Table 1). The highest mean values of the turbidity were observed in the upstream segment (at S1), whereas the lowest values were recorded downstream (at S5). The water was deeper during the rainy season than during the dry season: 5.9 ± 0.8 m and 5.5 ± 1.0 m at S1; 6.0 ± 1.0 m and 5.5 ± 1.0 m at S2; 7.3 ± 1.5 m and 6.5 ± 1.0 m at S3; 6.8 ± 1.6 m and 5.4 ± 0.7 m at S4; 8.0 ± 1.0 m and 6.3 ± 0.5 m at S5; 6.7 ± 0.5 m and 5.5 ± 1.0 m at S6, respectively. The transparency (Secchi disk) ranged from 0.56 m (S1) to 2.01 m (S4) during the period studied, which corresponds to a depth in the euphotic zone ranging from 1.65 m to 4.88 m.

The dissolved oxygen saturation level (% DO) fluctuated between 37% (station S2) and 122.5% (station S6). The dissolved oxygen ranged from 3.2 to 9.4 mg/l; the highest oxygenation level of the water occurring at S6. The low levels at S1 and S3 were attributable to the high macrophytic biomass at the stations in the upstream segment of the lake. The highest conductivity values were observed during the rainy season. The mineralisation declined overall from upstream (88 ± 15.6 mg/l at S1) to downstream (63 ± 7.9 mg/l at S5). The pH tended to be neutral to basic at S1, S2 and S6, and neutral to acidic at S3, S4, and S5. This latter group of stations displayed high concentrations of silicates ([SiO 2 í ] > 4 mg/l). The levels of nitrate [NO 3 ] were highest in the zones S4, S5 and S6, but remained below 3 mg/l. Nitrite [NO 2 ] concentrations were very low. Orthophosphate concentrations increased progressively from the point of entry into the lake (S1: 7.4 ± 8.9 mg/l) towards the dam (S5: 54 ± 18 mg/l at S5). The concentrations of ammonium ions were low at the entry point (S1: 0.09 mg/l and S2: 0 mg/l) and reached 3.05 mg.N-NH4+/l at the bottom of the lake at S4 (Table 1).

Define abbreviations and acronyms the first time they are used in the text, even after they have been defined in the abstract. Abbreviations such as IEEE, SI, MKS, CGS, sc, dc, and rms do not have to be defined. Do not use abbreviations in the title or heads unless they are unavoidable.

Effects of Place and Season

The physicochemical characteristics seem to have been strongly influenced mainly by the season, and secondarily by the station, but not by the sampling depth (Tables 1 and2).

The PCA revealed the importance of the first three axes: 44%, 15% and 13% respectively (Figure 3). Axes 1 and 2 were strongly structured by season. High values of temperature, dissolved nutrients and transparency were characteristic of the dry season, unlike the rainy season, which was characterised by high DO concentrations, a high percentage DO, high conductivity and high levels of total dissolved solids (TDS). Axes 2 and 3 reflect the spatial variations induced by a season effect. We can, therefore, make three main observations:

x The spatial variability was more marked during the rainy season. The stations fall into 3 main groups (group 1: S6, group 2: S1, S2, S4, S5, group 3: S3) according to axes 1 and 2 in the rainy season. During the dry season, the stations were less dispersed. x There was an upstream-downstream gradient along axis 3, which was characterised by the variability of the concentrations of DO, % DO, nitrates and of the transparency.

x Axis 2 was characterised by high values of turbidity and pH.

Finally axes 1 and 3, the downstream stations (S4, S5, S6), which were subject to the greatest anthropic pressure, contrasted with the most upstream stations in the lake (S1, S2) and S3.

Average concentration of chl-a were low, exceeding 10 µg/L on only occasion in S1, and some values were less than 6 µg/L. Furthermore, a downstream increase in chla was observed. The influence of tributaries on phytoplankton algal biomass is shown in Table 1.

DISCUSSION

This paper is based on a recent field study, and includes various data available about Lake Taabo. The goal was to provide an update on the trophic status of the lake, and to identify the main problems that limit the use of this water resource with a view to defining an appropriate protection and management program.

The low values of transparency found during the rainy season (<1 m transparency during high water, compared to 2 m on average during the low water period) were due to an increase in the concentration of suspended matter. This was essentially due to erosion in the catchment area of the lake and to the presence of large quantities of agricultural waste carried in by leaching from the fields [START_REF] Akpan | Seasonal variation in water quality of the Cross river, Nigeria[END_REF].

The temperature profile does not display any marked thermal stratification or much variability. Cooling throughout the entire water column during the rainy season is characteristic of tropical lakes [START_REF] Ryding | Le Contrôle de l'Eutrophisation des Lacs et Réservoirs[END_REF]. The temperatures in Lake Taabo are of the same order as those reported in Lakes Kossou [START_REF] Brunel | Evaporation des Nappes d'Eau Libre en Afrique Sahélienne et Tropicale[END_REF], Buyo [START_REF] Yapo | Evaluation de l'état trophique du lac de Buyo (côte d'Ivoire), aspects physicochimiques et biologiques[END_REF] and Ayamé [START_REF] Diomandé | Macrofaune bentique et stratégies alimentaires de Synodontis batiani et S. schall en milieu fluvio-lagunaire (bassins Bia et Agnébi; Côte d'Ivoire)[END_REF]. Lake Taabo is a hot monomictic lake with little difference in temperature between the epilimnion and the hypolimnion (mean temperature 26ÛC to 27ÛC) which makes it impossible to establish a true thermocline.

The vertical distribution of the dissolved oxygen is clinograde all year round. The surface masses of water display marked supersaturation. In the depths, a marked deficiency of local dissolved oxygen may occur. The 60% level of dissolved oxygen saturation that constitutes a critical biological threshold is often reached at a depth of just 2 m below the surface. This value constitutes a warning sign; if the increased pollution in the lake continues, the oxygen deficiency will gradually increase to the point where it threatens aquatic life. During the dry seasons, complete anoxia is often recorded at a depth of 3 m. Only high rainfall leads temporarily to a slight increase in the dissolved oxygen saturation. Similar observations have been made in Lakes Ayamé 1 and 2 [START_REF] Galy-Lacaux | Long-term greenhouse gas emissions from hydroelectric reservoirs in tropical forest regions[END_REF]. At depths of more than 3 m, anoxia is permanent throughout the year. Such a change in the concentration of dissolved oxygen in an environment containing high levels of organic substances is characteristic of highly eutrophic environments, and related to the marked development of phytoplankton and macrophytes [START_REF] Moss | Ecology of freshwater: Man and Medium, Past to Future[END_REF]. The high conductivity observed cannot be explained by the granite-type rocky substratum [START_REF] Leblond | Contribution aux Études Hygéologiques en Côte d'Ivoire. Région de Yamoussoukro, Station Expérimentale[END_REF] and must, therefore, be attributed to inputs linked to human activities, such as untreated waste water [START_REF] Helena | Temporal evolution of groundwater composition in an alluvial aquifer (Pisuerga River, Spain) by principal component analysis[END_REF]. The deoxygenation of the deep zones of the lake is the consequence of the breakdown of the input of organic matter. These zones, which lie beneath the euphotic zone (a zone with low transparency), do not benefit from the purifying activity of photosynthetic organisms. In these deepest layers, the low values of dissolved oxygen can compromise the survival of aquatic organisms [START_REF] Izonfuo | The effect of urban runoff water and human activities on some physico-chemical parameters of the Epie creek in the Niger Delta[END_REF]. These observations are corroborated by the high levels of ammoniacal nitrogen at the bottom of the lake, which reflects the incomplete mineralisation of organic substances. The extension of the anoxic zones in the water column can threaten the entire ecosystem. However, oxygen levels during the daytime were highest at station S6 during the rainy season (Groga, unpublished data). This is certainly to be related to enrichment in nutrients, which temporarily promotes primary production.

Coastal areas, which are subjected to significant changes in water levels, are sites where non-negligible quantities of sediments and mineralised organic matter are resuspended and redissolved during successive flooding events. Nitrate concentrations are low (<0.5 mg/l) are comparable to those reported by [START_REF] Uzoukwu | Monitoring of seasonal variation in the water quality of Ubu river in Ekwusigo and Nnewi local government areas of Anambra State, Nigeria[END_REF] in the River Ubu (0.1 to 3 mg/L NO 3 -). At the beginning of the rainy season, an increase in the concentrations of nitrates is observed due to leaching from the soil. However, heavy and continuous rain results in dilution, leading to lower concentrations [START_REF] El Samrani | Trace element carriers in combined sewer during dry and wet weather: An electron microscopie investiga-tion[END_REF]. This change in concentrations linked to pollution is attributable to the leaching of artificial fertilisers (N/P/K) used by smallholders along the lake shores [START_REF] Bartsch | Stream life and the pollution environment[END_REF]. These inputs in surface water run-off depend on their flow and their proximity to sources of pollution (in our case, downstream from towns and agricultural land). During the rainy season, the high levels of nutrient salts observed reflect the presence of a large area of cattle pasture in the case of station S3, a strongly agricultural zone in that of station S4, and a densely-populated urban zone with little agriculture in that of station S6. These concentrations are gradually diluted towards the exit from the lake (station S5). Nutrient criteria for streams may be needed to avoid direct toxicity, taste, and odor problems, alterations in biotic integrity, and interference with recreation [START_REF] Dodds | Establishing nutriaent criteria in streams[END_REF].

When a dam is built and its reservoir fills, human activities soon develop in the region served by the dam, which leads to severe environmental disturbance (Figure 4). Thus, the banks are subjected to anthropic pressure due to the development of urbanisation, roads, industries and agriculture. The effects of these pressures and of their environmental impacts are obvious in the case of Lake Taabo. Human pressure and current climate changes are leading to rapid changes in lake function. The use of water to generate hydroelectric power contributes to accelerating changes in lake function. Flooding of agricultural land during the rainy season contributes to washing large amounts of sediment and nutrients into the lake. Finally, in the case of Lake Taabo, we can identify five major causes of disruption (Table 3): run-off from agricultural land and urban areas, solid waste washed off by the rain from garbage dumps, changes in water levels related to electricity generation. The combined effects of these disturbances can not only impair the functioning of the ecosystem, but also lead to impaired water quality and restriction of water usage by local communities, which in the medium term could compromise the growth of new economic activities around Lake Taabo.

Usage conflicts, associated with the use of water as a receptor for pollution, are increasing. In developing countries, the technical means required to reduce or treat pollution are generally not available. Meanwhile, the development of urban areas is accompanied by an increasing demand for drinking water. In remote locations, wastewater treatment plants are still very sketchy or even nonexistent. Despite existing concerns about traditional usages, and the resulting pressure on water resources is an important potential source of tension and of conflict, we should not ignore two other significant factors: the development of agro-industries and intensive irrigation on one hand [START_REF] Paquerot | Un Monde sans Gouvernail: Enjeux de l'Eau Douce[END_REF], and the numerous industrial usages of water on the other hand.

Excess nutrients can also indirectly prejudice human interests as a result of increased water treatment costs. Algal and cyanobacterial blooms can lead to taste and odour problems, and increase toxin levels in river water [START_REF] Maier | Flow management strategies to control blooms of the cyano-bacterium, Anabaena circinalis, in the River Murray at Morgan, South Australia[END_REF][START_REF] Davis | Eutrophication in Australian rivers, reservoirs and estuaries-A southern hemisphere perspective on the science and its implications[END_REF]. Algal blooms can also interfere with recreational uses such as swimming, fishing, and tourism due to increases in the phytoplankton, periphyton, and/or macrophyte biomass. Finally, eutrophication of rivers can alter the composition of the biotic community and decrease biotic integrity [START_REF] Miltner | Primary nutrients and the biotic integrity of rivers and streams[END_REF]. Chl-a concentration in the Lake indicates an apparently markedly trophic state.

Many sources of renewable energy use water, most obviously in the case of hydroelectricity. Reservoirs can also lead to other severe ecological disruption: excessive evaporation of the bodies of water in periods of hot weather, the drying up of the downstream segments of rivers. The vital and multifunctional nature of water, which is a limited resource and unequally distributed, suggests that there will be an increase in conflicts which, if they are not resolved by rules perceived as being fair, will contribute to increasing tension, situations of violence, marginalization and exclusion, and humanitarian disasters. The priorities for water management appear to be as follows:

A biosafety policy followed by an action plan to prevent telluric pollution, Reinforcement of the operational framework for the management of agrobiodiversity, Development of a global policy regarding the management of genetic resources that are important for farming and nutrition, Improving the conservation of biological variety in the development of sectoral activities, Making the choices that reduce negative impact of sectoral activities, and reinforcing the capacities of the participants.

The current situation could lead in the near future to a progressive and irreversible deterioration of water quality in the Lake. Faced by this threat to the ecosystem, as well as to human and animal health and to the economic development of the region, it is reasonable to promote a programme that will ensure sustainable development and monitoring of the environment and of water use throughout the entire catchment area of the Lake. These conclusion and suggestions for future planning and management will be taken into account by the Ministry of Water and Forests (MEF) and SODECI (The Ivory Coast Water Distribution Company), the two agencies in charge of managing Lake Taabo and its watershed.

Figure 1 .

 1 Figure 1. Water usage conflicts are linked to the interdependence of socio-economic drivers, the services expected and problems directly related to water quality.

Figure 2 .

 2 Figure 2. Sampling sites in Lake Taabo (S1: Sahoua, S2: Ahondo, S3: Courandjourou, S4:Taabo-village Wharf, S5: Taabo-Dam, S6: Taabo-City) and the main cities around the study zone. Agence Nationale d'Appui au Développement Rural, Ivory Coast). More recently-introduced crops, such as hevea and oil palm, occupy a total area of about 300 hectares, with an estimated production of 2000 metric tonnes per year (data from the Agence Nationale de l'Environnement, Ivory Coast). The other subsistence crops (rainyseason rice, maize, peanuts, cassava, plantain, yam) occupy an area of 4292 hectares and produce 19,466 metric tons per year (data from the Agence Nationale d'Appui au Développement Rural, Ivory Coast).Agriculture is the main activity in the region. It occupies 85% of the land in the catchment area of the reservoir and uses large amounts of agrochemicals (10,400 metric tonnes per year). Furthermore, during the last ten years, the invasion of water hyacinth has resulted in the progressive eutrophication of the lake, leading to major problems for water use[START_REF] Véi | Suivi et évaluation de l'impact socio-temporel d'un projet d'aménagement du territoire en afrique de l'ouest. l'exemple du barrage de taabo en côte d'ivoire[END_REF][START_REF] Kouamé | Macroinvertebrate communities associated with macrophyte habitats in a tropical man-made lake (Lake Taabo, Côte d'Ivoire)[END_REF].

Figure 3 .

 3 Figure 3. Principal Component Analysis (PCA). A and D: specific values diagram. A, B and C: axes 1 (44%) and 2 (15%). D, E and F: axes 1 and 3 (13%). C and F: samples pooled by depth, revealing the absence of vertical structuring. B, C, E and F: rainy season in black and dry season in black.

Figure 4 .

 4 Figure 4. Main factors influencing the volume of water in Lake Taabo.

Table 1 .

 1 Mean, minimum and maximum values, and standard deviation (sd) of the abiotic parameters measured between July 2006 and June 2007 at the 6 stations on Lake Taabo on the basis of the five depths. M: main. Sd: standard deviation. R: range. DO: dissolved oxygen.

					Sahoua (S1)		Ahondo (S2)		Courandjourou (S3)
				M	Sd	R	M	Sd	R	M	Sd	R
	Temperature (ÛC)	29.58	0.71	28.7 -30.4	29.4	0.63	28.5 -30.2	29.05	0.33	28.6 -29.9
	DO (mg/l)	4.67	1.06	3.2 -5.74	4.82	1.09	3.5 -6	4.05	5.76	3.2 -4.78
	DO (%)	59.84	15.4	41 -75.8	61.42	16.02	37 -79	53.89	76.22	45 -62.5
	pH			7.28	0.24	7.01 -7.68	7.08	0.15	6.85 -7.28	6.65	6.9	6.55 -7.07
	Conductivity (µs/cm)	102	26.6	76 -143	92.5	20.19	73 -115	91.9	86.2	73 -126
	Total dissolved solids (mg/l)	64.6	15.63	49 -88	59.7	13.19	47 -80	59.3	55.5	47 -78
	Turbidity (NTU)	36.51	27.65	16.2 -100.2	21.45	3.35	16.5 -27	12.6	12.25	9.7 -17.4
	Transparency (m)	0.71	0.48	0.56 -0.76	0.82	0.38	0.69 -0.89	1.45	1.44	0.98 -1.91
	Chl-a (µg/l)	9.4	2.02	6.5 -14.1	12.8	2.51	8.5 -16.3	12.01	11.2	7.6 -16.01
	NO 3	-(mg/l)	0.71	0.45	0.12 -1.51	0.94	0.61	0.16 -1.81	1.16	1.78	0.31 -2.89
	NO 2	-(mg/l)	0.6	0.46	0.19 -1.37	0.49	0.56	0.04 -1.65	0.72	0.96	0.18 -2 .19
	NH 4	+ (mg/l)	0.38	0.55	0.06 -1.75	0.39	0.52	0 -1.57	0.29	0.73	0.04 -1.49
	PO 4	3-(mg/l)	7.4	8.9	0.8 -29	8.1	7.6	0.8 -18	6.1	10	1.1 -31
	SiO 2 (mg/l)	15.13	11.4	3.41 -26.69	16.9	12.56	4.11 -38.99	22.87	31.12	8.49 -46.81
				Taabo village Wardf (S4)		Taabo village dam (S5)		Taabo city (S6)
				M	Sd	R	M	Sd	R	M	Sd	R
	Temperature (ÛC)	28.89	0.31	28.5 -29.2	29.15	0.48	28.6 -29.7	29.18	0.53	28.5 -29.7
	DO (mg/l)	5.76	0.93	4.2 -6.79	5.92	0.6	5.35 -6.85	6.28	2.81	3.4 -9.4
	DO (%)	76.22	11.57	53 -87.4	65.48	20.83	45.8 -89.4	83.1	35.08	47 -122.5
	pH			6.9	0.09	6.82 -7.02	6.87	0.08	6.77 -7.01	7.57	0.16	7.33 -7.85
	Conductivity (µs/cm)	86.2	11.92	73 -98	85.1	12.59	72 -97	82.33	7.43	75 -92
	Total dissolved solids (mg/l)	55.5	7.17	47 -65	55	7.86	47 -63	53.33	4.72	49 -59
	Turbidity (NTU)	12.25	1.85	9.04 -15.9	10.74	1.16	8.9 -12.6	22.68	19.99	12.5 -75
	Transparency (m)	1.44	0.6	0.87 -2.01	1.28	0.57	0.73 -1.82	0.91	0.02	0.89 -0.93
	Chl-a (µg/l)	11.2	2.25	5.3 -14.8	11.9	1.93	7.7 -16.5	11.2	1.27	7.7 -13.6
	NO 3	-(mg/l)	1.78	1.01	0.66 -3.71	1.68	1.04	0.26 -3.81	1.62	0.16	0.51 -3.74
	NO 2	-(mg/l)	0.96	1.17	0.05 -3.62	0.96	1.17	0.05 -3.62	1.03	1.14	0.02 -3.39
	NH 4	+ (mg/l)	0.73	1.09	0.06 -3.05	0.73	1.07	0.05 -3.02	0.98	1.04	0.03 -2.68
	PO 4	3-(mg/l)	10	17	0.02 -52	10	18	0.04 -54	15	17	0.01 -49
	SiO 2 (mg/l)	31.12	15	9.06 -46.62	21.12	18	9.06 -44.42	23.27	16.73	4.64 -44.76

Table 2 .

 2 Wilcoxon test (season effect) and Kruskal-Wallis test (station and depth effects) on each of the physicochemical descriptors. Significantly differences are shown in bold (p < 0.05). DO: dissolved oxygen.

	Descriptor	Season effect Wilcoxon	p	Ȥ 2	Station effect	p	Ȥ 2	Depth effect	p
	Temperature	13.0	<0.001	6.7		0.247	1.1		0.901
	DO	464.0	<0.001	12.0		0.035	1.2		0.884
	% DO	464.0	<0.001	16.4		0.006	1.4		0.851
	Transparency	0.0	<0.001	31.1		<0.001	0.0		1.000
	Turbidity	369.0	0.005	43.3		<0.001	2.3		0.687
	Conductivity	465.0	<0.001	6.1		0.295	0.7		0.950
	TDS	465.0	<0.001	4.7		0.457	1.2		0.885
	pH	85.5	0.003	46.9		<0.001	0.8		0.940
	PO 4	0.0	<0.001	2.3		0.809	1.6		0.809
	NO 3	298.5	0.176	11.4		0.044	6.1		0.194
	NO 2	20.0	<0.001	3.0		0.707	3.0		0.563
	NH 4	5.0	<0.001	6.5		0.261	1.5		0.832
	SiO 2	1.0	<0.001	6.7		0.243	1.7		0.798

Table 3 .

 3 Management of water-use conflicts.

	Potential anthropogenic factor: impacting water quality	Relative none, 5 = strong) importance in Taabo lake. scale 0 to 5 (0 =	Causes	Consequence	Warning parameter	Key and solution management	Control and monitoring
						Reduce pesticide	Control of main
	Water quality: Chemical parameters and Contaminants	3	Industrial waste, urban waste water, leaching from agricultural soils	Use for drinking water and irrigation, deterioration of conditions for aquatic life	Contaminant concentrations, Nitrogen and phosphorus fluxes	and herbicide use, improve agricultural practice, collect and treat waste	tributaries and outlet from industrial areas and towns. Modify agricultural practices to reduce the use of
						waster	agrochemicals
				Use for drinking
	Water quality: bacteriology	4	Garbage dumps, urban waste water, farming waste	Water compromised, deterioration of conditions for	Bacteriology Intestinal enterococci and Escherichia coli	Collect and treat waste water , get rid of unauthorized dumps	Control bacteriology in the areas where water is extracted for domestic use
				aquatic life
	Water quality: physical parame-ters	3	Floods, flushing of garbage dumps	Harmful effects on fishing and leisure activities	Water level changes, suspended matter, floating objects	Avoid the flushing of dumps, modify agricultural practices to limit leaching from soil	Control of dumps, establishment of a strip of grass between cultivated areas and the receiving water courses
				Toxic effects due to	Chlorophyll a,
	Water quality: eutrophication (phytoplankton, macrophytes)	4	Urban waste and agricultural fertilizers	cyanobacterial blooms, inconvenience caused by	cyanotoxins, changes in dissolved oxygen, biomass and areas	Limitation of nutrient inputs	Regular monitoring of Chlorophyll a content and macrophyte areas
				macrophytes	of macrophytes
	Ecosystem deterioration, Wetland degradation	4	Pollution and changes in water level	Decline in biodiversity, degradation of water quality	Species loss, Invasive species, Decline in fish production	Prevent degradation of spawning zones and wetlands	Monitor wetlands and river bank degradation
	Marling Water level changes	3	Electricity production and flood control	Hampers leisure and fishing activities, impacts on spawning and wetlands	Limitation of the use of the docks, some areas dangerous for swimming, considerable local residents inconvenience for	Limit tidal range at certain times, shoreline development for swimming boats and docks for	Survey users and communities to identify priority areas to protect, and develop infrastructure
						Impose a minimum
	Fishing activity 3	Water quality, float-ing waste	Reduces the tonnage of fish catches	Species loss, decline in fish production	size for each fish species, improve water	Monitor water quality control throughout the lake
						quality
	Power production	3	Respect of low tidal amplitude	Lower power production, complex management of generation	Major inconvenience for local residents	Avoid excessive changes in water level	Management of power generation by smoothing changes in water level
			Water quality:
			bacteriology,
			biology (macrophytes
	Leisure	4	and phytoplankton) and physical appearance (too much waste floating on the surface of	Reduction of activities. swimming is dangerous if the transparency is low	Decrease in the number of users and low satisfaction index	Water quality control, improving and securing the shoreline access	A survey of users and communities to identify priority areas to protect and develop infrastruc-ture.
			water), difficult to
			access the banks of
			the River

ACKNOWLEDGEMENTS

We would like to thank the Committee for Scientific and Technological Cooperation [COMSTECH] of the Islamic Conference Organisation (Islamabad, Pakistan) and the International Foundation for Science [IFS/FIS] (Stockholm, Sweden) who funded this study under the auspices of the Challenge Program Project CGIAR/FIS-CNRA N° A/4007-1.


C Rabouille

D Amouroux
email: david.amouroux@univ-pau.fr

P Anschutz
email: p.anschutz@epoc.u-bordeaux1.fr

J.-M Jouanneau

F Gilbert
email: franck.gilbert@univmed.fr

D Cossa

F Pre

Biogeochemical and contaminant cycling in sediments from a human-impacted coastal lagoon e Introduction and summary

General purpose

A general decrease in the anthropogenic pressure on coastal ecosystems has been observed recently in developed countries. But coastal lagoon ecosystems are still undergoing major human impact [START_REF] Lotze | Depletion, degradation and recovery potential of estuaries and coastal seas[END_REF]. A major environmental concern is the enhancement of contaminant dispersion and algal production due to eutrophication, associated with an increase in the duration of intermittent seasonal periods of anoxia. Due to past industrial activity, large concentrations of contaminants have been accumulated in sediments of natural water bodies. One of the issues raised by environmentalists is the fate of these contaminants with changing environmental conditions, such as restoration of water bodies to their ''pristine'' state (water framework directives), or worsening of the environment due to climate change and nutrient/organic loading. The processes that govern the fate of contaminants in sediments are complex interactions with the biogeochemical cycles of major redox and biogenic elements, such as C, O, P, S, Si, Fe, and Mn [START_REF] Middelburg | The role of sediments in shelf ecosystem dynamics[END_REF]. Coupled together with the sources (rivers, water table, surface runoff, and atmospheric precipitation), and physical constraints of the water column, the biogeochemical dynamics in the sediment ultimately controls the variation of contaminant sources over inter-annual time scales. The microbial and phytobenthic activities taking place in surface sediments promote changes in oxidation state of porewaters and sediment, in porewater pH and in reduced chemical species. This modifies the recycling of carbon and nutrients, and alters the mobility of contaminants in sediments.

From 2001 to 2003, the Microbent project (''Biogeochemical processes at the wateresediment interface in eutrophic environment'') was carried out within the framework of the Programme National Environnement Co ˆtier, the French contribution to LandeOcean Interaction in the Coastal Zone (LOICZ).

The Microbent programme was focused on the study of sediment biogeochemical cycles of carbon, oxygen, sulphur, iron, nitrogen, and phosphorus in relation to the faunal activity in the sediment and their relation with the mobility of metallic contaminants at the sedimentewater interface (SWI) in a Mediterranean coastal lagoon (Thau lagoon, France; Fig. 1). The aim of Microbent was to set up an interdisciplinary study bringing together geochemists, sedimentologists, and biologists in order to understand and quantify the main reaction pathways, and the fluxes of contaminants at the SWI, including those related to benthic fauna. Work was focused on the processes which generate contaminant fluxes: (1) early diagenetic processes, which generate the chemical conditions of the environment; (2) processes leading to the transfer of contaminants from particles toward biofilms, water column, and organisms; and (3) processes of sediment mixing by organisms and sediment accumulation.

Summary of major results

Study site

The Thau lagoon is a shallow and microtidal coastal enclosed bay exposed to human activity (Fig. 1). The surface area of the Thau lagoon is about 75 km 2 , the mean depth 3.5 m and the maximum depth 11 m. Salinity varies between 31 and 39. Twenty percent of the surface is occupied by shellfish farming with a total production reaching 22,000 tons a year, mainly oysters Crassostrea gigas, and mussels Mytilus galloprovincialis.

Sampling site C4 is located at a central position in the lagoon (N 43 24.018', E 3 36.703'). Around this position, the station represents a circle of 100 m in diameter and is at a depth of 8 m. The station C5 (N 43 25.994', E 003 39.657') is located close to a mussel table, where organic carbon fluxes are higher. The C5 working area is about 10 m in diameter and the water column height is 9 m. Five campaigns were undertaken in order to document seasonal trends between December 2001and May 2003(M1: December 2001and January 2002;M2: April 2002;M3: August 2002;M4: January 2003;and M5: May 2003).

Overview of the early diagenetic processes

A combination of techniques was used to obtain an overall view of early diagenetic processes at the two main stations. Porewater extraction from sediment cores and in situ porewater sampling with dialysis devices (Metzger et al., Anschutz et al.) enabled us to determine the centimetre scale vertical distribution of dissolved species across the sedimente water interface, including major cations (sodium, potassium, magnesium, and calcium), minor cations (lithium, strontium, and barium), redox sensitive species (dissolved manganese, ferrous iron, sulphate, sulphide, ammonium, and nitrate) and other diagenetic parameters (pH, alkalinity, soluble reactive phosphorous, and dissolved silica). The high resolution distribution of dissolved oxygen and pH was acquired using in situ microsensors (Dedieu et al.). To complement the 1D analysis of porewaters, we used a new sediment 2D planar probe for measuring dissolved iron and sulphur, a device that is a combination of a DET (Diffusive Equilibration in Thin-films) device and a DGT (Diffusive Gradients in Thin-films) device (Jezequel et al.). Direct flux measurements of dissolved species across the sedimentewater interface were obtained by in situ incubation using benthic chambers (Thouzeau et al.).

The data collected from the Thau lagoon sediments followed a general sequence of early diagenetic reactions [START_REF] Froelich | Early oxidation of organic matter in pelagic sediments of the eastern Atlantic: suboxic diagenesis[END_REF][START_REF] Emerson | Sediment diagenesis and benthic fluxes[END_REF] in which oxygen was reduced near the sedimentewater interface, followed by the reduction of iron oxides, and sulphate, whereas reduced compounds accumulated in the sediment. However, a large difference was observed between the two main stations which were located in the centre of the lagoon (C4) and close to the oyster/mussel farm (organically enriched, C5). The organic enrichment linked to oyster farming was clearly visible on oxygen distributions and fluxes and led to an increased intensity of diagenesis (Fig. 2).

Oxygen profiles penetrated twice as much at the central site compared to the organically enriched (3e5 mm compared to 1e2 mm, Dedieu et al.) one. The effect of this different organic load was apparent in the exchange fluxes which were measured using benthic chamber. Oxygen fluxes were doubled near the oyster farm compared to the centre of the lagoon (45 versus 20 mmol m À2 d À1 , Thouzeau et al.). Sulphate reduction rates also differed between the two stations in the same proportion as the oxygen fluxes (20 mmol m À2 d À1 near the oyster farm versus 10 mmol m À2 d À1 in the central part of the lagoon Point et al.). These rates and oxygen distribution translated in a very different depth scale of the diagenetic sequence (Metzger et al., Anschutz et al.). In the central zone, the oxic and suboxic zones spread over 15e35 cm depending on exact location and season with the anoxic sulphidic zone starting around 20 cm depth, whereas the organically enriched site showed a rapid growth of sulphide concentrations below 3e5 cm. Therefore, the two stations (C4 and C5) displayed contrasted diagenetic situations which were favourable to the study of trace metal mobility (Fig. 2). Indeed, trace metals' speciation is dependent on redox conditions, sulphide concentration, and the presence of iron or manganese oxides. In this regard, small scale variability within the different sites was evidenced by the presence of microniches with specific bacterial activities using the 2D sensors of dissolved Fe(II) and S(-II) (Je ´ze ´quel et al.). This variability may create hot spots of mobility/trapping of trace metals which may locally alter the contaminant transfer.

Most studies presented in this volume focused on spatial and temporal variabilities of the benthic system. The drivers of seasonal variation of the oxygen uptake and DIC fluxes were identified: temperature variation (a difference of 15e20 C between winter and summer, with a Q 10 of 2.5), labile organic matter availability and the presence/absence of the macroalgal cover on the sediment (Dedieu et al.).

Nitrate reduction was very low as both nitrate concentrations in the water column (around 1 mM) and nitrification rates were insignificant. Concerning the phosphorus cycling, the precipitation of an authigenic phosphorus-bearing phase was proposed at the station near the oyster park (Anschutz et al.). This suggested that the sediment located below the shellfish farming tables acted as a sink for lagoon phosphorus. Profiles of dissolved calcium also showed that the anoxic sediment of the Thau lagoon is a location for calcium carbonate precipitation (Metzger et al.). Sediment porewaters were supersaturated with respect to calcite because of the alkalinity increase caused by sulphate reduction. The precipitation of authigenic calcite in the shallow Thau lagoon sediments suggested that this process probably plays a role in the carbon balance of coastal environment.

Mobilization and remobilization of trace metals at the sedimentewater interface (SWI)

Coastal sediments in industrialized area have been impacted by metallic contaminants which have been stored in sediments, with capability for remobilization, exchange with the water column and a possible transfer to the benthic and pelagic food webs. Different techniques such as in situ probes, benthic chambers and field and laboratory experiments have enabled the investigation of the reactivity and cycling of trace elements at the sedimentewater interface. Using the gradient in diagenetic conditions (see the above section), these studies illustrate the role of redox and diagenetic processes, microscopic and macroscopic biological activities in mediating benthic reactivity and exchanges of trace metals.

Benthic chamber fluxes were measured for redox sensitive trace metals (Cu, Pb, and Co). They exhibited a significant correlation with Mn fluxes (Point et al.) which can be used as a proxy for such suboxic to anoxic remobilization processes at the SWI in eutrophic benthic environments (Table 1). In the example of Table 1, the Mn fluxes show an opposite sign between the organic-rich site (C5) which was a source of Mn for the water column and the central zone (C4) which was a slight sink. Cu, Pb or Co displays a similar behaviour as Mn, with the organic-enriched zone being mostly a source of these metals and the low impact zone being mostly a sink. The uranium flux displayed a negative correlation with Mn fluxes. Indeed, processes dominating uranium behaviour are linked to sulphate reduction which acts as a sink for uranium by reducing the U-oxides and uranium flux entering the sediment was found to be positively correlated with sulphate reduction rates measured within the first centimetre of the top sediment layer (Point et al.). Such coastal sediments act as a uranium sink related to the intense microbial activity located in the upper sediment layers.

Cadmium displayed a different behaviour with a similar flux in the two diagenetic situations (C4, and C5). In sediments, suboxic to anoxic conditions lead to sequential immobilisation of dissolved cadmium by sulphide production whereas production of dissolved Cd is linked to the reductive dissolution of iron and manganese oxyhydroxides. This was investigated in surface sediments during contrasted seasonal campaigns for both the reference and the mussel farm sites using DGT and DET combined probes (Metzger et al.). These two opposite processes lead to little variation in dissolved Cd fluxes between the two sites despite the very different diagenetic intensities (Point et al.) suggesting that production and consumption processes were counterbalancing each other. The comparison between calculated diffusive fluxes (Metzger et al.) and benthic chamber fluxes (Point et al.) suggested the major contribution of advective processes in surface sediment, enhancing Cd mobilization. In addition, Cd presented the highest lability (compared to Cu and Zn) toward benthic bivalves and worms, and exhibited a proportional gradient between sediment contamination and bivalve bioaccumulation (Amiard et al.).

Partition, mobility and fluxes of total mercury and methylmercury were investigated on sediment, porewaters and overlying water (Point et al., Muresan et al., Montperrus et al.). In Table 1, total dissolved Hg showed a good correlation with Mn whereas methylmercury showed a weak negative correlation. Investigation of diagenetic processes (Muresan et al.) indicated that amorphous iron oxyhydroxides play a major role in controlling total mercury at the SWI, whereas HgT was in equilibrium between solid and solution in the sulfidic part of the cores. Calculated diffusive fluxes for total mercury (Muresan et al.) exhibited intensities 20e100 times lower compared to measured benthic chamber fluxes (Point et al.). These results indicate again that significant advective transport is involved in benthic exchanges of both mercury species. In addition, potential mercury methylation studies in surface sediments were performed under spring conditions. They exhibited complex pathways for methylmercury formation as influenced by both benthic and pelagic dynamics (Monperrus et al.), which explains the lack of relationship between methylmercury and Mn fluxes.

The intensity of benthic remobilization of most trace metals and organometals was found to be strongly driven and enhanced by macrofaunal density and potential activity (irrigation, reworking, and ingestion). Processes governing benthic exchanges and bioaccumulation of trace metals remain complex and results obtained for methylmercury and tributyltin benthic fluxes (Point et al.) or Zn and Cu bioaccumulation (Amiard et al.) demonstrate that further processes need to be considered and investigated. For example, additional experiments with benthic chambers with macroalgal cover demonstrated significant changes in benthic fluxes' intensity and direction for all redox sensitive trace metals between dark and light conditions (Point et al.). This result suggests that benthic metal processes may vary over diurnal time scale in shallow coastal environment.

Sedimentation, bioturbation and archiving

Sediment accumulation and mixing are important processes which govern the distribution and fate of particulate chemical elements before their archiving in the sediment column. These processes were investigated at the two stations using complementary analyses of radionuclides ( 234 Th, 7 Be and 210 Pb, Schmidt et al.), fluorescent particles (luminophores) and macrofaunal population (Duport et al.), which allowed the quantification of sedimentation and bioturbation rates. A seasonal model describing bioturbation and burial was specially designed to account for temporal variations of these processes and the deposition fluxes of radionuclides (Lecroart et al.).

As exemplified in the papers from Schmidt et al. and Duport et al., no significant differences between the two stations were found in the intensity of overall sediment reworking for both radionuclides' and luminophores' measurements.

Yet, the two contrasted sites with different organic loading and diagenetic settings hosted different functional groups of macrobenthos. A dominance of biodiffusors was observed at the site located in the centre of the lagoon where organic loading was low whereas gallery-diffusors were predominantly recorded at the site close to the shellfish farm (Fig. 3). Taking into account the dominant functional groups present, there was thus a higher potential in surface oxygen and suboxic conditions penetration at the low impact site due to a homogeneous and active reworking of the surface layer linked to the organisms' displacements. At the organic-rich site, where biogenic structures built by the organisms created irrigated spots in a scattered pattern, less efficient oxygen diffusion and solid oxidant penetration were present which explain the occurrence of sulphide at 3e5 cm.

Both sedimentary 234 Th and 7 Be showed seasonal variations in activities and in penetration, up to 8 cm, which indicated efficient and variable mixing of upper sediments with time (Schmidt et al.). Bioturbation rates (D b ) were calculated from these profiles as a diffusive process under steady-state. Sediment reworking was also determined using the luminophore method which clearly showed that radionuclide-D b values presented higher values than luminophore-D b . The two methods showed a similar trend with time, indicating seasonal variation in particle penetration and mixing within the sediment. The seasonal trend showed an increase of mixing intensity in summer which was related to the seasonal variability in functional bioturbation group composition (e.g., biodiffusors, and gallery-diffusors) within the macrofaunal populations (Duport et al.).

By developing a time-dependent model, Lecroart et al. tested the accuracy of the conventional method used to calculate bioturbation coefficients from 7 Be and 234 Th profiles at steady-state. The model simulated activity profiles influenced by either seasonal or episodic input of radionuclide flux at the SWI, and calculated the apparent bioturbation coefficient. The error induced by the steady-state approximation reached significant values (up to 163% with the 7 Be and 74% with the 234 Th) calculated with a set of parameters typical of coastal environments.

Conclusions

Contaminant mobility in sediments is largely dependent on early diagenetic processes. The oxiceanoxic transition which involves changes in oxidation state of metallic contaminants and their carrier phase is essential in metal speciation. In addition, several other parameters are crucial in generating transfer and trapping of trace metals in the sediment column: the availability of iron and manganese oxyhydroxides, the concentration of free sulphide partly controlled by reduced iron, the availability of organic substrates, and the intensity of bioturbation and irrigation. Thus temporal and spatial variations of biogeochemical cycling of elements such as C, O, Fe, and S imply differences in the fate of trace metals deposited or historically buried in sediments. During the Microbent programme, we showed that changes in labile organic loads in sediments (related to the shell farming activity in the Thau lagoon) and their consequences related to early diagenesis or faunal population could significantly alter the fate of trace metals in sediments such as Cu, Co, U, Pb, and Hg in a non-linear way.

The results presented in this special issue show that an interdisciplinary approach, coupled to time-series investigation can provide understanding of complex benthic processes. These advances help to resolve the coupling between metallic contaminant mobility, organic matter recycling and benthic ecosystem functionality in complex coastal environments such as lagoons.

Fig. 1 .

 1 Fig.2. A schematic view of early diagenesis at the two sites studied in the Thau lagoon. Organic matter recycling is enhanced in the sediments of the oyster farm leading to reduced conditions close to the sedimentewater interface.

Fig. 3 .

 3 Fig. 3. Schematic diagram showing the type of bioturbation organisms encountered in the sediments of the Thau lagoon. Biodiffusors dominate at the central site (C4) whereas gallery-diffusors dominate at the organically enriched site.

Table 1

 1 Average trace metal fluxes in the two diagenetic contexts in April 2002 (Hg species data are from May 2003, see Point et al.). Intensified suboxic/anoxic conditions near the oyster farm (showed by Mn efflux) promote a release of redox sensitive metals (station C5). Positive values indicate effluxes out of the sediment whereas negative values indicate influxes from the water column

	Fluxes of	Centre of the	Near the oyster
		lagoon: C4	farm: C5
	Mn (mmol/m 2 /d)	À11	þ115
	Co (mmol/m 2 /d)	À0.7	þ3.4
	Cu (mmol/m 2 /d)	À7	þ11
	Pb (mmol/m 2 /d)	þ1	þ4
	U (mmol/m 2 /d)	þ1.7	À2.7
	Cd (nmol/m 2 /d)	þ110	þ110
	HgT (nmol/m 2 /d)	À3.2	þ2.6
	MMHg (pmol/m 2 /d)	þ300	þ95

Acknowledgements

We greatly thank IFREMER for providing logistic support during the field experiments. Especially, the IFREMER Station of Se `te provided laboratory space and the research boat Chlamys. An extensive diving team operated submarine equipment and sampling among whom Bruno Bombled, J.-J. Bourrand, R. Graille, B. de Ligonde `s and F. Zuberer need to be specially acknowledged. The crew of the research boat Chlamys, J.-L. Guillou, was of tremendous help during field work, his sense of humour and professionalism allowed us to optimise our field work without any damage. We also thank all the reviewers for our special issue (around 50 international researchers) who provided thoughtful comments and criticisms which greatly improved the quality of the manuscripts. This work was supported by a grant from the Programme National ''Environnement Co ˆtier'' e ART1 for Microbent and the Laboratoire des Sciences du Climat et de l'Environnement (CEA-CNRS) for CR. This is LSCE contribution number 2377.

List of papers in this special issue Diagenesis Anschutz, P., Chaillou, G., Lecroart, P. Phosphorus diagenesis in sediment of the Thau Lagoon. Dedieu, K., Rabouille, C., Thouzeau, G., Jean, F., Chauvaud, L., Clavier, J., Mesnage, V., Ogier, S. Benthic O 2 distribution and dynamics in a Mediterranean lagoon: an in-situ microelectrode study. Je ´ze ´quel, D., Brayner, R., Metzger, E., Viollier, E., Pre ´vot, F., Fie ´vet, F. Two-dimensional determination of dissolved iron and sulfur species in marine sediment, pore-waters by thinfilms based imaging. Thau lagoon (France). Metzger, E., Simonucci, C., Je ´ze ´quel, D., Viollier, E., Sarazin, G., Pre ´vot, F. Benthic response to shellfish farming in Thau lagoon: pore water signature. Thouzeau, G., Grall, J., Clavier, J., Chauvaud, L., Jean, F., Leynaert, A., Longphuirt, S., Amice, E., Amouroux, D. Spatial and temporal variability of benthic biogeochemical fluxes associated with macrophytic and macrofaunal distributions in the Thau lagoon (France).

Metal mobility and fluxes

Amiard, J.-C., Geffrard, A., Amiard-triquet, C., Crouzet, C. Relationship between the lability of sediment-bound metals (Cd, Cu, Zn), and their bioaccumulation in benthic invertebrates.

Metzger, E., Je ´ze ´quel, D., Elbaz-Poulichet, F., Simonucci, C., Viollier, E., Sarazin, G., Seidel, J.-L., Pre ´vot, F. Influence of diagenetic processes in Thau lagoon on cadmium behavior and benthic fluxes. Monperrus, M., Tessier, E., Point, D., Vidimova, K., Amouroux, D., Guyoneaud, R., Leynaert, A., Grall, J., Chauvaud, L., Thouzeau, G., Donard, O.F.X. The biogeochemistry of mercury at the sediment water interface in the Thau lagoon. 2. Evaluation of mercury methylation potential in both surface sediment and the water column. Muresan, B., Cossa, D., Je ´ze ´quel, D., Pre ´vot, F., Kerbellec, S.

The biogeochemistry of mercury at the sediment water interface in the Thau lagoon, 1. Partition and speciation. Point, D., Monperrus, M., Tessier, E., Amouroux, D., Chauvaud, L., Thouzeau, G., Jean, F., Amice, E., Grall, J., Leynaert, A., Clavier, J., Donard, O.F.X. Biological control of trace metal and organometal benthic fluxes in a eutrophic lagoon (Etang de Thau, Mediterranean sea, France).

Sediment reworking and archiving

Duport, E., Gilbert, F., Poggiale, J.-C., Dedieu, K., Rabouille, C., Stora, G. Benthic macrofauna and sediment reworking quantification in contrasted environment in the Thau Lagoon. Lecroart, P., Schmidt, S., Jouanneau, J.-M. Numerical estimation of the error of the biodiffusion coefficient in coastal sediments. Schmidt, S., Jouanneau, J.-M., Weber, O., Lecroart, P., Radakovitch, O., Gilbert, F., Je ´ze ´quel, D. Sedimentary processes in the Thau Lagoon (South France): from seasonal to century time scales.