Le Vu

B J Lemaire

Brigitte Vinçon-Leite

High-frequency phytoplankton monitoring in stratified lakes

Keywords: monitoring, stratified lakes, time series, WFD, cyanobacteria, phytoplankton, metalimnetic fluorescence peak

Large, sub-alpine, stratified lakes are directly within the scope of the European Water Framework Directive (WFD) and need adapted monitoring systems. Moreover, anthropogenic eutrophication was frequently the main cause of their water quality degradation in the 20th century. This paper is primarily aimed at demonstrating how in situ sensor-equipped buoys could be the base of monitoring designs to support the WFD objectives. The core of this paper, mainly methodological, focuses on single-depth, high frequency (4 per hour) fluorescence measurements. It shows that the internal wave pattern provides additional information to the single-depth time series to assess phytoplankton dynamics in a stratified water column displaying strong, vertical biomass heterogeneity. The paper deals with the following three aspects: (1) definition of an indicator to determine whether or not the sensor actually detects the metalimnetic fluorescence peak, (2) vertical representation of chlorophyll distribution from single-depth series; (3) time evolution of the fluorescence peak (maximum value, depth and associated temperature, peak width).

Context and Objectives

Anthropogenic eutrophication was the main cause of the deterioration of the water quality in European sub-alpine lakes in the 20 th century; [START_REF] Jeppesen | Lake responses to reduced nutrient loading -an analysis of contemporary long-term data from 35 case studies[END_REF][START_REF] Gerdeaux | Fishery changes during re-oligotrophication in 11 perialpine Swiss and French lakes over the past 30 years[END_REF][START_REF] Salmaso | Phytoplankton as an Indicator of the Water Quality of the Deep Lakes South of the Alps[END_REF][START_REF] Tilzer | Interannual Variability of Phytoplankton Productivity and Related Parameters in Lake Constance -No Response to Decreased Phosphorus Loading[END_REF]Vinçon-Leite et al., 1995a), in Italy (e.g. Great Lakes), Switzerland (e.g. Lake Geneva), Germany (e.g. Lake Constance), France (e.g. Lakes Bourget and Annecy), Austria (e.g. Mondsee). In spite of exhaustive restoration works in their watersheds (e.g. improved sewage treatment, ring trunk sewer, reduced use of detergent phosphorus), eutrophication control remains an issue for many sub-alpine lakes, for example Lake Geneva (Tadonléké et al., 2009), Lake Garda (Salmaso, 2009) or Lake Bourget (Jacquet et al.,2005), where phytoplankton biomass remains high or cyanobacteria proliferations regularly occur.

Phytoplankton blooms, triggered by nutrient loading which may be discharged by urban drainage systems, industry or agriculture are a major threat for lakes. Among the phytoplankton classes, cyanobacteria, which aggregate into dense biomass piles and surface scum, are specifically targeted, since some species produce toxins which can cause animal as well as human illnesses (Chorus and Bartram, 1999;[START_REF] Oberholster | Toxic cyanobacterial blooms in a shallow, artificially mixed urban lake in Colorado, USA[END_REF]. Moreover, algal blooms in freshwater are frequently dominated by cyanobacteria (e.g. Dokulil and Teubner, 2000;Hudnell, 2009).

Sub-alpine lakes share common morphological features. They are narrow and elongated, with steep sides and generally a flat bottom; their thalwegs are roughly oriented north-south [START_REF] Salmaso | Phytoplankton as an Indicator of the Water Quality of the Deep Lakes South of the Alps[END_REF]. Internal waves or seiches are commonplace during the summer stratification period.

To these common morphological features often correspond similar phytoplankton assemblages [START_REF] Anneville | Phosphorus decrease and climate variability: mediators of synchrony in phytoplankton changes among European peri-alpine lakes[END_REF]. Cyanobacterium Planktothrix rubescens is frequently found in most of them [START_REF] Briand | Variations in the Microcystin Production of Planktothrix rubescens (Cyanobacteria) Assessed from a Four-Year Survey of Lac du Bourget (France) and from Laboratory Experiments[END_REF][START_REF] Ernst | Abundance and toxicity of Planktothrix rubescens in the pre-alpine Lake Ammersee, Germany[END_REF]): e.g. Lake Zürich [START_REF] Holland | Viability of the cyanobacterium Planktothrix rubescens in the cold and dark, related to over-winter survival and summer recruitment in Lake Zürich[END_REF][START_REF] Walsby | The Burgundy-blood phenomenon: a model of buoyancy change explains autumnal waterblooms by Planktothrix rubescens in Lake Zürich[END_REF][START_REF] Walsby | Light-dependent growth rate determines changes in the population of Planktothrix rubescens over the annual cycle in lake Zürich, Switzerland[END_REF], Lake Garda [START_REF] Salmaso | Phytoplankton as an Indicator of the Water Quality of the Deep Lakes South of the Alps[END_REF], Lake Nantua [START_REF] Feuillade | Studies on Lake Nantua. Lake surroundings and historical background. Studies on Lake Nantua (France). A Eutrophic Lake on the Way to Rehabilitation[END_REF][START_REF] Schwob | Occurrence of toxic Planktothrix rubescens blooms in lake Nantua, France[END_REF], Mondsee (Dokulil and Teubner, 2000). The microcystin-producing Planktothrix spp. is one of the most abundant genera amongst the widespread toxin-producing cyanobacteria in temperate climates [START_REF] Ernst | Oral toxicity of the microcystin-containing cyanobacterium Planktothrix rubescens in European whitefish (Coregonus lavaretus)[END_REF].

Large, sub-alpine lakes located in densely urbanized watersheds have essential functions for the city dwellers. Therefore, they are directly within the scope of the European Water Framework Directive and need adapted monitoring systems.

In the European Union, surface and groundwater aquatic systems are monitored and controlled according to directive 2000/60/EC (European Parliament and Council, 2000), also known as the Water Framework Directive (WFD).

The monitoring programmes described in the WFD have two objectives: to assess the ecological status of natural and artificial lakes inside homogeneous hydroecoregions and to survey the effect of restoration measures. Therefore, the three types of monitoring programmes (surveillance, operational, investigative monitoring programmes) described in the WFD vary in scope in time, space, and the quality elements which are monitored (WFD, Annex V; [START_REF] Ferreira | Monitoring of coastal and transitional waters under the E.U. Water Framework Directive[END_REF].

The surveillance monitoring on a network of reference sites provides a panorama of the general status of aquatic ecosystems across the European Union. It also has the objective of assessing long-term changes in natural conditions, and long-term changes resulting from widespread anthropogenic activity [START_REF] Ferreira | Monitoring of coastal and transitional waters under the E.U. Water Framework Directive[END_REF].

Operational monitoring is performed on lakes which undergo restoration programmes called programmes of measures or which may fail to meet the environmental objectives of the WFD (good ecological status or potential); their status, as well as the effects of the programmes of measures, is to be assessed.

To understand the cause of the lake deterioration or in the case of accidental pollution, a more reinforced control called investigative monitoring is carried out.

Phytoplankton is a key biological quality element for the establishment of the WFD ecological status in lakes: the composition, abundance and biomass of phytoplankton as well as the frequency and intensity of phytoplankton blooms are to be monitored. The commitment of monitoring phytoplankton blooms in numerous lakes requires reliable, cost-effective, easy-to-handle, monitoring systems [START_REF] Allan | A "toolbox" for biological and chemical monitoring requirements for the European Union's Water Framework Directive. Talanta 1st Swift-WFD workshop on validation of Robustness of sensors and bioassays for Screening Pollutants -1st SWIFT[END_REF][START_REF] Dworak | The need for new monitoring tools to implement the WFD[END_REF]. But the traditional inverted microscope examination of phytoplankton abundance and taxonomic composition [START_REF] Afnor | Water quality -Guidance standard on the enumeration of phytoplankton using inverted microscopy (Utermöhl technique)[END_REF] is cumbersome, time-consuming, and requires highly qualified personnel. Moreover, the sampling frequency must be complying with the relevant timescales of the dynamics of the phytoplankton communities, and this is really unaffordable for water masses with these traditional methods. Available methods based on the composition of phytoplankton assemblages and the abundance ratios of the main phytoplankton classes could be an alternative to sampling and phytoplankton counting [START_REF] Sarmento | Use of marker pigments and functional groups for assessing the status of phytoplankton assemblages in lakes[END_REF]. These methods, allegedly, do not meet the requirements of the WFD to take into account the taxonomic composition and the frequency of algal blooms. But it can be regarded as a restrictive interpretation of the WFD which stipulates that "Member States shall identify the appropriate taxonomic level required to achieve adequate confidence and precision in the classification of the quality elements" (WFD, annex V, §1.3).

For all monitoring programmes required by the WFD, high-frequency monitoring systems may be useful, since episodic measurements may not enable the understanding of the local determinants of ecosystem dynamics [START_REF] Carstensen | Statistical principles for ecological status classification of Water Framework Directive monitoring data[END_REF][START_REF] Kaiblinger | Central European water quality indices applied to long-term data from peri-alpine lakes: test and possible improvements[END_REF].

Amongst autonomous monitoring systems, satellite remote sensing entails different limitations: some blooms can be too localised to be detected, but above all, satellite detection is limited by clouds and to the water surface only, while a lake's vertical structure drives phytoplankton dynamics. Therefore, in situ monitoring stations remain necessary.

In situ autonomous monitoring stations offer further advantages: (1) for surveying remote sites (polar [START_REF] Palethorpe | Real-time physical data acquisition through a remote sensing platform on a polar lake[END_REF] or mountain lakes in particular); (2) for dealing with the scarcity of qualified personnel to perform sampling, chemical analysis and phytoplankton counting, (3) for reducing the cost due to repeated analyses. But there also limitations such as, for example the frequent field visits which are required for sensor maintenance and for data quality control. Moreover, difficulties can also appear from mismatch between concurrent on-site measurements and sensor time-series, collected during the same period (e.g. [START_REF] Garel | The autonomous Simpatico system for real-time continuous water-quality and current velocity monitoring: examples of application in three Portuguese estuaries[END_REF][START_REF] Vandenberghe | Application of Automated Measurement Stations for Continuous Water Quality Monitoring of the Dender River in Flanders, Belgium[END_REF].

In coastal waters, real-time monitoring systems have already been operating for some years: automated buoys developed for survey programme of coastal seas [START_REF] Mills | Smartbuoy: A marine environmental monitoring buoy with a difference[END_REF], phytoplankton bloom warning buoys for mariculture [START_REF] Lee | Real time observations of coastal algal blooms by an early warning system[END_REF], warning system for low sub surface oxygen events [START_REF] Bergondo | Time-series observations during the low sub-surface oxygen events in Narragansett Bay during summer 2001[END_REF], ®MAREL1 environmental monitoring buoys [START_REF] Blain | High frequency monitoring of the coastal marine environment using the MAREL buoy[END_REF]; [START_REF] Dur | Analysis of high frequency temperature time series in the Seine estuary from the Marel autonomous monitoring buoy[END_REF]. These systems are still rare in freshwater bodies and are mainly dedicated to research purposes [START_REF] Jones | Automatic in-lake monitoring in the English Lake District: The effect of lake size on stratification[END_REF][START_REF] Madgwick | Phytoplankton communities and antecedent conditions: high resolution sampling in Esthwaite Water[END_REF].

In this context, the primary objective of this paper is to demonstrate how in situ sensor-equipped buoys are valuable and consistent devices which can be incorporated into future monitoring designs to complete the WFD objectives. The core of this paper, mainly methodological, will focus on temperature and fluorescence measurements. It will show that the internal wave pattern can provide additional information to single-depth measurements: (1) an indicator determining whether the sensors detect the metalimnetic fluorescence maximum (fluorescence peak); (2) a vertical representation of fluorescence distribution from single-depth series; (3) the time evolution of the fluorescence peak (maximum value, depth and associated temperature, peak width).

The results presented in this paper have been established within the PROLIPHYC project. Since 2007 the PROLIPHYC consortium2 has been developing, validating and pre-industrializing a real-time, autonomous, monitoring buoy and a short-time forecasting and warning system for phytoplankton blooms, including cyanobacteria. The system is specifically adapted for lake stakeholders (drinking water producers, local health authority, etc.) 2 Material and methods

Study site

Lake Bourget (Figure 1) is a deep, well-stratified lake, located in the French Alps (231 m altitude) and can be seen as a representative study site for deep sub-alpine lakes. Lake Bourget is long (18 km), narrow (2.5 km) and deep (maximum depth 145 m, mean depth 80 m). The lake watershed contains an overall population of about 180 000 inhabitants (INSEE, 2008), the largest towns being Chambéry (57 800 inh.) and Aix-les-Bains (27 500 inh.). The wastewater production in the watershed corresponds to 410 000 population equivalent [START_REF] Paolini | Le suivi de la qualité des eaux du lac du Bourget[END_REF] and is responsible for major anthropogenic impacts on the lake. Despite this considerable reduction of the nutrient concentrations since the eighties due to the improvement of waste water treatment and to the diversion of the treated effluent by a ring sewer, algal biomass remains high and the proliferations of the cyanobacterium, Planktothrix rubescens, which have been occurring regularly since 1996 impair the use of the lake for water supply and recreational activities. Furthermore, Lake Bourget is one of the most regularly monitored lakes in France; research projects concerning its main functioning features have been conducted for many years (e.g. [START_REF] Groleau | Tracing calcite precipitation with specific conductance in a hard water alpine lake (Lake Bourget)[END_REF][START_REF] Dorigo | Spatial variability in the abundance and composition of the free-living bacterioplankton community in the pelagic zone of Lake Bourget (France)[END_REF][START_REF] Vinçon-Leite | Phytoplankton variability in Lake Bourget: Phytoplankton dynamics and meteorology Lakes & Reservoirs[END_REF].

Since 1996, cyanobacteria have been regularly observed [START_REF] Cereve | Analyse de l'évolution de la qualité des eaux du lac du Bourget[END_REF][START_REF] Humbert | Monitoring a toxic cyanobacterial bloom in Lake Bourget (France) and its consequences for water quality[END_REF][START_REF] Oberhaus | Influence of physical factors on Cyanobacteria development in a deep subalpine lake (Lake Bourget)[END_REF]. The prevailing species, Planktothrix rubescens, can develop into a very high biomass, 100 000 cell.mL -1 in August 2007. This stratifying species occupies the metalimnion at around 15 m in depth, from summer to autumn. In late autumn and winter, when the lake's upper layer mixes, its biomass can remain high.

Figure 1: Lake Bourget aerial view and the buoy position (M point)

A warning system, based on a fortnightly survey, at the lake's mid-point, has been in operation since 1999. The vertical distribution in the water column of temperature and chlorophyll-a, distributed into phytoplankton classes, are measured using a thermistance and a 5-wavelength fluorometer (http://www.bbe-moldaenke.de). In Lake Bourget, Planktothrix rubescens cellular quota of microcystin-LR is around 0.2 pg.cell -1 during the stratified season [START_REF] Humbert | Monitoring a toxic cyanobacterial bloom in Lake Bourget (France) and its consequences for water quality[END_REF][START_REF] Briand | Variations in the Microcystin Production of Planktothrix rubescens (Cyanobacteria) Assessed from a Four-Year Survey of Lac du Bourget (France) and from Laboratory Experiments[END_REF]. Therefore, the stakeholders are warned when the cyanobacteria concentration reaches 5 000 cell.mL -1 , which corresponds to a microcystin-LR concentration higher than 1 µg.L -1 , threshold defined by the World Health Organisation (Chorus and Bartram 1999). The local authorities are very committed to the development of a more reliable and more economic monitoring system for the lake.

Internal waves in Lake Bourget

Internal waves or seiches are frequently observed in lakes where a thermal stratification of the water column occurs (e.g. [START_REF] Lemmin | Tests of an extension to internal seiches of Defant's procedure for determination of surface seiche characteristics in real lakes[END_REF]). On the basis of long-term temperature records (1995 -1996 and 2004 -2005) the most energetic modes in Lake Bourget (V1H1 and V2H1), were characterized [START_REF] Bournet | Ondes internes du lac du Bourget: analyse des observations par des modèles linéaires[END_REF][START_REF] Bournet | Ondes internes du lac du Bourget: analyse des observations par des modèles linéaires[END_REF]Cuypers et al., 2006b). The V1H1 mode, a tilt of the thermocline along the North-South axis of the lake, displays a node in the middle of the lake. The V2H1 mode displays two vertical nodes at the depths of the upper and lower limits of the metalimnion. From one year to the next, the summer stratification and internal wave patterns are comparable. The periods of the dominant modes are about 24 hours for the V1H1 mode, 70-80 h for the V2H1 mode. Their amplitude goes up to 5 m [START_REF] Bournet | Ondes internes du lac du Bourget: analyse des observations par des modèles linéaires[END_REF].

Proliphyc prototype and final systems

The PROLIPHYC consortium is developing a real-time, autonomous monitoring station. According to the specifications, in the final buoy, the sensors will continuously measure temperature, chlorophyll-a concentration and dissolved oxygen concentration at a single depth. Moreover, a profiling device will record daily profiles of the same parameters between the surface and the fixed position depth. Meteorological data will also be measured. Once in the water, the station will be installed easily, moored by a small motor-boat. Sensor maintenance will be facilitated by a freshwater-adapted protection against biofouling. An in situ biofouling protection of the sensors will be achieved daily by a chlorination device. At every 3 months, a local inspection will be required for servicing the whole system (platform, sensors, solar panels…). Measured data will be transmitted by GPRS to the laboratory where they will be automatically processed and eventually integrated into a short-term predictive model. When a phytoplankton bloom is forecasted, a warning will be sent to the lake managers by e-mail and/or mobile phone text messaging. At present, this final station is under development. Therefore, at the beginning of the project, a prototype station was developed in order to test and improve the system and the highfrequency data treatment. This prototype station displays all the same features, particularly the same sensors, as the final buoy but it does not include the antifoulant device, the GPRS data transmission neither the meteorological sensors. The autonomous prototype buoy was deployed on 11 th July 2007 at Point M (45°42.725' N, 05°52.853' E) where water depth is around 90 m (Figure 1).

The sensors are located at a fixed depth (15 m). A CTD multiprobe (nke MPx) measures conductivity, temperature and pressure. An optode (Aanderaa 4175) measures dissolved oxygen. A fluorimeter (BBE-Moldaenke FluoroProbe II) measures fluorescence at 5 wavelengths (450, 525, 570, 590 and 610 nm) so as to distinguish among phytoplankton classes. In this device, the excitation spectra of chlorophyll fluorescence is used to differentiate 4 main phytoplankton groups: diatoms and dinoflagellates, blue-green cyanobacteria, green algae and cryptophyta. Each group is characterised by a specific composition of accessory light-harvesting pigments, which can be associated to specific fingerprints [START_REF] Beutler | A fluorometric method for the differentiation of algal populations in vivo and in situ[END_REF]. In Lake Bourget, previous studies have shown that an additional, specific fingerprint can be assigned to the red, phycoerythrin containing P. rubescens (Leboulanger et al., 2002). Therefore, in this work, a specific fingerprint has been attributed to P. rubescens. Furthermore, the fluorimeter data are checked against cell counting and chlorophyll analysis of discrete field samples collected at the sensor depth and against samples of P. rubescens laboratory culture. This validation step is currently still ongoing but the fingerprint used in this work can already be considered reliable (Humbert and Escoffier, data not shown). In the following, only the fluorescence data associated to the Planktothrix rubescens fingerprint are used.

During the summer thermal stratification, the water column displays both a very high temperature gradient and a deep chlorophyll-a maximum within the metalimnion as shown for example on 7 th August 2007 (Figure 2). The depth of 15 m was selected for the sensors, since maximal biomass of Planktothrix rubescens is likely to be observed.

The autonomy of the prototype is 3 months with a data acquisition at a 15-min time-step. Each month, the data are manually downloaded to a host computer from the buoy flash memory.

A pluri-annual time series has been recorded since 11 th July 2007. It will be presented, analysed and thoroughly discussed in another paper.

Time series treatment

The objective of the data treatment is to derive indicators of phytoplankton biomass: the maximum fluorescence value in the vertical profile (fluorescence peak) and other associated parameters such as temperature, dissolved oxygen, conductivity, etc. One of the key points is the reliability of the time series measured at a single depth, for inferring the phytoplankton dynamics in the water column. Actually, in Lake Bourget, as in most stratified lakes, the water column displays very high gradients of temperature, oxygen and fluorescence (Figure 2). A slight motion of the water layers, for instance due to internal waves, will cause a great variation of the data recorded at a fixed depth. Therefore it is not easy to determine whether or not the fluorescence sensor actually measures the fluorescence peak nor to quantify its variation. The method developed to process the single-depth time series takes advantage of the internal wave pattern to assess the fluorescence peak during the thermal stratification.

Conceptual description of the internal wave motion

The internal waves result in the vertical oscillation of the water column in front of the fixed-depth sensors. A conceptual representation (Figure 3) of this phenomenon, limited to first order V1H1, is presented. It is based on the temperature and fluorescence vertical profiles measured on 7 th August 2007. The depth variation of temperature displays a monotonic decreasing gradient whereas the depth variation of fluorescence displays a maximum around 15 m depth (Figure 2). Figure 4-a and b depicts the theoretical time evolution of temperature and fluorescence, due to a V1H1 internal wave of 24 h-period, at a 15 m depth. From midnight to 5 a.m. and after 7 p.m., the fluorescence peak sinks below the sensors; fluorescence and temperature vary with time in phase opposition. After the fluorescence peak surges above the sensors, from 5 a.m. to 7 p.m., fluorescence (F) and temperature (T) vary in phase. To track the position of the fluorescence peak relative to the sensors, the ratio ΔF/ΔT between the fluorescence and temperature variations on each time step is computed (Figure 4-c). When ΔF/ΔT > 0, the fluorescence peak is located above the sensors, when ΔF/ΔT < 0, below the sensors.

Finding the actual fluorescence maximum values

The method presented in the previous section enables us to validate whether the maximum fluorescence measured everyday can be regarded as the fluorescence peak. Temperature and fluorescence time series measured from 1 st to 3 rd August 2007 by the PROLIPHYC prototype are used to illustrate the functioning of the method (Figure 4-d ande).

At each time step, the ratio ΔF/ΔT is computed (Figure 4-f). In order to overcome small scale measurement fluctuations, a threshold-based pre-treatment is applied to the raw data.

When the fluorescence peak drifts by the sensors during the internal wave period (24 h) the ratio ΔF/ΔT changes sign. The maximum of the fluorescence during such a period can be regarded as the actual fluorescence peak value. If the peak fluorescence does not pass in front of the sensors, the ratio does not change sign, and the maximum measured fluorescence underestimates the fluorescence peak value. Therefore, the change in sign on an internal wave period provides an indicator that the fluorescence peak was detected.

Time evolution of the fluorescence peak characteristics

In order to demonstrate the soundness of the method, the treatment of the daily maximum fluorescence was performed from 1 st to 15 th August 2007 (Figure 5-a). Daily information related to the phytoplankton dynamics and the sensor position can be extracted from the time series following the method previously described: the estimated fluorescence peak value, the related temperature (Figure 5-b) and the fraction of the day during which the fluorescence peak is above (resp. below) the sensors, computed as the time during which ΔF/ΔT>0 (resp. ΔF/ΔT<0) (Figure 5-c).

On 12 th and 13 th August, the peak did not drift past the sensors and the maximum fluorescence measured underestimated the fluorescence peak value.

Inference of the vertical fluorescence profile with a 1-D thermal model

The vertical periodic water displacement due to internal waves also enables us to reconstruct the part of the vertical fluorescence profile scanned by the sensors. A consistent methodology has been developed and validated within the PROLIPHYC project to infer the spatialisation of the time series. Based on field observations, original data treatments provided a new concept named 'mean image', allowing new qualitative and quantitative interpretation of continuous measurements [START_REF] Bensoussan | Observing and integrating deep chlorophyll maximum of the metalimnetic cyanobacteria P.rubescens from single point measurements[END_REF][START_REF] Bensoussan | From in Situ Time Series to Mean Image: new Tools for Monitoring Harmful Algal Blooms[END_REF]. This new comprehensive approach will be presented and analyzed in a forthcoming paper. The present paper introduces a method for spatialising the fixed-depth time series, using a thermal model for computing the temperature profiles. This model is a deterministic, one-dimensional vertical thermal model [START_REF] Vinçon-Leite | Modélisation de l'évolution thermique saisonnière du lac du Bourget (Savoie, France)[END_REF]. It has been successfully applied on Lake Bourget for various meteorological conditions and over long time periods [START_REF] Tassin | Forecasting of water quality in lakes: a predictive use of a onedimensional model. Application to lake Bourget (Savoie, France)[END_REF]Vinçon-Leite et al., 1995b). It is based on the advective-dispersive equation of heat, integrated on the horizontal dimensions. The surface heat budget is described using a classical formulation of the radiative and convective fluxes [START_REF]Engineering limnology[END_REF], based on meteorological forcing variables (air temperature, humidity, cloudiness, wind velocity, air pressure). The vertical dispersion coefficient has a different formulation in the epilimnion and in the hypolimnion. The advective-dispersive equation is solved using a semi-implicit finite difference scheme with 1-m thick layers and a 3-h time step.

The daily mean temperature profiles in the water column are computed, disregarding the influence of internal waves. These profiles provide a one-to-one relation between temperature and depth (Figure 6-a). Measured temperature values can thus be linked to the depth at which they would be observed if there were no internal waves (Figure 6-b). Fluorescence can be plotted versus this equivalent depth. This treatment is performed for each internal wave period (24 h) (Figure 6-c). The daily update of the relation between temperature and depth yields the time evolution of the vertical distribution of fluorescence in the metalimnion (Figure 7). It can be noticed again that the fluorescence peak remains above the sensors on 12 th and 13 th August 2007.

Discussion

The instrumented buoy, presented in this paper belongs to the simplest category of in situ highfrequency monitoring systems. It performs measurements at a single depth. The prototype does not include any profiling system, which reduces its cost and energy requirements and increases mechanical strength (no need of a winch and cable winder). Working with fixed-depth sensors also considerably simplifies data processing. Among the off-the-shelf sensors (conductivity, temperature, pressure, oxygen and fluorescence) embedded in the station, the spectrofluorimeter was chosen for its high performance so that it can distinguish among phytoplankton classes. The measurement features (frequency, depth, etc.) can be optimised for local applications.

The design of the prototype might appear a very doubtful option to assess phytoplankton dynamics in a stratified water column featuring strong, vertical biomass heterogeneity. It was therefore essential to establish if such a straightforward device was able to provide reliable information, from which comprehensive indicators could be derived for operational purposes.

The method described in this paper enables us to follow the dynamics of cyanobacterium Planktothrix rubescens which prevails in Lake Bourget. It first determines whether the sensor position is well located to detect the fluorescence peak along the vertical. Following the internal wave periodic motion, the fluorescence peak regularly drifts in front of the sensors, generally twice a day. For some dates, the sensors are clearly located within the fluorescence peak as it was the case during the first 10 days in August 2007 (Figure 5). On 12 th and 13 th August 2007, the sensors never detected the fluorescence peak which remained above the sensors; the recorded maximal values underestimated the maximum peak fluorescence (Figure 5).

It has been shown that the single-depth time series can yield the actual maximal values of the fluorescence, even if the sensors are not located at precise depth of the fluorescence peak. This requires that the distance between the sensors and the mean position of the fluorescence peak does not exceed the mean internal wave amplitude.

Moreover, the fluorescence peak can also be detected even if the fluorescence peak depth changes on a daily or weekly scale. When stratification is established at a stable depth in summer, the sensor position is to be set at the middle of the metalimnion after measuring a temperature profile.

When stratification is beginning in spring or ending in autumn, phytoplankton is more homogeneously spread in the euphotic zone and the amplitude of internal waves is greater than in summer. The singledepth monitoring system could still follow phytoplankton dynamics, but a different data treatment would be necessary.

Secondly, the fluorescence values of the peak and parameters recorded at the same time step can be extracted, such as temperature, dissolved oxygen, conductivity. Their seasonal and year-to-year evolution can be studied, as well as the relation of the fluorescence peak to additional environmental parameters (e.g. light, mixing, nutrients, etc.). Amongst other applications, phytoplankton dynamics can be modelled more efficiently, for example as part of a warning system: in the phytoplankton growth equation, the optimal temperature of the phytoplankton community in the metalimnion of the studied lake can be derived.

The metalimnetic spatial distribution of phytoplankton biomass can also be derived, using a 1D vertical thermal model and using the particular time evolution of the single-depth time series due to the internal wave motion. Furthermore, these reconstructed metalimnetic profiles can be used as initial conditions for predictive modelling embedded in a forecasting warning system. This outcome of the time series treatment can only be obtained when the thermal model has been previously calibrated and validated with an independent set of vertical profiles. Moreover, vertical profiles must be measured regularly in order to check the ongoing model simulation and adjust the parameter values if necessary. However, the 3-month autonomy of the prototype batteries requires regular maintenance, and this can be the opportunity to calibrate the sensors and to collect additional data.

Within the scope of the European Water Framework Directive, high-frequency measurements lead to valuable monitoring improvement. In a compromise between the number of lakes to monitor and sampling and analysis costs, the WFD sets a minimum measurement frequency of four times a year for physicochemical parameters, twice a year for phytoplankton, etc. (WFD, Annex V, § 1.3.4). ). This requirement can be consistently increased to twelve times a year. National authorities can increase these frequencies (e.g. Environmental Protection Agency, 2006; Ministère de l'Ecologie et du Développement Durable, 2006). The very low required frequencies permit to follow roughly the evolution of lakes, but extreme events of natural or anthropogenic origins will not be detected, which are particularly necessary to assess in a risk management framework. The monitoring will not enable the lake stakeholders to understand the functioning of the lakes so as to adapt their restoration strategy. It is insufficient for the calculation of most indicators of trophic status as defined for example by Organisation for Economic Co-operation and Development (1982) or [START_REF] Bricker | An integrated methodology for assessment of estuarine trophic status[END_REF]. This is singularly true for phytoplankton. The WFD monitoring addresses two issues: (1) the assessment of phytoplankton biomass using chlorophyll-a measurement as a proxy and (2) the record of the frequency, the composition and the intensity of phytoplankton blooms. It is assumed that phytoplankton biomass may be used as a proxy for net production and chlorophyll a as an index of phytoplankton biomass [START_REF] Painting | Assessing the suitability of OSPAR EcoQOs for eutrophication vs ICES criteria for England and Wales[END_REF]. Considering the day-to-day variability of phytoplankton dynamics, it is clear that the sampling frequency generally performed in the WFD surveillance monitoring does not comply with its objectives [START_REF] Carstensen | Statistical principles for ecological status classification of Water Framework Directive monitoring data[END_REF], unlike a high-frequency monitoring system.

Moreover, high-frequency monitoring systems can also contribute to long-term surveillance programmes by recording the year-to-year variations of dynamics of phytoplankton succession, bloom temporal distribution, duration and intensity and associated parameters such as temperature, dissolved oxygen, conductivity.

In lakes which require restoration, high-frequency monitoring would enable a better understanding of their functioning and more efficient programmes of measures could be defined.

Moreover, in both cases of operational and investigative monitoring programmes, routine sample monitoring does not always coincide with episodic discharges of pollutants. Real-time, instrumented buoys could help to pinpoint particular pollutant sources with a high degree of accuracy, for example during periods of wet weather.

Real-time monitoring systems are mandatory when a short-term response is needed, which is the case of cyanobacteria blooms. Bathers and drinking water producers need to be warned soon enough to close the beaches and to adapt the water production process. This is the ultimate aim of the PROLIPHYC project.

Additionally, the adaptation of the system in order to monitor phytoplankton in the sea could be studied. Sites where single-depth measurements could prove useful should exhibit strong and regular internal waves of known time period at depths at which phytoplankton grows. This should be possible in two types of sites: (i) when internal seiches dominate over internal tidal waves, which is the case in semienclosed bays or estuaries, as in Gullmar Fjord, Norway, where the pycnocline intersects a sill (Arneborg and Liljebladh 2001;Davies et al. 2009); this is close to the case of the large lake described in this paper, since the periods of internal wave modes are imposed by the bay or estuary geometry; (ii) when internal tidal waves are large: internal tides have semi-diurnal frequency and are either freely propagating waves, or more seldom partly standing waves, as in Mamala Bay, Hawaii (Alford et al. 2006); large semi-diurnal internal waves are common in oceans; large tides are also observed in particular sites of closed seas, like Gabes Bay, Tunisia (Sammari et al. 2006), even if the local internal wave climate apparently remains to be studied.

Conclusion

The data presented in this paper have been collected by a sensor-equipped buoy performing measurements at a single depth in the metalimnion of deep sub-alpine Lake Bourget. This simple, autonomous monitoring system coupled with an original method of data filtering (temperature and fluorescence) is able to derive a vertical representation of the phytoplankton biomass from high-frequency, single-depth measurements. It provides a powerful tool for supplying reliable data which can be further used to calculate operational indicators of lake status.

The method described, based on the use of the internal wave periodic motion, enables us to determine whether the sensor position is convenient to detect the fluorescence peak along the vertical. Secondly, the fluorescence values of the peak and parameters recorded at the same time step can be extracted, such as temperature, dissolved oxygen, conductivity. Their seasonal and year-to-year evolution can be studied, as well as the relation of the fluorescence peak to additional environmental parameters (e.g. light, mixing, nutrients etc.).

The autonomous buoy is also a part of an early warning system, for example aimed at signalling an HAB. In this case, when a forecasting model can be run, profiles reconstructed from the single-depth time series can be used as initial conditions for the predictive simulations.

Within the scope of the European Water Framework Directive's commitments, the system presented provides a valuable tool for operational monitoring (screening and verification of the impact of a programme of measures aimed at reducing the pressure on the water body). The system can also improve surveillance monitoring for the long-term changes involving events such as harmful algae blooms (HABs) which must be fully monitored to be characterized (duration and intensity of blooms). It could also help to meet the requirements of comparable legislation around the world like the US Clean Water Act or Australia's Water Act 2007 or the Canadian Environmental Protection Act.

Figure 2 :

 2 Figure 2: Temperature and fluorescence associated to Planktothrix rubescens at point M (07 August 2007 around 12:00 UT) (data CISALB, 2007).

Figure 3 :

 3 Figure 3: Conceptual representation of internal wave field (V1H1 mode, Period = 24 h). Influence on: (a) the thermal structure of the water column based on the 7 th August situation; (b) the variation of the maximum fluorescence depth located in this case on the 13 °C isotherm. The white dashed line indicates the sensor position (15 m). Shaded areas indicate the stages when the fluorescence peak is located above the sensors.

Figure 4 :

 4 Figure 4: (a) temperature, (b) fluorescence and (c) corresponding ΔF/ΔT ratio at 15 m in the conceptual representation; (d) temperature and (e) fluorescence measured by the sensors from 1 August to 03 August 2007 and (f) corresponding ΔF/ΔT ratio. Shaded areas indicate the stages when the fluorescence peak is located above the sensors.

Figure 5 :

 5 Figure 5: (a) Daily estimated fluorescence peak values; (b) Temperature related to the fluorescence peak; (c) Fraction of the day spent by the fluorescence peak above and below the sensors.

Figure 6 :

 6 Figure 6: (a) Simulated temperature profile (grey line) and measured temperature (black dots) on 2nd August 2007; (b) Equivalent depth of measured temperature; (c) Corresponding fluorescence plotted versus equivalent depth (grey dots) and smoothed profile averaged on 0.5m layers (black line).

Figure 7 :

 7 Figure 7: Fluorescence contour image inferred from the fluorescence and temperature time series and the 1D thermal model.

The ®MAREL buoy (Mesures Automatisées en Réseau pour l'Environnement et le Littoral) is an automatic measurement system developed by IFREMER (French Research Institute for Exploitation of the Sea) for operational monitoring of coastal water quality.

The PROLIPHYC project is conducted by a joint team of public research laboratories (Cereve, INRA, LGE, MNHN), the INSU-CNRS technical department (DT-INSU), an environmental engineering company (Sogreah), a water instrumentation company (nke) and a local authority (CISALB).

Acknowledgements

The Proliphyc project is funded by the ANR-PRECODD Programme. We wish to acknowledge Pascal Perney (INRA) for the logistic support and Yannis Cuypers (LOCEAN, Université Pierre et Marie Curie) for fruitful discussions. The authors also express their gratitude to Jean Sutcliff for the revision of the English text.


'Brâhmins of the Pariahs 1 ': Peripheries

in Quest of Identity The Valluvar of Tamil Nadu and the Construction of Dalit Identity in South India ALEXIS AVDEEFF "E ¯n paraiya ¯ ! Enkiratai vit, valluva paraiya ¯ enkiratu me ¯l." 2 "Why [say] Paraiyan ? When [one can say

T his scornfully and obvious ironical old Tamil proverb highlights the ambiguity surrounding the questions of status and identity of the Valluvar ca ¯ti 4 in Tamil society. It suggests that, whatever the title or the status they claim, the Valluvar belong to the Paraiyar community. But beyond this rhetorical device, there is truth in that popular Tamil expression: Even in the nineteenth century, there existed a difference between the Valluvar and Paraiyar communities, and questions related to identity were already present. Since the beginning of the rise of dalit 5 assertion, the question of the identity of untouchable sub-communities has passed from the daily village life and the local sphere to become a central matter of concern in the contemporary Indian political arena.

The purpose of this article is to analyse the special place that the Valluvar community occupies within Tamil society and their current process of identity construction. Considered popularly as 'Brahmin of the Paraiyar', the Valluvar have relationships with caste people in their role of traditional rural astrologers and physicians, and they enjoy a particular status among untouchable communities due to their tozhil (traditional occupation). First of all, I will analyse the etic and emic discourses on the Valluvar community through the work of scholars and through local discourses, in order to draw the contours of this particular identity. Then, drawing on data collected during three years of fieldwork in rural Tamil Nadu, I intend to show how the special position of the Valluvar within Tamil society is related to the notion of knowledge. Finally, I will focus on Valluvar self-assertion and analyse their identity construction process.

Contours of a South Indian Untouchable Identity: Discourses on the Valluvar Community

Even if no exhaustive study has been made of this endogamous South Indian community, there are nevertheless some references to the Valluvar in literature since the middle of the eighteenth century. Thus, I have been able to compare these ethno-historical sources with the present situation, and with regard to the emic discourses of the social actors encountered during my fieldwork.

One of the first references to the Valluvar community (excepting epigraphy) 6 can be found in A Description of the Character, Manners and Customs of the People of India by Abbe J. A. Dubois,[START_REF] To | s research, the original text by Gaston-Laurent Coeurdoux was written in the middle of the 18th century[END_REF] published at the end of the nineteenth century. In one chapter, in which he describes superstitions and people known for their magic skills, Dubois presents, with contempt, the Brahmin and even some Paraiyar whom he calls 'Valluvar' (Dubois 1879: 251). He describes them as soothsayers, omen decipherers and palmistry specialists. Further, in footnotes relating to the Paraiyar, he describes the Valluvar as one of the 13 classes composing the Paraiyar population and the most respected one. According to the author, this high rank among the Paraiyar is due to the figure of Tiruvalluvar, whom tradition has assigned to them (ibid.: 330). He adds that they are priests of the Paraiyar, and often physicians who 'enjoy such a reputation that Sûdras often resort to them and allow them to feel their pulse, [START_REF]The pulse is 'In Siddha medicine, as in Ayurveda, Yunani, and traditional Chinese medicines[END_REF] but always through a thin silken covering' (ibid.: 330). Further, he states that they conduct Paraiyar marriage rites and, in his opinion, this is perhaps why they are called with derision 'Brâhmans of the Pariahs' (ibid.: 333). Even if he does not clearly call them astrologers, he describes them as decipherers of pañca ¯nkam (almanac) predictions, identifying auspicious or inauspicious times for every life enterprise, and adds that although they use the almanac, they are not allowed to edit it, which only Brahmins can do. At the beginning of the twentieth century, more than any other observer, Edgar Thurston gave many details about the Valluvar community, and his compilations remain a reference for any anthropological work on South India. In his chapter on the Paraiyar, he presents the Valluvar as the people who have sacerdotal rank within the Paraiyar community (1909: vol. VI, 107). He insists on their fame: 'Their position and authority depend largely on their own astuteness. Sometimes they are respected even by Brahmans for their powers as exorcists' (ibid.). They serve in the marriage ceremonies of the Paraiyar, and work as astrologers for the untouchable populations, finding auspicious days to hold a marriage ceremony or for agricultural activities. He also mentions that they are buried in their own place, apart from the Paraiyar (Thurston 1909: vol. VI, 108). Then, in the chapter devoted to them, Thurston turns to the possible origins of the community, quoting a legend telling the story of a 'morganatic union' between a Brahmin and a Paraiyar woman.

According to one account, the Valluvans are the descendants of an alliance between a Brâhman sage and a Paraiyan woman, whose children complained to their father of their lowly position. He blessed them, and told them that they would become very clever astrologers, and, in consequence, much respected. (Thurston 1909: vol. VII, 304) In a way, we can say that from that angle, the Valluvar appear to be 'fallen Brahmins'. He states that they might have been the priests of the Pallava Kings before the arrival of the Brahmin (Thurston 1909: vol. VII, 303). He presents different sections of different ranks within the community, and notes that although there are both Šaiva and Vaisnava in the community, they do not intermarry (ibid.: vol. VII, 304). All males of the community above 12 years old wear the sacred thread (ibid.). He quotes the Madras Census Report of 1891: 'Both men and women are employed as astrologers and doctors, and are often consulted by all classes of people' (Thurston 1909: vol. VII, 305). Then, he discusses extensively the cycle of life rituals (ibid.: vol. VII, 307) and describes the burial in sitting posture of the Šaiva Valluvar (ibid.: vol. VII, 308) as a special marker of this Valluvar section. He finally notes that they abstain from eating beef and adds that 'Though they mix freely with the Paraiyans, they will not eat with them, and never live in the Paraiyan quarter', without specifying where they live within the village entity (ibid.: vol. VII, 310).

The other important reference to the Valluvar community is M. Moffatt's monograph on social structure among the untouchables of a southern Indian village in the 1980s. Like Thurston, he presents the Valluvar as 'the highest-ranking Untouchable caste in Endavur' 9 (1979: 102). In Moffatt's 'replication and consensus' theory on the social structure of the untouchables' world, the Valluvar play an important role as they illustrate the replication [START_REF]We must moderate this concept of replication as formulated by Moffatt[END_REF] among the untouchables of the Brahminical function among the u ¯r castes. [START_REF]the u ¯r. This is a common layout of South Indian villages[END_REF] Describing the relations with the other communities, Moffatt stresses that:

The Valluvar Pandarams of Endavur do not just relate downward to the Untouchables; however, they also serve upward as astrologers to the non-Brahmin uur castes. They are thus divided almost evenly between Untouchables and non-Untouchables by their tozhil, but the balance of these ties is toward the Untouchables. (Moffatt 1979: 102) Although the purohit [START_REF]person who has in his charge the priestly functions of a group or of a family[END_REF] functions of the Valluvar are only related to the untouchable world, as astrologers (and physicians), they are related to the caste people of the u ¯r. He also offers a crucial detail about the spatial disposition of the Valluvar within the u ¯r and the ce ¯ri: 'The position of the Valluvar Pandaram street clearly expresses their medial position between the uur castes and the rest of the Untouchables' (Moffatt 1979: 103).

Drawing on my observations in different villages, I must relativise Moffatt's statement as well as Thurston's about the spatial settlement of the Valluvar within the village entity. Indeed, although the Valluvar Street is a common feature of many village layouts, standing as the interface, rather than a boundary, between the u ¯r and the ce ¯ri, such a distinct quarter does not exist everywhere, and sometimes the Valluvar live in a corner of the ce ¯ri with other untouchables. About the fame of the community, nowadays too, the ca ¯ti name 'Valluvar' is given much respect by all the strata of the village society. Furthermore, since the Valluvar have acquired such a reputation in the practice of co ¯ciyam (astrology), in rural areas the name 'Valluvar' is, in many cases, a synonym for co ¯ciyan (astrologer) and to a lesser extent for traditional physicians. If a village or a hamlet does not have a Valluvar family, village people always know where to find one nearby. Their forecasting and practice of astrology is recognised even by the Brahmin, who do not hesitate to see a Valluvar astrologer instead of one of their own. From an internal point of view, the Valluvar present themselves as the authentic old priests, astrologers and physicians of Tamil Nadu. According to them, when the Brahmin came to South India, they despoiled them (Valluvar) of their traditional knowledge and reduced them (Valluvar) to an inferior status. This assertion may seem paradoxical, with regard to the legendary claim to be 'fallen Brahmin'. Indeed, on one side, they establish and defend their particular status relying upon their 'fallen Brahmin-ness' and, on the other, they claim to be a high community despoiled and downtrodden by the Brahmin, a common topic in the South Indian Dravidian context. From this brief browse through the literature and the current discourses, it appears clearly that the Valluvar enjoy a special position among the Tamil untouchables, a medial position just like Moffatt demonstrates, as they share symbols and functions with Brahmin and are entirely part of the untouchables' world, nevertheless having constant relations with higher caste people of the u ¯r.

Knowledge as the Key to a Particular Position in Tamil Society

Literacy

It should be clear by now that, among the untouchable communities, the Valluvar occupy a special position. Revealed both by emic and etic discourses, this position reflects a social reality. Indeed, if we look at the literacy rate in Tamil, according to the data gained in the Census of India in Madras Presidency for the years 1911, [START_REF]Data collected from Table IX -'Education by Caste, Tribe or Race[END_REF] 1921, [START_REF]Data collected from 'Table IX -Education by Caste, Tribe or Race[END_REF] and 1931, [START_REF]Data collected from 'Table XIV -Literacy by Selected Communities[END_REF] it appears that throughout the first half of the twentieth century, the literacy rate of the Valluvar was significantly high compared to other nutouchable and even non-Brahmin communities. During my fieldwork in the villages, I noticed that many of my older informants, 60-85 years of age, were literate in written Tamil, some even in English; in short, they had a good education. This situation contrasted with that of the same generation in the other communities which I encountered in the ce ¯ri. So it seemed that among the ce ¯ri communities, the Valluvar were more literate than the other untouchables. Perhaps this was due to their tozhil. In order to test these observations, I consulted the censuses of India of the pre-Independence period, from 1911-31; due to the Second World War, no Census was held in 1941. I used the data available in the three censuses, taking into account the fact that the communities appeared under different names from one census to another, such as Palli for Vanniyar and Kammalar for Visvabrahman, and converted this figures in percentages. I selected three major Scheduled Castes, including Valluvar, and a Scheduled Tribe. I also chose the Brahmin community and four important non-Brahmin communities to produce a cross-section of Tamil society. These percentages allowed me to highlight the fact that between 1911 and 1931, the literacy rate of the Valluvar population was between 14 per cent and 17 per cent, whereas the literacy of other untouchable communities was considerably lower. For example, the literacy rate of the Pallar population was between 2 per cent and 3 per cent; that of the Paraiyar population was between 1.5 per cent and 2 per cent. Obviously, the literacy rate of the Valluvar population was strongly lower than that of the Brahmin population (which was between 42 per cent and 62 per cent), but was higher than that of the other non-Brahmin communities (such as the Idayar/Konar, the Maravar, the Vanniyar) and slightly equal to that of the Kammalar (Visvabrahman). These percentages corroborate the observations made during my fieldwork, showing that even before the big education campaigns of independent India and quota policy with regard to the Scheduled Castes, the Valluvar's literacy rate was quite high, even compared to non-untouchables. This factor throws light on the discourses and on the singular position of the Valluvar community in Tamil society.

Practice of Astrology

The high literacy rate is directly linked to the traditional occupation of the community. Indeed, more than the practice of popular medicine, astrology requires many skills linked to the use of writing. First of all, in the transmission of astrological knowledge, literacy is crucial to the training process, even if the oral tradition plays a significant role. Moreover, learning and practising astrology presupposes a wide range of abilities linked to writing. In a consultation for making a birth chart, for example, the astrologer has to go through different steps, each one requiring a particular skill. It begins with the calculations: from the personal data provided by the 'patient' (time and place of birth), the astrologer must calculate the planetary positions, according to his pañca ¯nkam. Then he has to draw and fill in a birth chart, ja ¯takam, which will be the basis of further work and predictions related to this patient. Depending on the kind of consultation, the astrologer must adjust his calculations, comparing his results regularly with the pañca ¯nkam in order to interpret them and to forecast. From this example, it appears clearly that the Valluvar astrologer must have a mastery of basic mathematical calculations as well as a good command of written language, since he has to read the pañca ¯nkam to write the birth chart of his 'patient'. Reading and writing Tamil is, in fact, an indispensable precondition for acquiring, practising and transmitting astrological and, to a lesser extent, medical knowledge; depending upon the kind of medical practice, literacy may be less important than for the practice of astrology.

Skills and Symbolic Position

Astrological and medical knowledge and their practical application are related to aptitudes and particular competences that are generally recognised as belonging to the Valluvar community.

As astrologers, they are seen as the time keepers of the day-to-day life of the village, since they can read and interpret the pañca ¯nkam. This mastery of time is one of the most important skills that villagers recognise in the Valluvar, since knowing the auspicious time is a central concern of the organisation of daily life, for example, finding the auspicious time for a marriage, for agricultural activities, etc. Another skill central to the practice of astrology is calculation.

As the Valluvar practitioners themselves admit, they are not clairvoyant; they just know how to calculate according to the astrological treatises, and then they 'see the horoscope' (in Tamil: ja ¯takam pa ¯kka). Because of that skill (calculation or kanitam in Tamil), they are often called kaniya ¯n. Closely related to the practice of astrology is the practice of va ¯stu, [START_REF]places (like temples) and harmonious houses according to directions and cosmic forces[END_REF] an architectural knowledge that requires the mastery of space, knowledge of the right directions and orientations, according to the cosmic forces, in order to erect a building. Last, the practice of medicine, maruttuvam, [START_REF]By 'practice of medicine' I mean local medicines: paccilai-maruntu, na ¯tumu ¯likkai and, to a lesser extent[END_REF] presupposes the knowledge of the Materia Medica: plants, leaves, herbals, roots, etc. as well as other organic substances and metals. It presupposes too, and this is perhaps the central point, mastering the manufacturing process of remedies, which cannot be described here, ranging from the simple use of the fresh juice of a leaf to the most complex and perilous mixtures difficult to realise. We may also add here the knowledge of sorcery (mantiram) and the making of talismans and amulets (entiram), although the practitioners I met are reluctant to speak about those practices. Independently of skills related to a specific practice, Valluvar practitioners are renowned for their skill seen as almost innate, or at least intrinsic to this particular community. They are seen as the best astrologers in Tamil Nadu -Valluvar is often used as a synonym of astrologer -and the best traditional physicians of rural areas, for their supposed accuracy in diagnosis and forecasting. As powerful soothsayers their speech, their words, are seen as pure and always true, like the words of their legendary ancestor Tiruvalluvar. Some of the older Valluvar astrologers I encountered give their forecasts in a singing or poetical manner. About this particular skill relating to speech, a Tamil proverb says: 'Ma ¯nikka kalle ¯ kallu/Valluvarutaiya colle ¯ collu, meaning: 'the speech of a Valluvar is as pure (brilliant) as a jewel'. The power of speech can to some extent be linked, in the case of diagnosis and forecasting, with the practice of mantiram, which requires the mastery of words to be efficient. Indeed, if people admire and revere the speech skills attributed to the Valluvar, they also fear them for their ability to curse or throw spells. Admired, sometimes feared, for their knowledge and their practice of 'sacred sciences', the Valluvar are at the centre of daily village life, both in the u ¯r and in the ce ¯ri. Through their knowledge and their skill, they occupy a special status in society, between two worlds: that of the untouchables and of the caste people. Due to this particular position in Tamil society, the contours of the community identity are blurred and, consequently, the way they assert and construct their identity is slightly different from that of other untouchable communities.

Self-assertion and Construction of Valluvar Identity

Sangam and Relations with the Regional Centres

The question of Valluvar identity is deeply embedded in the whole dalit quest for self-dignity and eradication of untouchability in India. Nevertheless, as pointed out by Ghanshyam Shah, this contemporary dalit struggle and movement for identity and equality is not a unified one (Shah 2007: 195). The reality is more complex, and the awakening of dalit self-consciousness comes with various movements related to specific ideologies or particular untouchable communities (due to the variety of socio-cultural contexts in the Indian subcontinent and the region-wise classification of the Scheduled Castes). The many movements pursue a common aim in many different ways. Even within a region like Tamil Nadu, the process of untouchable identity construction differs from one community to another. [START_REF]See, for example, the studies on the Nadars (Templeman 1996 and Hardgrave 1969) and the analysis[END_REF] Concerning the Valluvar community, the beginning of their self-assertion movement goes back to the first half of the twentieth century, during the pre-Independence period. The associative structure that they created to represent their social interests on the regional political scene was founded in 1935 and was named The Madras Valluvar Mahajana Sangam (Mohan 1993: 50). Through its provincial conferences, the Sangam pointed out issues concerning representation in local bodies, public services and the educational field, sending the claims of the Valluvar community to the relevant authorities. Two decades later, in 1955, the name of the Sangam was changed to South India Valluvar Mahajana Sangam, Madras (ibid.: 51). Today, there is still an active association defending the interests of the Valluvar community:

The International Valluvar Community Welfare, Chennai (Ulaka Valluvar Kulap Po ¯vai, Cennai), based in Chennai, which organises meetings of the Valluvar community and edits a pluri-annual gazette for the community Valluvam pe ¯cukiratu. Among the struggles led, in 1962 the association obtained from the government the recognition of Nayanar 19 people as full members of the Valluvar community, and, in consequence, the right for them to benefit from Scheduled Castes' Reservation Policy, which was not permitted for them before (see Figure 10.1). More recently, the association obtained the recognition of Valluvar people as Scheduled Caste from the state authorities of Andhra Pradesh, where they were classified as Forward Communities (FC), allowing them to benefit from the Reservation Policy inherent to the Scheduled Castes classification.

As an associative entity, the Valluvar Sangam concentrates its action on the political scene, to publicly assert the Valluvar identity and to support the community interests. Nonetheless, parallel with this process of political assertion, another identity construction process can be observed at work within a significant part of the community. Regarding the data collected both at the Sangam and in villages of Tamil Nadu and Pondicherry, it appears that an identity formation process is at work on a global communityscale. This identity transformation process observed within the Valluvar community goes through a series of three major and distinct phases.

Differentiation and Reunification

This self-assertion process begins with the delimitation of Valluvar identity. 20 As we saw earlier, the special status of the Valluvar community within the untouchables and its relations with highercaste people make this delimitation less clear than for other untouchable communities, like the Paraiyar. Indeed, even if they are aware of the dalit condition as a whole, differentiation from other dalit communities is the first step of the process and particularly from the Paraiyar, with whom they are very close. The Valluvar are, as noticed by Moffatt, generally quite emphatic about their distinctness from the other untouchables (1979: 107) and from the Paraiyar in particular. Although they admit to being very close to them, the Valluvar normally do not intermarry with the Paraiyar or other untouchable communities. They also explain, though admitting that nowadays there is a standardisation of life cycle rituals within the whole rural society, that they have kept some of their own rituals and especially those concerning death, which for the Valluvar is a strong identity marker with regard to the other communities. When we came to the question of community rituals, they were very talkative about the way the Šaiva Valluvar 21 bury their dead: in a sitting position and in a particular part of the cemetery, collai ka ¯tu, apart from the others. For the Valluvar, burying the deceased in a sitting position is directly linked with the Hindu god Shiva and marks their higher sacerdotal rank among all other untouchables, who bury their dead in a supine position. Despite the fact that they strongly assert their differences with the other untouchables, the Valluvar know that they do not pertain to the caste people. Indeed, even if they share symbols and attributions with higher castes, they are aware of their low status and of the impurity related to their sacerdotal functions among the other untouchable communities. But this process of differentiation is closely linked to the construction of internal cohesion. Differences between different ko ¯ttiram 22 (Nayanar or Alvar) and other subsections (Ta ¯dar, Pandaram) are ironed out to reinforce the feeling of community. Intermarriage within Valluvar community subsections is reinforced. In the cities and among educated Valluvar, the old restrictions between different ko ¯ttiram and sections have vanished. For example, the Valluvar who converted to Christianity, most of who live in Tirunelveli district, are now taken into account by the community. As an active member of the Sangam told me:

In some places near Papanasam, some of the Valluvar converted themselves to Christianity. And there is an adjustment among the community when it comes to the marriages. If the girls of that family, who have converted themselves to Christians, come to the Valluvan family who are still Hindus, she changes herself as a Hindu and vice versa.

Nevertheless, although the Valluvar try to unite their community under a same identity by smoothing over the differences previously made about the subsections, notably with more flexibility regarding intermarriage, they remain very strict regarding other communities by maintaining endogamy within their own community. As underlined by the Valluvar man previously quoted: Through this process of differentiation and unification, the Valluvar are able to form a clear boundary of their community, and, thus, on this foundational basis they can firmly assert what they are and start their identity assertion.

Symbolic Emphasis: Drawing on Symbolic Resources of the Community

The core of the identity construction process is the use of the symbolic resources of the community and the way in which the Valluvar are used to sustain an identity assertion. Among all the resources used in this process, the glorification of their role as a sacerdotal caste and as astrologers of kings of ancient kingdoms is perhaps the most visible fact. The claim to be the old authentic specialists of Tamil astrology and medicine, before the supposed arrival of the Aryans and Brahmin -which is yet a controversial theory but a common ideological feature of many Dravidian movementscomes as a logical consequence of such an assertion. Yet this claim is not really founded on strong criticism and a total rejection of the Brahmin, with whom they share the symbols upon which both build their social differentiation and, in a way, their superiority (scriptural learning, sacred knowledge and sacerdotal functions, and in some cases, a vegetarian diet). The Valluvar and the Brahmin also share a common symbolic feature: They wear the pu ¯nu ¯l, the sacred thread. However, whereas it is a strong symbol for Brahmin identity, the Valluvar do not use it as an important marker in their self-assertion process. Indeed, for many of my informants, wearing the pu ¯nu ¯l is more than anything a purity marker directly related to sacredness. For this reason, it could not be an ostentatious symbol worn for a profane use. Many Valluvar wear it only when it is required by the context -astrological and medical consultations, ceremonies, festivals, etc. -and are reluctant to wear it in everyday life, if this is not necessary. Finally, the most visible symbol adopted by the Valluvar community is the figure of the Tamil poet Tiruvalluvar. As a key figure in Dravidian literature, culture and identity, questions about Tiruvalluvar identity are the object of passionate debates among scholars and various communities, which claim his ancestry (Blackburn 2000: 452). The question itself highlights the problem of recognition of dalits by high-caste people; in fact, the main controversial point of this debate is focused on whether Tiruvalluvar was or not an untouchable. Even if the question about the author of the Kura and his origins remains unsolved, the Valluvar regard him as one of them, undoubtedly belonging to their community. It is common to see concrete manifestation of such a claim in villages. Indeed, some families, to mark their belonging to the Valluvar community and to differentiate their house from others, hang a depiction of Tiruvalluvar on the door lintel or on the front of the house. The Valluvar Sangam itself uses his image and his name for its pluriannual gazette Valluvam pe ¯cukiratu, on which he appears standing on the World, holding an elutta ¯ni in his right hand and an o ¯lai cuvati 23 in the left. Half a century ago these were still the work tools of Valluvar astrologers and physicians. Thus, Tiruvalluvar clearly appears as the embodiment of community pride.

Breaking with the System

Nowadays, it is hard to find Valluvar 'Pandaram' in villages. Many Valluvar are increasingly reluctant towards their purohit duties and The Paraiyar consider the Valluvar as Brahmin and invite us for all their functions like marriages and other ceremonies, where they need a Brahmin to do all the rituals. I used to go their houses, but now I am reducing it because they do not give good respect. In those days, people used to give respect to the Valluvar and invite them, but now there is no respect given to us.

[…] We do not participate in the [village] festivals.

If they come to us for donation, we give them some money. Apart from that we do not go and participate in the festival.

Others complained about the financial aspect; indeed, to be attached to and looking after a temple is a harsh task, paid little by the village community. In cases in which the temple duties are left to other community people, I observed that some Valluvar families possess their own familial temple adjacent to their house. These private temples belong exclusively to the family and their relatives, but some ceremonies can be organised for other community people, on request and in exchange of payment in kind or in money. Although it seems clear that the Valluvar, on a village scale, try to severe the traditional ties that linked them with the Paraiyar and other untouchable communities, it also appears that they want to develop the other side of their traditional occupation that links them with all the other strata of society. Due to their reputation and their traditional experience as astrologers, nowadays the Valluvar develop the practice of astrology as a profession and, in some cases, into a very profitable business, like in Vaitheeswarankoil, 24 where customers come from all over India and even from abroad. Excepting this large southern Indian centre for the commerce of astrology, the majority of Valluvar who maintain their traditional occupation are moving from the village to the urbanised areas to develop their professional activity. The most reputed ones do not hesitate to travel around Tamil Nadu and beyond to give their consultations. Breaking with the local system of services, the Valluvar, nonetheless, continue their traditional practice of astrology, and in many cases they use it as a social uplift among higher strata of the Tamil society and as a place in the modern globalised economic model.

Nevertheless, a part of the Valluvar is not involved by these changes and transformations. Generally these Valluvar are isolated families in remote villages or hamlets, less educated than the average community member; they work in the fields, carrying out agricultural tasks, and do not practise traditional occupations such as astrology or medicine. These forgotten ones live far from social changes and view the Sangam as a political entity, which is not representative of them, but instead far from their daily preoccupations and mainly focused on the more educated urban or peri-urban Valluvar -the 'elite' of the community, as pointed out by a 75-year-old Valluvar man living in a remote hamlet of Thanjavur district:

There is a Sangam. But we do not know where it is and what are the main functions of the local Sangam. Being different … coming from other communities [my interpret and I], you are coming to me to get some information, but our own community people do not invite us for any of the Sangam meetings, and we are not aware of anything happening in the Sangam.

These forgotten people feel rejected by an elite, which -from their point of view -works only for a small fraction of the community people and does not seem to be concerned by their condition, as denounced by the old man from Thanjavur district:

Definitely they [the Valluvar] are better than before in the financial condition and other things. But I would say that they are not united; they are not working together for the social uplift of the community. They are keen in their work, but they only work for the betterment of their family alone and not for the community as a whole.

Thus, the major criticism expressed towards this Valluvar 'elite' is the lack of unity in the community struggle, in spite of all the efforts made by the Sangam to unify a heterogeneous and scattered community.

Conclusion

Identity assertion is a very central concern among peripheries, not only for the tribal people but also for the untouchables. The case of the Valluvar community shows another facet of the untouchable world and its struggle for recognition within Indian society. Despite belonging to the untouchables, the Valluvar community enjoys a special status in Tamil society. Indeed, due to their sacerdotal rank among these populations and to their practice of astrology and medicine, the 'Brahmin of the Paraiyar'as they are colloquially called -are seen as the purest among the untouchable communities. This special status has consequently given a particular orientation to the way they assert their identity, one different from other untouchable communities. In this struggle for self-assertion it appears that each untouchable subgroup has had to find its own way in the quest for identity, drawing on its own symbolic resources and community history.

The Valluvar self-assertion identity process goes through a series of three distinct phases: (1) differentiation which goes hand in hand with delimitation and unification; (2) re-symbolisation by using the symbolic resources of the community; and (3) breaking with the system, permitting a new beginning in the society, far from the traditional ties. The core of this process is the root of the identity in the pan-Dravidian figure of Tiruvalluvar, a prestigious symbol for the Valluvar community, and beyond, for Tamil identity in its whole. For an external observer, this process can seem paradoxical, in the sense that, on the one hand, the Valluvar claim a particular identity dissociated from the other untouchable identities, as other untouchable communities also do to reinforce their own assertion. On the other hand, they ask for the total annihilation of untouchability.

As we have seen, if the Valluvar have acquired a certain visibility and rights from the authorities, some of them still remain in the shadow of this transitional process. These forgotten ones, who do not benefit from the social progress gained by the Sangam, feel rejected. This may be the weakness of all these dalit movements scattered at the peripheries of Indian society. What we observed is still transitional, and the dalit movements may have a lot to earn by unifying themselves under one banner to reinforce their weight in this struggle. This difficult task of coordinating all present forces might be the final struggle for the peripheries on the way to emancipation.

Notes

1. This expression is quoted from J.-A. Dubois 1879, p. 333). 2. Tamil proverb quoted from the compilation of H. Jensen (1897): proverb no. 1585, p. 174. 3. Translated from Tamil by the author. 4. Ca ¯ti is the Roman transliteration of the Tamil term ja ¯ti. Ca ¯ti is generally translated as 'birth group' or 'sub-caste'. It is an endogamous community based on traditional occupation. Each (linguistic) region has its own ja ¯ti, even though we can distinguish recurrent features from one region to another. For the sake of clarity, the term 'community' will be used in this article instead of the term ca ¯ti.

20. The delimitation of an identity can involve a process of reappropriation, drawing on various sources, some of which can be quite surprising for the anthropologist. In South Tamil Nadu, I attended a Valluvar girl's puberty ceremony that brought together many Valluvar families. The Valluvar priest for the ceremony, also in charge of the local community association, brought many printed booklets of the chapter 'Valluvar' (taken from Thurston 1909) and asked the audience to carefully read this booklet said to contain the essential features of Valluvar identity and tradition. 21. Thurston mentions them as 'Saivite lingam wearers' (Thurston 1909: vol. VII, p. 308). It is interesting to notice that this funeral practice among Šaiva Valluvar bears some similarities with the funeral practice of Linga ¯yat-V¯rašaiva (see Assayag 1987: vol. 27(103), pp. 93-112). 22. Ko ¯ttiram, also spelt go ¯tra, means family, lineage. But it is commonly used when speaking about the religious sections within a community or a caste.

In the case of Valluvar community, the Šaiva are called Nayanars and the Vaisnava are called Alvars (which name refers to the 12, sometimes ten, Tamil Vaisnava Saints, the A ¯lva ¯r 23. The elutta ¯ni is the stylus used to write on the palmyra leaf and the o ¯lai cuvati is the palm-leaf bundle. 24. Vaitheeswarankoil is a town situated in the Nagapattinam district, renowned for its Shiva temple dedicated to Vaidyanatheeswarar, as well as for its many astrologers, in fact nadi readers, who practice a particular form of astrology, Nadi Jothidam.

  Sub.: Harijan Welfare -Communities -Valluvar in the list of Scheduled Caste -Nayanar -Brasketing of. Ref.: Your representation dated 8-12-62 With reference to your representations cited, I am directed to state that if Nayanar is the title of Valluvan Community and that if fees are demanded by the institutions persons for entering the caste "Nayanar" instead of "Valluvan" the persons concerned have to produce the necessary certificate from the prescribed officer of the Revenue Department to the effect that they belong to Valluvan Community in order to claim full fee concessions. The Government does not consider it necessary to the list of Scheduled Caste in the regards. Yours faithfully, Sd. C.N. Singaravelu 2.1.65 for Deputy Secretary to Government.

Figure 10 . 1 :

 101 Figure 10.1: An official letter published in the Valluvar gazette 'Valluvam pecukiratu', June 2005, p. 14.

The

  Pandarams themselves, there are two types of Pandarams, one is the Aandi Pandaram and the other one is Pookkatti Pandaram. Excepting for the title they do not actually belong to the Valluvan community. They cannot enjoy the intermarriage with the Valluvar, Nayanars or the Alvars. These Pandarams belong to the Backward class.

Plate 10 . 1 :

 101 Symbol of the Valluvar Community pride -Tiruvalluvar standing on the World holding a stylus in his right hand and a palm-leaf bundle in the left, as he appears on the front cover of the Sangam's gazette. Source: Valluvam pe ¯cukiratu, Chennai, Tamil Nadu. Used with permission. priestly functions among the untouchables. The main reason stated by my informants is the loss of respect for the Valluvar shown by the Paraiyar, as confessed by a Valluvar living in the Paraiyar corner of a Pondicherry village:

Plate 10 . 2 :

 102 Valluvar Community's banner as it appears on the front cover of the Sangam's gazette, with a depiction of Tiruvalluvar on a white background.Source: Valluvam pe ¯cukiratu, Chennai, Tamil Nadu. Used with permission.