Mélanie Herteman

Luc Lambs

Effects of pretreated domestic wastewater supplies on leaf pigment content, photosynthesis rate and growth of mangrove trees: A field study from Mayotte Island, SW Indian Ocean

Keywords:

After 12 and 18 months of daily wastewater discharge into mangrove plots in Mayotte Island, SW Indian Ocean, leaf pigment content, photosynthesis rate and growth of Rhizophora mucronata and Ceriops tagal mangrove trees were evaluated and compared with similar individuals from control plots. Chlorophyll and carotenoid contents, measured using an HPLC analyser, were significantly higher in leaves of mangrove trees receiving wastewater discharges. Photosynthesis and transpiration rates, analysed using an LCi portable system, increased significantly for mangrove trees in impacted plots. Measurements of leaf areas, young branch length and propagule length showed significant increases in plots receiving wastewater. These results suggest a beneficial effect of domestic wastewater on R. mucronata and C. tagal mangrove tree functioning. Analyses and observations on mangrove ecosystems as a whole -taking into account water and sediment compartments, crab populations and nitrogen and phosphorus cycles -are nevertheless necessary for evaluation of bioremediation capacities of mangrove ecosystems.

Introduction

Mangroves and bioremediation

The utilisation of mangrove swamps as natural systems for wastewater treatment has been proposed as an efficient and lowcost solution for tropical coastal areas. Characterised by a high primary production and biomass and established as often as not on nutrient-poor sediments, mangrove ecosystems are considered able to absorb nutrients in excess contained in sewage, without any major structural or functional disturbance [START_REF] Saenger | Mangrove Ecology Silviculture and Conservation[END_REF]. [START_REF] Nedwell | Inorganic nitrogen-metabolism in a eutrophicated tropical mangrove estuary[END_REF] showed that the discharge of pretreated wastewater into a mangrove swamp in Fiji could be a means of reducing eutrophication in coastal waters, and therefore suggested that mangroves might be used as the final stage in sewage treatment. [START_REF] Clough | Mangroves and sewage-a re-evaluation[END_REF] published one of the first review articles dealing with the impact of sewage on mangrove ecosystems. These authors established that the capacity of mangroves to remove nutrients from sewage was largely determined by hydrodynamic factors in the short term and that the efficiency of the processes was largely dependent on the sediment properties and biological characteristics of the ecosystem in the longer term. [START_REF] Corredor | Nitrate depuration of secondary sewage effluents in mangrove sediments[END_REF] demonstrated that the excess nitrogen coming from a sewage treatment plant in Puerto Rico could be absorbed by the mangrove ecosystem through natural denitrification processes, without any damage.

In an exploration of the different aspects of the role of mangrove swamps as sinks for wastewater-borne pollutants through numerous experiments conducted in the Hong Kong and Shenzen area (South China), [START_REF] Tam | Effects of wastewater discharge on microbial populations and enzyme activities in mangrove soils[END_REF]Wong (1995, 1996) successively showed that mangrove soils are good traps to fix phosphorus and certain heavy metals from wastewater; that no significant change was observed in the plant community structure or in leaf nutrient content of a mangrove site receiving wastewater discharges for 1 year [START_REF] Wong | Effect of wastewater discharge on nutrient contamination of mangrove soils and plants[END_REF][START_REF] Wong | Mangrove wetlands as wastewater treatment facility: a field trial[END_REF]; and that litter production and decomposition were not perturbed [START_REF] Tam | Litter production and decomposition in a subtropical mangrove swamp receiving wastewater[END_REF]. The addition of wastewater to mangrove soils also seems to stimulate the growth of microbial populations, probably through nutrients and carbon components present in wastewater [START_REF] Tam | Effects of wastewater discharge on microbial populations and enzyme activities in mangrove soils[END_REF]. More recently, these authors showed that a mangrove plant community growing in constructed microcosms receiving wastewater was effective in removing organic matter, nitrogen and phosphorus [START_REF] Wu | Constructed mangrove wetland as secondary treatment system for municipal wastewater[END_REF][START_REF] Tam | Mass balance of nitrogen in constructed mangrove wetlands receiving ammonium-rich wastewater: effects of tidal regime and carbon supply[END_REF], Analysing a natural mangrove area in Thailand, [START_REF] Wickramasinghe | Multi-functional pollution mitigation in a rehabilitated mangrove conservation area[END_REF] arrived to similar conclusions, demonstrating the efficiency of mangrove ecosystem in waste treatment, with an enhancement of mangrove growth and abundance of invertebrate populations 1.2. Mangrove tree growth and nutrient enrichment While the use of mangrove ecosystems for removing pollutants from sewage discharges is becoming rather well documented, the response of mangrove plants themselves in terms of growth should be analysed and controlled, and results in this domain are still contradictory. [START_REF] Henley | An investigation of proposed effluent discharge into a tropical mangrove estuary[END_REF] reported that mangrove tree growth in the Darwin area, Australia, was not affected when they received sewage discharges, and [START_REF] Clough | Mangroves and sewage-a re-evaluation[END_REF] concluded that nutrient enrichment of a mangrove ecosystem through wastewater supply did not appear harmful and in some cases might have a beneficial effect on growth and productivity. [START_REF] Kelly | Effects of long-term discharges of treated sewage on the nutrient status of adjacent mangrove communities[END_REF], investigating the impact of sewage effluents on mangroves dominated by Avicennia marina in Australia, found that the N and P leaf concentrations were higher at impacted sites, but no clear growth-enhancing effects were noted at these same sites. From similar experiments concerning the two mangrove species Kandelia candel and Aegiceras corniculatum, [START_REF] Wong | Effect of wastewater discharge on nutrient contamination of mangrove soils and plants[END_REF] did not find any significant differences in plant growth after 1 year of sewage discharges, but noted that effects -positive or adverse -on vegetation functioning could become apparent only over a longer term. More recently, [START_REF] Lovelock | Nutrient enrichment increases mortality of mangroves[END_REF] established that nutrient enrichment (N and P) could increase the mortality of mangroves in sites characterised by low annual rainfall and high sediment salinity. These authors added that mortality rates were significant in landward scrub forests and no tree deaths occurred in fringe forests. [START_REF] Lovelock | The effect of nutrient enrichment on growth, photosynthesis and hydraulic conductance of dwarf mangroves in Panama[END_REF] and [START_REF] Martin | Nitrogen fertilization enhances water-use efficiency in a saline environment[END_REF] specified that N and P enrichment significantly increased mangrove tree growth, but in certain salinity conditions might alter the structure of mangrove forests.

Mangrove tree functioning and environmental stresses

Relationships between nutrient enrichment and metabolic processes in mangroves are still little documented. Peculiarly, data on photosynthesis rate in mangrove trees as a functional marker of their health state are rare; such data are generally linked to hydrological and salinity parameters and take into account propagule populations in greenhouse conditions [START_REF] Ball | Photosynthetic and stomatal responses of two mangrove species. Aegiceras corniculatum and Avicennia marina, to long term salinity and humidity condition[END_REF][START_REF] Youssef | Photosynthetic gas exchange and accumulation of phytotoxins in mangrove seedlings in response to soil physico-chemical characteristics associated with waterlogging[END_REF][START_REF] Kao | The photosynthesis and chlorophyll a fluorescence in seedlings of Kandelia candel (L.) Druce grown under different nitrogen and NaCl controls[END_REF][START_REF] Kao | Effect of NaCl and nitrogen availability on growth and photosynthesis of seedlings of a mangrove species, Kandelia candel (L.) Druce[END_REF][START_REF] Krauss | Influences of salinity and shade on seedling photosynthesis and growth of two mangrove species, Rhizophora mangle and Bruguiera sexangula, introduced to Hawaii[END_REF]. Some studies considered the links between mangrove structure (scrub vs. fringe mangrove), mangrove tree height and photosynthesis characteristics [START_REF] Lin | Comparative study of water uptake and photosynthetic gas exchange between scrub and fringe red mangrove, Rhizophora mangle L[END_REF][START_REF] Lovelock | The effect of nutrient enrichment on growth, photosynthesis and hydraulic conductance of dwarf mangroves in Panama[END_REF], and [START_REF] Naidoo | The effects of coal dust on photosynthetic performance of the mangrove, Avicennia marina in Richards Bay, South Africa[END_REF] established in a specific context that photosynthetic performance of A. marina was reduced when coal dust was deposited on the leaf surface of the mangrove trees.

In other studies, pigment content of mangrove leaves has been analysed in relation to the light environment of the mangrove forest canopy [START_REF] Lovelock | Influence of solar-radiation and leaf angle on leaf xanthophyll concentrations in mangroves[END_REF][START_REF] Moorthy | Influence of ultraviolet-B radiation on photosynthetic and biochemical characteristics of a mangrove Rhizophora apiculata[END_REF]. [START_REF] Rajesh | Growth and photosynthetic characteristics of Ceriops roxburghiana under NaCl stress[END_REF] established correlations between growth rate, photosynthetic and pigment characteristics, and salinity levels for Ceriops populations. [START_REF] Macfarlane | Photosynthetic pigments and peroxidase activity as indicators of heavy metal stress in the grey mangrove, Avicennia marina (Forsk.) Vierh[END_REF] showed that photosynthetic pigment concentration decreased in A. marina populations impacted by heavy metals, and MacFarlane (2002) suggested that photosynthetic pigments could be considered as biological indicators of stress for mangrove trees.

Mayotte Island context and bioremediation project

The Mayotte Archipelago, West Indian Ocean, is currently experiencing environmental degradation linked to a very important In this context, experiments have been launched at Malamani, SW Mayotte, to evaluate the bioremediation capacities of a mangrove swamp receiving, in controlled conditions, pretreated domestic wastewater. Water bodies, sediment, vegetation and fauna (crab populations) of mangrove ecosystems have been taken into account and analysed [START_REF] Herteman | Evaluation des capacités bioremédiatrices d'une mangrove impactée par des eaux usées domestiques[END_REF]Herteman et al., submitted for publication) and experiments are still in progress at the Malamani study site.

We now report investigations concerning mangrove vegetation functioning after 12 and 18 months of daily wastewater discharge. An aerial survey of the study site clearly showed a change in the colour of the mangrove canopy, turning from light green to strong green, corresponding to mangrove plots receiving wastewater (Fig. 1a). This change appeared 6 months after the first sewage discharges in the mangroves and persisted 12 months later; the change in leaf colour clearly corresponds to the discharge. Observations in the field confirmed the colour change of the mangrove leaves and also showed obvious differences in branch length between control and impacted plots (Fig. 1b andc). To analyse such changes in vegetation and evaluate the impact of wastewater, photosynthetic pigment concentrations, photosynthesis rate and growth of mangrove trees were followed in impacted and nonimpacted mangrove plots, in two different facies, respectively, dominated by Ceriops tagal (Perr.) C.B. Robinson and Rhizophora mucronata Lam.

Materials and methods

Study area

Mayotte Island is a dependent French overseas territory in the Comoro Archipelago, located in the Mozambique Channel, SW Indian Ocean (Fig. 2). The little volcanic island (376 km 2 ) is surrounded by an almost continuous barrier reef system enclosing one of the largest lagoons in the world (1500 km 2 ). Mangrove swamps are developed at the ends of bays on around 650 ha. The tide range is high for an oceanic island, reaching up to 4 m in spring tides. Mayotte's climate is maritime tropical, with a warm wet season from November to April (mean seasonal rainfall and temperature: 1200 mm and 27.2 • C, respectively) and a cooler dry season from May to October (210 mm and 25.1 • C).

The study area is located in Chirongui Bay, southwest of Mayotte (12 • 55 ′ S, 45 • 09 ′ E). A primary treatment unit sized for 400-equiv. inhabitants daily receives domestic wastewater from Malamani village. Wastewater is decanted and stored, and then carried through a pipe network to the mangrove area. Time delivery and discharge volumes are automatically controlled by a SOFREL processing system. Wastewater is then delivered every second low tide onto two mangrove plots respectively dominated by C. tagal and R. mucronata at the rate of 10 m 3 per 24 h on each 45 m × 15 m plot. A third 45 m × 15 m plot connected to the pipe network automatically receives wastewater in excess, particularly in the rainy season when discharge volumes exceed 20 m 3 per day (Fig. 2c).

Photosynthetic pigment concentration, photosynthesis rate and growth of mangrove trees were analysed 12 and 18 months after commencement of wastewater discharges in the two impacted plots, and in two equivalent control plots. The average composition of the wastewater is given in Table 1, and the vegetation structure of the four plots is presented in Table 2.

Photosynthetic pigment analyses

Mature and healthy leaves of 12 random patches in each of the four plots were collected in January (wet season) and April (beginning of dry season) 2009 and rapidly stored in a cold place (cooler box during transport, then -80 • C freezer in laboratory). Three disks 18 mm in diameter were cut from each leaf patch sample, crushed with 50 mg Fontainebleau sand, rinsed with 20 ml methanol, and then placed under ultrasound for 3 min. Mixtures were stored for 15 min at -20 • C, and then spin-dried (5 min in -1 • C centrifuge at 3500 rpm). Samples of the supernatent were taken (1 ml), filtered through 0.2 m syringe filters, and then analysed using HPLC.

Photosynthesis and transpiration rates

The net photosynthetic rate was measured on intact, mature C. tagal and R. mucronata leaves with a portable photosynthesis system (ADC Bioscientific Ltd portable), equipped with a 6.25 cm 2 leaf chamber. We measured 150 and 120 leaves, respectively, in each C. tagal and R. mucronata 45 m × 15 m plot. Three successive measurements were made for each sampled leaf at intervals of 25 s. All measurements were made between 10:00 and 13:00 h, on sunny days and under the following conditions: photosynthetically active radiance: 1000-2000 mol m -2 s -1 , relative humidity: 65 ± 5%, temperature: 30 ± 2 • C.

Growth rate measurements

Leaf measurements: In each control and impacted plot (C. tagal and R. mucronata stands), 90 mature and healthy leaves were randomly collected, and their lengths and widths measured. Fresh and dry weights were measured, and leaf areas were calculated using ImageJ software and leaf digitisation. Leaf area-weight relationships were determined. Measurements were made in April and October 2009.

Branch measurements: In each C. tagal plot, 60 branches were measured on 15 trees, i.e. four branches per tree, distributed in the upper, middle and lower parts of the control and impacted plots. In the R. mucronata plots, 39 branches were measured on 13 trees, i.e. three branches per tree, distributed throughout the control and impacted plots. Measurements were made in April and October 2009.

Propagule measurements: In each control and impacted plot (C. tagal and R. mucronata), 90 propagules were randomly collected from 9 trees, i.e. 10 propagules per tree, distributed in the upper, middle and lower parts of plots. Propagule length was measured in October 2009.

Statistical analyses

The Shapiro test was conducted on each dataset (pigment concentration, photosynthesis and transpiration rates, growth rate measurements) and showed that data were normally distributed. Mean values and standard deviation were calculated.

One-way ANOVA (for p ≤ 0.05 and p ≤ 0.01) was employed to test the significance of differences between control and impacted plots, between dates and between species, for each dataset except propagule lengths, which were analysed using Student's t-test (p ≤ 0.05).

All analyses were performed using the PAST software, version 1.94b [START_REF] Hammer | PAST: palaeontological statistics software package for education and data analysis[END_REF].

Results

Vegetation structure

Mangroves on the study site are developed over a length of about 600 m with a classical zonation according to inundation and salinity gradients, i.e. from landward to seaward: a degraded Heritiera littoralis Dryand. stand at the upper limit of tidal influence, followed by a barren salt flat or "tanne" surrounded by old A. marina (Forssk.) Vierh. trees, a dense and a low C. tagal stand progressively mixed with R. mucronata individuals, a high and important R. mucronata stand including scattered patches of Bruguiera gymnorhiza (L.) Lam., and finally on the lagoon side a well-developed Sonneratia alba J. Smith zone.

Experiments were conducted in two mangrove facies, chosen for their representativeness and their important development in most mangrove stands in Mayotte, namely C. tagal and R. mucronata facies. Structures are described in Table 2. The C. tagal facies were largely dominated by the eponymous species, which represented 90% of the specific composition, with 9% for R. mucronata and a few individuals of A. marina in the upper part of the stand and rare B. gymnorhiza in the lower part. Total density is very high with 69,500 ind ha -1 and 62,750 ind ha -1 for C. tagal. C. tagal individuals are small trees with 2.2 ± 1.1 cm trunk diameter and 1.7 ± 0.9 m in height. The second facies is dominated by R. mucronata (79%) with C. tagal individuals in the upper part (16%) and patches of B. gymnorhiza (5%). Total density is lower, with 7900 ind ha -1 and 6250 ind ha -1 for R. mucronata. The mean trunk diameter for dominant individuals of R. mucronata is 16.1 ± 5.2 cm with a height of 7.1 ± 2.1 m.

It is important to note that the vegetation structure was analysed successively in November 2006, before the first sewage discharges, and in November 2008, 6 months after discharges began. No significant difference was observed within the period. We also noted that the vegetation structure had not changed after 12 and 18 months of discharges when functional analyses were made, in terms of density or mortality rates. Regeneration seems to be enhanced in impacted plots and density of canopy as well. Analyses are currently underway to quantify these processes.

Photosynthetic pigment concentration

Table 3 and Fig. 3 show the results of analyses of chlorophyll a and b, carotene, and xanthophyll pigments extracted from C. tagal and R. mucronata leaves sampled in control and impacted plots (January 2009, April 2009).

Pigment content appears to be significantly higher in plots having received wastewater than in control plots, for all pigment types, for the two dates analysed and for both C. tagal and R. mucronata stands, except for chlorophyll b, for which results are not significant for R. mucronata in January 2009.

Pigment concentration increased around twofold between C. tagal control and impacted stands and for the two dates, i.e. from 1.47 to 2.88 mg g -1 dw (January 2009) and 1.23 to 3.28 mg g -1 dw (April 2009) for chlorophyll a. In comparison, the increase in R. mucronata is significant but moderate, i.e. from 2.08 to 2.87 mg g -1 dw (January 2009) and from 3.01 to 4.01 mg g -1 dw (April 2009) for chlorophyll a.

In C. tagal plots, we note that differences between the two control plots and between the two impacted plots are not significant between January and April, for all pigments. For instance, changes were from 1.47 to 1.23 (control plots) and 2.88 to 3.28 mg g -1 dw (impacted plots) for chlorophyll a, 0.43 to 0.36 (control) and 0.79 to 0.87 (impacted) for ␤-carotene, and 0.07 to 0.06 (control) and 0.13 to 0.15 (impacted) for xanthophylls. Similar comparisons for R. mucronata, however, show significant increases between dates, with 2.08-3.01 (control) and 2.87-4.01 (impacted) for chlorophyll a, 0.58-0.82 (control) and 0.76-1.04 (impacted) for ␤-carotene, and 0.1-0.14 (control) and 0.14-0.19 (impacted) for xanthophylls. Photosynthesis rate appears significantly higher in plots receiving wastewater than in control plots, in April (6.14 vs. 9.86 mol m -2 s -1 ) and October (5.82 vs. 9.53 mol m -2 s -1 ) for C. tagal plots and in October 2009 only (8.68 vs. 10.66 mol m -2 s -1 ) for R. mucronata plots.

Photosynthesis and transpiration rates

Comparisons between species show that the photosynthesis rate is significantly higher in R. mucronata than in C. tagal, in both control (12.52 vs. 6.14 mol m -2 s -1 in April, 8.68 vs. 5.82 mol m -2 s -1 in October, respectively) and impacted plots (12.62 vs. 9.86 and 10.66 vs. 9.53) and for each of the dates considered. The photosynthetic rate also appears slightly but significantly higher for both species at the end of the wet season (April) than at the end of the dry season (October).

If we compare transpiration rates between control and impacted plots, measurements indicate significant differences for both species in April with higher values in plots receiving wastewater (3.95 vs. 2.47 mmol m -2 s -1 for C. tagal, and 4.1 vs. 3.53 mmol m -2 s -1 for R. mucronata, respectively), while the differences are not significantly different in October (3.64 vs. 3.65 for C. tagal and 3.27 vs. 3.64 for R. mucronata).

Growth rate measurements

Results for leaf (length, width, weight, surface area), branch and propagule (length) measurements after 12 and 18 months (April 2009 andOctober 2009) are presented in Table 5 and Fig. 5.

Except for R. mucronata measurements in April, leaf length and width and consequently leaf area are significantly higher for samples collected in impacted plots for both species and dates, i.e. for leaf areas, respectively 15.9 (control plots) and 37.9 cm 2 (impacted The leaves of both species are slightly heavier (dry weight) for both species, and leaf area-to-weight ratios are significantly higher in impacted conditions than in controlled ones.

Concerning branch length, all results are significant with important increases, i.e. 4.68-13.38 cm for C. tagal and 16.04-20.06 cm for R. mucronata in October. No significant seasonal change appeared in branch length from April to October for either species or plot conditions.

Finally, the impact of wastewater supply in mangrove plots significantly increased propagule length in both species, i.e. 16.4-32.1 cm for C. tagal and 32.1-39.1 cm for R. mucronata.

Discussion

Leaf pigment concentration in mangrove trees and wastewater effects

In natural conditions in Mayotte Island, we established that pigment concentration was significantly higher in R. mucronata than in C. tagal leaves, particularly for chlorophyll a, the trigger element of photochemical processes. Chlorophyll a:b ratios, considered to be a significant index of photosynthetic functioning, exhibit very stable and similar values for both species and dates, corresponding to val-ues given by [START_REF] Das | Studies on pigments, proteins and photosynthetic rates in some mangroves and mangrove associates from Bhitarkanika, Orissa[END_REF] for R. apiculata and B. gymnorhiza, and by [START_REF] Basak | Chlorophylls, carotenoids, proteins and secondary metabolites in leaves of 14 species of mangrove[END_REF] for R. mucronata and C. decandra. As noted by these authors, carotenoid content is very low in the Rhizophoraceae family, as we observed in Mayotte Island in natural conditions.

The supply of wastewater to mangrove plots enhances pigment concentration in mangrove leaves, with clear increases for chlorophylls, ␤-carotene and xanthophylls in both species. While no data were found in the literature directly concerning the relationships between pigment concentration in mangrove leaves and wastewater supplies, many authors have considered pigment concentration in relation to environmental factors. [START_REF] Medina | Osmolality and delta C-13 of leaf tissues of mangrove species from environments of contrasting rainfall and salinity[END_REF] established that chlorophyll content appeared to be higher in mangrove leaves of riverine mangrove stands and lower in leaves of mangroves from dry sites, and added that N and P leaf concentrations and leaf areas varied in the same way between dry and wet sites. Authors interpreted such results as interactions between salinity and water stresses, in relation to nutrient supplies and photosynthetic productivity. [START_REF] Macfarlane | Photosynthetic pigments and peroxidase activity as indicators of heavy metal stress in the grey mangrove, Avicennia marina (Forsk.) Vierh[END_REF] and [START_REF] Macfarlane | Leaf biochemical parameters in Avicennia marina (Forsk.) Vierh as potential biomarkers of heavy metal stress in estuarine ecosystems[END_REF] showed links between leaf chlorophylls (a + b) and carotenoid content of A. marina and heavy metal concentration in mangrove sediment. [START_REF] Ye | Growth and physiological responses of two mangrove species (Bruguiera gymnorhiza and Kandelia candel) to waterlogging[END_REF] examining the effects of waterlogging on growth and physiological characteristics of B. gymnorhiza and Kandelia candel (Rhizophoraceae), showed that chlorophyll and carotenoid concentrations increased when waterlogging duration and intensity increased. Wastewater supply to impacted mangrove plots in Malamani contributes both to lower salinity level -fresh water is added to the ecosystem -and to increased N and P levels. The average composition of sewage (Table 1) indicates the amount and the nature of nitrogen and phosphorus compounds delivered daily to mangrove plots. Moreover, we established that wastewater delivered to mangroves at low tide rapidly seeps into sediment and is progressively absorbed by vegetation, and that N and P compounds are at least partially used by mangrove trees [START_REF] Herteman | Evaluation des capacités bioremédiatrices d'une mangrove impactée par des eaux usées domestiques[END_REF]Herteman et al., submitted for publication). The change in the colour of the vegetation of impacted plots (Fig. 1) also reflects these processes and corresponds to the increase in leaf pigment concentration. As proposed by the authors cited above, pigment concentration may thus be considered a marker of stress conditions for mangrove trees, or a marker of change in mangrove functioning, revealing pollution with heavy metals [START_REF] Macfarlane | Photosynthetic pigments and peroxidase activity as indicators of heavy metal stress in the grey mangrove, Avicennia marina (Forsk.) Vierh[END_REF][START_REF] Macfarlane | Leaf biochemical parameters in Avicennia marina (Forsk.) Vierh as potential biomarkers of heavy metal stress in estuarine ecosystems[END_REF] or an excess of nutrient, as we demonstrated in our Malamani experiments. From a more general point of view, studies of pigment content in higher plants as biomarkers are rare, and essentially concerned micro-algae, where pigment content is directly linked to biomass [START_REF] Wilhelm | The HPLC-aided pigment analysis of phytoplankton cells as a powerful tool in water-quality control[END_REF]. [START_REF] Brain | Biomarkers in aquatic plants: selection and utility[END_REF], in a recent review on biomarkers in aquatic plants, indicated advantages for considering pigment content as a biomarker: it is an easy-to-measure and robust parameter and, furthermore, visual observation, as in our Malamani experiments, may preclude the necessity of measuring pigment content. These authors added that chlorophylls and carotenoids were the primary light-capturing pigments in higher plants, absorbing light energy for photosynthesis. Nutrient status, with light intensity or temperature, is one of the factors affecting the content of photosynthetic pigments. At high nutrient availability, and particularly with excess N, pigment content increases and enhances carbon fixation.

Photosynthetic processes

Pigment concentration is directly linked to photosynthetic activity, and photosynthetic rates and pigment content, i.e. chlorophyll a:b ratio, have been found to be correlated [START_REF] Anderson | Thylakoid membrane organization in sun/shade acclimation[END_REF][START_REF] Das | Studies on pigments, proteins and photosynthetic rates in some mangroves and mangrove associates from Bhitarkanika, Orissa[END_REF].

Measurements of photosynthesis rate in control plots in Malamani clearly indicate differences between species, with the highest values obtained in R. mucronata, where the highest pigment concentrations were also found. [START_REF] Theuri | Photosynthesis and related physiological processes in two mangrove species, Rhizophora mucronata and Ceriops tagal, at Gazi Bay, Kenya[END_REF] obtained similar results with higher values for R. mucronata than for C. tagal in mangrove stands in Kenya. Nevertheless, these authors globally found lower values in Kenyan mangroves than in Mayotte (around 1.5 and 1.2 mol m -2 s -1 for R. mucronata and C. tagal, respectively) and important seasonal variation, with twofold values in the wet season (around 4.0 and 3.0 mol m -2 s -1 , respectively) while seasonal changes in Malamani were not significant. Transpiration rate levels are also greater in Mayotte (2.45-3.65 mm m -2 s -1 ) than in Kenyan mangrove (0.78-0.94 mm m -2 s -1 ). [START_REF] Clough | Estimating leaf area index and photosynthetic production in canopies of the mangrove Rhizophora apiculata[END_REF] and [START_REF] Clough | Mangrove forest productivity and biomass accumulation in Hinchinbrook Channel, Australia[END_REF] found high values of photosynthetic rates for R. apiculata (average rate for the whole canopy: 9.0 mol m -2 s -1 ) and different Rhizophoraceae (6.13-12.9 mol m -2 s -1 ), with higher values for Rhizophora spp. and lower values for C. australis. These authors added that rates of photosynthesis may be substantially lower in mangrove stands characterised by higher aridity and salinity conditions with values around 4-5 mol m -2 s -1 .

In the mangrove stands of Malamani, wastewater supplies clearly contribute to increased photosynthetic rates in impacted mangrove plots as they lead to an increase in pigment concentrations. As we noticed above, wastewater contributes to lower salinity rates, enriches the mangrove ecosystem in N and P nutrients, and consequently enhances photosynthesis rate. [START_REF] Sobrado | Relation of water transport to leaf gas exchange properties in three mangrove species[END_REF] and [START_REF] Li | Effect of NaCl on photosynthesis, salt accumulation and ion compartmentation in two mangrove species, Kandelia candel and Bruguiera gymnorhiza[END_REF] indicated similar relationships between salinity conditions and photosynthesis processes for different mangrove trees including Rhizophoraceae, and [START_REF] Kao | Effect of NaCl and nitrogen availability on growth and photosynthesis of seedlings of a mangrove species, Kandelia candel (L.) Druce[END_REF] showed that an increase in N availability increased photosynthetic rates for the Rhizophoraceae K. candel. [START_REF] Li | Effect of NaCl on photosynthesis, salt accumulation and ion compartmentation in two mangrove species, Kandelia candel and Bruguiera gymnorhiza[END_REF] added that high levels of Na concentration in mangrove trees inhibited electron transport in photosynthetic processes and consequently led to a decrease in photosynthetic efficiency.

Mangrove growth rates and wastewater supplies

Increased mangrove growth rates (leaf dimensions and surface areas, branch length) observed in impacted plots at the study site are a direct effect of enhancement in photosynthesis rate and of the increase in leaf pigment concentration. The supply of freshwater and nutrients (N and P), particularly through wastewater discharges, is known to induce an increase in mangrove tree growth [START_REF] Clough | Mangroves and sewage-a re-evaluation[END_REF]) by acting as a fertiliser supply in the ecosystem [START_REF] Boto | Nitrogen and phosphorus nutritional status of a northern Australian mangrove forest[END_REF]. [START_REF] Onuf | Interactions of nutrients, plant growth and Herbivory in a mangrove ecosystem[END_REF] also observed that a Rhizophora mangrove stand naturally enriched with guano from a bird colony exhibited significant enhancement of growth. [START_REF] Clough | Mangroves and sewage-a re-evaluation[END_REF], analysing all these results, concluded that "nutrient enrichment from fertilization or from sewage effluent is not likely to be deleterious to mangroves, and may be beneficial where the nutrient status of the mangroves is low". [START_REF] Lin | Comparative study of water uptake and photosynthetic gas exchange between scrub and fringe red mangrove, Rhizophora mangle L[END_REF], and more recently [START_REF] Lovelock | Photosynthetic performance and resource utilization of two mangrove species coexisting in a hypersaline scrub forest[END_REF] and [START_REF] Lovelock | The effect of nutrient enrichment on growth, photosynthesis and hydraulic conductance of dwarf mangroves in Panama[END_REF], while analysing functional differences between scrub and fringe mangroves, established that CO 2 assimilation rate and photosynthetic efficiency seem to be lower in scrub facies, also characterised by high salinity levels. Conversely, fertilisation by N and P supplies may induce significant shoot growth in dwarf mangrove stands.

Recent papers, however, have emphasised potential negative consequences of excessive nutrient enrichment in mangroves. They established, for instance, that excessive N supply might induce changes in root-to-shoot ratio (development of shoots at the expense of roots) and increase the vulnerability of mangrove stands in high-saline environments [START_REF] Martin | Nitrogen fertilization enhances water-use efficiency in a saline environment[END_REF], or even lead to the death of mangrove trees in high salinity and low rainfall conditions [START_REF] Lovelock | Nutrient enrichment increases mortality of mangroves[END_REF]. In this last study, nutrient enrichment seems to have been through a single, massive supply annually or biannually, i.e. 300 g of urea or phosphate into holes cored on either side of the tree stems. Notice that these amounts correspond to the total amount of N provided to our impacted plots in a whole year, but delivered daily every second low tide to our experimental mangrove stands. The kinetics of absorption and assimilation of nutrients is then certainly different in the two cases, and thus the consequences on mangrove tree metabolism will be different as well. While no negative effect on mangrove vegetation appeared after 18 months of wastewater supplies in the mangroves of Malamani, we will still require continuing control experiments to assess the long-term efficiency of bioremediation through mangrove ecosystem. In another domain, Penha-Lopes et al. ( 2010) indicated, from mesocosm experiments, that sewage contamination caused disturbances to gastropod populations (Terebralia palustris) associated with mangrove trees. In the Malamani study site, preliminary results did not show any change in crab populations impacted by wastewater [START_REF] Herteman | Evaluation des capacités bioremédiatrices d'une mangrove impactée par des eaux usées domestiques[END_REF], but further experiments are planned to evaluate potential effects over a longer term.

Conclusions

The present study showed that domestic wastewater discharges induced important changes in mangrove vegetation. In particular, the wastewater:

• increased leaf pigment content in C. tagal and R. mucronata stands impacted with 12-18 months of daily supplies; • enhanced significantly photosynthetic activity and transpiration rate; and • induced significant increase in leaf area and branch length of impacted mangrove stands.

At the same time, no evident modification appeared in general structure or functioning of mangrove vegetation.

If our results seem to demonstrate that pretreated domestic wastewater may have beneficial effects on mangrove functioning, a survey of the literature nevertheless shows that N and P excess, brought through domestic wastewater or experimental supplies, could in certain conditions and over a long term induce dysfunctioning in mangrove vegetation.

Further experimentation and analyses are necessary before we can clearly define the possible role of mangrove ecosystems in bioremediation of domestic wastewater.

Such experimentation is currently in progress in the Malamani study site, taking into account the different compartments of the mangrove ecosystem and their interactions, i.e. salty and freshwater bodies, sediment, crab populations and the structure and functioning of the mangrove vegetation. While the preliminary results in this paper show that wastewater is effectively absorbed by mangrove trees and induces enhancement of mangrove tree functioning, global N and P balances must be established for better quantification. Another avenue of research, also in progress, is to improve wastewater treatment in the primary treatment unit before its discharge into mangrove stands.

Fig. 1 .

 1 Fig. 1. (a) The study site, aerial photography from ultralight. The two impacted plots are characterised by a strong green colour (white circles mark the upper limit of plots). (b) Colour changes in Ceriops tagal leaves between control and impacted plots. (c) Growth differences in Ceriops tagal branches between control and impacted plots. All pictures: March 2009, i.e. after 12 months of daily wastewater discharge.

Fig. 2 .

 2 Fig. 2. The study area and the experimental site. (a) Mayotte Island, SW Indian Ocean. (b) The study site, SW Mayotte Island, between Malamani village and the lagoon. (c) Experimental setting: decanter collecting domestic wastewater from Malamani; pipe network; impacted and control plots in Ceriops tagal and Rhizophora mucronata mangrove facies; piezometer network for water analyses.

  Table 4 and Fig. 4 summarise results for both parameters, for measurements made in April 2009 (end of wet season) and October 2009 (end of dry season).

Fig. 3 .

 3 Fig. 3. Pigment concentration in Ceriops tagal and Rhizophora mucronata leaves, collected in control and impacted plots, Malamani study site, January and April 2009. (a) Chlorophyll a. (b) Chlorophyll b. (c) ␤-Carotene. (d) Xantophyll. Unit: mg g -1 dw.

Fig. 4 .

 4 Fig. 4. Photosynthesis rate and transpiration rate measured on Ceriops tagal and Rhizophora mucronata leaves, in control and impacted plots, Malamani study site, April and October 2009. Unit: mol m -2 s -1 .

Table 1

 1 Nutrient composition (mg l -1 ) of domestic wastewater after pre-treatment in decanter. Analyses realised on July 02, 2009, SIEAM Laboratory (Mayotte); April 01 and October 10, 2009, ARVAM Laboratory (La Réunion Island).

		NO3	NO2	NO3 + NO2	NH4	PO4
	July 02, 2009	1.40	0.17	1.57	-	8.40
	April 01, 2009	1.09	0.01	1.10	1.18	5.61
	October 10, 2009	0.01	0.02	0.03	1.95	12.55

Table 2

 2 Structural analyses of mangrove plots, before wastewater discharge (November 2006).

	Facies	Species	Specific	Density	Dbh	Dbh	Height (m)	Height (m)	Basal area	Dead ind.
			dominance (%)	(n ha -1 )	(<10 cm)	(>10 cm)	(Ø < 10 cm)	(Ø > 10cm)	(m 2 ha -1 )	(n ha -1 )
	C. tagal	A. marina	0.7	500	5.8	-	2.8	-	2.8	250
		B. gymn.	0.3	250	3.2	-	0.6	-	0.4	0
		C. tagal	90.0	62,750	2.2	-	1.7	-	31.1	3500
		R. mucr.	9.0	6000	4.8	11.1	2.7	3.5	18.2	250
	R.	B. gymn.	5.0	350	3.9	21.7	2.2	5.8	0.29	50
	mucr.	C. tagal	16.0	1300	4.1	-	2.6	-	2.04	50
		R. mucr.	79.0	6250	6.4	16.1	3.7	7.1	71.5	200

Table 3

 3 Leaf pigment content (mg g -1 dw) of C. tagal and R. mucronata, in control and impacted plots January and April 2009.

	Ceriops tagal

Table 4

 4 Photosynthesis rate (mol m -2 s -1 ) and transpiration rate (mmol m -2 s -1 ) in leaves of C. tagal (n: 150) and R. mucronata (n: 120), in control and impacted plots, April and October 2009 (mean ± Sd).

		Ceriops tagal				Rhizophora mucronata
		April 2009		October 2009		April 2009		October 2009
		Control	Impacted	Control	Impacted	Control	Impacted	Control	Impacted
	Photosynthesis rate	6.14 ± 1.9	9.86 ± 1.52	5.82 ± 1.3	9.53 ± 1.35	12.5 ± 2.7	12.62 ± 2.39	8.68 ± 4.02	10.66 ± 2.98
	Transpiration rate	2.47 ± 0.94	3.95 ± 0.82	3.65 ± 0.89	3.64 ± 0.89	3.53 ± 1.11	4.1 ± 1.18	3.64 ± 1.11	3.27 ± 0.75
	plots) for C. tagal and 63.2 (control) and 89.5 (impacted) in October
	for R. mucronata.

Table 5

 5 Shoot length (cm), internode number and leaf number per shoot for C. tagal and R. mucronata, in control and impacted plots, April and October 2009 (mean ± Sd, n = 60).

		Ceriops tagal				Rhizophora mucronata
		April		October		April		October
		Control	Impacted	Control	Impacted	Control	Impacted	Control	Impacted
	Shoot length	4.68 ± 2.5	13.38 ± 4.26	5.01 ± 3.44	12.82 ± 1.5	16.04 ± 2.11	20.06 ± 2.63	9.16 ± 6.19	8.88 ± 6.02
	Internodes number	1.3 ± 0.5	3.3 ± 0.9	2.1 ± 0.7	3.3 ± 0.9	3.7 ± 1.4	2.8 ± 0.5	17.3 ± 1.2	21.2 ± 2.0
	Leaf number per shoot	8.6 ± 5.5	9.6 ± 7.3	-	5.2 ± 1.3	5.9 ± 1.5	5.3± 1.0	4.7 ± 1.7	4.2 ± 1.1

Acknowledgements

This work was part of a programme on the role of the mangrove ecosystem in wastewater treatment in Mayotte Island, co-funded by the Water Syndicate of Mayotte (SIEAM, 2006(SIEAM, -2010) ) and the French National Research Centre (CNRS) through the Ecological Engineering Programme (2007).

The first author is a doctoral researcher funded by the Association Nationale de la Recherche Technique (ANRT) through the CIFRE Grant 250/2006.


Chantal Keller
email: chantal.keller@inria.fr

Jérémy Bluteau

Renaud Blanch
email: blanch@imag.fr

Sabine Coquillart
email: sabine.coquillart@inria.fr

Pseudo-Weight: Making Tabletop Interaction with Virtual Objects More Tangible

Keywords: Tabletop interaction, pseudo-weight. ACM Classification: H.5.2 [Information Interfaces and Presentation (e.g., HCI)]: User Interfaces -Graphical user interfaces, Input devices and strategies

In this paper we show that virtual objects manipulated on a tabletop interaction device can be augmented to provide the illusion they have a weight. This weight offers a supplemental channel to provide information about graphical objects without cluttering the visual display. To create such a pseudo-weight illusion on a passive device, the pressure applied with the fingers during the interaction has to be captured. We show that this pressure can be estimated without hardware modification on some touch sensitive tabletop setups (e.g., MERL's DiamondTouch). Two controlled experiments show that pseudo-weight is perceived effectively. The first one demonstrates that users, without training and without previous knowledge of the system, can accurately rank virtual objects according to their pseudo-weights, provided they are sufficiently distinct. The second controlled experiment investigates more formally the relation between the pseudo-weight and the actual perception of the users.

INTRODUCTION

Over the past few years, tabletop interaction turned from prototypes into commercial products (e.g., the MERL Diamond-Touch [START_REF] Dietz | DiamondTouch: a multi-user touch technology[END_REF], or the Philips Entertaible [START_REF] Hollemans | Entertaible: Multi-user multi-object concurrent input[END_REF]), and the software editor Microsoft Corp. has coined the term "surface computing" to refer its first touch-enabled products: the Surface table from 2007. These systems are significant steps towards the merge of the digital and physical worlds envisioned twenty seven years ago by Krueger et al. [START_REF] Krueger | VIDEOPLACE-an artificial reality[END_REF].

However the illusion of manipulating real objects on a table

is not yet complete. Some physical properties such as tactile and haptic feedback are still missing when users are interacting with virtual objects. To provide feedback through these mechanical perception channels, the output device would have to be actuated. The hardware involved would be more complex, consume more power, and be more prone to failure.

In this article, we investigate the psychophysics of a software technique that creates the illusion of a weight for virtual objects. This weight is a pseudo-perception: the channel by which the feedback is provided is not the regular one (tactile or haptic) but a substituted perception channel (namely vision) coupled to a passive (force) sensor.

We first review previous works related to our research. Then we present how we have implemented the pseudo-weight on a tabletop interaction device. Two experiments are then reported: the first one demonstrates that users perceive the pseudo-weight and can use it to accurately order virtual objects. The second experiment characterizes more formally the relation between the pseudo-weight assigned to objects and its perception by the user.

RELATED WORK

Previous works have introduced the notion of pseudo-haptic feedback: haptic properties such as stiffness, friction, or feeling forces, bumps or holes, can be simulated without haptic devices. Pseudo-haptic friction/stiffness feedback was initially obtained by coupling a force sensor with a perturbed visual feedback [START_REF] Lécuyer | Pseudo-haptic feedback: Can isometric input devices simulate force feedback?[END_REF]. Sugimoto et al. and Lecuyer et al. proposed to simulate forces and bumps or holes by simply varying the control to display ratio of the cursor [START_REF] Sugimoto | Enhancement of virtual interaction devices using pseudo force sensation[END_REF][START_REF] Lécuyer | Feeling bumps and holes without a haptic interface: the perception of pseudo-haptic textures[END_REF]. With tabletop interaction, such techniques can not be used because of the colocation of the cursor and the finger.

Early work on using the pressure on a touch sensitive device dates back to the eighties (e.g., [START_REF] Buxton | Issues and techniques in touch-sensitive tablet input[END_REF]). But even recent works (e.g., [START_REF] Benko | Precise selection techniques for multi-touch screens[END_REF][START_REF] Ramos | Pressure marks[END_REF]) use the pressure as a supplemental input channel only. The coupling of the pressure with a tactile feedback is investigated by Rekimoto & Schwesig [START_REF] Rekimoto | PreSenseII: bi-directional touch and pressure sensing interactions with tactile feedback[END_REF], but they used an actuated device to provide the feedback. Physical constraints on tabletop interaction is investigated by Patten & Ishii [START_REF] Patten | Mechanical constraints as computational constraints in tabletop tangible interfaces[END_REF], but with mechanically enforced constraints. More recently, physical models have been used to add plausible collision and friction [START_REF] Hilliges | Interaction on the tabletop: Bringing the physical to the digital[END_REF] but without using the pressure as input. ShapeTouch [START_REF] Cao | ShapeTouch: Leveraging contact shape on interactive surfaces[END_REF] explored the design space for interaction using virtual forces but does not deal with pseudoweight.

PSEUDO-WEIGHT

In order to substitute a sensory channel with a pseudoperception, we have to understand the actual perception. For doing so, we have observed users moving real objects with different weights (e.g., sheet of paper, books, etc.) on the surface of a table using one finger. The actual physics in- volved is surprisingly complex, drawing upon tribology, the science of friction. In our particular case, we observed two distinct modes: either the pressure on the object is sufficient, the object sticks to the finger and follows its movements, or the pressure is not sufficient, and the object sticks to the table and does not move regardless of the finger movements.

To simulate this behaviour, we have solved three problems, two of them were identified a priori while we did not anticipate the third one:

• capture the pressure applied with the finger to the table during the interaction (technological challenge);

• create the pseudo-weight illusion by simulating the behaviour of the virtual objects according to this pressure and to the weight assigned to them (design challenge); and

• provide insights that the system is reliable to avoid misinterpretation of an immobile object (design challenge).

Capturing Pressure

To conduct this study, we have used a MERL DiamondTouch coupled with a video projector. The DiamondTouch does not include any pressure sensor but we found that pressure can be estimated from the raw data captured for finger tracking. Fingers position is achieved by detecting variations of the capacitance caused by the fingers in an array of antennas embedded in the table [START_REF] Dietz | DiamondTouch: a multi-user touch technology[END_REF]. We observed that the capacitance varies according to the surface of the finger in contact with the table. In turn, this surface varies according to the squeeze applied to the table. Since the DiamondTouch API gives access to the row values read from the antennas (Figure 1), we have access to a quantity that varies according to the pressure. This quantity is not a real measure of the pressure, but it allows us to create the pseudo-weight illusion as the experiments described below show (measuring the real relation is not an option because placing a force sensor between the finger and the table disturbs the capacitance measure).

The DiamondTouch uses two orthogonal arrays of antennas that produce two monodimensional images of the table projected on the x-axis and y-axis. These images consist of 128 (resp. 96) values measured periodically by the vertical (resp. horizontal) antennas. Figure 1 shows the vertical antennas that give a projection of the table on the horizontal axis. The reported values are integers ranging from 0 to 255. We consider the x-axis array only since it provides sufficient data to compute the pressure. We define the pressure, in arbitrary units, as the sum of the values that pass a threshold of 10. This threshold is necessary because the signal reported by the antennas is noisy, and is never a true 0 even in the ab-sence of contact on the table. The sum performs a spatial integration of the signal so as to cover the whole contact surface (the spatial resolution of the antennas grid is 5 mm, and a finger typically activates 4 successive antennas).

Creating the Pseudo-Weight

Based on our preliminary observations, we have implemented the pseudo-weight by adding a weight attribute to each draggable object. At the beginning of an attempt to drag an object (when a finger touching the table enters an object) the current pressure is compared to the weight of the object. The object starts to move only when the pressure exerted exceeds its own weight. This makes heavy objects hard to move whereas light objects are easily dragged as in real life.

Pressure is monitored continuously during dragging. Dragging ends when the pressure falls below the weight of the object reduced by a constant. This constant is introduced so that the noise in the signal does not affect the interaction. It also models the hysteresis observed in the physical world known as static friction: friction forces are stronger when the object is immobile than when it has started to slide.

The objects are thus made sensitive to the pressure: according to their weight, the user has to drag them more firmly if they are heavier. Since more effort is needed to move the heavy object, the user should perceive it and associate this perception to a physical attribute of the virtual objects.

Providing Visual Feedback of the System Reliability

When we first confronted naive users to the system previously described, some of them did not perceive anything.

They interpreted the fact that some objects did not move when they tried to drag them as a malfunction of the finger tracking. We did not anticipate that our users are used to tactile systems, like automatic dispensers, that often miss user inputs. We solved this issue by providing an additional feedback: a circle is projected at the position of the tracked fingers. Since the finger tracking is very robust, these circles follow the fingers in real time without noticeable lag. This makes the user unconsciously aware of the reliability of the tracking system and re-enables the illusion.

EXPERIMENTAL VALIDATION

To test our hypothesis, we have conducted two experiments. 12 master students volunteered to take part in this study, they all did both of them. The first experiment is designed to check that the pseudo-weight is perceived and that it can be used as an output channel for a digital table. The second experiment is designed to explore more deeply the relationship between the pseudo-weight and its perception.

Proof of Concept

Our initial informal experiments convinced us that the pseudo-weight was actually working, but we wanted to evaluate if (and how) a user would perceive the effects of the system if she is not aware of the system internals. The participants were only told that they "would take part in an experiment designed to evaluate a new interaction technique on a digital table". Each one was familiarized individually with finger interaction on the tabletop display by using a non pseudo-weight enhanced photo-shuffler application until she was able to manipulate the photos easily. Setup When the participant is ready, the experiment begins. A text displayed on the table invites him to sit conformably and provides the following instructions: "In the next screens, 4 squares looking identical will be presented together with 4 places to put them. The squares behave differently while interacting with them. You can try to manipulate the square below."

Four draggable squares with different weights are placed at this place. The instructions then continue:

"Please move the squares into the places, and arrange them from left to right in growing order. When you are done, press the next button to start a new task."

As our goal was to test if the pseudo-weight is perceived by untaught people, the text was intentionally left vague and did not explicitly state what is the difference between the squares nor which criterion should be used to sort the squares.

When the participant depresses the go button, a series of 10 trials starts. Each one presents the same visual aspect: 4 blue squares grouped in front of the user and 4 places horizontally aligned on the top of the screen (see Figure 2 left). For each trial, the weights of the square are evenly distributed between 2 random extremal values. These values are constrained in a range compatible with the interaction determined a priori.

Results After the tests, the participants were asked to name the criterion they used to order the squares. Two of them did not notice any effect and could not find a name, but the 10 others cited: "force", "friction", "power" or "surface". Those terms are all related to the action of the user rather than to a property of the objects. When we suggested "weight" as a characteristic of the objects, the participants agreed upon it. We choose this term because it also has an abstract meaning.

Despite involving only 120 trials, the qualitative results are interesting. All of the 10 participants who felt something ordered spontaneously the objects in ascending weights from the left to the right: the object on the left is lighter than the one on the right in more than 70% of their trials. When considering all the participants, the heaviest object is correctly placed 66.67% of the time, the lightest one 60%. There is more confusion for the average objects: the second heaviest (resp. lightest) object is correctly placed 54.17% (resp. 51.67%) of the time. The whole 4 objects are positioned in the correct order 39.17% of the time.

These results depend highly on the difference between the weights of the objects presented simultaneously. If we redo the same analysis considering only the half of the data produced by the trials where the weights were the most separated, the order is correct 48.33% of the time. The heaviest (resp. lightest) object is correct 81.67% (resp. 68%) of the time (61.67% and 58.33% for the two other objects).

These results show that users perceive the pseudo-weight without being taught and that some information can be transmitted through this channel. It also shows that the more the weights are separated, the more they are discernible.

Controlled Experiment

The goal of the second experiment is to better understand the perception of the pseudo-weight.

Setup The experiment is roughly similar to the previous one, but only two squares are presented to the participant at each trial, and a directed axis is displayed at the top of the screen (Figure 2 right). The axis is depicted to the user as an axis "where the lightest object should be on the left side, and the heaviest on the right side". The participant is then instructed to place the two squares on the horizontal axis according to their respective weights.

Two factors are used to determine the weights of the squares: the median weight (w) and the difference between the two weights (∆w). The two variables have three possible values: w 0 = 80, w 1 = 95, w 2 = 110 (arbitrary units); and ∆w 0 = 4, ∆w 1 = 8, ∆w 2 = 12. Nine possible couples of weights are then possible (w i ± ∆ j /2, (i, j) ∈ [0, 2]2 ). To balance order effects, a pseudo-random series of 81 trials consisting of 9 time each couple was constructed using a 9×9 latin square.

Results

The final abscissas of the light and heavy squares (x l and x h ) are recorded and two dependent variables are computed: the median abscissa around which the two objects have been positioned x = (x l + x h )/2; and the signed deviation that separates them ∆x = x h -x l .

We first test the hypothesis that the squares are placed in the right order (i.e. ∆x > 0). As in the first experiment, there is a lot of inter-subject variability: the least effective participant is only slightly better than random (55,56% of correct ordering), whereas the highest success rate was 90,12%. On average, the success rate is 68.52 ± 9.56%1 . A repeated measure analysis of variance (ANOVA) shows that the only significant effect on the success is the difference between the weight of the two objects ∆w (F = 10.69 2 ).

The mean position x also depends highly on the user. A repeated measure ANOVA shows that the main factor affecting the position x is the weight w (F = 175.12). Figure 3 shows x as a function of w with the means connected. They form a quasi-straight line (r 2 > 0.998 for the linear fit). Similarly, ∆x is essentially a function of ∆w (F = 11.62). The linear fit is also very good (r 2 > 0.98).

These results show that the discriminability of two pseudoweights is directly linked to the amplitude of the difference separating the weights, which was expected. A more notable result is that the mapping between the weight assigned to the objects and the position where the participants put them is linear. It means that the arbitrary weight scale created by our software implementation of the pseudo-weight is the same as the psychophysical scale the user has modeled after her experience of the real world. This result validates a posteriori our definition of the pressure as a function of the capacitance signal, and the physical model on which we have based the pseudo-weight implementation.

CONCLUSION & FUTURE DIRECTIONS

We have shown that virtual objects can be augmented with a pseudo-weight on a passive touch sensitive surface provided that pressure can be monitored. We have shown that on a regular DiamondTouch table, this information can be deduced from the raw capacitance. We have shown that this input channel, coupled with an adequate visual feedback, can provide a pseudo-perception. In addition, pseudo-weight conveys information: our experiments show that it allows users to rank objects which are visually identical.

The technique has some weaknesses. The first one is that the real tactile signal is contradictory with the simulated behaviour: the real object below the finger (the table) is immobile, even when the virtual objects is dragged. This limits the palpability of the illusion, but this limitation seems impossible to overcome without introducing tangible objects [START_REF] Patten | Mechanical constraints as computational constraints in tabletop tangible interfaces[END_REF] or active feedback [START_REF]3D-Press: haptic illusion of compliance when pressing on a rigid surface[END_REF]. The second weakness is that the perception scale depends on the user. Several factors affect the range of pressure computed from the capacitance: a different finger size or a dryer skin changes the capacitance while the same real pressure is exerted. A sensor really responsive to the pressure would solve this problem.

This technique is nonetheless promising since it offers a supplemental channel to provide information about graphical objects without cluttering the visual display. It can be used to transmit weights in a broad sense i.e. any attribute that can be mapped on a continuous scale (e.g., for an icon, the size of the file it represents). We have observed that, contrarily to other interaction techniques involving a smart coupling in the perception-action loop, the pseudo-weight is noticeable for spectators of the interaction. They easily interpret the visual feedback provided by the degree of flexion of the finger as it is totally natural. This property is valuable since tabletop devices are often used for collaborative work.

In the future, we would like to study how the pseudo-weight could be exploited in collaborative environments (e.g., making objects harder to move if they are in someone else focus could improve awareness of such situations). On a more theoretical side, studying how the visual clues (e.g., size, shape or texture of the objects) interfere with the pseudo-weight could provide interesting results.

Figure 1 :

 1 Figure 1: Typical signal reported by the MERL Dia-mondTouch antennas when a finger touches the table.

Figure 2 :

 2 Figure 2: Experimental setups. (left) first: 4 places at the top, 4 blue squares to order at the bottom; (right) second: 2 squares to place on an axis.

Figure 3 :

 3 Figure 3: Position vs. weight (diamonds show 95% confidence intervals, box plots .25 and .75 quantiles).

m ± σ : the mean (m) and standard deviation (σ ) across users.

Reported F statistics all have a p value smaller than 0.0001.

http://gil.imag.fr, GIL: a Toolkit for Novel Human-Computer Interactions.

ACKNOWLEDGEMENTS

The authors would like to thank F. Bérard for the GIL toolkit 3 used to conduct the experiments.