N Groga
email: groga7@yahoo.fr

A Ouattara

A Koulibaly

A Dauta

C Amblard

P Laffaille

G Gourene

Dynamic and structure of phytoplankton community and environment in the lake Taabo (Côte d'Ivoire)

Keywords: Phytoplankton community, diversity, structure, nutrient, environmental factors, pollution

This study evaluated some parameters of Water quality and phytoplankton growth in order to ascertain its seasonality in the lake Taabo (South East of Côte d'Ivoire). Seasonality was mainly due to significant variations in parameters such as conductivity, turbidity, transparency and nutrients (mean values of nitrate and orthophosphate were respectively equal to 1.62 and 10 mg/L). The lake downstream presented some features: oxygenation and mineralization were higher than upstream, at the time. Chlorophyll a ranged from 4.8 to 16.5 mg/L. For algal species composition, 118 taxa distributed into 5 main groups and 45 genera were identified. The algal assemblage was dominated by Chlorophyceae (30.5%), followed by cyanobacteria (27.1%). Conditions prevailing in this biotope favoured a massive phytoplankton growth, dominated by Chlorophyta and Cyanobacteria. Spatial and temporal analysis of nutrient and the composition of phytoplankton community revealed a seasonal variability in the ecosystem of the lake of Taabo which was under anthropogenic stress. This hydro system is growing towards a dystrophic state, thus implies an urgent need for restoration, followed by rehabilitation and process of elimination of solid materials in the lake catchment.

INTRODUCTION

Lake ecosystems are not only fresh water, but they are also tourist attractions and fish can stimulate regional economies [START_REF] Burton | Le plus grand lac du Mexique peut-il être sauvé ?[END_REF]). Given the current population explosion, we realize that fresh water resources are finite, and human activities are one of the major causes of stress in aquatic ecosystems [START_REF] Vasquez | Status of the health conditions of subtropical Atezea lake[END_REF][START_REF] Dokulil | Anthropogenic impacts to large lakes in China: the Tai Hu example[END_REF][START_REF] Tazi | Impact de la pollution sur l'unique réseau hydrographique de Casablanca[END_REF]. Around the world, many water bodies are thus irreversibly damaged by pollution and/or eutrophication, the most vulnerable are those located nearby large human settlements [START_REF] Zohary | Interannual phytoplankton dynamics of a hypertrophic african lake[END_REF].

The consensus seems to emerge on the role played in the process of eutrophication by phosphorus [START_REF] Lacaze | L'eutrophisation des eaux marines et continentales[END_REF], but the vulnerability of established tropical lake ecosystems calls for greater vigilance and hydrobiological investigations investigations in that region to look for factors that may be specific.

Phytoplankton communities consist of assemblages of species with different morphological (size, shape ) and physiological (nutrition mode, reproduction) characteristics and whose organization is a key thing in the understanding of the dynamic of any ecosystem.

During a seasonal cycle, the phytoplankton composition follows the general trends, as described by [START_REF] Sommer | The PEG model of seasonal succession of planktonic events in lakes[END_REF]. These trends vary in response to changes in environmental conditions such as temperature, light, and nutrients [START_REF] Reynolds | Vegetation Processes in the Pelagic. A Model for Ecosystem Theory[END_REF][START_REF] Grover | Seasonal dynamics of phytoplankton in two warm temperate reservoirs: association of taxonomic composition with temperature[END_REF]; and various biological interactions namely grazing, parasitism, viral lyses [START_REF] Griffin | Zooplankton grazing dynamics: top-down control of phytoplankton and its relationship to an estuarine habitat[END_REF]; [START_REF] Brussaard | Optimization of procedures for counting viruses by flow cytometry[END_REF][START_REF] Gleason | The Influence of Patient and Companion Cure expectations on treatment decisions[END_REF]. At tropical lake reservoir scale, a main variable may be important for hydrological seasonal variations. That is likely to play a major role in the stability of the water column, which can have a significant impact on the composition of the phytoplankton community.

Wind is also an environmental factor that can significantly alter the physical structure and form of the reservoir and by the same exerts forcing on the control of phytoplankton dynamics [START_REF] Pannard | Phytoplankton species turnover controlled by physical forcing at different time scales[END_REF]. Wind speed and direction that induce movements of water mass and the current that is generated by the arrival of water in the inlet channels may also be responsible for the horizontal heterogeneous distribution of phytoplankton [START_REF] Moreno-Ostos | The spatial distribution of different phytoplankton functionnal groups in a Mediterranean reservoir[END_REF]. The human activities is dominated by agriculture and the influence of anthropogenic activities on phytoplankton was studied in the characteristic zone of Côte d'Ivoire near the Lamto region [START_REF] Koulibaly | Vegetation characteristics and changes under cash crop cultivation in forest-savanna mosaics in Côte d'Ivoire[END_REF], the Taabo region where, the invasion of water hyacinth has resulted in the progressive eutrophication of the lake, leading to major problems for water use during the last ten years. Véi 2005 revealed the invasion of the lake of Taabo by Macrophytes which was described by [START_REF] Kouamé | Macroinvertebrate communities associated with macrophyte habitats in a tropical manmade lake (Lake Taabo, Côte d'Ivoire)[END_REF] The aim of this study was to investigate the influence of anthropogenic nutrient inputs, both on water quality and phytoplankton community in the Lake of Taabo, which is located in a tropical climate characterized by two rainy seasons and two dry seasons; and relatively high temperatures throughout the year. This study pointed out the species composition and spatial and temporal variations of phytoplankton with respect to environmental physicochemical variables, which may account for the level of degradation of the hydro system.

METHODS

Study site

The lake of Taabo (6° 25'-56' N and 5° 07'-33'W; Figure 1) is the regulating lake of the Kossou dam. It is located downstream from the confluence of the White Bandama and the Marahoué (or Red Bandama). The lake of Taabo has a catchment area of 58.700 km². The lake covers an area about 80 km², and is generally subjected to a daily fluctuation of the water level about 3 meters depending on the functioning of Kossou dam and wind stress. A hydroelectric reservoir was installed on the lake since 1975 (Table 1) what explains the presence of inhabitants in the vicinity of the lake which is estimated to 140.000 inhabitants. The main activity in the region is agriculture occupying 85% of the land in the catchment area of the lake reservoir, and it uses large amounts of agrochemicals.

Sampling

Water samples were collected during five campaigns from July 2006 to June 2007, using an opaque PVC Van Dorn-type bottle with a capacity of 2.5 L, mounted horizontally in order to maintain the sampling depth. Samples were collected through the water column at depths as follow: subsurface 40 cm, 1 m, 2 m, 3 m, and 4 m. Six stations were defined taking into account the succession of aquatic habitats and human activities from upstream to downstream (Figure . 2): furthest upstream, the reference station Sahoua (S1), the upper flow at Ahondo (S2), the deep segment of the left bank of the lake at Courandjourou (S3), an agricultural zone at Taabo-village wharf (S4), an urban zone on the right bank at Taabo-City (S6), and finally the furthest downstream at the Taabo-village (S5). A GPS MLR SP 12X satellite navigator was used for the location of the stations. Water transparency was measured in situ, from the depth (Zs) at which the Secchi disk disappeared. Since 15% of the incident light reached this depth, the depth of the euphotic layer (Zeu) could then be evaluated using the following equation: Zeu = 2.43 x Zs [START_REF] Wetzel | Limnological analysis[END_REF]. Measurement of temperature (°C), conductivity (µS.cm -1 ), total dissolved solids (TDS; mg.L -1 ), dissolved oxygen (DO; mg/L), pH and the turbidity (NTU) of water were carried out on the field, using a multiparameter WTW 340i instrument.

Nutrients analysis

Water samples were analysed in the laboratory in order to determine the concentrations of ammonia (mg/L of NH4 + ), nitrate (mg/L of N-NO3), nitrites (mg/L of N-NO2), phosphate (mg/L of PO4 3-) and silica (mg/L of SiO2 -) [START_REF] Wetzel | Limnological analysis[END_REF]. Samples were filtered through Whatman GF/C fibreglass filters (25 mm diameter). Then, in accordance with standard operating procedures (APHA, American Public Health Association, 1989), the concentrations of these parameters were

Biological analysis

For Chl a concentration measurement, 250 ml of each river water sample was passed through glass fibre filters of 0.7 µm size (Whatman GF/C, diameter of 25 μm) no later than 10 hours after collection. Because sampling was carried over 2 days time, filters were stored at -80°C for 48h, following which, chl a was extracted in 97% methanol at 75°C. The extinction of extracts was measured at 665 nm and 750 nm before and after HCL addition. The chl a content was calculed according to [START_REF] Herbland | Size structure of phytoplankton in the Equatorial Atlantic Ocean[END_REF].

Samples for the determination and counting of phytoplankton were collected near the surface and at the different depths. These samples were fixed in formalin 5% (neutralized with borax). These samples previously agitated, settled in sedimentation chambers of 25 ml capacity. Microscopic observation was then performed on an Olympus AX70 microscope, using the method developed by [START_REF] Utermöhl | Zur Vervollkommnung der quantitativen Phytoplankton-Method[END_REF]. The counting was performed using an inverted microscope. Counts were conducted on two orthogonal bands using the objective (X40). The number of counted cells was reported to the whole surface of the chamber.

Phytoplankton composition and structure were assessed by the taxonomic richness (the number of taxa), diversity (the Shannon-Wiener index, Ish = -Σ ((ni/N) × log2 (ni/N))), uniformity (Pielou index, E '= H'/log 2 S) [START_REF] Magurran | Ecological diversity and its measurement[END_REF]) and abundance. However, our analyses were carried out on the standardized data. To obtain such results, data were compiled and passed to the density using the formula of the total number N of phytoplankton cells contained in one liter of sample: N = (n * S/s) x (1000/V) n: Number of cells counted in the two bands, S: total area of the settling chamber, s: surface of the two bands, V: volume of the settling chamber. Taxa identification was possible by using keys, monographs and books ( Bourrelly, 1966( Bourrelly, , 1968( Bourrelly, , 1970;;[START_REF] Streble | Das Leben im Wassertropfen, Frankhische Velagsshandlung[END_REF][START_REF] Carter-Lund | Freshwater Algae. Their microscopic world explored[END_REF][START_REF] Ouattara | Les micro-algues de deux cours d'eau (Bia et Agnébi) de Côte d'Ivoire[END_REF][START_REF] Desikachary | Cyanophyta[END_REF][START_REF] Hendey | An introductory account of the smaller Algae of british coastal waters. Part V : Bacillariophycae (Diatoms)[END_REF]Compere, 1967. and Krammer and Lange-Bertalot;1986[START_REF] Magurran | Ecological diversity and its measurement[END_REF], 1991a, 1991b.

Data analysis

R package available at the following address: http://cran.r project.org was run for statically analysis. Variations of physical, chemical and biological parameters were tested by both Kruskal-Wallis and Wilcoxon tests for significant differences between the selected stations with respect to depth of the water and seasons. The spatial and temporal analysis were carried out through a normalized principal component analysis (PCA) processing analyzes between the groups (Doledec and Chessel 1987) to test the significance of the effects of three factors on the multidimensional model. The test procedure was based on 9999 random permutations of the rows of data file [START_REF] Manly | Randomization and Monte Carlo methods in Biology[END_REF].

RESULTS

Physical and chemical features of the Lake of Taabo

Physical and chemical parameters of water quality of the lake of Taabo varied significantly from the surface of the water to depth (Wilcoxon test, p <0.05). Thus, water transparency ranged from 0.56 to 2.1 m. The lowest values of transparency were measured during the rainy season (June 2007 andJuly 2006) and the highest in dry seasons (December 2006 andMarch 2007).

Water temperature ranged from 26 to 32°C. The spatial and temporal fluctuations of water temperature showed a slight cooling near the lake floor. However, no remarkable thermal stratification was noted during the study time even if that cooling was important throughout the water column in October 2006 and March 2007. The pH values were measured between 6.55 and 7.85, indicating slightly acid waters and alkaline temporarily. Conductivity as a whole was measured between 73 and 143 μS.cm -1 . These values varied slightly from the surface of the water to 2 m of depth. Then, there were significant elevated values, leading to an accumulation of conductive elements in the deep water masses. Vertical profile of dissolved oxygen decreased throughout the year. Unlikely the surface water bodies which showed well marked super saturations. Deep waters instead exhibited strong deficit in dissolved oxygen so that the rate of oxygen saturation that characterizes the critical biological was reached at a depth of 2 m. During the dry seasons, it was not uncommon to meet anoxia at 3 m of depth. Only sporadically heavy rains cause slight elevation of dissolved oxygen concentrations at that depth. Concentrations of ammonia had undergone large temporal variations and increase with depth (0.03 a 3.05 mg.L-1). Vertically, relatively poor surface layer (up to 2 m) could be distinguished from a deep richer zone (4 m). This distinction was rarely noticeable during the rainy season when the lowest values were generally measured. Its occurrence was mainly regulated by changes in dissolved oxygen levels at these depths, which displayed the significant negative correlations (p <0.05) between ammonia and the percentage of oxygen saturation. In raw water (unfiltered), concentrations of total dissolved phosphorus ranged from 0.01 to 54 mg/L. The highest levels were encountered during the rainy seasons (June and July). Their vertical distribution showed negative trend which was more pronounced during the rainy seasons. This would denote its dilution in the lake at the sediment-water interface. Nitrate concentrations were measured between 0.12 and 3.81 mg/L. The levels of nitrite (NO2 -) fluctuated between 0.02 and 2.19 mg/L. Elevated levels of that variable were recorded during the rainy season. Its vertical distribution presented an orthograde profile. Silica concentrations ranged from 3.41 to 46.81 mg/L. The high nutrient concentrations were measured at stations S4, S6, and S5 located in the enclosing channel of the lake, while low values were noted at S1 and S2 which are upstream of the lake. Intermediate values recorded at station S3 of the lake showed the purifying role played by the water. These values varied slightly from the surface to depths. Chlorophyll a concentrations were generally quite high and ranged between 0.84 and 16.52 μg/L. Spatial and temporal variations of Chlorophyll a concentrations revealed that the highest values were located at 0.5 m depth, while the lowest values were measured at 4 m depth. That feature revealed two periods of high concentrations in chlorophyll a in the lake ecosystem. The first occurred from December 2006 to January 2007 and the second period started in March to end in May 2007. These high chlorophyll growths were consecutive to intense multiplication of volvocales Eudorina elegans and Pandorina sp. The start of the rainy season significantly reduced their biomass. Thus, physical and chemical features firstly appeared to be strongly influenced by seasons, and secondarily, accordingly to stations (Table 2 andFigure. 2). Variations recorded in depths showed minor heterogeneity of the water column.

Analyses of phytoplankton concentrations

Qualitative analysis

Analyses of composition of the lake water revealed the identification of 118 taxa distributed in 45 genera from which only some were periodically and particularly dominant at certain stations. Taxa were divided into the five following classes: 36 Chlorophyta (Chlorophyceae, and Conjugatophyceae), 30 Cyanophyceae, 28 Diatoms, 17 Euglenophyceae, 6 Dinoflagellates, 1 Chrysophyte (Table 3).

The percentage of the latter group did not exceed 1% of the total density recorded at each station, thus was neglected in the different graphs representation. Chlorophyta were dominated by the subclass of Desmids (over 15% of total species), mainly Closterium strigosum species, Closterium gracile and Staurastum volans. However, these species represented an important part of the phytoplankton biomass. On the contrary, the Centrophycideae were always more abundant despite the low number of observed taxa, among the species that have most marked the various developments of Chlorophyta, we quoted:

-Eudorina elegans, with exceptional growth in October, was regularly present in the waters of Lake of Taabo, but at relatively low densities. Its growth primarily occurred during the campaign in December and March, when it reached approximately 30% of the total density (80.10 6 cells/L) at station S4 (Figure . 3).

Staurodesmus subulatus is being common species in the neritic plankton and freshwater environments. S. Subulatus bloom primarily occurred in December, with a maximum at station S6 and S4 in March, with a rate exceeding 35% of the total density. Cyanophyceae showed high species richness, compared to that of Chlorophyta, and was always expressed as the dominant group in the lake. The genus Scenedesmus, in particular Scenedesmus quadricauda, was detected in 60% of sampling stations (Figure . 3).

Cyanophyceae (Chroococcales), represented by five species of Microcystis, never appeared as a dominant class. However, the genus Microcystis aeruginosa was present in elevated numbers within 75% of samples of the annual campaign. On the contrary, the Hormogonales like Anabaena spiroides and Anabaena affinis were mainly present in October 2006 (rainy season) and December 2006 (dry season). The other classes, smaller, such as Euglenophyceae and mainly represented by Strombomonas, Euglena and Trachelomonas were very rare for Phacus taxa.

Chromophyta (Diatoms) were essentially dominated by the subclass of Pennatophycid (over 30% of total taxa), including species as Pinullaria acrosphaeria, Pinullaria brebissonii, Fragilaria crotonensis, Navicula placentula, and Gomphonema augur var. turris. However, these perennials were never significant portions of phytoplankton biomass. The Centrophycideae despite the low number of observed taxa, were always more abundant. Among the species that have most marked the various developments of diatoms, we mainly observed Acanthoceras sp, Lyngbya granulata and Fragilaria virescens.

Pyrrhophyta, represented by six species, had never appeared as a dominant class. They were counted within 55% of samples during the campaigns in December Phytoplankton in the Lake of Taabo was characterized by significant species richness of 118 taxa. We noted an average of 41 species per station, with a maximum richness of 57 taxa at Station S4 in the dry season. The maximum richness (32 taxa) was observed at 1 m depth at S5. Below 2 m depth, richness was significantly lower (Wilcoxon test, p = 0.001). However, no significant differences in species richness were observed between stations and seasons (Wilcoxon test, p = 0.163 and p = 0.230 respectively). The Shannon index (H) and equitability (E) are based on the proportions of observed species. In the Lake of Taabo, the yearly average values were respectively: S1 (H = 1.48, E = 0.60), S2 (H = 0.39, E = 0.31), S3 (H = 1.72, E = 0.61), S4 (H = 3.46, E = 0.78), S5 (H = 1.83, E = 0.66), S6 (H = 1.99; E = 0.67). The minimum values of H and E were observed at station S2 (H = 0.39, E = 0.31) while the maximum was obtained at S4 (H = 3.46, E = 0.78). The variability index was high at stations S4, S5 and S6 unlikely stations S1 and S3 which presented relatively low variability (Figure . 5).

The Shannon index and equitability reached the lowest values in June and July. In October, the diversity increased rapidly due to the development of several Chlorophyceae, Diatoms and Euglenophyceae species. In March, communities presented their maximum diversity. Changes in diversity indices and Shannon equitability between the stations of study were not significant (Kruskal-Wallis, p> 0.05). The principal component analysis showed the Eigen values A, B, C as follow: axis 1 (40.95%) and 2 (6.37%); axis 1 showed the depth gradient. D, E, F: axis 1 and 3 (4.90%), Axis 2 showed the season/depth interaction, the effect being marked at surface and disappeared at depth (Figure .  7). That graph shows the large dispersion of taxa in the medium area.

Quantitative analysis

The seasonal peak in phytoplankton growth occurred in March with a large share of small species (nanoplankton) belonging mainly to the centric Diatoms. That was followed in June by a collapse as samples displayed large presence in zooplankton. Sharp increase in transparency was observed as result of a phase of clear water. In December, phytoplankton exhibited low growth again which was mainly consisted of large species easily consumed by zooplankton. Grazing can be considered as a decisive factor in the decline or collapse of the phytoplankton populations. Thus, phytoplankton growth in the Lake Taabo started in October, ending in the rainy season. When thermal stratification shapes the surface layer of the lake in july, algae already take advantage of intense lighting and high nutrient concentrations. Algae migrate to the euphotic zone during the vertical water mixing which takes place in December. Transparency in turn is very low (0.65 m).

In July, Cyanobacteria were weakly represented in the lake (Figure 4). On the contrary, we observed their important presence from December to March (25%). Maximum growth was observed for Chlorophyceae in June (30%), Euglenophyceae in December (11%) and Dinoflagellates in October (8%). Regarding the phytoplankton dynamic, the stations of that study in the Lake of Taabo were divided into two major groups of phytoplankton densities as follow: S4, S3, S6 and S1, S2, S5 (Figure . 4).

The average biomass in chlorophyll a concentration was 10.7 μg.L -1 . Peaks were observed in October, December and March with respective chlorophyll a concentrations of 16.4, 16.02 and 16.52 μg.L -1 . These concentrations were consecutive of intense growth of volvocales Eudorina elegans and Pandorina sp, which coincided with rains. The lowest concentrations in phytoplankton biomass were measured in July (4.84 μg.L -1 ). Those concentrations resulted of water clarification leading to a much higher transparency (2.01 m) (Figure. 6).

Phytoplankton community of the Reservoir of the Lake Taabo presented a taxonomic spectrum dominated by Chlorophyta, Cyanobacteria and Euglenophyta. Those phytoplankton communities are characteristic of rich and polluted environments in putrescible organic substances.

The continuous nutrient input in the environment is causing the instability of phytoplankton populations. In addition, the development of large Euglenophyceae that dominated the number of species which were restrained to the middle by their biomass and the extent of areas of the lake overgrown with macrophytes, are signs of advanced functional degradation of the ecosystem.

DISCUSSION

Apparent changes in the lake ecosystem could be related to natural causes, anthropogenic activities and management issues. Natural influences such as climatic variations in the West African region had caused changes in levels of the lake. Regulations of the lake outflow for hydroelectric power generation at the Taabo dam located downstream may also contribute to changes in the lake levels. Inundation of farmlands that occur during such periods may contribute to large loads of sediments and nutrients to the lake, whereas, decrease in water levels can modify nearshore habitats and change breeding areas available to many fish species.

Overall, Phytoplankton communities consist of assemblages of species with morphological and physiological characteristics with different ecological organizations. The Phytoplankton diversity in lake of Taabo was dominated by Chlorophyta, against Diatoms when the site is small and severely disturbed [START_REF] Moshood | Phytoplankton assemblage of a small, shallow, tropical African reservoir[END_REF][START_REF] Haouati | Distribution spatio-temporelle de la flore planctonique en fonction du degré de pollution dans le barrage de Boukourdane (Tipaza-Algérie) Dynamics and Biodiversity of the terrestrial and aquatic Ecosystems -ALGERIA[END_REF]. Their densities were relatively high in Lake Taabo, low both downstream and upstream. Accordingly to the longitudinal gradient upstream-downstream, phytoplankton was differently distributed at each station. Indeed, the population mainly changed with respect to the number of species extinction, at least in the cashing of the lake. That is an essential component to the understanding of ecosystem functioning (Klug and Tiedje 1993). Thus, knowledge of community taxonomic compositions, such as Diatom indices, is a source of key information concerning pollution [START_REF] Descy | Utilisation des diatomées benthiques pour l'évaluation de la qualité des eaux courantes[END_REF][START_REF] Karr | Biological integrity: a long-neglected aspect of water resource management[END_REF]). In the case of this study Diatoms (22%) indicated that the changes in environmental conditions have to be considered in regard to the other studies in Africa [START_REF] Moshood | Phytoplankton assemblage of a small, shallow, tropical African reservoir[END_REF][START_REF] Haouati | Distribution spatio-temporelle de la flore planctonique en fonction du degré de pollution dans le barrage de Boukourdane (Tipaza-Algérie) Dynamics and Biodiversity of the terrestrial and aquatic Ecosystems -ALGERIA[END_REF].

In terms of structure, the phytoplankton community was not different between stations. Changes in phytoplankton biocoenosis could be explained by the hydrological changes that occurred and the influence of surrounding anthropogenic inputs. Indeed, phytoplankton densities were relatively higher downstream the Lake of Taabo than upstream. In that river-lake system, flows were more or less rapid and turbulent, and phytoplankton communities were very scarce. On the contrary, phytoplankton densities were relatively high in the lacustrine system of Taabo. Indeed, phytoplankton communities found many favorable conditions for their development in relation with a stagnant state and urging water transparency of 0.56 m to 2.01 m.

Gradient in dissolved oxygen concentrations resulted from the organic matter oxidization. Indeed, high concentrations of ammonia recorded at the bottom of water bodies reflected an incomplete mineralization of organic substances. Nitrifying bacteria are aerobic and cannot influence the conditions of the site because nitrogen in its ammonium form predominates (Guilford et al., 2000). Highly eutrophic environments are characteristic of organic substance rich environments [START_REF] Moss | Ecology of freshwater: Man and medium, past to future[END_REF], in which deoxygenated deep waters are produced by biological means. These low levels in oxygen concentration were due to suspended solids overload, and photosynthetic activity at the Lake surface was weak to offset the consumption of dissolved oxygen. On the contrary, phosphorus predominated in some stations (S4, S5, and S6) during the rainy season. Most of the initial detritus are incompletely mineralized. Therefore, those detritus sink and accumulates on the lake bottom, resulting in the extension of areas of anoxia in the water column through the entire ecosystem. Release into the ecosystem of large amounts of untreated domestic sewage lead to the growth of taxonomy dominated by Chlorophyta, the Cyanobacteria and euglena. Those genera are characteristic of polluted and organic substances rich environments [START_REF] Akpan | Seasonal variation in water quality of the Cross river, Nigeria[END_REF]. Representatives groups, including the genera Scenedesmus, Microcystis and Lepocinclis, are known for their preference for eutrophic sensu lato [START_REF] Reynolds | The regulation of phytoplankton population dynamics in the world's largest lakes[END_REF]. Algal bloom occurred 1-2 months after heavy rainfall which sluices inputs in phosphorus from surrounding farms and lands, and also causes complete mixing of the lake. Phytoplankton communities had experienced significant changes in biomass as well as in their composition. Episodic phytoplankton blooms were regularly reported in the middle of the lake on calm and sunny water [START_REF] Cogels | Premiers effets des barrages du fleuve Sénégal sur le lac de Guiers[END_REF]. Those authors reported that these blooms were mainly composed of Cyanobacteria including Anabaena spiroides, Woronichinia sp and Microcystis aeruginosa, typical algae of eutrophic waters. These species were previously identified by [START_REF] Dia | Le phytoplancton du lac de Guiers : approche qualitative et quantitative[END_REF]. The occurrence of these algal blooms in these periods of the annual cycle is therefore related to the tropical climate and highlights the regulatory action of rain on the photosynthetic activity within the water body. Besides the supply in organic matter from the bottom, the rain action leaded larger flow discharges, and backlash by more efficient solids transport. The structure of the phytoplankton community is partly characterized by a small number of species of Bacillariophyceae, also noted by Carl [START_REF] Bro | Etude bathymétrique et limnologique du lac de Guiers[END_REF]. Therefore, the predominance of green algae and higher number of species of cyanobacteria showed changes occurring in the phytoplankton community. Analysis of vertical structure also indicated that this lake was not actually in green estate according to [START_REF] Margalef | Valeur indicatrice de la composition des pigments du phytoplancton sur la productivité, composition taxonomique et propriétés dynamiques des populations[END_REF]. That was with phases of rapid and ephemera growth of various species. The constant nutrient enrichments from the watershed and sediment following high frequency of episodic mixing water [START_REF] Dokulil | Anthropogenic impacts to large lakes in China: the Tai Hu example[END_REF], and the shallowness of the water, favor the maintenance of this type of community.

Conclusion

This paper is based on a recent field study, and includes various data available about Lake Taabo. The physicochemical analysis of water and the study of the phytoplankton populations based upon sampling performed during an annual cycle were used to determine trophic status of the lake Taabo. The effects of environmental pressure and subsequent impacts are evident in the Lake of Taabo. This Lake is under a disturbance of its ecosystem at physical chemistry and phytoplankton diversity levels, due to the anthropological influence at its proximity. This water body is engaged in an eutrophication process. The imbalance of the ecosystem from a continuous nutrient inputs and domestic untreated discharges is causing the instability of phytoplankton communities, the extent of flooded areas of the lake by macrophytes. These observations are signs of advanced functional degradation of the ecosystem. Therefore, the results of this study could help in pollution management and project for preservation of the Lake of Taabo. The rapid evolution of phytoplancton communities related to the environmental changes increase the interest of their ecological follow-up. Next studies should allow a precise approach which will take into account the interactions between the diversity of phytoplankton, the levels of the trophic network and the physico-chemical parameters of these sites that are uses for multipurpose.

Figure 1 .

 1 Figure 1. Location of the sampling stations of Taabo reservoir (S1, S2, S3, S4, S5, S6).

Figure 1 :

 1 Figure 1: Location of the sampling stations of Taabo reservoir (S1, S2, S3, S4, S5, S6)

Figure 2 .

 2 Figure 2. Seasonal variations of physical and chemical parameters in different stations. (SP): rainy season and (SS): dry season.

Figure 2 :

 2 Figure 2:Seasonal variations of physical and chemical parameters in different stations. (SP): rainy season and (SS): dry season

  2006 and March 2007 (dry season) as showed on figure 4. The Chrysophyceae, mainly represented by Dinobryon, were sporadically observed in the samples collected in July 2006,

Figure 4 :

 4 Figure 4: Seasonal variations with relative density of dominant phytoplankton groups in different stations

Figure 5 .

 5 Figure 5. Average diversity index of Shannon (H) and equitability (E) at sites S1, S2, S3, S4, S5 and S6

Figure 6 :

 6 Figure 6: Annual change in total biomass Chlorophyll (µg / l) in the six stations of Lake Taabo for flood periods (= rainy season from June to October) and low water (= dry season from December to March).

Figure 7 :

 7 Figure 7: Principal Component Analysis of phytoplankton taxa at various depths, stations and seasons F1 (40.95%), F2 (6.37%) and F3 (4.90%)

Table 1 .

 1 Morphometric and hydrographic characteristics of the reservoir hydroelectric Taabo

	Geography coordinates		5°07'6°40'N
	Date of construction		1975-1979
	Date of impoundment		1978
	Date of commissioning		1979
		Catchment area	58700km 2
		Annual medium flow	278m 3 / S
	Hydrology	Coast of normal reserve	124m
		Surface at the coast 124m	69km²
		Minimal level of exploitation	121m
		Volume of the useful section	185×10 6 m³
	Reservoir	Coast of maximum rising	124.8m
		Volume with the maximum rising	163×10 6 m³
		Minimum rating of exploitation	120m
		Type of water retention	Earth and rock fill
		Coast of the peak	127m
	Dam	Maximum height	34m
		Length of the peak	7.5m
		Height of the hydrant	35m
		Width of the catch	35m
	Hydrant	A number of valves(coach)	3
		Diameters of valves force	6.6m
		Side of the lower edge of the penstock	99.4m
		Types of vannes	Segmented valve
		Numbers	5
		Capacity of evaluation	4460m 3/s
	Spillways	Side of the lower edge of the penstock	113m
		Working installed capacity	210MW
	Power and Energy	Annual productivity	1050GWH

Table 2 .

 2 Wilcoxon test (effect of season) and Kruskal-Wallis (stations and depth effect) applied to each descriptor.). DO: dissolved oxygen. Bold values significantly different (p <0.05

	Descriptor	Season effect	Station effect	Depth effect
		wilcoxon	P	χ²	P	χ²	P
	Turbidity(NTU)	369.0	0.005	43.3	<0.001	2.3	0.687
	Transparency	0.0	<0.001	31.1	<0.001	0.0	1.000
	Temperature (°C)	13.0	<0.001	6.7	0.247	1.1	0.901
	pH	85.5	0.003	46.9	<0.001	0.8	0.940
	Conductivity(µS.cm -1 )	465.0	<0.001	6.1	0.295	0.7	0.950
	Dissolved oxygen(mg.L -1 )	464.0	<0.001	12.0	0.035	1.2	0.884
	(%)O2	464.0	<0.001	16.4	0.006	1.4	0.851
	NH4 -N(mg.L -1 )	5.0	<0.001	6.5	0.261	1.5	0.832
	NO3 -N(mg.L -1 )	298.0	0.176	11.4	0.044	6.1	0.194
	NO2 -N(mg.L -1 )	20.0	<0.001	3.0	0.707	3.0	0.563
	PO4 -P(mg.L -1 )	0.0	<0.001	2.3	0.809	1.6	0.809
	SiO2(mg.L -1 )	1.0	<0.001	6.7	0.243	1.7	0.798

Table 3 .

 3 Frequencies of phytoplankton taxa

	code	Taxa	Present(%)
		cyanobacteria
		chroococcales
	Apho	Aphanoc aspa ho Isatica (lemm) Cron berg & Komarek.	0.41
	Apin	Aphanoc aspa Incerta (lemm) Cron berg & Komarek.	0.37
	Clim	Chroococcus Limneticus Lemm	0.49
	Cmin	Chroococcus Minicus (Kutz) Nag.	0.31
	Ctur	Chroococcus turgidus (Kutz) Nag. Var. Maximus Nyg.	1.02
	Cpus	Coelomoron pusillum (Van Goor) Kom.	0.52
	Cosp	Coelosphaerium sp.	1.01
	Mele	Merismopedia elegans A. Br. ex kutz. var. elegans	0.41
	Mgla	Merismopedia gluaca (Ehr.) Nag.	1.97
	Mpun	Merismopedia punctata Meyen.	0.88
	Mten	Merismopedia tenuissina Lemm.	2.59
	Maer	Microcystis aeruginosa (Kutz) Kutz.	2.52
	Mden	Microcystis densa W. & G. West.	1.28
	Mflo	Microcystis flos-acquae (Wittr.) Kirchan.	1.09
	Mrob	Microcystis robusta (Clark) Nyg.	0.42
	Mwes	Microcystis wesenbergii. Kom.	0.78
	Syca	Synechococcus capitatus A, E, Bailey-Watt & J, Komarek,	0.52
	Wosp	Woronichinia sp,	1.27
		Hormogonales
	Aaff	Anabaena affinis Lemm.	1.10
	Acta	Anabaena c atenula. var. affinis (Lemm) Geitler.	0.57
	Aflo	Anabaena aflos-aquae Breb. Ex. Born. & fl. var. aflos-aquae	1.49
	Aspi	Anabaena spiroides Kleb.	0.33
	Crac	Cylindrospermopsis raciborskii (Wolosz) Seena and Raju	0.20
	Lgra	Lyngbya granulata (Ehrenb.) Simonsen.	0.36
	Lgra	Lyngbya granulata (Ehrenb.) Simonsen.var.angustissinaf spirallis	0.85
	Lmag	Lyngbya magnifica Gardner.	0.15
	Opri	Oscillatoria princeps Vaucher ex Gom	0.31
	Porn	Phormidium omatum (Kutz) Anagnostidis & Komarek.	0.35
	Pcon	Plantolyngbya contoria (Lemm) Anagostidis & Komarek.	0.31
	Plsp	Plantolyngbya sp.	1.02
	Pgra	Plectonema graccilimun Hansg.	0.55
	Rhsp	Rhabdoderma sp.	0.24
		Euglenophyta, Euglenophyceae
		Euglenales
		Chlophyta, Conjugatophyceae
		Desmidiales
	Clac	Closterium acutum Breb.	1.37
	Clacu	Closterium acutum Breb. Var. Variable (Lemm.) Krieger	1.81
	Clgr	Closterium gracile Breb.ex. Rafls.	2.07
	Cllin	Closterium lineatum Ehrenb.ex Rafls.	1.25
	Clpa	Closterium parvulum Nag.	0.98
	Clps	Closterium pseudolunula Borge.	1.07
	Clsu	Closterium subulatum (Kutz.) Breb. Var. Subulatum	1.07
	Clst	Closterium strigosum Breb.	1.43
	Cobr	Cosmarium brebissonii Rafls.	1 .1
	Ccon	Cosmarium contractor Kirchn.	1.47
	Corps	Cosmarium psuedodecoratum Schmidle.	0.92
	Eude	Euastrum dentriculatum F. Gay.	1.18
	Euel	Euastrum elegans (Turp.) Rafls.	0.7
	Stbr	Staurastrum brachiopromines Borg. var acherianum Bohl.	0.71
	Stpo	Staurastrum polymorphum Breb. ex Rafls.	0.55
	Stvo	Staurastrum volans W. & G. S. West.	0.81
	Stsu	Staurastrum subulatus (Ehrenb.ex Rafls) Teil.	0.92
	Xasu	Xanthidium subtrilobum W. & G.S West var. inornatum	0.48

Table 3 .

 3 Cont.

		Chlorophyceae
		Chloroccocales
	Ahib	Ankistrodesmus bibraianus (Reunch) Korsch.	0.66
	Afal	Ankistrodesmus falcatus (Corda) Rafls var. falcatus	0.63
	Agra	Ankistrodesmus gracilis (Reinch) Korsch.	0.87
	Coin	Coelastrum indicum Turn.	1.54
	Cosp	Coelastrum sp.	0.7
	Dpul	Dictyosphaerium pulchellum Wood.	0.77
	Grad	Golenkinia radiate (Chod) Wille.	0.98
	Kcon	Kirchneriella contorta (Schmidle) Bohl.	1.96
	Pdup	Pediastum duplex Meyen var. duplex.	1.5
	Pgra	Pediastum duplex Meyen var. gracillimum W. & G. S. West.	1.53
	Psim	Pediastum simplex.	1.76
	Sbic	Scenedesmus bicaudatus (Hansg.) Chod.	0.99
	Sper	Scenedesmus perforatus Lemm.	0.44
	Scua	Scenedesmus quadricauda (Turp.) Breb sensu Chod	0.45
	Sspi	Scenedesmus quadrispina Chod.	0.86
	Tmin	Tetraedron minimum (A. Br.) Hansa f tetralobulatum (Reinsch.)	0.78
		Chlorophyceae
		Volvocales
	Euel	Eudorina elegans Ehrenb.	1.57
	Pspe	Pandorina sp	1.41
		Pyrrhophyta, Dinophyceae
		Peridiniales
	Peac	Peridinium aciculiferum Lemm.	0.54
	Peci	Peridinium cinctum (O.F.Mull.)Ehrenb.	0.56
	Pein	Peridinium inconspicuum Lemm.	0.54
	Peum	Peridinium umbonatum F. Stein	0.43
	Pevo	Peridinium volzii Woloszynska.	0.61
	Pesp	Peridinium sp.	0.7
		Chromophyta, Bacillariophyceae, Diatomophyceae
		Centrales
	Cyme	Cyclotella menaghhiniana Kutz.	0.26
	Cysp	Cyclotella sp.	0.21
	Temu	Terpsinoe musica Ehrenb.	0.18
		Pennales
	Acsp	Acanthoc eras sp.	0.61
	Eupe	Eunotia pectinalis (Dyll.) Rabenh.var. pectinalis	1.02
	Fcap	Fragilaria capucina Desm azies.	0.85
	Fcro	Fragilaria crotonens is kitton.	0.12
	Fuln	Fragilaria ulna (Nitzsch.) Lange-Bert.	0.28
	Fvir	Fragilaria virescens Rafls.	0.43
	Guag	Gomphonema augur Ehrenb .var. turis (Ehrenb.) Lange -Bert.	0.23
	Ggra	Gomphonema gracile Ehrenb.	0.42
	Goli	Gomphonema olivaceum (Hom.) Breb. var olivaceum	0.34
	Gosp	Gomphonema sp.	0.22
	Mesp	Melosira sp.	0.38
	Ncus	Navilcula cuspidata Kutz . Gomphonema sp.	0.23
	Npla	Navilcula placentula (Ehrenb.) Grun.	0.31
	Npup	Navilcula pupulla var. pupulla Kutz.	0.42
	Nisc	Nitzchia scalar is (Ehrenb.) W. Smith.	0.28
	Nisp	Nitzschia sp.	0.37
	Piac	Pinnularia acrosphaeria Rabenh.	0.16
	Pbra	Pinnularia brauniana (Grun.) Kram.	0.21
	Pbre	Pinnularia brebissonii (Kutz) Rabenh.	0.51
	Pdiv	Pinnularia divergens W. Smith.	0.43
	Pigi	Pinnularia gibba Ehrenb.	0.17
	Pieo	Pinnularia neomajor Kram.	0.28
	Pivi	Pinnularia viridis (Nitzsch.) Ehrenb.	0.32

Table 3 .

 3 Cont.

		Chromophyta, Chrysophyceae, Ochromonadales
	Dser	Dinobryon sertularia Ehrenb.	0.13

Figure 3 : Distribution of the phytoplankton density in different stations

ACKNOWLEDGEMENTS

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


Emmanuel Le Guen

Anne-Claude Tarot

Jean-Luc Mattei

Half-Massive Ceramics for Antenna Downsizing: Improvement of a Smart Magneto-Dielectric Material with Matching Permeability and Permittivity, and with Enhanced Low-Loss Frequency Range

Keywords: half-massive ceramics, magnetodielectric materials, antenna, miniaturization. I

Complex permeability and permittivity of Ni 0.5 Zn 0.3 Co 0.2 Fe 1.98 O 4-x half-ceramic ferrites (porosity ~50%), obtained from various annealing temperature T A (500°C, 800°C, 900°C), are measured and compared. Their characteristic impedance Z, magnetic and dielectric loss tangents are presented. Bandwidth BW and radiation efficiency η values of patch antennas are obtained through numerical simulations and compact formulations. The value of the annealing temperature used for the substrates determines those of BW and η. BW and η are themselves used as criteria to decide which one among the various substrates is proper to be used at given frequency. It is concluded that whereas the sample annealed at 500°C is far to meet the required properties in reason of detrimental magnetic losses, the half massive ceramics got after annealing at T A =800°C and 900°C show appropriate performances for further applications. The electromagnetic properties of half ceramics materials sound to be very competitive at frequencies up to 0.8GHz.

INTRODUCTION

Miniaturization of electronic devices for mobile communication has led to increasing demands for the reduction of antenna dimensions, so that antenna downsizing is one of today's most important challenges for antenna designers [START_REF] Skrivervik | PCS antenna design: The challenge of miniaturization[END_REF]. The use of high-permittivity substrates allowed size reduction, but causes a considerable decrease in antenna efficiency and bandwidth [START_REF] Hwang | Planar inverted F antenna loaded with high permittivity material[END_REF]- [4]. For applications in the low-UHF frequency band (300-862 MHz), the use of magnetodielectric substrates is expected to overcome these limitations by offering larger bandwidths and improved efficiency [START_REF] Hansen | Antennas with magneto-dielectrics[END_REF]- [START_REF] Bae | A small meander VHF & UHF antenna by magneto-dielectric materials[END_REF]. Indeed it has been shown [START_REF] Niamien | Compact Expressions for Efficiency and Bandwidth of Patch Antennas Over Lossy Magneto-Dielectric Materials[END_REF] that, although the permittivity (ε') increases the impact of losses, as well as the stored energy, the permeability (µ') plays the opposite role by decreasing them. As a consequence, the negative impact of ε' on radiation efficiency (η) and bandwidth (BW) is counterbalanced by the positive impact of µ'. This was expressed through relations ( 1) and ( 2), derived for lossy magnetodielectric materials [START_REF] Niamien | Compact Expressions for Efficiency and Bandwidth of Patch Antennas Over Lossy Magneto-Dielectric Materials[END_REF].

The interesting point in these relations is that, unlike previous works, they take both magnetic and dielectric losses into account. Moreover, if loaded with magnetodielectric material antenna's dimensions can be reduced, theoretically, by a miniaturization factor n (n= ' ' µ ε is the refractive index) while its electrical dimension remains unchanged [START_REF] Mosallaei | Magneto-dielectrics in electromagnetics: Concept and applications[END_REF]. The magnetodielectric materials should also present a characteristic impedance Z (Z=Z 0 ' ' ε µ , where Z 0 is free space impedance) close to that of free space, that requires matching µ' and ε'. Spinel ferrites, which have ε'>1 and µ'>1, together with low loss tangents (both dielectric and magnetic), are among the best candidates for antenna downsizing [START_REF] Mattei | Suitability of Ni-Zn Ferrites Ceramics With Controlled Porosity as Granular Substrates for Mobile Handset Miniaturized Antennas[END_REF][START_REF] Ji | Miniaturized T-DMB antenna with a low-loss Ni-Mn-Co ferrite for mobile handset applications[END_REF][START_REF] Mattei | Dense and half-dense NiZnCo ferrite ceramics: their respective relevance for antenna downsizing, according to their dielectric and magnetic properties at microwave frequencies[END_REF][START_REF] Zheng | Low loss NiZn spinel ferrite-W-type hexaferrite composites from BaM addition for antenna applications[END_REF][START_REF] Kong | Recent progress in some materials and structures for specific electromagnetic applications[END_REF] . However most of the recent published studies focus on bulk ferrites, that show detrimental magnetic losses at frequencies higher than 300MHz, thereby limiting at frequencies lower than 300MHz their possible applications: recent work [START_REF] Mattei | Dense and half-dense NiZnCo ferrite ceramics: their respective relevance for antenna downsizing, according to their dielectric and magnetic properties at microwave frequencies[END_REF] report for a NiZnCo ferrite the following values: tgδ µ =0.09, 0.5, 1.47 at frequencies f=0.2, 0.5, 1 GHz respectively. The sintering process that is commonly used to obtain dense ferrite substrates involves elevated sintering temperature (usually about 1100°-1200°C) [START_REF] Zheng | Low loss NiZn spinel ferrite-W-type hexaferrite composites from BaM addition for antenna applications[END_REF], [START_REF] Kong | Recent progress in some materials and structures for specific electromagnetic applications[END_REF]. The grain growth to which this procedure leads is associated with a multi-domain magnetic grain structure, which ensured high permeability values. Then, however, undesirable magnetic losses associated with domain-wall bulging remain high from the quasistatic frequencies until several 100 MHz. In contrast, it has been shown that magnetodielectric materials in porous or composite forms, with moderate magnetic losses and low dielectric losses, have potential applications in antenna miniaturisation up to 700 MH [START_REF] Mattei | Suitability of Ni-Zn Ferrites Ceramics With Controlled Porosity as Granular Substrates for Mobile Handset Miniaturized Antennas[END_REF][START_REF] Souriou | Antenna miniaturization and nanoferrite magneto-dielectric materials[END_REF][START_REF] Canneva | MINIATURE RECONFIGURABLE ANTENNA WITH MAGNETO DIELECTRIC SUBSTRATE FOR DVB-H BAND[END_REF]. With the aim of avoiding low frequency losses we first suggested using porous (i.e., half-massive) ferrite ceramics, got from low annealing temperature and in which the grain size is small enough [START_REF] Mattei | Suitability of Ni-Zn Ferrites Ceramics With Controlled Porosity as Granular Substrates for Mobile Handset Miniaturized Antennas[END_REF], [START_REF] Souriou | Antenna miniaturization and nanoferrite magneto-dielectric materials[END_REF]- [START_REF] Huitema | Frequency tunable antenna using a magnetodielectric material for DVB-H application[END_REF]. Both porosity, that is a measure of the void space in a material, and concentration of cations Fe 2+ in the ferrite chemical composition, have a strong influence on permeability and permeability. An experimental process that allows to obtain such half-massive ferrite was detailed elsewhere [START_REF] Mattei | Dense and half-dense NiZnCo ferrite ceramics: their respective relevance for antenna downsizing, according to their dielectric and magnetic properties at microwave frequencies[END_REF], [START_REF] Thakur | Low-loss spinel nanoferrite with matching permeability and permittivity in the ultrahigh frequency range[END_REF]. The magnetic phase of the so got porous nanostructured material is constituted by nanograins of ferrite. It was also demonstrated that µ and ε are greatly sensitive to many parameters, and first of all to the annealing temperature T A applied during material heat treatment. Actually, value of T A drives both Fe 2+ and Fe 3+ iron cations concentrations and the material porosity, to which µ and ε strongly depend. Typically, when T A >1050°C the obtained ceramic is densified at a rate closed to 100%, and Fe 2+ concentration is close to zero. Whereas when T A <900°C, the material shows porosity close to 50%. Obviously, to increase T A may involve variations of electromagnetic properties. When T A changes from 800°C to 900°C, the variations in µ and ε might be attributed, for a very large part, to the decrease of the rate Fe 2+ /Fe 3+ , and, but only for a few part, to the limited decrease of the porosity fraction value (that remains about 50% for T A =800°C as for T A =900°C) [START_REF] Mattei | Dense and half-dense NiZnCo ferrite ceramics: their respective relevance for antenna downsizing, according to their dielectric and magnetic properties at microwave frequencies[END_REF]. The upper limit for frequency applications of ferromagnetic materials is fixed by the ferromagnetic resonance frequency F R (The observed maximum of µ" in the µ"-f curve obtained on polycrystalline materials, accords with the resonance frequency; this feature defined the ferrimagnetic resonance F R [START_REF] Nakamura | Control of high-frequency permeability in polycrystalline (Ba,Co)-Z-type hexagonal ferrite[END_REF]). There is a great increase in magnetic losses and a decrease in permeability even only slightly below F R . Furthermore, it should be noted that antennas then need to be used at frequency values much lower than F R . As stated by Snoëk's law [START_REF] Snoek | Dispersion and absorption in magnetic ferrites at frequencies above one Mc/s[END_REF], F R and the static permeability µ S are related to each other: the higher the static permeability µ S , the lower the spin resonance frequency F R . It is written (µ S -1).F R =2/3 γ M S , where M S is the magnetization at saturation and γ is the gyromagnetic ratio ( γ =35.185 MHz/kA.m -1 ). Snoëk's law is useful for estimating the microwave performances of a bulk polycrystalline material. Then it can be seen that µ S and F R cannot be as high as possible at the same time. Therefore the electromagnetic properties (including permeability µ=µ'-jµ", permittivity ε=ε'-jε'', spin resonance frequency F R and static permeability µ S , miniaturization factor, reduced impedance) of half-massive ferrites can be easily tailored by choosing the appropriate temperature T A . On the basis of our previous results the composition Ni 0.5 Zn 0.3 Co 0.2 Fe 1.98 O 4-x was selected for this study, and annealing temperatures were 500°C, 800°C, 900°C. The abilities of the obtained materials to be used as magnetodielectric substrate up to 1GHz are compared together, by computing radiation efficiencies and bandwidth of patch antennas.

II.

ELECTROMAGNETIC PERFORMANCES OF HALF-MASSIVE Ni 0.5 ZN 0.3 CO 0.2 FE 1.98 O 4 FERRITE CERAMICS Torroidal samples were obtained from nanosized ferrite particles (the particles mean size was equal to 12nm) by moulding before heat treatment. The experimental set up for measuring the complex dielectric constant and the complex permeability constant in the frequency range from 0.1-6GHz consisted of a coaxial transmission line (APC 7 standard), and a HP 8753S network analyzer. The method used was based on the measurements of the scattering parameters of a torroidal sample placed into the coaxial line. Dielectric permittivity and magnetic permeability were derived using the transmission/reflection method based on the algorithms of Nicolson, Ross, and Weir [START_REF] Weir | Automatic measurement of complex dielectric constant and permeability at microwave frequencies[END_REF]. The complex permeability dispersion of the ferrite annealed at different temperature is shown in Fig. 1. Permeability reveals the degree of magnetization that a material obtains in answer of an applied magnetic field. The complex permeability of ferrite may be related to two different mechanisms: the spin rotational magnetization, and the domain wall motion. The later mechanism, to which undesired high losses are attached since the quasi-static range, arises when the grain size is larger than a certain critical size. In order to avoid this detrimental domain wall contribution to the permeability, grain size should be kept below the critical size by using moderate annealing temperature. As a counterpart, the permeability value, when originating from spin rotation only, cannot reach very high values. In the present study, the fairly low value of permeability is attributed: first to the rotational contributions to magnetization in the nanoferrites, and second to the porous nature of the samples. Permittivity is determined by the ability of a material to polarize in response to an electric field. Because of their porous structure, the dielectric constant of the half-massive ferrite (Fig. 3) is quite low (in the range 5-7) as compared to ε'~16 for usual dense ferrites [START_REF] Shannon | Dielectric polarizabilities of ions in oxides and fluorides[END_REF]. It should be underlined that both permeability and the permittivity show noticeable constant values in an unusual wide range of frequency starting from 0.1GHz up to 1GHz (inset in Fig. 1, and Fig. 3). This is an interesting feature, that is related to the uncomplete sintering of the ferrites samples Figures 4a-4c show, at given frequencies (f=0.4GHz, 0.8GHz, 1GHz), the values of µ and tgδ µ , ε and tgδ ε , and the overall losses tgδ µ +tgδ ε . Owing to its poor electromagnetic properties (low permeability, high dielectric and magnetic losses), sample S1 will be no more considered in the following.

III.

PATCH ANTENNA PERFORMANCES Tables I and II show values of the miniaturization factor (n) and of the reduced impedance (Z/Z 0 ) at given frequencies, for the samples S2 and S3. For the sample annealed at T=900°C, considering the range of frequency up to 0.4-0.5GHz, reduced impedance is close to unity, the miniaturizing factor is constant (with a mean value ~5.6) and the overall loss tangent value remains moderate. Whereas beyond 0.4-0.5GHz, and up to 1GHz, the preferred annealing temperature should be T=800°C. For a more detailed discussion, the example of a square patch antenna (width W) printed over lossy magnetodielectric materials, (thickness d=3mm) is considered in the following. The width W of the patch is obtained from the relationship :

𝐵 = 𝜆 𝑔 2 = 𝜆 0 2𝑡 = 𝑐 2𝑡𝐹 0 (3)
Where c=3.10 8 m/s represents the velocity of light, n the miniaturization factor and F 0 , the operating frequency (0.4, 0.8, or 1GHz).

Permittivity-Real part ε'

Frequency (GHz)

T A =900°C T A =800°C 0.2 0. 7 1.2 1.7
Permittivity-Imaginary part ε" between radiation efficiency and bandwidth is observed: it is not possible to get high values for both of them at the same time. As an example, let us consider a patch antenna intended to be used at f=0.4GHz. Because at this frequency the reduced impedance is closed to unity for T A =900°C, and also because efficiency is higher for T A =900°C than for T A =800°C, one could conclude that the ferrite Ni 0.5 Zn 0.3 Co 0.2 Fe 1.98 O 4-x should be retained, after annealing at T=900°C. But if bandwidth should be favored, instead of efficiency, then the recommended annealing temperature for the same ferrite material should be T A =800°C, and not 900°C. As a counterpart the reduced impedance is then Z/Z 0 =0.73, far from unity. Then it appears that the value of annealing temperature of a given material influences deeply its electromagnetic properties, and therefore the ability of the antenna to be used at given frequency. Interest in using half-massive ferrites as substrates for antennas miniaturization, instead of bulk materials, lies in the easiness to tailor their electromagnetic properties, in particular through the value of the applied temperature during heat treatment. The case of patch antenna with W=71.3mm was simulated over sample S3 (T A =900°C) with HFSS software. The reflexion coefficient is presented in Figure 5: it exhibits a resonance frequency of 397 MHz with a bandwidth of 29.5 MHz (7.4%). These results agree with those of Table 3 (F 0 = 400MHz -BP = 7.3%). IV. CONCLUSION Ni 0.5 Zn 0.3 Co 0.2 Fe 1.98 O 4-x ferrites were synthetized and annealed at various temperatures T A . Depending on the value of annealing temperature T A , the obtained half-massive ferrite ceramics (with porosity fractions ~50% in volume) shows competitive electromagnetic properties in the range of frequency up to 1GHz. Patch antennas realized with these halfmassive ferrites as substrates were considered. It was shown that the value of annealing temperature for a given material affects deeply its electromagnetic properties, and therefore it influences the ability of the antenna to be used at given frequency: antennas performances (bandwidth BW and radiation efficiency η), calculated through numerical means, and through compact expressions as well, are sensitive to T A . The value that must be selected for T A depends on which one among the two antenna parameters considered in this study has to be favored (BW or η). In the field of the present study, T A =800°C is a good choice for application up to 0.8GHz, whereas T A =900°C leads to better results at frequencies below 0.5GHz. Interest in using half-massive ferrites as substrates for antennas miniaturization, instead of bulk materials, lies in the large facility to tailor their electromagnetic properties, in particular through the value of the applied temperature during heat treatment.

T A =800°C T A =900°C ε' ε" T A =500°C T A =500°C 0

Fig. 1 .Fig. 2 .Fig. 3 .

 123 Fig.1. Complex permeability spectra for samples S1 (TA=500°C), S2 (TA=800°C), S3 (TA=900°C) over the frequency range 0.1-6 GHz. Inset: focus on the real permeability over the frequency range 0.2-1 GHz

Fig. 4a .

 4a Fig.4a. Permeability, permittivity, magnetic losses, dielectric losses, and overall losses at given frequencies for sample S1 (TA=500°C).

Fig. 4b .

 4b Fig.4b. Permeability, permittivity, magnetic losses, dielectric losses, overall losses at given frequencies for sample S2 (TA=800°C).

Fig. 4c .

 4c Fig.4c. Permeability, permittivity, magnetic losses, dielectric losses, and overall losses at given frequencies for sample S3 (TA=900°C).Tables I and II also provide calculated values of bandwidth and radiation efficiency, obtained from formulas (1) and (2), at given frequencies (0.4, 0.8, 1GHz). The classical trade-off

Fig. 5 .

 5 Fig.5. Reflexion coeffcient for a patch antenna (width W=71.3mm) printed over sample S3 (TA=900°C) of thickness d=3mm.

Ni 0,5 Zn 0,3 Co 0,2 Fe 1,98 O 4 T A =500°C

						permittivity, permeability, overall losses	0 0.2 0.4 0.6 0.8 1 1.2 1.4	0,052 losses:0,104 0,055 @400MHz	0,040 losses:0,120 0,080 @800MHz	0,034 losses:0,194 0,160 @1GHz	Ni 0,	µ"/µ' ε"/ε'
							2.E+08	7.E+08	1.E+09	2.E+09
								f(Hz)
	permittivity, permeability, overall losses	0.2 0.4 0.6 0.8 1 1.2 1.4	0,055 @400MHz 0,110 losses:0,165 µ'=2,20 ε'=6.96	0,047 µ'=2,12 0,120 ε'=6.82 losses:0,167 @800MHz	0,047 µ'=2,90 0,155 ε'=6.75 losses:0,202 @1GHz	µ"/µ' ε"/ε'
		2.E+08	7.E+08	1.E+09	2.E+09
					f(Hz)

5 Zn 0,3 Co 0,2 Fe 1,98 O 4 T A =800°C

					µ'=3,56
				µ'=3,40	ε'=6.0
			µ'=3,33	ε'=6.04
			ε'=6.20
	permittivity, permeability, overall losses	0 0.2 0.4 0.6 0.8 1 1.2 1.4	0,030 losses:0,091 0,061 @400MHz	0,017 losses:0,647 0,630 @1GHz 0,016 losses:0,226 0,210 @800MHz	Ni 0,	µ"/µ' ε"/ε'
		2.E+08	7.E+08	1.E+09		2.E+09
					f(Hz)

5 Zn 0,3 Co 0,2 Fe 1,98 O 4 T A =900°C

			µ'=6,32
		µ'=6,46	ε'=5.04
	µ'=5,43	ε'=5.06
	ε'=5.11