INTRODUCTION

Primary production by coastal phytoplankton contrib-utes almost 15 % of global oceanic production (Biswas et al. 2010). Moreover phytoplankton plays the central role in the functioning of food webs and ecosystems (Pilkai-tytë et al. 2004). Ongoing climate change and anthropic activities are introducing stressors to the coastal envi-ronment, which affect the ecology and biology of phy-toplankton. Changes of land use and vegetation cover have impacted fresh water flow, sediment transport, and nutrient dynamics of coastal ecosystems (Walsh, 1991). Anthropogenic input of excess nutrients to many coastal watersheds has increased dramatically over the last three to four decades, resulting in changes in ecosystem struc-ture and function (Emmanuel & Onyema 2007). These changes have major effects on phytoplankton dynamics and primary production (Castel et al. 1983), especially in shallow estuaries where phytoplankton blooms are linked to fluctuations in river flow, stratification of the water column, grazing pressure by zooplankton, nutrient dynamics and light availability. In general, phytoplankton communities are characterized by their rapid response to alterations in environmental conditions (Pilkaitytë et al. 2004) such as anthropogenically induced eutrophication of coastal water (Nwankwo et al. 2008). The latter char-

acteristic makes phytoplankton sensitive indicators of change in aquatic ecosystems (Vuorio et al. 2007).

The south-eastern Ivory Coast Ebrié Lagoon is a dynamic environment with a complex of coastal features and biochemical properties (Koné 2009). The most recent researches conducted on its phytoplankton are three decades old (Maurer 1978, Iltis 1984), which is outdat-ed and needs to be updated. The high human population density and rapid economic growth of this region makes the coastal environment vulnerable to a whole range of anthropogenic stress factors (Seu-Anoï 2012). The sur-rounding water bodies are highly eutrophicated, leading to frequent oxygen depletion, massive fish kills and repel-ling sulphuric smells (Scheren et al. 2004), and have been included in the recent compilation of coastal “dead zones” (Diaz & Rosenberg 2008). The objective of the present study was to relate information on the phytoplankton structure of the Ebrié Lagoon water with environmental factors. We tested the following hypotheses. (1) Sites with similar environmental characteristics are also similar with respect to species composition (environmental signal). This is based on the assumption that community com-position is determined by species-specific responses to environmental gradients. (2) We also tested for a possible variance of species occurrence due to seasonal changes

Vie et milieu - life and enVironment, 2013, 63 (3/4): 181-192

PhyTOPLANKTON DISTRIBuTION AND ITS RELATIONShIP TO ENVIRONMENTAL fACTORS IN ThE EBRIé LAgOON,

IVORy COAST, WEST AfRICA

n. m. Seu-anoï *1, Y. J. m. Koné 2, K. n. Kouadio 1, a. ouattara1, G. Gourène 1

1 université nangui abrogoua, laboratoire d’environnement et de Biologie aquatique, 02 BP 801 abidjan 02, ivory Coast2 Centre de recherche océanographique (Cro), BP V 47 abidjan, ivory Coast,

* Corresponding author: nettomiranoy@yahoo.fr

ABSTRACT. – We report on the composition, structure, abundance, biomass variations of the coastal phytoplankton communities and their relationship with respect to environmental param-eters in the Ebrié Lagoon, south-eastern Ivory Coast. The studies reveal the environmental parameters reflecting seasonal changes related to rainfall distribution pattern and tidal seawater incursion. Surface water temperature (26.3-31.1 °C), ph (6.54-8.11), transparency (0.15-1.7 m), salinity (0-27.5), SRP (0.04-2.76 µmol L-1) show increased values in the dry compared to the wet season. On the other hand nitrate (0-21.56 µmol L-1), SRSi (20.72 to 143.31 µmol L-1) show higher values in the wet season. The 122 identified species were represented mainly by Bacil-lariophyta (36.1 %), Chlorophyta (23.8 %), followed by Cyanoprocaryota (22.1 %) and Eugle-nophyta (14.7 %). Species composition of phytoplankton was typical of eutrophic conditions and was frequently characterized by the presence of Bacillariophyta. Phytoplankton standing crop attained the highest counts during short or long dry season (19-500 x 106 cells L-1) due to the dominance of Cyanoprocaryota species (Phormidium sp., Cylindrospermopsis raciborskii, oscillatoria tenuis, o. limosa, lyngbya martensiana and microcystis aeruginosa). The Shan-non-Wiener and Equitability indices indicated pollution stress and dominance by a few species. Recorded chlorophyll a values give the Ebrié Lagoon the eutrophic status. It is suggested that increasing tidal influence associated with reduced rain events may have enhanced elevated salinities and created conditions for the development of more algal cells, hence higher chloro-phyll a records.

PhyTOPLANKTON STRuCTuREENVIRONMENTAL PARAMETERS

RESTRICTED LAgOONfREShWATER fLOW

182 N. M. SEu-ANOï, y. J. M. KONé, K. N. KOuADIO, A. OuATTARA, g. gOuRèNE

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(temporal signal) and disturbance of the system (distur-bance signal).

MATERIALS AND METHODS

Study area description: Ebrié lagoon system is located in the far east of the coast of Ivory Coast between 5°02’-5°42’N and 3°47’-5°29’W (fig. 1). The lagoon system consists in Potou Lagoon, Aghien Lagoon and the Ebrié Lagoon. The main char-acteristics of these lagoons and tributary rivers are shown in Table I. The Ebrié lagoon system is connected to the sea by an artificial channel (Vridi Channel) and is the largest lagoon in West Africa with a total area of 524 km2 (Adingra & Arfi 1998). This lagoon system falls under the “restricted lagoon” class according to the Kjerfve (1985) classification. It extends over 140 km of the coastline with a mean depth and a width of 4.8 m and 7 km, respectively (Koné 2009). The Ebrié lagoon system is surrounded by mangrove forests and the annual freshwater inputs from Comoé River, which is estimated to be ~7 km3 rep-resenting ~3 times the total volume of the lagoon system, while the flow of seawater is ~14 times this volume (Durand & guiral

1994). The lagoon system is strongly polluted by domestic and industrial waste water inputs (Adingra & Arfi 1998). The waters around Abidjan are highly eutrophicated, leading to fre-quent oxygen depletion, massive fish kills and repelling sulphu-ric smells (Scheren et al. 2004), and have been included in the recent compilation of coastal “dead zones” (Diaz & Rosenberg 2008). It is important to highlight that the Ebrié lagoon system is a restricted lagoon system where marine influence is more important. The climate in the study area is close to equatorial, having two rainy seasons separated by two dry seasons (Durand & Skubich 1982). The long rainy season (LRS) occurs from May to July and is followed by the short dry season (SDS) from August to September. The short rainy season (SRS) starts from October to November while the long dry season (LDS) occurs from December to April. The annual rainfall is about 2000 mm.

Sampling site: Thirteen stations were chosen that were the same as the monitoring stations of the system Ebrié lagoon (fig. 1). Based on nutrient sources and salinity gradient, the 13 stations represent four main regions: eastern waters (1-7), which are under the influence of Mé River, Aghien Lagoon (stations 1-3) and Comoé River (stations 4-7); central waters (8-10),

fig 1. – Location of the study stations in Ebrié Lagoon, Ivory Coast.

PhyTOPLANKTON DyNAMIC IN EBRIé LAgOON, IVORy COAST, WEST AfRICA 183

Vie milieu, 2013, 63 (3/4)

which are close to Abidjan city and dominated by high anthro-pogenic pressure, high salinity coastal/shelf seawater; western waters (11-13), which are under the influence of Agnéby River.

analysis of physico-chemical variables: Temperature, salin-ity, ph and transparency were measured in situ using a WTW COND 340-i conductivity meter for temperature and salinity, an ORION 230-A meter for ph and a Secchi disc for water trans-parency. The nutrient concentrations such as nitrates (NO3

-), soluble reactive phosphate (SRP) and soluble reactive silicate (SRSi) were analyzed in a series of four samples collected in July, September, November and february, which are considered representative of the different seasons of the year. Water sam-ples for nutrient measurements were filtered through Sartorius cellulose acetate filters, re-filtered through 0.2 µm pore size polysulfone filters and preserved with hgCl2 for NO3

- and SRP, and with hCl for soluble reactive Si. NO3

- concentrations were determined following the Auto Analyser II (Tréguer & Le Corre 1975), with an estimated accuracy of ± 0.1 µmol L-1 and a mini-mum detection limit of 0.05 µmol L-1. SRP and SRSi concen-trations were analyzed according to the standard colorimetric methods (grasshoff et al. 1983), with an estimated accuracy of ± 0.01 µmol L-1 and ± 0.1 µmol L-1, respectively.

Sampling and analysis of biotic variables: A bolting silk (20 µm) plankton net was used to collect phytoplankton in June, September and November 2006 and in february 2007 at all sta-tions during the four seasons. The net was dragged horizontally for 6 m in the surface water to obtain a sample of phytoplank-ton. The total volume of water that passed through the plankton net was calculated as V = r 2 × π × d, where r is diameter of the plankton net; π, 3.14 and d, distance covered by the plankton net.

for quantitative estimation of phytoplankton, duplicate water samples were stored in polyethylene bottles and preserved with 5 % buffered formalin. After 24 h of sedimentation, plankton was identified in the laboratory using an Olympus BX40 micro-scope equipped with a calibrated micrometer.

Samples for diatom (Bacillariophyta) analyses were treated with 10 % nitric acid on a hot plate for 10 min and then left to cool. Then, after several rinses with distilled water, 1 ml of the sample was spread on a cover slip and left to dry at room temperature before being permanently mounted using Naphrax, a highly refractive mounting medium.

The classification system followed Round et al. (1990) for diatoms and Van Den hoek et al. (1995) for other algal divi-

sions. The different species encountered were identified based on morphological criteria, after consultation of various works on identification. The tests and journal articles used most frequent-ly to aid in taxonomic identification were Desikachary (1959), Komárek & Anagnostidis (2005) for Cyanobacteria; huber-Pes-talozzi (1955) for Euglenophyta; Chapman (1961) and Komárek & fott (1983) for Chlorophyta; Tomas (1995) for Dinophyta; Krammer & Lange-Bertalot (1988, 1991), Tomas (1995) and hartley et al. (1996) for Bacillariophyta.

The phytoplankton counts were conducted using an inverted microscope, following the utermöhl method (utermöhl 1958). The abundance of taxa was expressed as cells L-1 (utermöhl 1958, Aktan et al. 2005). The total number of cells was count-ed according to the Stirling (1985) formula. The structure of the phytoplankton assemblages was studied by calculating the Shannon-Weaver diversity (h’) and evenness (J’) according to the equations of Shannon (1949) and Pielou (1966), respectivi-ly. Water samples (250 ml) for chlorophyll-a analysis were fil-tered through Whatman gf/f filters (0.7 µm). These filters were stored for 24 h at -40 °C and then pigments were extracted with acetone (90 %) for 24 h in the dark. The extracts were centri-fuged and chlorophyll-a concentrations were measured by spec-trophotometry before and after acidification with hCl (0.1 N). Chlorophyll-a concentrations were estimated using the equa-tions given by Lorenzen (1967).

Statistical analyses: Parametric and non-parametric tests were conducted in order to determine whether the results obtained from different statistical treatments were significantly different (p < 0.05). Prior to testing, normality and homoscedas-city of data were checked. for the abiotic parameters, density and chlorophyll-a concentration, differences among data were tested for statistical significance (p < 0.05) with the Kruskal-Wallis method (Zar 1999) because normal distribution could not be assumed for their data. using One-way Anova followed by the Tukey test was computed for Shannon-Weaver diversity (h’) and evenness (J’). All these analyses were carried out using the STATISCA 7.1 computer software.

To evaluate the joint influence of several parameters on phy-toplankton, a multivariate analysis was performed by ReDun-dancy Analysis (RDA) using the program CANOCO 4.5. The P-value was obtained by a Monte Carlo permutation test (5000 permutations), carried out for all canonical axes. In this analy-sis, 17 phytoplankton taxa having a relative abundance > 1 % in at least 5 samples (as shown in Table II) and 7 environmen-tal variables were taken into account. Abundance values were transformed by log ([100 × abundance] + 1).

Table I. – Main morphometric characteristics of Ebrié system lagoon and of the main rivers (Mé, Comoé and Agnéby) flowing into this system lagoon (unpubl data from Koné 2009 and Durand & Chantraine 1982).

EbriéLagoon system

Area (km2)Volume(km3)

Meandepth (m)

RiversTotal length

(km)Drainagearea (km2)

Mean waterdischarge (m3s-1)

Aghien 22 – –Mé 140 4300 47

Potou 21 0.03 2.7

Ebrié 523 2.6 4.8Comoé 1160 78000 224

Agnéby 200 8900 27

184 N. M. SEu-ANOï, y. J. M. KONé, K. N. KOuADIO, A. OuATTARA, g. gOuRèNE

Vie milieu, 2013, 63 (3/4)

Tabl

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PhyTOPLANKTON DyNAMIC IN EBRIé LAgOON, IVORy COAST, WEST AfRICA 185

Vie milieu, 2013, 63 (3/4)

RESULTS

Environmental variables

The temperature at the water surface varied signifi-cantly across seasons in all stations (fig. 3). During the survey, the temperature fluctuated between 26.3 and 31.1 °C. The higher values were obtained during the long dry season in all regions. generally, the short dry season was characterized by lower temperature values. The tem-perature did not differ significantly between the regions while significant differences were noted between LRS and LDS. A clear temporal and spatial variation in ph and salinities was observed in all seasons. Those parameters were higher during the long dry season and lowest during the rainy season at all sites. The ph oscillated between 6.54 to 8.11 (fig. 3), and there was a significant differ-ence between these two seasons and between the regions (p < 0.05). The salinities values fluctuated between 0 and 27.5. Notable differences of this parameter were observed between the sector IV and other sectors and between LRS and LDS (p < 0.05). Transparency was lowest (close to 0.15) in all stations during the long dry season and high

(up to 1.7) in sector II during the short dry season. Nota-ble difference was noted between sectors IV and II and between LRS and LDS. Nitrate concentration exhibited a clear seasonal variation in all stations, with the lowest (0 µmol L-1) during the long dry season and the highest (21.56 µmol L-1) during the long rainy season (fig. 3). Significant difference was found between LDS and LRS (p < 0.05). Soluble reactive phosphate concentration showed no clear spatial or temporal pattern. however, soluble reactive phosphate concentration was low (0.04-0.2 µmol L-1) in regions I and II during the long dry sea-son. generally, the soluble reactive silicate concentration was significantly higher during the long rainy season in all regions (p < 0.05), while the lowest values were recorded in region IV during the short rainy season. The concentra-tions ranged between 20.72 and 143.31 µmol L-1.

Phytoplankton community, abundance and biomass

We identified 63 phytoplankton genera, 122 species belonging to five taxonomic groups: Diatoms (36.1 %), Chlorophyta (23.8 %), Cyanoprocaryota (22.1 %), Eugle-nophyta (14.7 %), and Dinophyta (3.3 %). Species com-

fig 2. – Seasonal and spatial variation in abiotic parameters, abundance and biomass of phytoplankton at the study area (LRS: Long Rainy Season, SDS: Short Dry Season, SRS: Short Rainy Season, LDS: Long Dry Season).

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fig 3. – Seasonal and spatial variation in diversity (h’) and evenness (J’) of phytoplankton at the study area (LRS: Long Rainy Season, SDS: Short Dry Season, SRS: Short Rainy, LDS: Long Dry Season).

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position was typical of eutrophic conditions and was fre-quently characterized by the presence of bacillariophyta. Compared to the other taxonomic groups, Diatoms was the most important group in terms of species number. however, the most diversified genera were: Strombomo-nas (5 %), followed by trachelomonas and aulacoseira (4.1 % each). According to their habitat association, 89 % of these taxa were freshwater taxa while 10.6 % were marine affinity taxa. The highest (43 taxa) and the lowest (25 taxa) richness values were recorded in region I (east part) and IV (westward), respectively. however, species richness presented small modifications during almost the entire period from one sector to another (Table II). Phy-toplankton density presented different patterns accord-ing to the sector (fig. 3), and varied from 1 106 to 500 106 cells L-1

. The highest density values were observed in all regions during the long dry season, except region IV where high values were noted during the short dry sea-son. The lower values were obtained in the long rainy season in all regions. Besides, shifts were observed in the dominant species during each season. In the period of high density, for instance, the dominant (more than 50 %)

species were Cyanobacteria taxa (Phormidium sp., Cylin-drospermopsis raciborskii, oscillatoria tenuis, o. limosa, lyngbya martensiana and microcystis aeruginosa). No significant difference in density was found between dif-ferent sectors (p > 0.05). however, notable difference in density was found between the long rainy and the long dry seasons (p < 0.05). Concerning the Chlorophyll-a concentrations (fig. 3), it showed no clear spatial or tem-poral pattern and varied between 0 and 22 µg L-1. howev-er, the Chlorophyll-a concentrations were low (0 µg L-1 to 4.3 µg L-1) in stations in the vicinity of the Comoé River mouth in the East part (region II) during all seasons, and relatively high (4.3 µg L-1 to 22 µg L-1) in region IV dur-ing the short dry and long rainy seasons (fig. 3).

Diversity evaluation

The values of the species diversity index varied between 0.001 and 2.35 (fig. 2). The lower values were noted during the long dry season in region III, while high-er values were obtained during the short rainy season in sector II. Concerning the Pielou evenness index (J’), the

fig 4. – Triplots obtained through the RDA of physico-chemical variables, stations and phyto-planktonic abundance in Ebrié Lagoon (see table I for abbrevia-tions). Circles (I and II) represent the groups identified based on the seasonality.

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values fluctuated from 0.001 to 0.95 during our study. The minimum (region III) and maximum (region II) values were observed during long dry season and short rainy sea-son, respectively. During the short dry season, these indi-ces varied slightly from one sampling station to another. Notable significant differences in the Shannon-diversity values were observed (p < 0.05) between the region III and the others, and between LDS and SRS. Concerning the Pielou evenness values, no significant differences were detected between sectors and seasons (p > 0.05).

Phytoplankton and environmental variables

The Monte Carlo permutation tests (n = 1000 permuta-tions) indicated that the results of the redundancy analysis performed were significant (p < 0.01). The first and sec-ond axes of the RDA analysis performed with species and environmental variables explained 36.1 % and 16.8 %, respectively, of the total variance of the species matrix. The first axis, principally defined by the nitrates and temperature, presented the strongest correlation between species and environmental variables. The second axis was defined by the nutrients SRP, transparency and salin-ity (fig. 4). Two groups of samples and taxa can be dis-tinguished in the graph: (I) samples from the LRS, SRS and SDS; characterized by high abundance of taxa such as lyngbya martensiana (Lyma), oocystis gigas (Oogi), microcystis aeruginosa (Miae), merismopedia elegans (Meel) and anabaena planctonica (Anpl). These species were associated with the period of higher concentration of nitrates, SiRD and PRD; (II) samples from LDS dominant by the taxa were Cylindrospermopsis raciborskii (Cyra), Komvophoron minutum (Komi), anabaena sp. (Ansp), oscillatoria limosa (Osli) and Planktothrix agardhii (Plag). These taxa were associated with high temperature, salinity and transparency.

DICUSSION

Phytoplankton community

The phytoplankton species composition recorded for the entire study was similar to those reported previously in Ebrié Lagoon, Ivory Coast (Maurer 1978, Iltis 1984, Couté & Iltis 1988), in Aby Lagoon system, Ivory Coast (Seu-Anoï et al. 2011), in Bizerte Lagoon, Tunisia (hlaili et al. 2007) and in Qua Iboe Estuary mangrove swamp, Nigeria (Essien et al. 2008). however, the phytoplankton communities found in Ebrié Lagoon were high compared to those recorded in Izmit Bay, Turkey (Aktan et al. 2005) and in Laguna de Rocha, uruguay (Aubriot et al. 2004). This might be attributed to the relatively high nutrient (SRP and nitrates) status (Conde et al. 2007). Indeed, the Ebrié Lagoon system receives enormous quantities of anthropogenic wastes (domestic and industrial), such as

raw human from its surroundings. These wastes increase the nutrient capabilities of the lagoon. These nutrients (soluble reactive phosphate and nitrates) stimulate phyto-plankton (algae) growth (Pilkaitytë et al. 2004). however, the number of phytoplankton taxa observed (122 specific and subspecific taxa) was not exhaustive because taxa under 20 µm were not collected in the plankton net.

In Ebrié Lagoon, Diatoms (36.1 %) were the most important group in terms of species number compared to other taxonomic groups. This result could be due to fresh-water inputs from rivers that brought these diatoms into the lagoon system, as indicated by Seu-Anoï et al. (2011). In general, the phytoplankton is mostly represented by freshwater species, due to the fact that Ebrié Lagoon is strongly influenced by freshwater from rivers Comoé, Mé and Agnéby, which allowed it to contain diverse organ-isms from freshwater. Moreover, most of the dominant taxa (microcystis aeruginosa, aphanizomenon flos-aquae, Phormidium sp., Cylindrospermopsis raciborskii, oscillatoria tenuis, o. limosa, and lyngbya martensiana) were indicative of eutrophic conditions. The recorded dominant species could be considered as a result of rela-tive high PRS concentration and organic pollutants in these wastes.

Seasonal and spatial variation of phytoplankton structure

In the Ebrié Lagoon, phytoplankton experienced significant seasonal variation in their growth rates, lead-ing to changes in their abundance. The low phytoplankton abundances observed during the long rainy season were more likely related to dilution processes rather than nutri-ent inputs from the rivers (Pikaitytë et al. 2004). In addi-tion, the brown water of the rivers in the long rainy sea-son enriched the lagoon water in organic matter and thus reduced the transparency. This most likely decreased the algal growth rates during this season. According to Bon-illa et al. (2005), water discharged during rainy seasons into coastal environment can wash out phytoplankton biomass, preventing the development of blooms. In fact, in Ebrié Lagoon, during the long rainy season, the fresh-water inflow is too strong to allow biomass to build up, causing everything to be flushed out to sea. Low phyto-plankton abundances during the long rainy season in the Ebrié Lagoon are contrasted to those generally observed in other tropical lagoons where phytoplankton abundanc-es are positively correlated to nutrient inputs from the riv-ers (hlaili et al. 2007).

The redundancy analysis indicated that the phytoplank-ton species distribution was significantly correlated with nitrates, SiRD and PRD during the LRS, SRS and SDS and with temperature, salinity and transparency during the LDS. In fact, samples from the LRS, SRS and SDS were characterized by high abundance of taxa such as lyngbya martensiana (Lyma), oocystis gigas (Oogi), microcystis

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aeruginosa (Miae), merismopedia elegans (Meel) and anabaena planctonica (Anpl). Samples from LDS were dominated by taxa such as Cylindrospermopsis racibor-skii (Cyra), Komvophoron minutum (Komi), anabaena sp. (Ansp), oscillatoria limosa (Osli) and Planktothrix agardhii (Plag). According to Kadiri (1993) the distribu-tion of phytoplankton species is influenced by changes in the physical and chemical properties of the water which can be dependent on rainfall. Similarly, salinity is known to regulate the occurrence and distribution of biota in the lagoon (Nwankwo, 2004).

Ebrié Lagoon was characterized by proliferation of the taxa N2-fixing filamentous and filamentous non-het-erocystous Cyanobacteria (gas vesicles species specially) during the long dry season because these species are able to growth in low NO3

- conditions and have the capacity to fix atmospheric nitrogen (Walsby 2001). Abundance values (1 106 to 500 106 cells L-1) of phytoplankton in the Ebrié Lagoon are close to those observed in the tropical and temperate lagoons that varied from 1 106 to 934 106 cells L-1 (Lehman et al. 2010, Seu-Anoï et al. 2011).

During this study, phytoplankton biomass showed slight variations between sectors and seasons. The Chlo-rophyll-a concentrations were relatively high during the short dry season in sector IV. During this period, the Cyanobacteria were also numerous, due to the very high concentrations of anabaena flos-aequa (Anfl), a. planc-tonica, oscillatoria princept, which contributed > 70 % of total phytoplankton biomass in this sector. Indeed, the high transparency during the short dry season in this sector leads to a good light penetration. This resulted in increases in the algal growth rates, leading to elevation in their biomass. This explains significant relationship (p < 0.05) between transparency and growth, during this season, of eutrophic Cyanobacteria (anabaena flos-aequa, a. planctonica, oscillatoria princept).

In Ebrié Lagoon system, the lower diversity (h’) and evenness (J’) values obtained during the long dry season in sector III were attributed to the dominance of six spe-cies, Phormidium sp., Cylindrospermopsis raciborskii, oscillatoria tenuis, o. limosa, lyngbya martensiana and microcystis aeruginosa, constituting about > 50 % by number to the total count. In fact, the diversity index depends on the dominance of species, not on the total phytoplankton counts, i.e. when many species shared the phytoplankton bloom, higher values of diversity were recorded and vice versa (Mohamed et al. 2005). gener-ally, Ebrié Lagoon showed diversity similar throughout the study period, when compared with previous records in Ebrié Lagoon system (Dufour 1984), Aby Lagoon sys-tem, Ivory Coast (Seu-Anoï et al. 2011), Mediterranean Sea Lagoon, Egypt (Mohamed et al. 2005). Diversity and evenness indices are a useful approach to estimate biolog-ical quality through the structure of species community (Aktan et al. 2005).

CONCLUSION

The present study was the first to examine the distri-bution of phytoplankton communities in Ebrié Lagoon system in relation to anthropogenic nutrient input. The composition and distribution of the phytoplankton com-munity in the surface waters of the Ebrié Lagoon were clearly related to seasonal trends. The study contributed to identify 122 taxa, which were dominated by fresh-water species, due to the fact that the Ebrié Lagoon sys-tem is influenced by high freshwater inflow from riv-ers. Because of this high freshwater input from rivers, whatever the season, the phytoplankton community was mostly represented by Bacillariophyta (36 %). Of the periods sampled, abundance of microalgae was greatest during the long dry season, when the chlorophyll-a was greater than during the short dry season. The diversity of the phytoplankton community also varied seasonally and was low in sector II during the long dry season. The Ebrié Lagoon was numerically dominated by Cyanoprocaryota species (Phormidium sp., Cylindrospermopsis racibor-skii, oscillatoria tenuis, o. limosa, lyngbya martensiana and microcystis aeruginosa). Compared to the taxonomic list of similar other ecosystems, phytoplankton communi-ty in the Ebrié Lagoon appears to be diversified and abun-dant. however, the observed assemblage composition indicates that anthropogenic disturbance not only affects system descriptors, such as species richness, abundance, and diversity, but may also alter the system dynamics. We draw attention to the importance of conserving estuar-ies, especially the ébrié Lagoon which will in turn help ensure the persistence of phytoplankton assemblages. Appropriate habitats for phytoplankton taxa still remain in the ébrié Lagoon, highlighting the need for additional surveys.

Acknowledgments – The authors are grateful to Dr JE Niamien-Ebrottié, AT Kouassiblé and MP Adon, université d’Abobo-Adjamé, for their help with phytoplankton identifica-tion. y. J-M Koné, in collaboration with university of Liège, received financial support from the government of Ivory Coast and from Agence universitaire de la francophonie (Auf).

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received on July 23, 2013 accepted may 22, 2014

associate editor: C fernandez