Spatio-temporal distribution of phytoplankton in fourcoastal rivers of southeastern of Ivory Coast (Soumi�e,Eholi�e, Ehania and No�e)

Julie E. Niamien-Ebrottie1*, F�elix K. Konan2, Allassane Ouattara1 and GermainGourene1

1Laboratory of Aquatic Environment and Biology, Department of Sciences and Environment Management, University Nangui Abrogoua Abidjan,

Abidjan 02 BP 801, Ivory Coast and 2Department of Environnement, University Jean Lorougnon Gu�ed�e Daloa, Daloa BP 150, Ivory Coast

Abstract

Spatio-temporal dynamics of phytoplankton and their

relation to abiotic environmental factors in four rivers of

south-eastern Ivory Coast (Soumi�e, Eholi�e, Ehania and

No�e) was analysed from July 2003 to March 2005. The

pelagic zone of each river was sampled upstream and

downstream. Phytoplankton abundance was higher in

No�e River (154.3 104 cells l�1) and lower in Eholi�e river

(23.05 104 cells l�1). Dominant taxa were Microcystis

aeruginosa (K€utz.) Lemmerm. and Aphanocapsa incerta

(Lemmerm.) Cronberg & Kom�arek. In general, minimum

densities of phytoplankton were observed during the rainy

season, while maximum were observed in dry season in

the whole stations of the rivers studied, such periods

corresponding to low and high concentrations of nitrates.

Among the rivers surveyed, Eholi�e river seems to be the

least disturbed because of its higher species diversity.

Seasonality fluctuations of algae abundance appear to be

influenced by the flow of water and nitrate levels. This

work is a useful starting point for future research on micro

algae in Ivory Coast.

Key words: abundance, coastal Rivers, Ivory Coast,

Phytoplankton, species diversity, structure

R�esum�e

Nous avons analys�e la dynamique spatio-temporelle du

phytoplancton et sa relation avec des facteurs environn-

ementaux abiotiques dans quatre cours d’eau du sud-est de

la Cote d’Ivoire (Soumi�e, Eholi�e, Ehania et No�e) de juillet

2003 �a mars 2005. Des �echantillons furent pr�elev�es dans

la zone p�elagique de chaque cours d’eau, en amont et en

aval. Le phytoplancton �etait plus abondant dans la

No�e [(154,3 104 cellules l�1) et moins dans l’Eholi�e

(23,05 104 cellules l�1). les taxons dominants �etaient

Microcystis aeruginosa (K€utz.) Lemmerm, et Aphanocapsa

incerta (Lemmerm.] Cronberg & Kom�arek. En g�en�eral, nous

avons observ�e les densit�es minimales de phytoplancton

pendant la saison des pluies et les densit�es maximales

pendant la saison s�eche dans toutes les stations des cours

d’eau �etudi�es, ces p�eriodes correspondant respectivement �a

des concentrations faibles et �elev�ees de nitrates. De toutes

les rivi�eres �etudi�ees, l’Eholi�e semble etre la moins perturb�ee

vu sa plus grande diversit�e d’esp�eces. Les fluctuations

saisonni�eres de l’abondance d’algues semblent etre influ-

enc�ees par le d�ebit de l’eau et les taux de nitrates. Ce travail

est un point de d�epart utile pour toute recherche future sur

les micro-algues de Cote d’Ivoire.

Introduction

Algal community is, in terms of abundance and biomass, a

critical component of aquatic environments. It is by its

concentration on substrates or suspension in the water

column, a food source for many aquatic organisms such as

zooplankton, insects and some fishes (grazers and filter)

(Large et al., 1993 and Round, 1993). Indeed, Carmichael

(1994) and Kotak et al. (1996) have showed that in

stability conditions and favourable nutrient enrichment,

algal population can grow, and depending on the case, it

may be beneficial or harmful to human. Strong growth of

human activities impact on watershed in West Africa, in

particular agricultural activities which use extensive*Correspondence: E-mail: brouestelle@yahoo.fr

© 2013 John Wiley & Sons Ltd, Afr. J. Ecol. 1

inputs such as fertilizers or pesticides, leading to water

quality disturbance. This is due to nutrients accumulation

from agricultural run-off, which has a direct impact on

organisms that live there, especially algae.

These micro-organisms are generally poorly studied in

Africa, especially in freshwater ecosystems where the most

significant scientific works are those conducted by

Comp�ere during the 1970s (see Comp�ere, 1977). In Ivory

Coast, studies concern some systematic aspects (Da, Traor�e

& Ass�emien, 1999 and Ouattara et al., 2000) and ecology

of algal community (Iltis, 1982; Ouattara et al., 2003;

Ouattara, Podoor & Gour�ene, 2007; Niamien-Ebrotti�e

et al., 2013). However, only few studies have been

performed on phytoplankton of south-eastern rivers of

Ivory Coast. Thus, the structure of algal community is not

well known in areas of high agricultural activities as this

part of the country. Indeed, this region is known, over the

past few decades, for its high agricultural productivity

related to industrial and village plantations of cash crops

(coffee, cocoa, pineapple and palm oil). These plantations

cover 44% of the surface area (Anonymous, 2001).

However, these crops productivity is strongly supported

by the use of chemical products such as fertilizers and

pesticides. Such practices expose to disturbances of water

systems quality and ecology.

The general objective of this work is to study phyto-

plankton dynamics from four rivers in south-eastern Ivory

Coast. More specifically, it is to describe the spatial

structure of these community, following their temporal

evolution and development in relation to physico-chemical

and hydrological variables to enable a better understand-

ing of the rivers ecological functioning.

Materials and methods

Study area

The south-eastern Ivory Coast has a relatively dense

hydrographic network consisting of several coastal rivers,

which include Soumi�e, Eholi�e, Ehania and No�e. Soumi�e

River, a tributary of River Bia, measures 41 km with a

catchment area of 458 km2. Its mean annual flow is

11.8 m3 s�1 with a slope of 0.33%. With 35 km length,

Eholi�e River drains a catchment area of 435 km² with a

slope of 0.3% and a mean annual flow of 11.4 m3 s�1.

Ehania River extends over a distance of 87 km and covers

a catchment area of 747 km2. The slope and the mean

annual flow are, respectively, 0.24% and 15.7 m3 s�1.

No�e River has a length of 30 km, an average annual flow

of 9.6 m3 s�1, a slope of 0.15% and drains an estimated

282-km2 watershed. Two sampling stations were selected

in each river as follows: Soumi�e [stations S1 (05°N 29′44′′

and 03°W 22′15′′) and S2 (05°N 24′53′′ and 03°W 16′56′

′)]; Eholi�e [stations E1 (05°N 28′36′′ and 03°W 08′25′′)

and E2 (05°N 23′58′′ and 03°W 08′44′′)]; Ehania [stations

Eh1 (05°N 24′39′′ and 02°W 55′43′′) and Eh2 (05°N 16′

71″and 02°W 50′02″)]; No�e [stations N1 (05°N 16′71″

and 02°W 50′02″) and N2 (05°N 17′73′′ and 02°W 46′

99′′)] (Fig. 1). Those stations were chosen on the basis of

their accessibility and hydrological zonation. Stations

surveyed in waters’ course are surrounded by plains

except station E1 (Akakro), which is a U-shaped valley and

that of Eh1 (Affi�enou) in an asymmetrical valley. Stations

are characterized by the presence of cocoa plantations.

Stations Eh1 (Affi�enou), N1 (M’Possa) and N2 (No�e) are

located near villages. Aquatic vegetation (Nymphaea lotus

and Eichhornia crassipes) only present at Affi�enou, cover

30% of water.

Measuring of abiotic parameters

Eight sampling campaigns were carried out from July

2003 to March 2005. These campaigns cover the four

climatic seasons (long dry season, long rainy season, short

dry season, short rainy season) on the basis of two

sampling by season. Physico-chemical parameters were

measured using various devices. A GPS MLR SP12X served

Fig 1 Location of rivers and sampling stations (modified by

Konan et al., 2006.) S = Soumi�e, E = Eholi�e, Eh = Ehania,

N = No�e, 1 = upstream, 2 = downstream

© 2013 John Wiley & Sons Ltd, Afr. J. Ecol.

2 Julie E. Niamien-Ebrottie et al.

of location of stations. Conductivity was measured using a

HACH conductivity type, model 44600. A pH meter WTW

was used to measure pH and temperature of the water.

Measurement of dissolved oxygen was taken using a pulse-

type WTW. A Secchi disc has enabled determination of

transparency. Five floats, a stopwatch and a tape of

measurement were used to determine water velocity.

These measurements were taken in situ. For nitrate

concentration, a sample of surface water was taken and

kept in a bottle of one litre at a temperature of 4°C. In the

laboratory, nitrate concentration was determined accord-

ing to standard T90-110 (AFNOR, 1994).

Sampling, observation, identification and plankton counting

Phytoplankton was collected using a Van Dorn bottle of

2.5 l capacity for quantitative investigations and a plank-

ton net (10 lm of mesh) for qualitative one. Samples were

stored in 30-ml pill and fixed in formalin 5%. Qualitative

sample was separated in two, one part was oxidized to

cleaned diatoms frustules and the over one was used to

identified over phytoplankton. Oxidation of the raw

material was carried out boiling material of diatoms with

technical nitric acid during 1 h through the method of

Prygiel and Coste (2000). Observation of taxa was

performed using a Trinocular microscope type Olympus

BX40. Slides and cover glass were used for mounting.

Identification of taxa was made at specific or infraspecific

level using various works (keys and/or description) such as

those of Foged (1966), Comp�ere (1977, 1991), Krammer

and Lange-bertalot (1991), Cocquyt (1998), Kom�arek and

Anagnostidis (2005), Ouattara et al. (2000).

Counting of phytoplankton was performed after homog-

enization of samples. Only samples collected using hydro-

logical bottle were taken into account. A fraction was

taken, mounted between slide and cover slip and observed

under a Trinocular microscope. Number of considered

mounts was determined by the method Uehlinger men-

tioned in Lazzaro (1981). In this study, the number of

mounts per sample was set at five. Phytoplankton density

is expressed as number of cells per volume unit (cells l�1).

Characterization of stand algal

Species richness, diversity index of Shannon–Wiener (H‘)

and evenness (E) were calculated to characterize the

structure of the stand. Species richness is a good indicator

of a station’s capacity. Shannon–Wiener index measures

the degree of organization of settlement and fairness to

study regularity of species distribution.

Diversity is minimum when H’ tends to 0 and maximal

when H’ tends to infinity (Washington, 1984).

Low value of E indicates that population is dominated by

few species. Evenness tends to 1 when all species have the

same abundance.

Statistics processing

An algorithm of self-organizing maps (SOM) or Kohonen

maps (Kohonen, 1995) was used to order samples from

species assemblages. The SOM Toolbox (version 2) inter-

face for Matlab� (MathWorks, Natick, MA, USA) used was

developed by the Helsinki University of Technology and is

available at: http://www.cis.hut.fi/projects/somtoolbox/.

Multiple stepwise regression was used to determine phys-

ical and chemical parameters that significantly influence

taxa distribution. This test was performed with the

software STATISTICA 7.1 (StatSoft, Inc, 2005). Nonpara-

metric tests, Kruskal–Wallis test and Mann–Whitney U-

test, were used to measure the degree of biological indices

calculated variation. Tests are significant at P < 0.05.

Results

Physico-chemical variables

Spatial and temporal variations of physical and chemical

parameters of water in the four rivers surveyed (Table 1)

indicate very low pH changes from upstream to down-

stream and from one season to another. These values have

the same magnitude and oscillate on either side of

neutrality (6.6–7.5). In all rivers, dissolved oxygen con-

centrations (3.6–9.8 mg l�1, P < 0.05) were significantly

higher downstream than upstream. Regarding the flow, it

has a low variation from one station to another and from

one season to the next in all rivers. The highest rates

(0.7 m s�1) were recorded during the short rainy season

in all rivers and the lowest rates (0.1 m s�1) during the

high rainy season and high dry season. Nitrate concen-

trations in all the streams are lower upstream

(1–1.5 mg l�1) relative to the downstream stations (2.2–

3.4 mg l�1). High concentrations of nitrates are noted

during the high rainy season at all stations of the rivers

except the downstream of No�e River (high dry season). As

for conductivity, values obtained are higher upstream

(60.8–68 lS cm�1) of all rivers except Eholi�e River where

© 2013 John Wiley & Sons Ltd, Afr. J. Ecol.

Phytoplankton dynamics in four coastal rivers of Ivory Coast 3

the variation is lower (52.3 and 60 lS cm�1) from

upstream to downstream. The seasonal variation is not

well marked (P > 0.05).

Abundances of phytoplankton

One hundred and ninety-three (193) species and infraspe-

cies were identified. The absolute abundances of phyt-

plankton of Soumi�e, Eholi�e and Ehania are high during the

short dry season in both upstream (respectively, 20.7

104cellules l�1, 27.7 104 cells l�1 and 62 104 cells l�1)

and downstream (respectively, 49.7 104 cells l�1, 18.4

104 cells l�1 and 7.8 104 cells l�1). Abundances remain

low during the other seasons. Among the different groups

that make up the population, cyanobacteria have high

proportions over (50%) in this period. Dominant species

recorded are Microcystis aeruginosa (Miae) (K€utz.) Lem-

merm. and Aphanocapsa incerta (Apin) (Lemmerm.) Cron-

berg & Kom�arek. This group is followed by Chlorophyta,

which represent more than 10% of taxa identified. In the

lower reaches of river Eholi�e, Rhodophyta are most

abundant (42%). Audouinella hermannii (Auhe) (Roth)

Table 1 Physical and chemical parameters values (dissolved oxygen, transparency, flow, nitrate concentrations, conductivity and pH) of

rivers

Rivers Stations Saisons Temperature (°C) pH

Dissolved

oxygen

(mg/L) Transparency (m)

Flow

(m3/s)

Nitrate

(mg/l)

Conductivity

(lS/cm)

Depth

(m)

Soumi�e S1 LRS 24.40 7.48 4.46 0.61 0.33 1.93 60.80 1.23

SDS 24.47 7.13 4.17 0.58 0.34 1.64 58.47 0.66

SRS 26.40 6.87 3.55 0.82 0.38 1.49 55.90 0.53

LDS 25.75 6.76 5.27 0.54 0.28 1.33 53.20 0.89

S2 LRS 25.30 6.77 5.99 0.64 0.21 3.42 45.35 1.49

SDS 24.43 7.04 4.50 0.75 0.33 2.39 42.23 1.22

SRS 27.40 6.94 5.05 0.48 0.34 2.27 45.00 1.45

LDS 25.60 6.58 5.83 0.39 0.26 2.14 39.10 1.59

Eholi�e E1 LRS 25.00 6.87 6.08 0.44 0.30 1.91 52.30 1.71

SDS 25.40 7.06 5.98 0.55 0.30 1.57 57.40 1.09

SRS 26.30 6.88 5.04 0.47 0.38 1.62 55.50 0.70

LDS 26.75 7.02 6.42 0.60 0.30 1.88 24.55 1.38

E2 LRS 25.05 7.11 8.43 0.56 0.40 2.97 59.95 1.65

SDS 25.37 6.89 6.79 0.55 0.53 2.74 57.23 2.09

SRS 26.60 6.94 8.71 0.47 0.63 2.62 55.90 2.1

LDS 27.10 6.94 5.46 0.49 0.41 2.61 54.15 1.74

Ehania Eh1 LRS 24.60 7.09 5.28 0.79 0.14 1.57 67.80 1.25

SDS 25.27 6.92 6.09 0.76 0.44 1.03 64.17 1.29

SRS 26.20 7.16 4.92 0.62 0.17 1.51 60.40 0.80

LDS 26.10 7.15 4.05 0.72 0.22 1.59 62.15 0.97

Eh2 LRS 25.15 6.74 7.98 0.48 0.25 2.16 51.75 2.56

SDS 25.20 7.05 6.78 0.50 0.34 1.46 56.27 2.25

SRS 26.70 6.74 9.82 0.45 0.40 1.95 49.60 2.25

LDS 26.95 6.90 6.52 0.53 0.18 2.16 58.50 2.11

No�e N1 LRS 24.95 7.13 6.29 0.60 0.29 1.60 68.00 0.78

SDS 25.03 7.00 7.63 0.59 0.53 1.39 63.73 0.64

SRS 26.10 6.58 7.85 0.52 0.28 1.36 64.80 0.70

LDS 25.95 7.12 7.17 0.60 0.30 1.61 60.90 0.67

N2 LRS 25.10 6.92 7.32 0.44 0.49 2.11 54.60 2.25

SDS 25.50 6.67 6.80 0.46 0.54 2.10 54.47 2.13

SRS 27.60 5.76 9.82 0.37 0.30 2.18 53.30 3.86

LDS 26.40 6.83 6.67 0.44 0.23 3.15 52.95 2.15

S = Soumi�e, E = Eholi�e, Eh = Ehania, N = No�e; 1 = upstream; 2 = downstream; LRS = long rainy season, SRS = short rainy season,

SDS = short dry season, LDS = long dry season.

© 2013 John Wiley & Sons Ltd, Afr. J. Ecol.

4 Julie E. Niamien-Ebrottie et al.

Duby appears as the most abundant species in this group.

In contrast to the three rivers above, No�e River is

characterized by a high density during the dry season in

the upstream (150.9 104 cells l�1) and downstream

(157.6 104 cells l�1) in long rain season. Cyanobacteria

(80% of taxa collected) are most represented in the

population of these two periods. The dominant species of

this group is Microcystis aeruginosa (K€utz.) Lemmerm.

Diversity of phytoplankton population

The Shannon diversity index is similar than the evenness

in the rivers studied (Fig. 2). The amplitude variations of

the Shannon index in Eholi�e and Ehania are generally low.

In addition, evenness has high values during the long dry

season in the upstream of the two rivers (higher than

0.80). Unlike these two streams, fluctuations between

Shannon index and evenness are significant (P < 0.05) in

Soumi�e and No�e. The highest values of the Shannon index

are recorded during the short rainy season in No�e River

(1.65) (upstream and downstream) and upstream river

Soumi�e (2.17). The evenness has high values in the

upstream of the river Soumi�e (above 0.87) and No�e

(N1: 0.96).

Ordination of samples according to the algal density

Self-organizing maps method has been used to classify the

samples according to abundance of algae. Samples were

projected on a card of 40 cells (eight rows 9 five columns)

to minimize quantization error (QE) and topographic error

(TE), thus a good type of game input data map is

preserved.

A hierarchical cluster analysis of cells in the SOM

with the Ward method (Fig. 3) was used to classify the

cells of the self-organizing map into three groups (I, II and

III) according to the similarity of their taxonomic

assembly.

The majority (81.3%) of samples from two stations (E1

and E2) were grouped in the lower part of the card,

specifically in groups II and III (Fig. 3). Samples from other

rivers were classified in the upper part, particularly in

group I. This group includes 37, 28 and 26% of samples

collected, respectively, in Ehania, Soumi�e and No�e.

Soumié

0

0.5

1

1.5

2

2.5

GSP PSS PSP GSS GSP PSS PSP GSS

S1 S2

0

0.2

0.4

0.6

0.8

1Eholié

0

0.5

1

1.5

2

2.5

GSP PSS PSP GSS GSP PSP PSS GSS

E1 E2

0

0.2

0.4

0.6

0.8

1

Ehania

0

0.5

1

1.5

2

2.5

GSP PSS PSP GSS GSP PSS PSP GSS

Eh1 Eh2

0

0.2

0.4

0.6

0.8

1Noé

0

0.5

1

1.5

2

2.5

GSP PSS PSP GSS GSP PSS PSP GSS

N1 N2

0

0.2

0.4

0.6

0.8

1Eq

uita

bilit

y

Equi

tabi

lity

Equi

tabi

lity

Equi

tabi

lity

Shan

non

inde

xSh

anno

n in

dex

Shan

non

inde

xSh

anno

n in

dex

Shannon index Equitability

Fig 2 Diversity indexes of Shannon and equitability in different rivers (S = Soumi�e, E = Eholi�e, Eh = Ehania, N = No�e, 1 = upstream,

2 = downstream, LRS = long rainy season, SRS = short rainy season, SDS = short dry season, LDS = long dry season)

© 2013 John Wiley & Sons Ltd, Afr. J. Ecol.

Phytoplankton dynamics in four coastal rivers of Ivory Coast 5

Pattern distribution of phytoplankton was emerged.

Distribution is made from the analysis of each taxon

contribution in each group of the typology. It allows

highlight assemblies taxa that characterize each of the

three groups. This profile yielded distribution of taxa in the

different groups (Table 2). Group I is characterized by an

abundance of cyanobacteria (Planktolyngbya contorta (Lem-

merm.) Anagn. & Kom�arek, Microcystis aeruginosa (K€utz.)

Lemmerm. and Aphanocapsa incerta (Lemmerm.) Cronberg

& Kom�arek, Euglenophyta (Phacus sp. and Trachelomonas

volvocina (Ehrenb.) Ehrenb) and Diatoms (Aulacoseira

granulata (Ehrenb.) Simonsen, Gomphonema sp., Eunotia

sp., E. monodon Ehrenb. and Ulnaria biceps (K€utz.)

Comp�ere). The second group is characterized by a high

diversity of diatoms with 75% of taxa (Planothidium

lanceolatum (Br�eb. ex K€utz.) Lange-Bert., G. parvulum K€utz.,

Capartogramma crucicula (Grunow ex Cleve) Ross, Gyro-

sigma acuminatum (K€utz.) Rabenh. Hantzschia amphioxys

(Grunow ex Cleve) Ross. Group III is characterized by a

predominance of diatoms such as Seminavis strigosa (Hust.)

Danielidis & Econ.-Amilli, E. pectinalis (K€utz) Rabenh.,

Navicula sp., U. biceps (K€utz.) Comp�ere and Nitzschia sp.

This analysis reveals that group II is more diverse than

groups I and III.

Determinism of stand rivers

In Soumi�e river, the most abundant taxa (6) are signifi-

cantly correlated with abiotic parameters (Table 3). Pin-

nularia sp., Navicula sp. and Microcystis aeruginosa (K€utz.)

Lemmerm are, respectively, related positively to nitrate

concentration, flow and pH. The depth is negatively

correlated with Gomphonema sp. density. As for the

abundance of Cocconeis euglyptoides (Geitler) Lange-Bert.,

it is correlated with the change in temperature and

dissolved oxygen. Density of Phormidium sp. is positively

correlated with the flow. In Eholi�e River, pH and depth are

positively and negatively correlated with respective abun-

dances of Aulacoseira granulata (Ehrenb.) Simonsen and

Nitzschia sp. (Table 3). Dissolved oxygen and flow are

negatively associated with Lyngbya sp. Flow and pH had

positive correlation with Eunotia sp. density. Abundance of

Eh28N21

N11

N15N25

E27N27

S11E16N14

E14E12E23E26N26

N17

E22

S21Eh12N28

S27Eh17Eh22Eh24S18

Eh16

Eh23Eh26E11

E17

Eh13E28

E15E25

E24

Eh11S22Eh25N18N22

E21

S16

N23

S28N12

S15S17Eh18 S26

Eh21N24

S12S25

S14Eh15

E18S24Eh14Eh27

S13 S23E13N16

II

III

I

Fig 3 Distribution of samples on the Kohonen map-based

assemblages of taxa of algae full river water studied; S = Soumi�e,

E = Eholi�e, Eh = Ehania, N = No�e, 1 = upstream, 2 = down-

stream, the index numbers (1–8) correspond to different

samples

Table 2 Distribution of taxa in each group defined by the SOM

(dark:light colour and high abundance:low abundance)

Taxa

Groupe I Planktolyngbya contorta, Microcystis aeruginosa,

Aphanocapsa incerta, Scenedesmus quadricauda,

Phacus sp., Trachelomonas volvocina,

Aulacoseira granulata, Gomphonema sp.,

Eunotia sp., E. monodon

Groupe II Lyngbya sp., A. incerta, Phacus sp.,

T. volvocina, Phormidium sp., Closterium sp.,

C. kuetzingii, Amphora commutata,

Planothidium lanceolatum, A. granulata,

Cocconeis euglyptoides, Encyonema silesiacum,

Gomphonema sp., G. parvulum, Gyrosigma

acuminatum, Cymbella sp., E. monodon,

Hantzschia amphioxys, Luticola muticoides,

Navicula sp., Ulnaria biceps, U. ulna, Nitzschia

palea, Pinnularia sp., Placoneis cf. densa,

Sellaphora bacillum, Capartogramma crucicula,

Stauroneis anceps

Groupe III Lyngbya sp., Phormidium sp., A. commutata,

S. strigosa, C. euglyptoides, E. silesiacum,

G. parvulum, G. acuminatum, Cymbella sp.,

Eunotia sp., E. monodon, E. pectinalis,

Navicula sp., U. biceps, U. ulna, Nitzschia sp.,

Pinnularia sp., P. mesolepta, Placoneis cf.

densa, S. bacillum

© 2013 John Wiley & Sons Ltd, Afr. J. Ecol.

6 Julie E. Niamien-Ebrottie et al.

eight taxa in Ehania River is significantly correlated with

abiotic parameters (Table 3). The temperature is positively

correlated with the abundance of Navicula sp. and Encyo-

nema silesiacum (Bleisch) D.G. Mann, while the flow is

associated with density of Aulacoseira granulata (Ehrenb.)

Simonsen and Ulnaria biceps (K€utz.) Comp�ere. Dissolved

oxygen influences positively the abundance of Gompho-

nema parvulum K€utz. Parameters pH and temperature are

negatively correlated with the abundance of Lyngbya sp.

and Seminavis strigosa (Hust.) Danielidis & Econ.-Amilli.

Density of Nitzschia sp. is positively influenced by dissolved

oxygen and transparency. Regarding No�e River, transpar-

ency is positively correlated with abundance of Navicula

heimansioides Lange-Bert. and Surirella sp. (Table 3).

Abundances of Gyrosigma acuminatum and Nitzschia sig-

moides (Nitzsch) W. Sm. are correlated with temperature.

The flow is positively and negatively correlated with

densities of Planothidium lanceolatum (Br�eb. ex K€utz.)

Lange-Bert. and Navicula sp., respectively. Abundance of

Cocconeis sp. is positively associated with transparency

and nitrate concentration. Nitzschia palea (K€utz.) W.Sm.

has positive correlation with the flow and nitrate

concentration.

Discussion

The distribution of planktonic algae in rivers considered is

far from uniform. Studies on the spatio-temporal distribu-

tion of algae (Reynolds, 1989; Cabioc’h et al., 1992; Large

et al., 1993; Angelier, 2000) have shown that several

factors often interactive influence this distribution in

running water. Among these factors, the current velocity

Table 3 Multiple regression step linking the physical–chemical and the most abundant taxa in Soumi�e, Eholi�e Ehania and No�e rivers

Taxa Parameters t R2 F P

Soumi�e Pinnularia sp. Nitrate (mg/l) 2.54 0.40 F1,14 = 6.43 *

Navicula sp. Flow (m/s) 3.02 0.42 F1,14 = 6.32 *

Microcystis aeruginosa pH 2.33 0.38 F2,13 = 3.99 *

Gomphonema sp. Depth (m) �2.37 0.29 F1,14 = 5.60 *

Cocconeis euglyptoides Temperature (°C) 2.25 0.40 F3,12 = 2.69 *

Dissolved oxygen (mg/l) 2.28 0.50 F4,11 = 2.72 *

Phormidium sp. Flow (m/s) 3.37 0.38 F1,14 = 8.61 **

Eholi�e Aulacoseira granulata pH 2.25 0.27 F1,14 = 5.06 *

Nitzschia sp. Depth (m) �3.14 0.41 F1,14 = 9.88 **

Lyngbya sp. Flow (m/s) �2.97 0.45 F2,13 = 5.38 *

Dissolved oxygen (mg/l) �2.53 0.55 F3,12 = 4.02 *

Eunotia sp. Flow (m/s) 2.50 0.44 F2,13 = 5.17 *

pH 2.31 0.52 F3,12 = 4.40 *

Ehania Lyngbya sp. pH �3.06 0.27 F1,14 = 5.16 **

Temperature (°C) �2.44 0.50 F2,13 = 6.48 *

Seminavis strigosa pH �3.02 0.39 F1,14 = 9.11 **

Temperature (°C) �2.21 0.66 F3,12 = 7.74 *

Nitzschia sp. Dissolved oxygen (mg/l) 3.26 0.44 F2,13 = 5.20 **

Transparency (m) 2.48 0.67 F3,12 = 4.19 *

Gomphonema parvulum Dissolved oxygen (mg/l) 2.21 0.26 F1,14 = 4.88 *

Navicula sp. Temperature (°C) 2.70 0.34 F1,14 = 7.30 *

Encyonema silesiacum Temperature (°C) 2.64 0.43 F1,14 = 6.95 *

Ulnaria biceps Flow (m/s) 2.40 0.29 F1,14 = 5.76 *

Aulacoseira granulata Flow (m/s) 3.57 0.48 F1,14 = 12.72 **

No�e Ulnaria biceps Conductivity (lS/cm) 2.21 0.40 F4,11 = 1.82 *

Nitzschia palea Flow (m/s) 2.38 0.29 F1,14 = 5.66 *

Pinnularia sp. Flow (m/s) 2.47 0.30 F1,14 = 6.11 *

Nitzschia sp. Depth (m) �2.33 0.30 F2,13 = 2.72 *

R2 = coefficient of determination, F = coefficient of inclusion, t = regression coefficient, P = probability.

*P < 0.05.

**P < 0.01.

© 2013 John Wiley & Sons Ltd, Afr. J. Ecol.

Phytoplankton dynamics in four coastal rivers of Ivory Coast 7

and nutrient intakes appear larger (Mercado, 2003).

Stations surveyed absolute densities appear quite similar,

except for No�e River where particularly high densities are

recorded. These densities are observed upstream and

downstream, respectively, during the long dry season

and long rainy season. This increase in density upstream

can be correlated with the physico-chemistry of water.

Indeed, this station has, in periods of high density (long dry

season), high nitrate concentration and low flow that

would promote the development of phytoplankton. This

result corroborates observations of Angelier (2000), who

noticed that the growth of algae is linked to the availability

of nutrients and the flow velocity in temperate regions. In

addition, maximum density observed downstream during

the long rainy season could be explained by the existence

of small isolated ponds in the immediate vicinity of that

station. During great rainy seasons, a part of water in

these ponds is discharged into the river overflow, inducing

algal enrichment of these waters. These are just as

stagnant ecosystems, environments conducive to algal

blooms (Ouattara, 2000). These results are similar to those

of Huang et al. (2004). He observed a high density of

phytoplankton during the season rainfall in the Pearl River

in China. Unlike No�e River, densities are important in

small dry season in the stations of other streams. This

observation may be attributed to the addition of nutrients

in the environment by run-off abundant in main rainy

season, which precedes the short dry season. The Shannon

diversity index and evenness indicate that algal population

is quite diverse and evenly distributed in most of the

surveyed sites. This observation shows that rivers are

habitats for an algal flora, which varied following ecolog-

ical requirements (Wetzel, 1983). In addition, the physico-

chemical conditions of the areas quite similar for the four

basins contribute to the regularity of taxa distribution.

From the composition, stand structures are relatively

similar. There is a high proportion of cyanobacteria (other

50%) in all rivers studied. Our results are in agreement

with those of Iltis (1982), L�eveque, Dejoux and Iltis (1983)

and Ouattara et al. (2003), who found that cyanobacteria

dominate the phytoplankton communities in rivers of

Ivory Coast. Presumably, cyanobacteria, at times, have

reproductive activity that far exceeds that of other algal

classes inventoried. Nevertheless, akinetes, resting cells of

planktic Nostocales cyanobacteria, help to withstand

unfavourable conditions and play a key role in their

invasion from lower to higher latitudes (St€uken et al.,

2006). Diatoms are the most divers algal group in the

rivers studied. This is consistent with the observation of

Round (1993). In fact, diatoms have high reproduction

rate, and individual species have high sensitivity towards

different levels of organic polluted waters (Reid, Tibby &

Penny, 1995). Among diatoms abundantly encountered in

environments studied, we note the presence of Aulacoseira

granulata (Ehrenb.) Simonsen, Planothidium lanceolatum

(Br�eb. ex K€utz.) Lange-Bert., Gomphonema parvulum K€utz.

and Nitzschia palea (K€utz.) W.Sm. These species are known

for their link with eutrophication and pollution (Prygiel &

Coste, 2000; Ndiritu et al., 2003). Their presence and

especially their abundance illustrate the fragility of sur-

veyed rivers. However, size of Cocconeis euglyptoides

(Geitler) Lange-Bert. of Frustulia crassinervia (Br�eb.)

Lange-Bert. & Krammer and Gyrosigma acuminatum (K€utz.)

Rabenh. in the rivers studied does not allow concluding

that environments have reached a considerable degree of

pollution. According to their ecology (Coste, 1996), these

diatoms are very sensitive to pollution. Their frequency

decreases with pollution. Therefore, Soumi�e, Eholi�e, Eha-

nia and No�e rivers are weakened by the hydro-mode

occupations of their watersheds.

Acknowledgements

We thank the Representative Resident of WSA (Water and

Sanitation for Africa) Office of Ivory Coast, Professor

Theophile GNAGNE. We are grateful to him for adding us

to the project entitled ‘Study of transfers of pollutants in

the Aby-Bia-Tano�e river-lagoon system’. We also thank all

members of the Laboratory of Environment and Aquatic

Biology of Nangui Abrogoua University (Abidjan, Ivory

Coast).

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(Manuscript accepted 15 October 2013)

doi: 10.1111/aje.12132

© 2013 John Wiley & Sons Ltd, Afr. J. Ecol.

Phytoplankton dynamics in four coastal rivers of Ivory Coast 9