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Le fonctionnement hydrologique du lac de Guiers, principal reservoir d’eau douce du pays, est totalement controle depuis la mise en service des barrages de Diama (1985) et de Manantali (1987) sur le fleuve Senegal. Le statut trophique du lac a ete considerablement affecte par ces changements majeurs. Les parametres physicochimiques et biologiques ont ete suivis pendant quatre annees (2002 a 2005) afin de caracteriser cette evolution. Nos resultats decrivent un etat eutrophe de 2002 a 2004 et hypertrophique en 2005. Les mesures du disque Secchi sont restees inferieures a 100 cm. La conductivite de l’eau a augmente significativement en 2004 et 2005 par rapport aux deux annees precedentes. Les concentrations des sels nutritifs ont varie de 0,3 a 7,8 μgP/L (PO 4 3−), de 0 a 14,5 μgN/L (NO 3 −) et de 1,3 a 28,9 μgN/L (NH 4 +). Les biomasses chlorophylliennes ont quant a elles fluctue annuellement entre 5 et 50 μg/L de 2002 a 2004, puis entre 20 et 105 μg/L en 2005. Cent onze especes de phytoplancton appartenant a sept classes ont ete recensees au cours du cycle annuel de 2002 a 2003 avec une dominance des classes de cyanophycees (25 %) et des chlorophycees (32 %). Les pressions multiples et croissantes subies par ce lac contribuent a la degradation continue de la qualite de ses eaux (eutrophisation) : un monitoring regulier de l’etat de sante de cet ecosysteme emblematique et strategique constitue une exigence.

Introduction Le nord de la Côte d'Ivoire est caractérisé par un climat relativement humide (1 200 mm par an), mais avec des périodes très sèches qui alternent avec des périodes de fortes précipitations. Le contrôle de l'eau à des fins d'intensification des activités agricoles et pastorales y est apparu comme une nécessité. Plusieurs centaines de petites retenues d'eau (surface < 1 km2) souvent formées à partir d'une digue barrant un bas-fond ont ainsi été créées au cours des dernières décenni...

Introduction Les recherches sur la productivité planctonique des réservoirs visent à en améliorer la production piscicole (Mc Queen et al., 1992). En Côte d'Ivoire, les petits barrages agropastoraux font l'objet d'une exploitation croissante et incontrôlée (Idessa, 1992), mais jusqu'ici, les études étaient surtout focalisées sur les barrages hydroélectriques (Yte, 1982 ; N'Douba et al., 1985 ; Noba, 1998 ; Ouattara, 1998). Notre travail, réalisé lors d'une vaste prospection spatiale conduite ...

International audience; In this work, two field campaigns were performed in July 2008 (wet season) and March 2009 (dry season) to produce original data on the concentration, partition and distribution of mercury and butyltin compounds along the tropical Bach Dang Estuary located in North Vietnam (Haiphong, Red River Delta). The results demonstrate that mercury and butyltin speciation in the surface waters of this type of tropical estuary is greatly affected by the drastic changes in the seasonal conditions. During high river discharge in the wet season, there was a large estuarine input of total Hg and tributyltin, while the longer residence time of the waters during the dry season promotes increasing MMHg formation and TBT degradation. Although most of the Hg and TBT is transported into the estuary from upstream sources, tidal cycle measurements demonstrate that this estuary is a significant source of TBT and MMHg during the wet (∼3 kg TBT/day) and dry (∼3 g MMHg/day) seasons.

Seasonal variations of zooplankton communities and their interactions with phytoplankton and environmental parameters were studied over a twelve-month period in Lake Guiers (Senegal, West Africa). Zooplankton was most abundant during the warm and rainy season corresponding to the flood event (July to October) with peaks for rotifers and cyclopoid copepods in July–August and September–October, respectively. Filamentous cyanobacteria were the dominant phytoplankton during the warm season (August to November), while diatoms were dominant during the cool season (December to April). Temperature increase and river flooding both played an important role in these successions, favoring intense development of cyanobacteria and cyclopoid copepods. Trophic interactions also seem to be an important factor in structuring of the plankton community. Overall, the plankton composition displayed a trend toward a eutrophication state. Our study confirms the existence of seasonality in zooplankton communities and the importance of using biological indicators such as phyto- and zooplankton to monitor water quality, in particular in tropical freshwater ecosystems. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Understanding how human-induced management interacts with and affects the structure and functioning of large estuarine ecosystems is a major research challenge. In West Africa, human intervention on the Senegal River Estuary was intended to reduce the impact of major flooding by opening a new mouth in October 2003, 25 km to the north of the existing mouth. This study describes the effects of the new environmental conditions on the physical and biochemical characteristics of the water column and on microbial communities (bacteria, phytoplankton by size class and heterotrophic nanoflagellates (HNF)) in comparison with the situation in 2002. In 2006, seventeen stations were sampled, during both neap and spring tides, at a depth of 0.5 m along a salinity gradient from freshwater to marine conditions. Inorganic nutrient levels were often low but there were high levels of chlorophyll a in the estuarine area (mean of 13.7–20.7 μg L−1 in spring and neap tide conditions, respectively) producing a eutrophic status in this estuary. Average HNF abundances were lower (mean of 108 and 174 cells l−1 during neap and spring tides, respectively) compared to the situation in 2002 (mean between 2.5 and 6.7 × 104 cells l−1). Three biological indicators for assessing environmental changes are discussed: ratio of bacteria to heterotrophic flagellate abundances, ratio of picophytoplankton to nanophytoplankton, and the density of thermo-tolerant coliforms (TTC) and faecal streptococci. It is demonstrated that man-made alteration of the hydrologic regime can modify the microbial community structure and cause the health status of the estuary to deteriorate.

Summary 1. We experimentally compared the impacts of visually feeding zooplanktivorous fish and filter-feeding omnivorous fish in shallow tropical Dakar Bango reservoir, Senegal. We provoked a cyanobacterial Anabaena bloom under mesotrophic to eutrophic N-limited conditions in 18 enclosures assigned to six Nile tilapia life-stage treatments, at typical biomasses: fishless control (C), zooplanktivorous fry (Z), omnivorous juveniles (O), herbivorous fingerlings (H) and two combinations (OZ, OH). 2. All fish grew well, but as prevalent inedible phytoplankton dampened fish effects, community-level trophic cascades did not occur. Planktivore types acted independently and affected differentially the biomasses of total zooplankton, cyclopoids, nauplii, cladocerans, invertebrate carnivores, large herbivores, colonial cyanobacteria and Chlorophyta. They neither influenced the total biomass of phytoplankton, nor most water chemistry characteristics. Responses were apparently not fish-biomass related. The bloom collapsed synchronously in all enclosures, coinciding with enrichment ending, with a return to clear water within 12 days. 3. Our results support the hypothesis that excess nutrients and prevalent inedible cyanobacteria inhibit the cascading effects of natural biomass levels of both visually feeding zooplanktivores and filter-feeding omnivores. In N-limited meso-eutrophic shallow tropical lakes with predominantly small herbivorous zooplankton, neither the type nor the biomass of planktivorous fish present seems likely to prevent the transient outburst of cyanobacterial blooms. Such fragile ecosystems may thus not sustain a trophic state suitable for drinking water production, unless human impacts are restricted. The generality of restoration approaches based on ecological engineering should be further explored.

The chemical and biological characteristics of Lake Guiers (Senegal) have changed markedly since the impoundment of the Senegal River (Diama and Manantali dams) and subsequent development of irrigated agriculture in the nineteen eighties. On a longitudinal transect of 10 stations (from south to north), the environmental characteristics and the spatial variability of physicochemical variables, phytoplankton and zooplankton communities were studied. Within a marked south-north gradient, the southern stations were characterized by the highest conductivity and pH and by the lowest values of suspended solids, chlorophyll-a concentrations and phytoplankton abundance (mainly Chlorophycea, Cyanobacteria and Bacillariophycea). The spatial distribution of zooplankton showed a clear distinction between the southern zone, characterized by the presence of the rotifers Brachionus falcatus and Conochiloides sp., the cladoceran Bosmina longirostris, the cyclopoid Thermocyclops neglectus and the calanoid Pseudodiaptomus hessei. A co-inertia analysis clearly showed that environmental factors and phytoplankton drives the spatial distribution of zooplankton communities. The comparison of our data with previous studies suggests a marked change in the biological communities since the impoundment of the Senegal River, with rarefaction of P. hessei and proliferation of the cyclopoid Mesocyclops ogunnus. Several hypotheses are discussed to explain these biological changes. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The trophic relationships between metazooplankton and natural particles were studied in May 2002 in the Senegal River Estuary (16 degrees N, 16 degrees W) in low water conditions (dry season). Environmental factors, micro-organism and metazooplankton were analyzed through sampling at a fixed station. Gut fluorescence measurements of and field experiments on zooplankton metabolism were also performed. Chlorophyll a concentrations ranged from 6.5 to 10.2 mu g 1(-1). The phytoplankton was dominated by picoplanktonic cells (83-94% of total numbers). The particulate organic carbon (1.2-2.7 mg l(-1)) originated for a large amount from organic detritus (20-70%). The zooplankton biomass was dominated by Cirripedia larvae and calanoid copepods (Acartia clausi, Temora stylifera and Paracalanus spp.). These taxa showed diel vertical migrations and maximal gut fluorescence at night, independently of tidal effects. Metabolic budgets show that their daily ingestion rates on phytoplankton (27-55% of body carbon weight) were insufficient to balance their respiration needs (40-51% of body carbon) and suggest that a selective feeding upon micro-heterotrophs (Heterotrophic NanoFlagellates, HNF) and/or detritus would be necessary to complete their energetic needs. The daily grazing pressure of metazooplankton represented only 5% of the in situ chlorophyll a and 14% of the primary production, but this pressure was mainly orientated towards nanophytoplankton. The daily recycling of nutrients by the metazooplankton excretion was rather high (83 and 46% of the in situ NH4-N and PO4-P concentrations, respectively). Therefore, the impact of metazooplankton on phytoplankton through top-down (grazing) and bottom-up (nutrient recycling) processes seemed substantial in this tropical estuary. (c) 2005 Elsevier Ltd. All rights reserved.

We conducted experiments to determine whether bacterial growth, with or without predators, is limited by inorganic (N and P) and organic (C) substrates in Selingue, a mesotrophic reservoir located in Mali, West Africa. Significant increases (relative to controls) in bacterial cell vol- umes and thymidine incorporation rates were observed after 24 h incubation only for samples amended with the combination +PC and +CNP. The data revealed a colimitation of bacterioplankton growth by organic carbon during the dry season. Flow cytometry discriminated 3 groups of bacteria (Bact I, Bact II, Bact III) differing in increases in nucleic acid content and cell size. The Bact I group, comprising cells with low nucleic acid content and of small size, was the dominant population in all experiments. In the absence of bacterial predators, only the Bact II group showed significant differ- ences between the control and the +PC and +CNP treatments, indicating that this bacterial group was the most sensitive to nutrient additions. The Bact II group, corresponding to cells with high DNA content, are active members of the bacterioplankton community. The Bact III group did not increase in any treatment but the proportions of these cells always increased in the presence of bacterial predators. These cells may be, at least partly, grazing-resistant bacteria.

This work investigates the effects of climate and hydrology as factors controlling the trophic status of Selingue, a monomictic reservoir in Mali, West Africa. Environmental (water transparency, hydrochemistry, nutrients) and biological (chlorophyll a concentrations) descriptors were studied on water samples collected biweekly from November 2000 to November 2001 in a station representative of the northern part of the water body. Statistical methods were used to estimate the existence and significance of breaks in the time series and environmental data were used to explain these breaks. In Selingue Reservoir, the water column is stratified from March to May as a result of the cooling induced by the harmattan (North-East trade winds). Stratification lasts a few weeks, until the beginning of the monsoon (South-West trade winds), after which progressive warming allows vertical mixing of the lake. In harmattan or monsoon situations, the winds do not play a significant role except for storm events during the rainy season. Such events can change transiently, but markedly, the physical conditions prevailing in the whole water column. The water quality closely follows the hydrological cycle, characterized by a unique flood, which lasts a few weeks, from mid-July. Input of nutrients during the flood events is limited and local mineralization of organic matter in the hypolimnion during the stratification period seems to play a major role in the availability of dissolved inorganic nitrogen and soluble reactive phosphorus. This availability is subject to vertical mixing which is only possible when the water height is sufficiently low and the wind-induced energy is sufficiently high. Dry (dust during the harmattan period) and wet deposition (linked to rains) play a role in supplying nutrients to the lake and can be factors in the observed increased chlorophyll a concentrations. Thus, environmental factors in Selingue change with the current hydrological conditions. Selingue Reservoir can be regarded as oligotrophic during the high water period, but as meso-eutrophic during the low water period. Each annual flood initializes the ecological conditions in this water body. This pattern limits the risks of trophic change (which can lead to algal blooms) to short periods of very low water levels immediately preceding flood events.

Phytoplankton biomass, productivity and assemblages were studied during the main hydrological seasons in two shallow reservoirs in Ivory Coast (West Africa). In this area, the hydrological pattern begins with the flood from July to October, that is the main change in environmental condition. The flood is followed by a dry period from November to March; then the rainy season resumes from April with numerous and intense storms. Chlorophyll-α concentrations ranged from 5.0 to 27.4 μg l -1 in mesotrophic Brobo reservoir and from 15.1 to 123.0 μg l -1 in eutrophic Nambengue reservoir. Despite their different trophic states phytoplankton production varied between 0.2 and 1.8 g C m -2 d -1 in both reservoirs. Both Chlorophyceae and Cyanobacteria largely dominated the phytoplankton communities. Opportunist species (mainly Chlorophyceae), with medium sized cells (10-15 μm equivalent spherical diameter, ESD), were observed during the flood. Slow growing/slow sinking species, with relatively large cells (> 15 μm ESD, mainly Cyanobacteria and Chlorophyceae), were present during the dry season. Small species (< 10 μm ESD), dominated by Chlorophyceae and Cyanobacteria were abundant during the rainy season. At this late stage of the hydrological year, N 2 fixing Cyanobacteria like Anabaena and Anabaenopsis were also present. Eight functional associations (H I, J, K, L M , N, P, S 1, X 1) from the thirty-one proposed by REYNOLDS et al. (2002) were outlined in the two systems surveyed; these functional groups were representative of warm, shallow, turbid, nutrient-enriched and frequently mixed habitats. A large part of the biomass was composed of cells hardly matching the needs of superior consumers, in size as well as in shape. These conditions could explain the inefficient transfer of biomass within the pelagic food web. resulting in trophic dead end and contributing to the eutrophication of the sites.

Zooplankton samples were collected from 49 small reservoirs of northern Ivory Coast in April 1997. Thirty taxa were identified, including 20 rotifers, 3 copepods and 6 cladocerans. The number of taxa per lake ranged between 12 to 22 and decreased with the total abundance of zooplankton. Copepods dominated standing biomass. Coinertia analysis suggested the role of seston food abundance, oxygen depletion and turbidity for zooplankton abundance and community structure. Rotifers, and particularly Brachionus angularis, Polyarthra and Filinia, were more abundant than copepods in the most eutrophic, turbid and deoxygenated reservoirs. The role of oxygen as a determinant of community structure is probably linked to the specific tolerance of taxa, but turbidity role could not be evaluated with certainty in the absence of information on visual predators.

The cladocerans Moina micrura and Diaphanosoma excisum and the copepod Thermocyclops decipiens were studied in microcosms (0.8 m3) under semi-controlled experimental conditions at 25–29 °C for 32 days, by daily sampling after an initial monospecific inoculation. For each species, the time series began with an exponential population growth phase. M. micrura showed a higher daily population growth rate (mean = 1.19) than D. excisum (0.78) and T. decipiens (0.45). The growth phase of M. micrura coincided with bacterial and phytoplanktonic peaks while the growth phase of the two other species followed these peaks. After this phase, M. micrura quickly disappeared (day 10), while D. excisum biomass decreased but showed a second increase, followed by a stabilization sequence. T. decipiens biomass had a slower increase and stabilized after day 17. The passage to sexual reproduction in relation to crowding was the main regulating factor for M. micrura, whereas food limitation was important for D. excisum. For T. decipiens, population growth was limited by decreased recruitment to copepodite stages which could have resulted from cannibalism exerted by older stages.

A systematic study of some pollution indicators [ammonia (NH4-N), faecal coliforms counts] has been carried out for three years (1993-95) in the urban area of the Ebrié lagoon, evaluating the impact of a sea outfall project. This project, for collecting and forwarding wastewaters of the city of Abidjan to the Atlantic ocean in order to reduce the pollutant load in the lagoon, was initiated in 1994. It failed a few months after the first operation and there is now an overall increase in pollution due to continuous use of the lagoon for dumping domestic as well as industrial and agricultural wastes as a consequence of the malfunctioning of the outfall, which was poorly designed, probably because of insufficient funding. (Résumé d'auteur)

Gut fluorescence, feeding and egg production rates of zooplankton assemblages were measured in a shallow, eutrophic brackish-water pond for 24 days. Brachionus plicatilis, Hexarthra intermedia and Apocyclops panamensis successively developed and exhibited differences in food selectivity. Rotifers selected small particles but also had a preference for larger particles (15–21 μm, Equivalent Spherical Diameter, ESD). B. plicatilis appeared less selective than H. intermedia, which fed mostly on particles

Biomass and production of bacterial and phytoplanktonic communities were measured during diurnal cycles at different stations in 3 shallow tropical reservoirs (Ivory Coast). Investigations were conducted in 1995 during 2 typical hydrological seasons (dry season in March and following rainy season in December). Bacterial production in the plankton ranged from 1.2 to 26.2 microg C/l/h and bacterial biomass ranged from 11 to 163 microg C/l. A slope of 0.625 (n = 93) for the regression of log-transformed bacterial biomass versus log-transformed production suggests that the bacteria were strongly controlled by bottom-up processes. Ratios between net primary production and bacterial production averaged 67% (range 38 to 140%), indicating that the reservoirs studied can be considered as meso-eutrophic ecosystems. Average bacterial carbon demand corresponded to 97% of the net primary production, suggesting that the biological systems studied are based on autotrophic metabolism. These relationships are the result of a close metabolic coupling between bacterioplankton and phytoplankton, with a large fraction of primary production routed through heterotrophic bacteria and the microbial loop. (Résumé d'auteur)

Tlic s i x . coniposition and distribution of p;irticlcs in the water column wcrc surveyed i n ;I shiillow iirc;i ( I ni depth) of ;I tropicd Iiigoon (CStc d’Ivoire) during ;I scqucncc of wind-induccd rcsuspcnsion. \Viiter s;iniplcs wcrc collected hourly near the surf;lcc during one tidal cyclc. Thrcc c1i;ir;tctcristic periods wcrc distinguishcd: n c;ilni period with low wind spccd (iivcriigc I .Z ni s-’I). ;I windy period with wind spccd >3 i11 s-’ (riingc bctwccn 4 ;ind 0 m s-I) inducing scdimciii rcsuspcnsion ;ind ;I rcl;ix;ition period during tlic dccrc;isc of wind velocity. I‘roni the ;innlysis of scvcr;ii p;ir;inictcrs (pnrticlc sizc iiiitl voltinic. Ixictcria. pico;ind ii;inopIiyropl;ili~toii. ciliiitcs a n d dctritus), scdiiiicnt rcsuspcnsion c;iuscd ;I regular injcction o f p;irticlcs f r o m tlic lwd. The finest particles ( 15-0 pni: clilorophytcs such ;is Clllorcllr spp.. picocyano1)actcri;i such :IS .S~~rrcr/rorocrr~s) were ttic first to be iiffcctcd by wind-induccd turbulence. whereas large p;irticlcs (6-12 pi: diatoms. cyiinohx1cri:i such ;IS Lprr~hicr spp.) wcrc dispcrscd i n t o the water column ;it the highest wind spccd. Thc fate of rlic different scston components differed according to thcir sizc. Therefore. wind-induccd rcsuspcnsion could greatly influence the food wch orgiiniz;ition through tlic quantity. qwility and sizc ,of cdihlc piirticlcs ;iviiil;ihlc ;it ;I given tinic. ’I

The effects of the water quality on food and nutritional characteristics of stomach contents of Oreochromis niloticus in semi-intensive aquaculture ponds were investigated in dry and rainy seasons. Nutrient concentrations mainly ammonium and orthophosphates in water presented significant seasonal variation. The chemical composition of the total suspended solids, the main trophic source in pond is characterized by a high proportion of mineral (89%) during the two seasons. This is reflected in the low quality of food ingested by fish: the indigestible materials (fiber and minerals) represented 70 and 61% of the diet in the dry and rainy season, respectively. Seasonal variation was not observed in the digestibility of the dietary organic matters (ash free dry matters, AFDW) and total amino acids (TAA). Analysis of these results showed that anthropogenic factors, including fertilizer application and changes in environmental conditions have affected enormously the quality of fish food and the productivity of these ponds. Key words: Seasonal changes, water quality, semi-intensive aquaculture, Nile tilapia, food quality, Mali.

In shallow environments, under certain conditions of fetch, wind velocity, bathymetry and bottom characteristics, resuspension can be generated by wind induced waves. In the tropical Ebrie lagoon, austral trade winds are dominant almost all year long, and their velocity shows a marked diel pattern with maximum speed between noon and midnight. Only austral trade winds with a speed >3 m s−1 allow particle resuspension which is effective for depths 1.5 m, particle sinking is permanent in depressions which are spontaneously transformed into anoxic systems. At the lagoon scale, sedimentation is significantly modified by wind induced resuspension. According to the bathymetry and the distance from a river, three sedimentary facies are recognized. Their grain size distributions are parabolic in areas where resuspension occurs, logarithmic in areas where no resuspension is possible and hyperbolic in the hollows and the main channels. Finally, a large part of the allochthonous inputs (from drainage and rivers) and autochthonous pelagic production is trapped into the Ebrie lagoon and less than 10% of the particles entering the lagoon are exported toward the Atlantic Ocean.

The aim of this study was to determine bacteria could be a substantial source of carbon for zooplankton and whether the grazing pressure of these metazoan filter-feeders could influence the fate of bacterial production. Eight grazing experiments using natural bacteria labelled with 3H thymidine were conducted in a tropical pond (Ivory Coast) during various phases of biological colonization (rotifer-dominated and copepod-dominated phases of the colonization). Higher grazing and clearance rates were observed with rotifers (#Brachionus plicatilis$ and #Hexarthra intermedia$), while very low values where obtained when the cyclopoid copepod #Apocyclops panamensis$ was dominant. Less than 1% of the bacterial production was harvested when copepods were dominant, while #B. plicatilis$ consumed up to 36% of this production. However, this consumption of bacteria appeared to contribute only to an insignificant proportion of the daily carbon intake (e.g. 0.9. to 7.1% of body carbon for rotifers). The low contribution of bacteria in the nutrition of zooplankton is discussed in terms of their cell size and their relative abundance in the total amount of seston available. (Résumé d'auteur)

Chlorophyll a and pheopigment standing stocks and fluxes were used during a two weeks colonization experiment in a productive tropical pond (Layo, Cote d'Ivoire) in order to establish a chlorophyll budget. The experiment started from an azoic state (the pond was dried, limed and progressively filled with ground water). Algal production was the only input to the phytoplanktonic system, while grazing and algal sedimentation were the main outputs. Chlorophyll a reflected the algal biomass, and degradation pigments were considered as an index of grazing by zooplankton (here, protozoans and rotifers). An estimation of the input through the algal growth rate was performed for the two main biological events observed during the study. The first algal bloom, with a large picoplankton participation, was mainly regulated by microzooplankton (increase of the peak) and rotifers (decrease of the peak). The second bloom (exclusively nanoplankton) was regulated by rotifers (increase) and by sedimentation of living cells (decrease). This last process was related to a sudden exhaustion of ammonia in the water column. Because of the time-lag between algal proliferation and zooplanktonic bloom, the phytoplanktonic biomass was able to be adjusted according to the availability of nutrients. This self-regulation took the form of sinking of active algal cells, resulting in a transient reduction of the food available for rotifers. This process had drastic consequences in these shallow waters, since a major part of the phytoplankton produced was removed from the pelagic system. For an optimal exploitation of the natural resources of an aquaculture pond, a study of the equilibrium nutrients-phytoplankton-zooplankton would provide a basis for artificial intervention, with a view to limit the impact of this mode of natural regulation.

The Selingue Reservoir is a monomictic lake in Mali (West Africa). Previous studies on zooplankton only concerned rotifers; the present work is thus the first also concerning crustacean zooplankton. Environmental variables (transparency, temperature, oxygen, pH, alkalinity, nutrients, suspended solids, and chlorophyll) and zooplankton were investigated once, during the high water period (November 2000), at 9 stations. Thirty-one zooplanktonic taxa were recorded: 23 rotifers of which 5 species (Brachionus patulus, Conochilus unicornis, Lindia torusola, Proales decipiens, Pompholyx sp.) are first records for this ecosystem, 4 cladocerans (Bosmina longirostris, Ceriodaphnia cornuta, Diaphanosoma excisum, and Moina micrura), and 3 cyclopoid copepods (Thermocyclops incisus incisus, Mesocyclops ogunnus, and Eucyclops sp.). Calanoid copepods were totally absent. A clear relationship between environmental conditions and zooplankton was shown using a multivariate analysis (coinertia), which discriminated three ecological zones on the basis of both datasets. Several zooplankton species were associated with alkaline and turbid waters, near the mouths of the two tributaries (the Sankarani and Bale rivers). Other species characterized more transparent and nanophytoplankton-rich waters at the confluence zone, far from the flood influence. Our results clearly show the shaping role of environmental conditions and confirm the suitability of zooplankton as an indicator of water quality.

In shallow environments, particle resuspension can induce large ecological effects. Under some certain conditions of fetch, wind velocity, bathymetry and bed roughness, resuspension is generated by wind induced waves. During December 1991, a shallow station (1 m depth) in the north shore of a tropical lagoon (Cote d'Ivoire) was investigated in order to study the impact of wind induced resuspension on the ecosystem. In this area, Austral Trade winds are dominant almost all year long, and their velocity shows a marked diel pattern. During the survey, three sequences were distinguished: a period of Austral Trade winds (with possible resuspension), a period of Boreal Trade winds (no wind induced waves at the station) and a period of transitional Trade winds. Only Austral Trade winds with a speed >3 m s-1 allowed particle resuspension. For chlorophyll, mineral seston and ammonia, significantly higher values were noted during the windy sequences. Conductivity and water colour varied in relation to tides. Granulometric and mineralogical analyses showed that only the 0-3 cm superficial level of the sediment was involved in resuspension. This process induced several effects: (1) an increase of suspended matter concentration in the water and thus a light attenuation due to a higher turbidity, (2) a distribution in the whole water column of nutrients from the pore water, (3) a modification of the sediment granulometric characteristics and (4) an increase in the food available for planktonic filter feeders since algal cells were periodically resuspended in the whole water column. Wind induced resuspension occurred in 10% of the Ebrie lagoon. In this area, the daily alternate of resuspension-sedimentation sequence is then a major factor controlling the productivity of a system which is potentially highly productive (high nutrient load, favourable climatic conditions) yet characterized by high turbidity. These observations can be generalized to comparable systems in the tropical area.

Most regions in the tropics undergo high seasonal precipitation that produces cyclic patterns of riverine discharge, resulting in periods characterized by low and high water levels. Many chemical and bio-logical factors are affected by this hydrologic seasonality, and it therefore appeared to be very likely that aquatic food webs would also differ during the low and high water periods. Available carbon sources for fish are thought to be less varied during low water periods, but flooding during high water periods could bring fish into contact with a greater abundance and diversity of food sources such as terrestrial plants or the biofilms that grow on submerged terrestrial plants. At low water levels, higher fish densities may lead to more piscivory and less omnivory when compared with the high water periods. Therefore, trophic links within the fish communities may then be modified by water level changes in tropical reservoirs. To address this prediction, we performed stable isotope analyses of the most common species in Sélingué and Manantali, two large reservoirs in Mali (West Africa). Allochthonous and littoral carbon sources were shown to support fish production to a significant extent, even during low water periods. However, the allochthonous or littoral carbon contributions that sustained the top-predators production were indeed greater during the high water periods as expected. The expected higher omnivory in the high water period might have shortened the food chain when compared with the low water period. Some carnivorous fish species were shown to feed at lower trophic levels during high water periods in both reservoirs, but this was not a general pattern. Flooding did not, therefore, necessarily result in a shorter food chain when water levels were high.

Coastal ecosystems are frequently submitted to anthropogenic pressure but little is known about how the dynamics of aquatic communities can be altered. The impact of urban and industrial discharges on microbial plankton was studied in Hann Bay near Dakar (Senegal) on the Atlantic Ocean. Spatial patterns were studied using three transects, with a total of 20 stations, during two periods in May and November (before and after the seasonal upwelling), revealing a clear contrast between near shore stations and the intermediate and seaward stations. In November, phytoplankton were associated with dissolved nutrient availability (e.g. with nitrate, r = 0.76) whereas in May, phytoplankton were more correlated with microbial variables (e.g. with heterotrophic nanoflagellates, r = 0.63). Most samples (40) failed to meet the quality levels for the faecal indicator bacteria (FIB) defined by the European Union bathing water quality directive. The topography plays a major role in water circulation explaining the presence of FIB at the seaward stations. The high prevalence of FIB during the two periods suggests chronic pollution and a potential risk to recreational swimmers and fish consumers in Hann Bay. Thus, as demonstrated in various temperate systems, the decline of water quality constitutes a serious problem in many West African countries.

Coastal ecosystems are frequently submitted to anthropogenic pressure but little is known about how the dynamics of aquatic communities can be altered. The impact of urban and industrial discharges on microbial plankton was studied in Hann Bay near Dakar (Senegal) on the Atlantic Ocean. Spatial patterns were studied using three transects, with a total of 20 stations, during two periods in May and November (before and after the seasonal upwelling), revealing a clear contrast between near shore stations and the intermediate and seaward stations. In November, phytoplankton were associated with dissolved nutrient availability (e.g. with nitrate, r = 0.76) whereas in May, phytoplankton were more correlated with microbial variables (e.g. with heterotrophic nanoflagellates, r = 0.63). Most samples (40) failed to meet the quality levels for the faecal indicator bacteria (FIB) defined by the European Union bathing water quality directive. The topography plays a major role in water circulation explaining the presence of FIB at the seaward stations. The high prevalence of FIB during the two periods suggests chronic pollution and a potential risk to recreational swimmers and fish consumers in Hann Bay. Thus, as demonstrated in various temperate systems, the decline of water quality constitutes a serious problem in many West African countries.

La presente etude porte sur deux series de donnees recoltees en baie de Bietri (lagune Ebrie, Cote d'Ivoire). La premiere concerne un programme d'echantillonnage a basse frequence (octobre 1985–mars 1986), au cours duquel ont ete mises en evidence les conditions hydroclimatiques necessaires a la suppression dans ce milieu du gradient de densite (augmentation de la salinite de surface, refroidissement des eaux lagunaires consecutif a une remontee cotiere). La seconde se rapporte a un echantillonnage a haute frequence (janvier–fevrier 1987), sur le meme site, afin de definir les mecanismes et les consequences ecologiques de la destratification. Lorsque les conditions hydroclimatiques sont reunies, la destratification proprement dite se realise a l'occasion d'une sequence de vive-eau, lors d'une basse basse-mer exceptionnelle, par l'acceleration des deplacements d'eau de surface dans la baie, traduite par un nombre de Richardson minimal. Cette destratification permet une homogeneisation des profils verticaux des concentrations en sels nutritifs, mais surtout, elle entraine une disparition de 90% de la charge en N-NH4 et de 60% de celle en P-PO4. Une modification de l'environnement chimique des eaux de l'hypolimnion lors de la reoxygenation est suggeree pour expliquer cette brutale et transitoire disparition. A l'oppose, la repartition verticale des biomasses semble peu affectee par la suppression du gradient de densite, la topographie des fonds interdisant l'etablissement d'importants mouvements convectifs. Ainsi, les consequences biologiques de la destratification dans ce milieu estuarien eutrophe sont peu marquees, et sans comparaison avec les modifications hydrodynamiques et hydrochimiques observees a cette occasion.

The coastal waters along the Atlantic Sahara are characterized by intense hydrodynamic phenomena (upwellings, water mass alternations) throughout the year that deeply influence their productivity. Deep-water arrivals at surface levels are closely related to the local prevailing winds dominating all year long, the Northeast Trade winds. Working on a time series of 28 yr shows that far from being regular and constant during that period, the Trade winds were very variable. The coastal upwellings induced by these winds showed important intensity variations: after years of strong activity (1955–60) there was a period of minimal activity (1961–69), which was followed by a period of great irregularity (1970–82), showing a progressive return to the initial conditions. These important variations inducing deep hydrological consequences and modifications of water trophic potentialities can be related to the biological anomalies observed during the same time in the Northwest Africa fisheries.

Thanatocœnoses of recent benthic Foraminifera have been studied in Ebrie Lagon (Ivory Coast), in the mangrove swamps of coastal Guinee, on each side of Kaloum peninsula, in the estuaries and lagons of Sénégal, in the lagoons surrounding Cap Timiris (Mauritania). One hundred and sixty six species were recognized in these paralic ecosystems and near their opening to the ocean. Specific diversity, high near the open sea (α, Les thanatocœnoses de Foraminifères benthiques récents ont été inventoriées dans la lagune Ebrié (Côte d'Ivoire), dans les mangroves de Basse-Guinée, de part et d'autre de la presqu'île duKaloum, dans les estuaires et lagunes du Sénégal et dans les lagunes bordant le cap Timiris (Mauritanie). Cent soixante six espèces ont été reconnues dans ces écosystèmes et au niveau de leur débouché sur l'Océan Atlantique. Dans chaque écosystème, la diversité spécifique, forte à proximité de la mer (α, Debenay J. P., Arfi Robert, Konate S. Foraminifères récents des milieux paraliques des côtes d'Afrique de l'Ouest. In: Géologie Méditerranéenne. Tome 14, numéro 1, 1987. Xe colloque africain de micropaléontologie. A – Le Cenozoique. Rabat – Université Mohamed V – Maroc. 29 septembre-1er octobre 1987. pp. 5-13.

The annual cycles of nutrients in the Etang de Berre were studied in 1985. From measurements twice a month at the input points, daily data for the freshwater flow and from daily measurements at the output point, an estimation is proposed for a nutrient budget. Seasonal variability is important, and a biological control of nutrient availability (locally remineralized products) results in a hydrologically controlled system (nutrients carried by continental waters) which produces the very high microalgal biomass observed in the lagoon. The seasonal halocline brings the euphotic layer very close to the remineralization site: nutrients are then constantly available to the phytoplankton.

Surrounded by a heavily industrialized area, subject to natural fresh water dilution (Etang de Berre, Rhone river), the Gulf of Fos is an unbalanced neritic zone. Its distrophic character is accentuated by numerous continental discharges; furthermore, the climatic instability which characterizes this region (irregular periods of winds, fluvial risings) reinforces the changing ecological conditions in the gulf waters. The present paper shows the major ecological conditions in that neritic area, and the main events occured during the 1976–1978 period, through and extensive sampling network. The principal regulating features are thus described by the mean of a mathematical data processing (Principal Components Analysis).