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The factors controlling estuarine phytoplankton diversity and production are relatively well known in temperate systems. Less however is known about the factors affecting phytoplankton community distribution in tropical estuaries. This is surprising given the economic and ecological importance of these large, deltaic ecosystems, such as are found in South East Asia. Here we present the results from an investigation into the factors controlling phytoplankton distribution and phytoplankton-bacterial coupling in the Bach Dang Estuary, a sub-estuary of the Red River system, in Northern Vietnam. Phytoplankton diversity and primary and bacterial production, nutrients and metallic contaminants (mercury and organotin) were measured during two seasons: wet (July 2008) and dry (March 2009). Phytoplankton community composition differed between the two seasons with only a 2% similarity between July and March. The large spatial extent and complexity of defining the freshwater sources meant that simple mixing diagrams could not be used in this system. We therefore employed multivariate analyses to determine the factors influencing phytoplankton community structure. Salinity and suspended particulate matter were important factors in determining phytoplankton distribution, particularly during the wet season. We also show that phytoplankton community structure is probably influenced by the concentrations of mercury species (inorganic mercury and methyl mercury in both the particulate and dissolved phases) and of tri-, di, and mono-butyl tin species found in this system. Freshwater phytoplankton community composition was associated with dissolved methyl mercury and particulate inorganic mercury concentrations during the wet season, whereas, during the dry season, dissolved methyl mercury and particulate butyl tin species were important factors for the discrimination of the phytoplankton community structure. Phytoplankton-bacterioplankton coupling was also investigated during both seasons. In the inshore, riverine stations the ratio between bacterial production and dissolved primary production was high supporting the hypothesis that bacterial carbon demand is supported by allochthonous riverine carbon sources. The inverse was true in the offshore stations, where BP:DPP values were less than 1, potentially reflecting differences in primary production due to shifting phytoplankton community diversity.

This is a study of the environmental conditions and primary phytoplankton production in a Sahelian shallow lake of Senegal, West Africa. Environmental descriptors (nutrient, water transparency, temperature and hydrochemistry) and their effects on primary production were studied. Samples were collected monthly from March 2002 to February 2003 in 3 stations of the central part of water body. Climatic variables show a marked annual pattern following two dominant wind directions: NNE (North-North-East) and NNW (North- North-West). This cyclic evolution affects hydrobiological variables. In this central part of the lake, water body was not stratified with same measures between surface and bottom. Lake water was turbid with lower values during the maritime trade winds period characterized by high water temperatures and conductivity. Water pH was relatively higher than seven (07) associated with high contents of oxygen (approximately 90%) in all stations. The station Typha is marked by a low pH and high fluctuations in the rates of oxygen (20 to 120%). Nutrient concentrations were low, with high variability (from 0 to 30 μg.l -1 for DIN and from 0 to 18 μg.l -1 for PO 4 ). The primary production in Guiers Lake was high; it showed a vertical gradient with production of 2400 mgC.m -3 .days -1 in surface and a low of 200 mgC.m -3 days -1 at the bottom. In this shallow system, environmental conditions regulate the primary production. Keywords : Environmental descriptors, primary production, shallow lake, Senegal.

In mesotidal lagoons of the Indian Ocean, the coral reef barrier may be temporarily submerged at high tide and partially exposed at low tide, and this may cause unusual lagoon dynamics. A field measurement campaign was conducted in the north-east Mayotte Lagoon in order to understand these processes. An experimental approach was used, combining measurements taken by 1) a side-mounted Acoustic Doppler Current Profiler (ADCP) on a moving boat along transects through the reef passages (17 transects) and 2) by more conventional high-resolution moored ADCP measurements. A specific tidal analysis methodology was used to determine the spatial variability of the velocity. The tidal hydrodynamics within the lagoon were determined using a numerical model and then analyzed. The tide acted as a quasi-progressive forced wave in the lagoon: at low tide, water entered through the south passage, over the reef and left the lagoon through the north passage. This flow was reversed at high tide. The tide-driven quasi-progressive wave created a specific lagoon dynamics. Contrary to most other channel lagoons, the flow over the reef was mainly outward. This increases the inflow through the passages, which renews the water in the lagoon as shown by the indicators of age and origin of the water inside the lagoon. This study also showed the importance of these indicators for better understanding the variations and levels of plankton biomass (with chlorophyll concentration as proxy) which is quite high in this lagoon.

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.

International audience; Size fractionation was performed using water from the Great Reef of Toliara (Madagascar) taken from two different habitats (ocean and lagoon) during the dry and wet seasons, to study the growth and mortality rates of bacterioplankton. Experiments were conducted with 1 and 100% of heterotrophic nanoflagellate (HNF) concentrations and virus-free water was obtained by tangential filtration (10 kDa). During the dry season, in both environments, bacterial abundance and production were significantly lower than values recorded during the wet season. Bacterial growth rates without grazers were 0.88 day–1 in the lagoon and 0.58 day–1 in the ocean. However, growth rates were statistically higher without grazers and viruses (1.58 day–1 and 1.27 day–1). An estimate of virus-induced bacterial mortality revealed the important role played by viruses in the lagoon (0.70 day–1) and the ocean (0.69 day–1). During the wet season, bacterial growth rates without grazers were significantly higher in both environments than were values obtained in the dry season. However, the bacterial growth rates were paradoxally lower in the absence of viruses than with viruses in both environments. Our results suggest that changes in nutrient concentrations can play an important role in the balance between viral lysis and HNF grazing in the bacterial mortality. However, virus-mediated bacterial mortality is likely to act simultaneously with nanoflagellates pressure in their effects on bacterial communities.

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.

La dynamique des particules dans les systemes aquatiques est regit par l’interaction entre la resuspension et la sedimentation. Cette etude est consacree a la determination des processus responsables de la resuspension des particules dans le lac de Guiers situe au nord du Senegal. Des echantillons ont ete collectes au cours d’un cycle annuel au niveau de trois stations prises dans la partie centrale du lac. Dans ce site, la resuspension des particules est liee a l’action des vents mais aussi a la crue du fleuve Senegal. La resuspension sous l’effet des vents se produit pendant l’harmattan, alors qu’en periode d’alizes maritimes, c’est la crue du fleuve Senegal qui est responsable de la remise en suspension des particules. Le taux de particule en suspension est plus important en periode d’harmattan qui est marquee par des vitesses du vent plus elevees (moyenne de 3,6 m.s -1 fluctuant entre 1 et 7 m.s -1 ), mais surtout par un fetch fort (24 km). Cette evolution saisonniere de la resuspension affecte la turbidite de l’eau, les taux de matiere en suspension, la biomasse phytoplanctonique, le coefficient d’attenuation lumineuse, le flux des particules et la structure de la colonne d’eau. La resuspension sous l’effet des vagues creees par le vent associe au fetch eleve constitue dans ce systeme peu profond le principal processus controlant la dynamique des particules. Mots cles : Vagues, crue du fleuve, resuspension, particule, turbidite, lac de Guiers.

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.

Salinity in estuaries is influenced by a variety of processes including tidal advection and diffusion and river discharge. The effect of hydrodynamic features on salinity was studied in the Senegal River Estuary (SRE). This estuary is of strategic importance for large populations, but it has been greatly affected by human action (regulation of the freshwater inflow, change in the location of the river mouth ...), which has caused major changes in salinity and ecological functioning. To analyse the impacts of these changes and to determine the spatial and temporal variations in salinity in the SRE, we used a combination of three-dimensional modelling and field measurements. The overall salinity depends on freshwater inflow and tidal fluctuations. Salinity variation is mainly driven by ebb and flood near the mouth and by fortnightly cycles upstream. The enlargement of the mouth increases salinity, whereas its shifting location changes the location of the salinity front and creates a slack water zone downstream. Connection and disconnection of the tributaries also affects the salinity. The present study explains how the recent modifications have increased spatial variation and reduced seasonal differences of salinity and provides a tool for managing the water in the estuary.

Introduction L'un des axes de recherche du programme Petits Barrages dans sa composante limnologique est, en relation avec les différents facteurs agissant sur les écosystèmes, l'identification des éléments structuraux et fonctionnels qui en expliquent la productivité (Cecchi, 1998). Tout écosystème, d'une façon générale, est en équilibre entre l'homogénéité et l'hétérogénéité (Legendre et Troussellier, 1993) et cela sous la pression de trois forces : exogènes, comme les énergies auxiliaires,...

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

International audience; In the Bach Dang-Cam Estuary, northern Vietnam, mechanisms governing cohesive sediment aggregation were investigated in situ in 2008-2009. As part of the Red River delta, this estuary exhibits a marked contrast in hydrological conditions between the monsoon and dry seasons. The impact on flocculation processes was assessed by means of surveys of water discharge, suspended particulate matter concentration and floc size distributions (FSDs) conducted during a tidal cycle at three selected sites along the estuary. A method was developed for calculating the relative volume concentration for the modes of various size classes from FSDs provided by the LISST 100X (Sequoia Scientific Inc.). It was found that all FSDs comprised four modes identified as particles/flocculi, fine and coarse microflocs, and macroflocs. Under the influence of the instantaneous turbulent kinetic energy, their proportions varied but without significant modification of their median diameters. In particular, when the turbulence level corresponded to a Kolmogorov microscale of less than ∼235 μm, a major breakup of flocs resulted in the formation of particles/flocculi and fine microflocs. Fluctuations in turbulence level were governed by seasonal variations in freshwater discharge and by the tidal cycle. During the wet season, strong freshwater input induced a high turbulent energy level that tended to generate sediment transfer from the coarser size classes (macroflocs, coarse microflocs) to finer ones (particles/flocculi and fine microflocs), and to promote a transport of sediment seawards. During the dry season, the influence of tides predominated. The turbulent energy level was then only episodically sufficiently high to generate transfer of sediment between floc size classes. At low turbulent energy, modifications in the proportions of floc size classes were due to differential settling. Tidal pumping produced a net upstream transport of sediment. Associated with the settling of sediment trapped in a near-bed layer at low turbulent energy, this causes the silting up of the waterways leading to the harbour of Haiphong.

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.

High-latitude coral reefs may be a refuge and area of reef expansion under climate change. As these locations are expected to become dryer and as livestock and agricultural yields decline, coastal populations may become increasingly dependent on marine resources. To evaluate this social-ecological conundrum, we examined the Grand Récif of Toliara (GRT), southwest Madagascar, which was intensively studied in the 1960s and has been highly degraded since the 1980s. We analyzed the social and ecological published and unpublished literature on this region and provide new data to assess the magnitude of the changes and evaluate the causes of reef degradation. Top-down controls were identified as the major drivers: human population growth and migrations, overfishing, and climate change, specifically decreased rainfall and rising temperature. Water quality has not changed since originally studied, and bottom-up control was ruled out. The identified network of social-ecological processes acting at different scales implies that decision makers will face complex problems that are linked to broader social, economic, and policy issues. This characterizes wicked problems, which are often dealt with by partial solutions that are exploratory and include inputs from various stakeholders along with information sharing, knowledge synthesis, and trust building. A hybrid approach based on classical fishery management options and preferences, along with monitoring, feedback and forums for searching solutions, could move the process of adaptation forward once an adaptive and appropriately scaled governance system is functioning. This approach has broad implications for resources management given the emerging climate change and multiple social and environmental stresses.

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.

This paper presents the outline of an estuary modeling study encompassing a study of the biogeochemical model on its own. It includes a preliminary validation of the new mechanistic features using mesocosm data, and a study of a coupled physical and biogeochemical model of the Senegal River Estuary (SRE) from flood to low-water conditions. The originality of this work is that it distinguishes the marine and freshwater origins of some of the organisms modeled and includes additional mortality terms owing to the osmotic stress experienced by the freshwater organisms. The mechanistic model implemented in the Eco3M modelling tool, and partially designed for this study, succeeded in representing the dynamics of the planktonic food web in two mesocosm experiments and in the SRE under very different conditions using a single set of model parameters. Among the important results, it reveals that distinguishing between marine and freshwater origins provides substantial additional information on the distribution of organisms along the estuary. Finally, failure to take account of the osmotic stress and the origin of the organisms can give rise to substantial errors in the calculation of the biomass in the SRE. (C) 2011 Elsevier B.V. All rights reserved.

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.

International audience; The study of lysogeny in aquatic systems is an often overlooked aspect of microbial ecology, especially in tropical environments. Herein, the fraction of lysogenized cells (FLC) was detected in the surface waters of 20 coastal stations distributed from the eutrophicated shoreline to seaward waters of Hann Bay (Senegal). Concurrently, viral lytic infection rates were extrapolated from the frequency of visibly infected bacterial cells (FVIC), as determined from transmission electron microscopy observations. The experimental induction of prophage was observed in less than 3% of indigenous marine bacteria, suggesting that lysogenic stages of infection are rare in Hann Bay. Similarly, only 0.5-4.7% of bacteria showed visible signs of lytic infection. However, the positive correlation between the fraction of lysogenic and lytic cells (r = 0.67, p < 0.05, n = 20) may actually indicate that the coexistence of both lifestyles may be due to the massive and rapid induction of lysogens, potentially from the high levels of local UV radiation. Overall, we suggest that the determination of FVIC and FLC to examine the predominance of one type of cycle versus the other may be a source of misinterpretation in some particular aquatic environments.

Phytoplankton species composition and seasonal changes were investigated monthly in Lake Guiers, one of the largest lakes in West Africa. Taxonomic composition, diversity, biovolume and abundance of phytoplankton were studied at 3 representative stations from March 2002 to March 2003, in relation to various climatic factors (wind, insolation), physical and chemical factors (temperature, conductivity, nutrients) and biological factors (zooplankton grazing). A total of 111 spe- cies of phytoplankton belonging to 7 algal classes were identified, predominately Chlorophyceae (32% of species) followed by Cyanobacteria (25%) and Bacillariophyceae (diatoms, 15%). From December to March, a period characterised by low temperature, solar insolation and north-easterly winds that mixed the water column, the community was dominated by the diatom Fragilaria sp. and cyanobacterium Anabeana miniata. From April to September, during higher temperatures, solar insolation and predominant north-westerly winds that mixed the water column less effectively, the community was dominated by cyanobacteria Cylindrospermopsis raciborskii and Lyngbya versi- color. A statistical approach based on co-inertia analysis was used to describe the phytoplankton/ environment relationships, in particular the factors governing the presence of C. raciborskii. The phytoplankton community was characterised by a clear succession of species groups: Fragilaria sp.-A. miniata (November to March) → Microcystis aeruginosa (March to May) → C. raciborskii- L. versicolor (June to November). Finally, a functional approach originally designed for temperate zones was applied to the algal groups (11 functional groups), which gave valuable information on algal strategies in a shallow tropical lake.

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.

International audience; Seasonal and depth-related variability in the grazing activity of heterotrophic nanoflagellates (HNF) on viruses was examined in the oligo-mesotrophic Lake Pavin and in the eutrophic Lake Aydat, French Massif Central, between May and November 2000. Ingestion rates (IR) were determined using 50 nm diameter fluorescent microspheres, as virus analogues. In both lakes, highest grazing activities on virus-sized particles were recorded in the metalimnion, at the beginning and the end of the thermal stratification period. Estimated IRs in Lake Pavin (mean = 0.4 viruses HNF-1 h-1, CV = 38.0%) and in Lake Aydat (mean = 0.3 viruses HNF-1 h-1, CV = 35.6%) were not significantly different, in contrast to clearance rates (CR), which were significantly higher in the oligomesotrophic (2.3 x 10-2 nl HNF-1 h-1) than in the eutrophic lake (0.7 × 10-2 nl HNF-1 h-1). CRs for viruses were correlated with CRs for bacteria in Lake Aydat but not in Lake Pavin, suggesting a greater abundance within the HNF assemblages of virus-sized particle feeders in the less productive lake. We estimated that 4.1 and 0.8% of viral production were grazed by HNF in Lake Pavin and Lake Aydat, respectively. Finally, although viruses seem to represent a minor food source for HNF (i.e. compared to bacteria), they may not be inconsequential in their diet, especially in oligotrophic lakes.

Grazing of the copepods Acartia clausi and Pseudodiaptomus hessei on natural particles was studied from on board experiments during several 24 h time series performed between 1993 and 1997 in four sites of the Ebrie Lagoon (Cote d'Ivoire). Ingestion rates of both species increased linearly with food concentration until a concentration threshold (5.5×109 μm3 l−1 for A. clausi and 5.2×109 mm3 l−1 for P. hessei) beyond which the relation presented a plateau. Both species poorly selected the peak of available particles (range 3–6 mm equivalent spherical diameter, ESD) but A. clausi seek preferentially smaller particles (6–21 μm ESD) than P. hessei (9–33 μm ESD). When the proportion of the preferred particles in the food offered decreased, A. clausi extended its selectivity towards both smaller and larger particles whereas P. hessei extended its selectivity towards larger particles only. As a consequence of these patterns, the useful particle concentration (UPC) was higher for A. clausi than for P. hessei. In addition, the ratio of the UPC for the two species showed a positive relationship with the ratio of their respective biomass. The significance of these results for the adaptation capacities of the two species and for the ecosystem functioning are discussed. A. clausi which is more suited than P. hessei to exploit smaller particles (3–6 μm) which dominate the seston, has a food competitive advantage. The inadequacy between the seston food-size composition and the selective patterns of the two main zooplankton species of the Ebrie Lagoon explains that they could be food limited despite the high trophic level of the lagoon. It could also partly explain the low transfer efficiency between phytoplankton and zooplankton in this ecosystem.

L'operation de recherche decrite ci-dessous a ete realisee dans le cadre du projet Gihrex de l'IRD, dont l'objectif etait de modeliser de la productivite du delta interieur du Niger en fonction de la variabilite hydrologique et des strategies d'exploitation. Cela implique, entre autres, d'avoir une meilleure connaissance des flux de matieres qui participent au fonctionnement biohydrogeochimique du delta en relation avec son exploitation par l'homme. D'une superficie d'environ 40 000 km2, le delta interieur du Niger est situe au centre du Mali ou il designe un ensemble de plaines inondables, de lacs, de mares et de chenaux alimentes par le fleuve Niger. Dans cette zone sahelienne, l'arrivee des eaux fluviales a la faveur des crues annuelles est a l'origine d'un fort developpement biologique donnant lieu a une forte production en ressources naturelles renouvelables, exploitees par trois systemes d'exploitation majeurs se partageant l'espace et le temps selon le rythme saisonnier impose par le cycle hydrologique du fleuve Niger : peches saisonnieres, cycles de culture de riz pluvial et transhumances des troupeaux. Les premieres mesures de qualites chimique, organique et biologique de l'eau montrent que cet ecosysteme d'une grande richesse apparente serait un systeme plutot oli-mesotrophe, ou toute activite trophique est sous-tendue par l'acces a la ressource primaire. Cette etude de la repartition spatiale au cours du temps des teneurs en nutriments dans une plaine inondable du delta interieur du Niger se propose d'apprehender le role des plaines d'inondation saheliennes sur le cycle des nutriments en utilisant des bilans de masse sur les exportations dissoutes d'azote et de phosphore. (Resume d'auteur)

Diel changes in vertical distribution and gut pigment contents of Acartia clausi andPseudodiaptomus hessei were studied during several 24-h time series performed between 1993 and 1997 in four sites of the Ebrie Lagoon (Cote d'Ivoire). The sites differed by their morphology and their hydrological structure and by the vertical distribution of chlorophyll biomass. Both species showed classical diel vertical migrations (DVM). Copepodites and adult stages of P. hessei were almost benthic during the day and evenly distributed through the water column at night. The amplitude of DVM of A. clausi increased from copepodites I–III to adults. Copepodites and adults of A. clausi increased significantly their gut fluorescence at night, whereas those of P. hessei showed no clear diel feeding rhythm (DFR). These results suggest that A. clausi feed mostly at night on phytoplanktonic particles and P. hessei feed mostly on benthic algal particles during the day and on sestonic particles at night. No relationship was observed between DFR and DVM because both patterns occurred when food was either vertically homogeneous or vertically stratified. The daily average gut fluorescence of A. clausi increased with ambient chlorophyll concentration until around 12–15 μg l−1, whereas no relationship was found for P. hessei. The implication of these patterns on the adaptation capacities and the behaviour of the two species are discussed. The DVM of P. hessei should explain its rarity in the estuarine area. The comparison of our results with previous ones suggests an evolution of A. clausi DFR between 1981–1982 and 1996–1997, in relation to an intensification of eutrophication.

Les zones inondables des fleuves tropicaux sont caractérisées par une forte saisonnalité hydrologique (Talling, 1992 ; Lévêque, 1999). Une période de basses-eaux, pendant laquelle le lit mineur du cours d’eau constitue l’essentiel du milieu humide, succède à une période de hautes-eaux, pendant laquelle le fleuve déborde largement du lit mineur, inonde la plaine en alimentant un lit majeur et remplit de nombreux lacs et mares (Lemoalle, 1999). Ces zones inondables sont réputées pour leur forte...

In the Ebrié Lagoon (Côte d'Ivoire), growth of periphyton on bamboo stuck in the sediment is at the basis of the acadja, a low-cost system developed in order to enhance aquaculture of omnivorous fish like the tilapia #Sarotherodon melanotheron$. A survey based on hydrological, bacterial and algal data (including phytoplankton and periphyton) was conducted from November 1992 to June 1993 in an experimental structure. Climatic and hydrological seasonality was marked during the study. Succeeding to a rainy season, the oligohaline situation starting in November was characterized by low bacterial and chlorophyll biomass attached to the bamboo (respectively 1 and 18 mg/m2). The mesohaline situation observed from January featured a sharp increase in periphyton biomass, with a maximum in April (16 and 177 mg/m2 for bacterial and algal biomass, respectively). The flood of the Agnéby river, induced in June by the local rains, originated again oligohaline conditions. The biomass decrease observed in May and June resulted from a shift towards low salinity (from 9.8 to 1.8 psu), a decrease in light availability (combined effects of a decreasing solar radiation during the rainy season and an increase of water turbidity due to the flood) and a lower phytoplankton biomass (therefore limiting the secondary epiphytism potentialities). In this shallow tropical environment characterized by high nutrient concentrations (due to local hydrology and organic nature of the substrate), the combination of seasonal variations of climatic (light availability), hydrological (salinity) and biological (abundance or lack of epiphytic algae) seems to control the periphyton biomass growing on bamboo. Therefore, marked seasonality in the production of resource available for the target fish would limit the interest of the acadja as an aquaculture system in brackish ecosystems. (Résumé d'auteur)

Activity of heterotrophic bacteria is recognized as an important process governing the biochemical functioning of many aquatic ecosystems. The aim of this study is to compare bacterial isolates of pelagic heterotrophic bacteria from two sites, each with a different trophic status (4 microg/l at Kpass and 23 microg/l at Layo for chlorophyll index) located in the Ebrié lagoon, and the bacteria associated with periphyton growing on an artificial reef of bamboos stuck into the sediment of the two sites. Hierarchical classifications were performed from morphological and biochemical tests of 343 heterotrophic bacterial strains isolated from the two sites and for each habitat. Despite differences in trophic status, numerical taxonomic analysis of the pelagic bacterial strains (n = 165) showed a dominant cluster (72% of strains) of mixed origin (close to 50% of isolates from each site). The majority of these strains were non-fermentative rods producing neither amino-acids decarboxylases, nor acid from any carbohydrates. In the more eutrophic site (Layo), few differences of properties were observed between pelagic and periphyton-associated bacteria. On the contrary, a large part of bacteria isolated from bamboo at Kpass was characterized by specific particularities such as the degradation of arginine, tryptophan and citrate. Numerical taxonomic analysis allowed the distinguishing of specific characteristics of bacteria according to the habitat. Our results suggest that hydrological and biological conditions (especially photosynthetic exudates from periphyton) may induce an obvious specialization of the periphytic bacteria in the less eutrophic ecosystem (Kpass) compared to the bacteria isolated from the more eutrophic environment (Layo). (Résumé d'auteur)