Wind induced resuspension in a shallow tropical lagoon (GBR)

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 (Côte 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 Borela 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. (D'après résumé d'auteur

Natural recolonization of a productive tropical pond : day to day variations in the photosynthetic parameters

Chlorophyll pigments (CHL), primary productivity (PP) and particulate nitrogen (Np) in relation to several environmental factors were monitored during planktonic colonization of an aquaculture pond (Layo, Côte d'Ivoire). How interactions between the organisms are established in an initially azoic environment were investigated ... Overall, photosynthetic activity appeared to be closely linked to algal biomass. The study of autotrophic biomass and activity in different size classes in relation to the other parameters allowed us to precise the origin of the biomass fluctuations. The first bloom appeared to be controlled by selective grazing on small algae. The second algal development ended when N requirement represented at least 69% of N supply (in the N - NH4 form). This control was enhanced by the appearance of rotifers, leading to a more complex equilibrium. (D'après résumé d'auteur

Diel fluctuations of bacterial abundance and productivity in a shallow eutrophic tropical lagoon

Abundance and productivity (3H thymidine incorporation) of free-living and attached bacteria from surface waters were studied from hourly samples during typical hydrological conditions (rainy and flood seasons) in three different bays of the Ebrié lagoon (Côte d'Ivoire). Several additional biological variables (particulate organic carbon, phytoplankton biomass, zooplankton density) were also recorded in order to establish possible relation with bacterial parameters. (D'après résumé d'auteur

Conséquences biogéochimiques de l'atténuation de stratification dans une baie eutrophe : la baie de Biétri en lagune Ebrié (Côte d'Ivoire)

L'influence de l'affaiblissement annuel du gradient de densité sur l'hydrologie et la géochimie de la baie de Biétri (lagune Ebrié, Côte d'Ivoire) a été étudiée en suivant l'évolution des caractéristiques physiques et chimiques des eaux au cours de 45 sorties réparties sur 5 mois. A chaque sortie, température, salinité, transparence, oxygène dissous, potentiel redox et concentrations de N-NH4, N-NO2 et N-NO3 et P-PO4 ont été déterminés sur un profil vertical, au centre de la baie. La confrontation des données recueillies lors de cette étude avec les cycles de marées et la température de surface de la mer a permis de préciser les conditions requises pour un affaiblissement maximal du gradient de densité. Elles correspondent à l'atténuation du gradient de salinité, au début de la grande saison sèche, lors de la période de forte influence marine, et à l'installation d'une remontée côtière océanique refroidissant les eaux de surface, par le jeu des marée

Communautés, biomasses et productions algales au sein d'un récif artificiel (acadja) en milieu lagunaire tropical

Dans un secteur oligohalin et turbide de la lagune Ebrié (Côte d'Ivoire), les communautés phytoplanctonique et périphytique d'une structure d'aquaculture extensive, localement dénommée acadja (bambous plantés verticalement dans le sédiment), sont étudiées en termes de biomasses, productions, rendements énergétiques et composition spécifique. Lors de cette étude, la biomasse algale au sein de l'acadja constituée à 73% par du périphyton était 4 fois supérieure à celle des eaux libres lagunaires. L'assimilation autotrophe totale estimée à 9,9 g C.m-2.J-1 dans l'acadja était 4,5 fois supérieure à celle des eaux lagunaires et 80% de cette activité sont dus au peuplement périphytique... Compte tenu des forts rendements photosynthétiques des communautés fixées (5,2% pour le périphyton contre 0,8% pour le phytoplancton), l'acadja permet une augmentation sensible de la productivité naturelle du milieu lagunaire. Au sein d'une telle structure, l'élevage de poissons à régime alimentaire phytophage constituerait un mode de valorisation possible de ce surcroît de production autotrophe. (D'après résumé d'auteur

Role of bacteria as food for zooplankton in a eutrophic tropical pond (Ivory Coast)

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$was dominant. Less than 1$ 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

Ecological organization and succession during natural recolonization of a tropical pond

The structure of a planktonic community was studied in April 1990 for 24 days (D1 to D24) during the natural recolonization of a tropical pond (Côte d'Ivoire) made azoic by empting and liming (D0). Abundance of bacterioplankton, phytoplankton, protozoans and zooplankton were studied twice daily, whereas hydrological descriptors (temperature, conductivity, oxygen, pH, dissolved nutrients, sestonic weights) were recorded several times daily. After the pond treatment, the natural refilling from groundwater began immediately. The microheterotrophic (bacteria, flagellates, ciliates), phytoplanktonic (#Coelastrum microsporum) and zooplanktonic (the rotifers #Brachionus plicatilis and #Hexarthra intermedia$) communities were first based on opportunist species favored by the initially large nutritive availability. (D'après résumé d'auteur

In situ spawning rate of the calanoid copepod Acartia clausi in a tropical lagoon (Ebrié, Côte d'Ivoire) : diel variations and effects of environmental factors

rate of the calanoid copepod Acartia clausi in a tropical lagoon (Ebrié, Côte d Ivoire): diel variations and effects of environmental factors. Zoological Studies 43(2): 244-254. Diel changes in the spawning rate of Acartia clausi were studied during several 24 h time series performed between 1993 and 1997 at several sites in Ebrié Lagoon (Côte d Ivoire) at which the morphology and hydrological structures differed. The daily egg production rate ranged from 8 to 60 eggs female-1 d-1 (5 % to 45 % of body carbon) and varied according to stations and periods. A correlation analysis, performed with the present results, revealed no significant effect of either envi-ronmental factors (salinity, temperature, and chlorophyll a biomass) or individual weight of females. However, the roles of salinity and food abundance (chlorophyll a) appeared when grouping the present data (1993-1997) with results from the same sites obtained previously (1981-1982) by Saint-Jean and Pagano (1984). This grouping showed the importance of inter-annual variations as compared to seasonal variations. The rhythm of egg laying was characterized by a night maximum which occurred earlier in the estuarine zone (22:00-01:00) than in the western oligohaline zone (04:00-06:00). These peaks of egg laying always occurred after the peak of gut fluorescence which was consistently observed at the beginning of the night (data from Kouassi et al