Eléments nutritifs et production primaire dans les lagunes du Côte d'Ivoire : cycle annuel

This paper collects most of the information gathered between October 1974 and July 1976 within the framework of a research program concerning the hydrobioclimate of ivorian lagoons and especially the Ebrié lagoon. Monthly surveys concerning the latter were carried out during 1975. The following parameters - and their vertical distribution wherever it had a meaning - were systematically gathered in a system of fifty-five stations: Transparency (Secchi), Temperature, Salinity, Chlorophylla, Dissolved oxygen, phosphate, nitrate, nitrite. These data provide an outline of the annual cycle of nutrients and primary production of the Ivoirian lagoons

Méthodes simples de determination du CO2 total et du carbone organique dissous en eau saumâtre

Total CO2 of brackish water (Ebrié lagoon) is measured with a simple method: degassing and trapping CO2 in NaOh solution, the conductivity of which is a function of the trapped CO2 quantity. A relation CO2 versus salinity is deduced; it is nearly the same as the one deduced from CO2 measurements performed from alkalinity. Dissolved organic C is evaluated with a CHN analyser by evaporating 0.5 ml of acidified sample. Variation coefficient is near 10% for the lagoon values.

Étude comparative de la macrofaune associée aux racines de Pistia stratiotes L. (Araceae) du lac de barrage d'Ayamé II et du fleuve Comoé (Côte d'Ivoire)

Abundance and species variations of the macrofauna associated with the roots of Pistia stratiotes of Ayamé II lake and Comoé river were studied during one year. Samples of plants were collected by hand once a month within a sampler of 1 m2 of surface.

Évaluation de la production primaire et de la chlorophylle à partir des données hydrologiques. Application au Golfe de Guinée

Quantitative relationships between nitrate distribution, chlorophyll and primary integrated values have been used to evaluate the phytoplankton abundance in the Gulf of Guinea. Data of Guinee I cruise (May-July 1968) of the R.V. Jean Charcot have been used. They show a large oligotrophic convergence area (< 250 mg C/m2/J). A relationship previously found between the depth of the nitracline (first level where the nitrate appears) and the depth of the thermal maximum gradient is confirmed. From a practical point of view, this relationship is very useful since it allows, when the biological or chemical data are not available, a rough estimation of phytoplankton integrated biomass and production in the water column, from a temperature profile.

Étude du crabe rouge profond Geryon quinquedens en Côte d'Ivoire. 2 - Éléments de biologie et d'écologie avec références aux résultats obtenus au Congo

Geryon quinquedens is present along the West African continental slope at depths from 300 to 1000 m, on silt-clay sediments. Geryon is a cold and rather poorly oxygenated water loving species. It is easily caught by traps as it is a scavenger and predatory crustacea. In a given area its distribution does not appear to be homogeneous: for example, densities of red crabs are higher in the eastern and western region of Côte d'Ivoire than in the central zone. Similar observations can be made off Congo, Angola and United States. It can be assumed that there is a relation between the abundance of Geryon and the productivity level of the area. Geographical variations of sex ratio are suspected to be correlated with the density distribution. Males and females have not the same bathymetric distribution: females are only common in the shallower waters (300-500 m) whereas males are present in the whole biotope. Seasonal migrations occur down and up the slope in both the sexes and are certainly related to the reproductive biology. Knowledge of the reproductive biology is also necessary to understand fishing-trap catch rate: egg maturation extends over several months and ovigerous females are exceptionally caught by traps; males also are less available during the same period (March to August) when migrations are less important; in this period, mean size increases and probably this happens at the end of a moult. From September to February the catch-rates increase. Growth is slow compared with other littoral Guinean Crustacea (Peneides). Females become sexually mature at a size of 80 mm (carapace width): modification in the allometric relations of abdomen and carapace are then conspicuous.

Méthodes des mesures des biovolumes, poids secs, poids secs sans cendres et des éléments C, N, P du mésozooplancton utilisées au C.R.O. d'Abidjan

Methods for the estimation of zooplankton biomasses, used in the Oceanographic Research Center of Abidjan are presented. They deal with settled and displacement volumes, dry weight and ash-free dry weight, elementary carbon, nitrogen and phosphorus composition. The dry weight method is detailed: elimination of salt by a fresh water stream, preservation of dry samples at -20 degrees Celsius, rehydration during weighing. A few comments on the 'CHN' analysed values are made: at 1,100 degrees Celsius, most of the carbon is organic, only 10% of the mineral fraction being analysed.

Biologie de Penaeus duorarum notialis en Côte d'Ivoire. 4 - Rélations entre la répartition et les conditions du milieu. Étude des variations du sex-ratio

The analysis of the geographic and bathymetric distribution of Penaeus duorarum and, particularly P. d. notialis off Côte d'Ivoire and in its whole distribution area leads to the definition of the adult ecological requirements (temperature, salinity, grain size and sediment composition, organic matter) and the importance of the thermocline in the bathmetric distribution. The population structure study shows: (1) variations of size with depth, (2) variations of sex ratio, with size, depth and seasons.

Biologie de Penaeus duorarum en Côte d'Ivoire. 3. Étude des variations quotidiennes des rendements et leur relations avec l'hydroclimat. Essai de généralisation au Golfe de Guinée

Hydroclimatic conditions in the Gulf of Guinea between Senegal and Nigeria are briefly described emphasizing the seasonal variations of transparency. Analysis of the Abidjan based shrimp fleet allowed to the description of the seasonal variations of activity rhythms for Côte d'Ivoire, Ghana, and Nigeria. These rhythms are different between seasons, between fishing grounds, and sometimes even between depths on a given ground. These variations follow the turbidity ones. Diurnal activity is observed in very turbid waters, nocturnal and transition activity in clearer ones. The authors assume that the basic behaviour is a nocturnal one, but that the shrimp-trawlers catches reflect some apparently different ones resulting from diel variations in the stock availability. To explain the apparently diurnal behaviour observed most of the year over the whole Gulf of Guinea it is suggested that these generally benthic shrimps become nectonic at night when turbidity is very high. The results obtained in Ivory Coast, Ghana and Nigeria are compared to those from Senegal where hydroclimatic conditions are different. The similarities are emphasized. The differences in observed behaviour are supposedly caused by the cold season water temps which are sufficiently low to disturb the nor mal activity rhythm.

Production primaire, secondaire et régéneration des sels nutritifs dans l'upwelling de Mauritanie

Primary and secondary productions and nutrient regeneration in the Mauritanian upwelling area were studied by following a drogue for 9 days, from the point of upwelling till the water mass dives under offshore waters. The lag between phytoplanktonic bloom, zooplanktonic peak and bacterial activity is very short and may be explained by a well-settled biological cycle connected with an undercurrent. Organic production was estimated in two ways: (1) from chlorophyll 'a' values, considering a C/Chla ratio of 25 during the 5.5 day phytoplankton growth period, primary production computed by this method reaches 13.5 g C/m2; (2) from 14C values net primary production calculated for the same period reaches 10.5 g C/m2 and total organic production (net production + organic excretion) reaches 19.5 g C/m2. Organic production computed ratios, delta O/ delta C/ delta N/ delta Si/ delta P are equal to 130/43/11/7.4/1. Secondary production and 'grazing' are estimated from mesozooplankton respiration values and have a huge increase during the bloom. Net secondary production is assessed to be 1.0-4.2 g C/m2 for 6 days. Evidence of nutrient regeneration as ammonia, phosphate and silicate is given and regeneration rates are calculated. Zooplankton excretion plays an important part in nitrogen and phosphorus regeneration. Bacterial activity is induced by zooplankton organic excretion, then increased by phytoplankton decomposition at the end of the bloom.

Distribution et abondance des larves de thonidés dans l'Atlantique tropico-oriental. Étude des données de 1963 à 1974

From several sources of data, the authors study seasonal variations of larval abundance for five species of tuna in Eastern Tropical Atlantic and its relation to the hydrological conditions. Distributional maps and TS graphs are given.