The aim of this study was to evaluate the physicochemical and microbiological quality of the waters of Franceville lakes, which are real receptacles for several pollutants. The physico-chemical parameters in situ were measured by means of a multiparameter and other chemical elements as phosphate, sulfate and ammonium were highlighted and measured by a colorimetric and spectrophotometric method. The microbiological parameters were determined by the colony-forming unit (CFU) method on selective agar media. Finally, strains of Escherichia coli were identified by biochemical tests. The results of the physico-chemical parameters show that the waters of Lake Lacaisse have the highest temperature, conductivity, salinity and dissolved oxygen values respectively, 26.71 °C, 398.67 μs/cm, 190 mg/l and 376.4 mg/l. On the other hand, these waters have neutral pH. The results of the bacteriological parameters show that the lake Makana contains the strongest rates of coliformes totals (4.84.105 UFC/100 ml) and of presumptive Escherichia coli in 37°C (3.54.105 UFC/100 ml). Eosin methylene blue medium is the best for counting total coliforms. In addition, (6.45%) colonies of Escherichia coli were identified. So, other coliformes thermotolérants (Entérobacter Sakazakii, Entérobacter cloacae, klebsiella ozaenae, klebsiella Pneumoniae. pneumoniae). In sum, this study shows that the water in Lake Makana appears to be more polluted than Lac Lacaisse.

Phoma sabdariffae Sacc. (Sphaerioidaceae) is the main pathogenic fungus agent of roselle (Hibiscus sabdariffa L. var. sabdariffa), in Gabon. It does induce wet rot on every part of the infected plant's organs. This work was initiated to evaluate in greenhouse the impact of this pathogenic fungus agent on 13 roselle cultivars through the study of some morphological and biochemical parameters of the fruits. Morphological parameters evaluated have been fruit's number, weight and contamination rate. Concerning biochemical study, parameters measured have been roselle fruit's acidity, proteins and phenolics compounds levels. The results revealed that excepted 3 cultivars VV1, RV1 and RR1, Phoma sabdariffae caused significant decrease in the fruits number and weight of most of the roselle's cultivars. The decrease rates have been higher than 60% and 20% respectively. The improvement in acidity, protein and phenolics compounds levels were not significant, in contrary to those of the 3 cultivars above mentioned. In these 3 roselle's cultivars, biochemical parameters (pH, oxalic acid, ascorbic acid, proteins and phenolics compounds) levels have been superior to 20%. This work showed the disastrous impact of this fungus on the roselle's fructification and revealed that, some of the cultivars of roselle bright develop resistance mechanisms against Phoma sabdariffae.