Diversité saisonnière du phytoplancton en relation avec quelques facteurs abiotiques de la zone côtière de Kribi (Sud Cameroun)

Abstract

Phytoplankton, an unknown microalgae group that sustains the planet, is made up of a diversity of microorganisms that live freely in all aquatic ecosystems. Its biodiversity along the Cameroonian coasts remained poorly understood until the start of this study, despite the key role played by these microalgae in their functioning. The present study aimed to investigate the taxonomic composition of this important biological component of Atlantic Ocean at Kribi, as well as the influence of seasons on some abiotic factors and the dynamics of the taxa. It was conducted from February 2020 to February 2021 with a monthly sampling frequency at 13 sampling points (K1 to K13) chosen according to appropriate selection criteria. The abiotic factors were measured in the field using a composite to obtain overall information on the water layer from the surface to depth. This composite was carried out in a 20 L bucket by taking water samples at the surface using a 5 L bucket, and at depth in the euphotic and aphotic layers using a Van Dorn bottle. Samples for laboratory analyzed were taken in 250 and 1000 cc polyethylene bottles, stored in a refrigerated chamber and analysed using standard analysis protocols. For phytoplankton, the water was taken from the same sections of the water column as the abiotic factors samples, then filtered through a 40 µm mesh plankton sieve and the resulting retentate was fixed directly with Lugol. Phytoplankton taxa were identified and counted using a Sedgewick-Rafter cell and an Olympus optical microscope, as well as appropriate keys and books. The results were grouped by season and analyzed using appropriate software. Data recorded using the Kruskall-wallis test applied for abiotic factors analyses, showed their significant variations between points K2 and K7 located at the mouths of the Kienké and Lobé rivers respectively and the other sampling points on the one hand, and between the different seasons on the other. This result illustrates the influence of these rivers and the seasons on the physico-chemical quality of the water. Temperature, pH, conductivity, salinity, dissolved O2, nitrates and orthophosphates varied around the general mean values of 29.46±0.48 °C; 8.53±0.11; 26.1±3.33 mS/cm; 15.5±2.13 mg/L; 89.56±1.13 %; 1.61±0.47 mg/L; 1.86±0.29 mg/L respectively. With regard to the phytoplankton biodiversity studied, a diversity of 274 taxa including 192 taxa of Diatoms, 60 taxa of Dinoflagellates and 22 taxa Other grouped into 08 Phyla, 12 Classes, 46 Orders and 73 Families were collected. Among the Diatoms, the classes Bacillariophyceae and Coscinodiscophyceae were the most diverse, with 80 and 70 taxa each respectively, while among the Dinoflagellates, represented by 02 Classes, Desmophyceae was the most diverse, with 34 taxa, and Dinosaphyceae, with 25 taxa. This diversity varied between the different sampling points, with 89 taxa recorded at K5 and 129 taxa at K9, and xxvi between seasons, with 142 taxa identified during the Short Rainy Season (PSP) and 197 taxa during the Long Dry Season (GSS). In terms of densities, species belonging to the Diatoms were the densest spatially, except at K3, K4, K6, K8 and K12, where the density of species belonging to the Desmophyceae, the most diverse class of Dinoflagellates, was highest. Seasonally, phytoplankton densities increased overall from PSP to GSS. Grouping the densities of species into Classes on a seasonal basis shows that the densities of the Diatom Classes were higher in all seasons. However, the dominance of Desmophyceae was higher than that of certain Diatom Classes during the PSP and GSP. Spearman correlation tests show that temperature, electrical conductivity, salinity, TDS, levels of various forms of nitrogen and orthophosphates are the physico-chemical parameters that have a significantly greater positive or negative influence on taxa densities during this study. This work has enabled us to identify 20 taxa that are harmful and toxic to the health of the marine ecosystem, of which 03 are globally ubiquitous (Coscinodiscus wailesi, Ceratium furca and Dinophysis caudata); 5 are regular (Bacteriastrum hyalinum, Chaetoceros perivianus, Pseudosolenia calcar-avis, Pyrodinium bahamense, Prorocentrum micans); 3 are constant (Thalassiosira hendeyi, Odontella sinensis, Chaetoceros lorenzianus). However, a taxon used as a food supplement and in cosmetology (Odontella aurita) has also been identified in the coastal area of Kribi. It is clear from this study that the high representativeness of diatoms, which are a predominant component of marine phytoplankton, shows that the Atlantic Ocean at Kribi is not yet experiencing any major disturbance. Nevertheless, the presence of harmful and toxic species, as well as the enrichment in nutritive elements, reveal a progressive degradation of the environment. From the point of view of future research, strict and regular monitoring of nutrients and the taxonomic composition of phytoplankton should therefore be considered.