Structure et biodiversité zooplanctonique des eaux de quelques mangroves de la côte littorale camerounaise et rôle potentiel des facteurs abiotiques

Abstract

With the increase of human activity and pollution of aquatic ecosystems, the question of optimal management of these hydrosystems becomes increasingly important. It is in the context of the search for solutions to the degradation of mangrove forest ecosystems that the present work takes place. Its aim is to evaluate the structure and diversity of the zooplankton population in the waters of a few mangroves on in Cameroonian coast, in relation to a number of abiotic factors. Sampling was carried out from November 2019 to November 2020 on a monthly basis. Fourteen (14) sampling stations were selected, with six (06) stations in the Mouanko marine influenced mangroves (EMMo), four (04) stations in the Manoka marine-influenced mangroves (EMMa) and four (04) stations in the Mouanko freshwater-influenced mangroves (EDMo). Physicochemical analyses were carried out in situ and in the laboratory using standard methods. Variables such as Temperature, pH, Electrical Conductivity, Salinity, TDS, TSS, Color, dissolved oxygen (O2), Turbidity and heavy metals (Manganese, Iron, Nickel, Lead, Zinc…) were analyzed. For biological analysis, 100 liters of water were filtered through a 64 µm mesh at each station. The concentrate obtained was fixed with 96° alcohol, then zooplankton species were identified and counted in the laboratory using an optical microscope and a binocular magnifying glass, and appropriate keys and books. Analysis of abiotic variables shows that the mangrove waters studied are slightly basic (7.59 ± 0.04 UC), moderately oxygenated (65.36 ± 1.21 %), low in Nitrite (0.08 ± 0.008 mg/L), Nitrates (6.82 ± 0.27 mg/L) and Orthophosphates (0.05 ± 0.01 mg/L) with high concentrations of TSS (14.09 ± 1.35 mg/L), Turbidity (30.85 ± 2.66 mg/L) and Color (213.73 ± 18.1 Pt. Co). Stations located in EDMo are respectively less mineralized and less salty (1354.3 ± 34 µS/cm and 74.25 ± 20.12 mg/L) than those located in EMMo (8236.46 ± 898.6 µS/cm and 4627.09 ± 419.9 mg/L) and EMMa (12580.42 ± 1012.36 µS/cm et 6985.46 ± 595.5 mg/L). The metal element levels measured are below the relevant environmental standards (SEQ-Eau, 2003). In terms of fauna, 81 species of zooplankton were identified, including 72 Holozooplankton and 9 Merozooplankton. The Holozooplankton identified include 30 species of Rotifera belonging to 12 families, the richest and most abundant of which is the Brachionidae ; 22 Copepoda belonging to 11 families, the richest and most abundant of which is the Paracalanidae ; 16 Cladocera belonging to 5 families, the richest of which is the Chydoridae ; 03 Ostracoda belonging to 2 families, the richest and most abundant of which is the Cypridae ; and 01 Chetognath belonging to the Sagittidae family. Merozooplankton includes 06 decapods xxvii belonging to 06 families, the most abundant of which is the Parthenopidae ; 02 Urochordae belonging to 02 families, the most abundant of which is the Oikopleuridae ; and 01 Mysidacea belonging to the Mysidacae family. The EDMo were the richest, followed by the EMMo and EMMa with 64, 61 and 47 species respectively. On the other hand, EMMo was the densest, followed by EMMa and EDMo with 722.94, 104.25 and 36.75 ind./L respectively. Overall, copepods were the most abundant (83.71% in EMMo, 94.53% in EMMa and 61.96% in EDMo), represented mainly by larval stages and the species Paracalanus indicus, Parvocalanus elegans, Oithona nana and Oithona sp. ; while Mysidaceans (0.07%), represented only by the species Mysis oculata in EMMa, were the least abundant. With the exception of Rotifers, which were most abundant in the dry season, the other zooplankton groups harvested were most abundant in the rainy season. The Shannon-Weaver diversity and Piélou equitability indices showed that the EDMo stations had a more diverse (H’ = 4.19 ± 0.05 bits/ind) and better distributed fauna (J’= 0.79 ± 0.005). Specific diversity and taxon evenness were also higher at low tide (3.81 ± 0.11 bits/ind and 0.82 ± 0.01 respectively) and in the rainy season (4.07 ± 0.07 bits/ind and 0.85 ± 0.01 respectively). Canonical Redundancy Analysis (CRA) and Spearman's correlation test revealed that high levels of salinity, electrical conductivity, TDS and Potassium ion favored the proliferation of Parvocalanus elegans, Paracalanus indicus and Oncaea clevei species in EMMo and EMMa, while temperature and nitrates favored the proliferation of Oncaea sp. and Oithona sp. in EDMo. Species distribution in the EDMo depends on water from the Sanaga and Kwa-Kwa rivers, which enrich the environnement with freshwater species, while the EMMo and EMMa are strongly influenced by tidal movements, which bring in species with a marine affinity from the Atlantic Ocean. Overall, these results show that the waters studied are of good ecological quality, and the species Oithona nana and Paracalanus parvus are those that best characterize this environment. However, some stations are already showing signs of advanced degradability.