Assemblage Structure and Population Dynamics of Phytoplankton in a Brackish Coastal Creek, Badagry, Southwest Nigeria

Aims: The objectives of the study were to determine phytoplankton assemblage structure in Badagry creek; to examine spatial-seasonal variations of abiotic factors and to assess their effects on it.

Study Design: Stratified random sampling.

Place and Duration of Study: Nine sampling stations were selected in Badagry creek to represent its three different areas/zones (upper, middle and lower). Three stations randomly chosen in each zone and sampled on bimonthly basis for two years beginning from November 2011 between 9.00 and 12:00 hours.

Methodology: Water and phytoplankton samples were collected and analyzed bi-monthly for two years from each station using standard methods. Water samples were analysed for temperature, pH, salinity, conductivity, turbidity, dissolved oxygen, water depth, nitrate and phosphate. Phytoplankton was identified to species level using relevant texts and counted under a Microstar IV Carl Zeiss binocular microscope calibrated at different magnifications. Diversity was determined using Shannon - Weiner (H), Evenness (e^H/S) and simpson (1-D) indices. Data were analysed using descriptive statistics, ANOVA, cluster and Canonical correspondence analysis (CCA) at α = 0.05.

Results: Overall, 242 phytoplankton species of six main classes were identified. Phytoplankton assemblage was dominated by Baccillariophyceae (133 species; 95.02%) specifically Aulacoseira granulata var. angustissima (Ehrenberg) Ralfs and Actinoptychus undulatus (Bailey). Other Algae groups were green algae (59 species), blue-green algae (27 species), euglenoids cell (17 species), dinoflagellates and Chrysophytes had three species each. Few phytoplankton species were reported to be potentially harmful species. Diversity indices values were highest (H = 2.633; E = 0.138; 1-D = 0.896) for st.8 (Iyagbe) and lowest (H = 1.221; E = 0.036; 1-D = 0.589) at Ajido (st.5). Comparatively, phytoplankton species (184) and individuals (541,655) recorded in the rainy season were higher than the dry season of 174 species and 446,516 individuals. Mean values ranged from 1.12 (st.2) - 7.83 psu(st.9), 2259.33 (st.2) - 13642.00 µScm-1(st.9), 19.67 (st.7) - 36.00 FTU(st.9), 1.38 (st.7) - 6.72 m (st.2) and 3.75 (st.1) - 12.53 µmol/L(st.9) for salinity, conductivity, turbidity, water depth and nitrate, respectively. Seasonal mean variations were pH 7.47; 7.75 and PO4 6.54; 4.80 µmol/L for dry and rainy seasons, respectively. Correlations between species and variables, suggests the importance of abiotic factors in phytoplankton assemblages. In CCA plot, Phosphate and water temperature have higher correlation ordination axis, closely followed by conductivity (salinity) and water depth.

Conclusion: Abiotic factors evaluated showed some influences on abundance and distribution of phytoplankton assemblage in Badagry creek. This study, however, provides baseline information for establishing predictions of phytoplankton population changes in Badagry creek.