Abstract

In this study, a field experiment was set up to examine the seasonal dynamics of bacterial population and to investigate the relationship between bacterial abundance and dissolved organic carbon (DOC) produced by phytoplankton in a tropical eutrophic fishpond located in Calabar, South-East Region of Nigeria. Sampling lasted for six months: January to March in the dry season and April to June in the wet season. The highest concentration of DOC released during the dry season was 4.440 mg/L in January, while during the wet season; the highest concentration of 4.992 mg/L was released in April. The patterns of nutrients (nitrate and phosphate) released during both seasons were similar. In the dry season, the range of bacterial count varied between 7.307 x 10⁴ and 0.025 x 10⁴ CFU/ml while during the wet season it was from 7.909 x 10⁴ to 0.019 x 10⁴ CFU/ml. The bacteria; Vibrio, Moraxella and Bacillus species were dominantly present throughout the dry season but during the wet season, the bacteria; Pseudomonas, Flavobacterium, Streptococcus, Bacillus, Corynebacterium and Aeromonas species dominated throughout. Bacterial species succession also took place within seasons. Our results suggest that, during growth and decomposition of phytoplankton, the bacterial community involved shows a successive change not only in generic composition but also in terms of the heterotrophic activity of the bacteria in the community. The correlation analysis between dissolved organic carbon and bacterial count showed a positive correlation (r = 0.95 for the dry season and r = 0.82 for the wet season). There was a significant difference (p<0.05) between season in the case of DOC and bacterial count.

Key words: Dissolved organic carbon (DOC), bacteria, nutrient, species, and succession.