Abundance and distribution of bacterioplankton in the Gambia River, West Africa

Abstract

Four ecological zones of the Gambia River were sampled during four different hydrologic seasons for determination of microbial, nutrient, and physical parameters. A Greco-Latin Square experimental design was used to define the particular transect, station, depth, and tide/time-of-day of samples taken. Ranges of total bacterioplankton densities (10⁶ cells/ml) were similar to those of tropical and temperate environments. Numbers of free bacteria were similar temporally, whereas attached bacteria numbers were greater during periods of high stream flows when suspended solids concentrations were higher. Free bacteria were usually twice as numerous in the freshwater zones than in the estuarine zones. Attached bacterial densities were approximately four times greater in the estuarine zones than in the freshwater zones. Uptake of³H-glucose on both a sample volume and per-cell basis increased from the early stages of the flood (6.95±SE 1.37 ng/liter/hour and 3.8 pg/hour/10⁶ cells, respectively) and reached observed annual maximums during the dry season (21.01±SE 3.05 ng/ liter/hour and 13.0 pg/hour/10⁶ cells, respectively). Spatially,³H-glucose uptake per sample volume and per cell was highest in the upper river zone and lowest in the lower estuary zone. The lower estuary zone consistently acted out of concert with the other river zones in terms of³H-glucose and¹⁴C-bicarbonate uptake. Analysis of variance (ANOVA) indicated that free and attached bacterioplankton densities were not homogeneous among transects, stations, depths, and tide/time-of-day at the different zones during the four hydrologic seasons. The results suggested that heterotrophy overshadowed autotrophy in the river and that the bacterial abundance, distribution, and glucose uptake activity in this tropical floodplain river were greatly influenced by the annual flood and the presence of extensive mangrove forests in the estuary.