Heterotrophic bacterial activity and primary production in a hypertrophic African lake

Abstract

The relationship between heterotrophic bacteria and phytoplankton in the epilimnion (0–10 m) of hypertrophic Hartbeespoort Dam, South Africa, was examined by statistically analyzing three years of parallel measurements of heterotrophic bacterial activity (glucose uptake) and phytoplankton particulate and dissolved organic carbon production. Algal biomass ranged between 4.0 and 921.1 mg Chl a m-3 at the surface. Primary production varied between 69.5 and 3010.0 mg C m-2h-1 while algal production of dissolved organic carbon (EDOC) ranged from 2.5 to 219.2 mg C m-2h-1. Bacterial numbers reached a summer peak of 44.23 × 106 cells ml-1 in the first year and showed no depth variation. The maximum rate of glucose uptake, Vmax, reached a peak of 5.52 μg C l-1h-1. Vmax, maximum glucose concentration (Kt + Sn) and glucose turnover time (Tt) were usually highest at the surface and decreased with depth concomitant with algal production. At the surface, Vmax was correlated to EDOC (r = 0.59, n = 67, p < 0.001) and primary production (r = 0.71, n = 70, p < 0.001). At 5 and 10 m, Vmax was correlated to integral euphotic zone (~ 4 m) algal production and bacterial numbers. Glucose turnover time was inversely related to integral algal production (r = -0.72, n = 70, p < 0.001) and less strongly to bacterial numbers. The data indicated that although bacterial numbers and biomass were low relative to algal biomass in this hypertrophic lake, the heterotrophic bacteria attained high rates of metabolic activity as a result of enhanced algal production of available organic carbon.