Heterotrophic Bacterial Production and Its Dependence on Autotrophic Production in a Hypertrophic African Reservoir

Abstract

The incorporation of [methyl-3H]thymidine (TdR) into bacterial DNA in Hartbeespoort Dam, South Africa was measured over 16 mo and at nine depths. Bacterial numbers at the surface ranged between 2.45 and 32.20 × 106 cells∙mL−1[Formula: see text] while bacterial production varied between 1.0 and 251 pmol TdR∙L−1∙h−1 (0.01 to 1.9 mg C∙m−3∙h−1). At the bottom, production ranged between 0 and 26.7 pmol TdR∙L−1∙h−1 (0–0.2 mg C∙m−3∙h−1). The fastest bacterial doubling time was 59 h. At the surface, bacterial production was dominantly correlated to chlorophyll a (6.6–6530 mg∙m−3) and phaeopigments (0.9–378 mg∙m−3) (r = 0.81) followed by primary production (26.6–8886 mg C∙m−3∙h−1) (r = 0.77) (n = 30–34, p < 0.001). However, below 5 m, water temperature and bacterial numbers were the dominant correlates. Bacterial production for the water column averaged 2% of daily, areal primary production. The data demonstrated a close coupling between autotrophic production and heterotrophic bacterial production. However, the low bacterial production compared with primary production, together with the small size of the bacteria (usually 0.09–0.25 μm width), suggest substrate supply was a major limiting factor of bacterial growth.