Bacterioplankton growth, grazing mortality and quantitative relationship to primary production in a humic and a clearwater lake

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Abstract
Bacterial growth and grazing mortality were estimated from May to October in two south Swedish oligotrophic lakes, one being a clearwater lake (water colour 5–10 mg Pt l −1 DOC 2.9–3.4 mg l −1 , Secchi disk depth 5.0–9.4 m) and the other a humic, brownwater lake (water colour 105–165 mg Pt l −1 , DOC 13.7–22.7mg l −1 , Secchi disk depth 1.3–2.1 m). Specific rates of growth and grazing mortality were generally similar for both lakes. However, the abundance of bacteria was consistently 2–3 times higher in the water of the humic lake, suggesting that the total production and consumption of bacterial cells were also higher than in the dearwater lake. The ratio of bacterial secondary production to primary production was higher in the humic lake than in the clearwater lake, indicating that the bacterioplankton of the humic lake utilize allochthonous substrates, in addition to substrates originating from autochthonous primary production. Most of the bacterial loss in both lakes could be attributed to small protozoan grazers. This implies that allochthonous and autochthonous organic carbon fixed by bacterioplankton is less important in terms of carbon flow to higher trophic levels than would be expected if macrozooplankton were the dominant bacterivores, providing a more direct and efficient transfer of carbon to larger organisms.