Bacterial utilization of imported organic material in three small nested humic lakes

摘要

Lakes import organic carbon in the form of dissolved humic substances and other terrigenous compounds. This allochtho-nous organic carbon is consumed by heterotrophic bacteria, with significant consequences for the biostructure and energy pathways of freshwater ecosystems (Jansson et al. 2007). Laboratory studies generally show that a minor share of terrigenous material (～10%) is available to bacterial consumption on short term scales (weeks) and that the degradation thereafter proceeds at slow rates (Tranvik 1998). Although natural exposure to solar UV radiation may increase the availability (Lindell et al. 1995, Tranvik 1998), it is not certain that this can compensate the loss of bioavailable carbon from bacterial consumption. We hypothesized that as allochthonous organic carbon is processed and aged during its residence time in lakes, its potential to support bacterial growth, both in terms of rates and efficiency, decreases successively. We employed a short term (12 day) bioassay approach to study 3 small nested lakes in Northern Sweden, almost entirely dominated by allochthonous organic carbon. The first 2 lakes drained almost directly into the third, making it possible to examine a downstream gradient from inlet to outlet and from lake to lake.