Allochthonous and autochthonous carbon sources of lake bacterioplankton

Abstract: Organic substrates for pelagic bacteria are derived from dissolved organic carbon (DOC) in the water column that originates either from primary production from within the lake itself (autochthonous), or import of organic matter from the terrestrial watershed (allochthonous production). This thesis addresses the utilization of allochthonous versus autochthonous carbon (C) sources by lake bacterioplankton. In addition, I examine the connection between the source of the organic matter and the structure and function of the bacterial community. In spite of low allochthonous C inputs, high respiration:primary production and bacterial:primary production suggested that five oligotrophic clear-water lakes on the Faroe Islands were net heterotrophic (respiration exceeded primary production) and that the heterotrophic plankton were subsidized by allochthonous organic C. While the occurence of net heterotrophy does not reveal anything about the fate of the allochthonous C, the importance of autochthonous versus allochthonous DOC in supporting bacterial production was examined by whole-lake additions of 13C (Forest lakes in Northern Wisconsin), and tracing its subsequent uptake by phytoplankton and on to bacteria. Bacterial production in lakes ranging from eutrophic and low-humic to oligotrophic and humic, consisted of 40 ? 80 % allochthonous C, confirming the often stated hypothesis that autochthonous carbon alone does not support bacterial production. Yet, autochthonous DOC was preferentially utilized relative to terrestrial DOC, although the preference for autochthonous C was not complete. Furthermore, based on a correlative study of ten lakes in the same area, bacteria seemed to grow more efficiently on autochthonous C, suggesting that this C is more effectively transferred to higher organisms. The results of the lake survey and the whole-lake isotope additions were simulated by a simple steady state model of bacterial utilization of autochthonous and allochthonous dissolved organic C. This showed that a higher preference and greater growth efficiency of bacteria on autochthonous C could explain why bacterial production was coupled to autochthonous production also in net heterotrophic ecosystems where the use of allochthonous C by bacteria was high. Finally, the source of the organic substrate (autochthonous and allochthonous) appeared to influence the bacterial community composition as well as general bacterial functions (production, respiration, growth yield and substrate utilization). Hence, bacterial sub-populations growing at different rates on substrates of differing origin, might explain why we observe a selective utilization of autochthonous C by the total bacterial community.