Spatial and Temporal Response of Stream Bacteria to Sources of Dissolved Organic Carbon in a Blackwater Stream System

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1. We hypothesized that changes in bacterial colony growth would be correlated to shifts in riparian vegetation (via leachate quality) along a river continuum of a south-eastern, blackwater stream (U.S.A.). Spatially, we expected bacterial assemblages from downstream reaches to utilize more sources of leachate and at higher concentrations than bacteria collected from headwater reaches. Temporally, we predicted higher colony growth on leachate from autumn-shed (senescent) leaves compared with leachate from fresh, green leaves.

2. We examined spatial differences in assemblage growth by culturing bacteria sampled along the stream continuum on gradient plates using leachates from four common riparian species (Taxodium distichum, Carya spp., Acer rubrum and Decumaria barbara). Bacteria from the lowest site were able to use all sources provided and at all concentrations, whereas bacteria from upper reaches could not. Colony density was correlated to relative leachate concentration at all sites along the continuum.

3. Leachates from fresh and senescent A. rubrum leaves were used to determine temporal differences. Winter assemblages of bacteria could not grow on fresh leaf leachate at any concentration but grew well on autumn leaf leachate at higher concentrations. Differential response of bacterial assemblages indicated local adaptation to potential sources of dissolved organic matter.

4. Growth response of stream bacterial colonies appeared to be dependent on the timing and source of leachate as well as on sources of dissolved organic carbon from further upstream. Growth of bacterial assemblages exhibited ‘generalist’ characteristics in headwater reaches and ‘specialist’ characteristics at the mouth of our study stream drainage. Thus, our findings lend support to the argument that variable resource habitats favour a small, generalist assemblage, while environments with stable resource supplies allow for highly diverse assemblages dominated by specialists.