SPATIAL AND TEMPORAL RESPONSE OF STREAM BACTERIA TO SOURCES OF DISSOLVED ORGANIC-CARBON IN A BLACKWATER STREAM SYSTEM

1. We hypothesized that changes in bacterial colony growth would be co rrelated to shifts in riparian vegetation (via leachate quality) along a river continuum of a south-eastern, blackwater stream (U.S.A.). Spa tially, we expected bacterial assemblages from downstream reaches to u tilize more sources of leachate and at higher concentrations than bact eria collected from headwater reaches. Temporally, we predicted higher colony growth on leachate from autumn-shed (senescent) leaves compare d with leachate from fresh, green leaves. 2. We examined spatial diffe rences in assemblage growth by culturing bacteria sampled along the st ream continuum on gradient plates using leachates from four common rip arian species (Taxodium distichum, Carya spp., Acer rubrum and Decumar ia barbara). Bacteria from the lowest site were able to use all source s provided and at all concentrations, whereas bacteria from upper reac hes could not. Colony density was correlated to relative leachate conc entration at all sites along the continuum. 3. Leachates from fresh an d senescent A. rubrum leaves were used to determine temporal differenc es. Winter assemblages of bacteria could not grow on fresh leaf leacha te at any concentration but grew well on autumn leaf leachate at highe r concentrations. Differential response of bacterial assemblages indic ated local adaptation to potential sources of dissolved organic matter . 4. Growth response of stream bacterial colonies appeared to be depen dent on the timing and source of leachate as well as on sources of dis solved organic carbon from further upstream. Growth of bacterial assem blages exhibited 'generalist' characteristics in headwater reaches and 'specialist' characteristics at the mouth of our study stream drainag e. Thus, our findings lend support to the argument that variable resou rce habitats favour a small, generalist assemblage, while environments with stable resource supplies allow for highly diverse assemblages do minated by specialists.