Heterotrophic prokaryotic production in ultraoligotrophic alpine karst aquifers and ecological implications

Spring waters from alpine karst aquifers are important drinking water resources. To investigate in situ heterotrophic prokaryotic production and its controlling factors, two different alpine karst springs were studied over two annual cycles. Heterotrophic production in spring water, as determined by [ ³H]leucine incorporation, was extremely low ranging from 0.06 to 6.83 pmol C L^-1 h^-1 (DKAS1, dolomitic-karst-spring) and from 0.50 to 75.6 pmol C L^-1 h^-1 (LKAS2, limestone-karst- spring). Microautoradiography combined with catalyzed reporter deposition-FISH showed that only about 7% of the picoplankton community took up [ ³H]leucine, resulting in generation times of 3-684 days. Principal component analysis, applying hydrological, chemical and biological parameters demonstrated that planktonic heterotrophic production in LKAS2 was governed by the respective hydrological conditions, whereas variations in DKAS1 changed seemingly independent from discharge. Measurements in sediments recovered from LKAS2, DKAS1 and similar alpine karst aquifers (n=12) revealed a 10 ⁶-fold higher heterotrophic production (average 19 μmol C dm ^-3 h^-1) with significantly lower generation times as compared with the planktonic fraction, highlighting the potential of surface-associated communities to add to self-purification processes. Estimates of the microbially mediated CO₂ in this compartment indicated a possible contribution to karstification.