The microbial segment of food webs playsa crucial role in lacustrine food-web functioning andcarbon transfer, thereby influencing carbon storageand CO2 emission and uptake in freshwater environments.Variability in microbial carbon processing(autotrophic and heterotrophic production and respirationbased on glucose) with depth was investigatedin eutrophic, methane-rich Lake Rotsee, Switzerland.In June 2011, 13C-labelling experiments were carriedout at six depth intervals in the water column underambient light as well as dark conditions to evaluate therelative importance of (chemo)autotrophic, mixotrophicand heterotrophic production. Label incorporationrates of phospholipid-derived fatty acid (PLFA)biomarkers allowed us to differentiate between microbialproducers and calculate group-specific production.We conclude that at 6 m, net primary production(NPP) rates were highest, dominated by algal photoautotrophicproduction. At 10 m —the base of theoxycline— a distinct low-light community was able tofix inorganic carbon, while in the hypolimnion,heterotrophic production prevailed. At 2 m depth,high label incorporation into POC could only be tracedto nonspecific PLFA, which prevented definite identification,but suggests cyanobacteria as dominatingorganisms. There was also depth zonation in extracellularcarbon release and heterotrophic bacterial growthon recently fixed carbon. Large differences wereobserved between concentrations and label incorporationof POC and biomarkers, with large pools ofinactive biomass settling in the hypolimnion, suggestinglate-/post-bloom conditions. Net primary production(115 mmol C m-2 d-1) reached highest values inthe epilimnion and was higher than glucose-basedproduction (3.3 mmol C m-2 d-1, highest rates in thehypolimnion) and respiration (5.9 mmol C m-2 d-1,highest rates in the epilimnion). Hence, eutrophicLake Rotsee was net autotrophic during our experiments,potentially storing large amounts of carbon.
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