Abstract
The microbial segment of food webs plays a crucial role in lacustrine food-web functioning and carbon transfer, thereby influencing carbon storage and CO2 emission and uptake in freshwater environments. Variability in microbial carbon processing (autotrophic and heterotrophic production and respiration based on glucose) with depth was investigated in eutrophic, methane-rich Lake Rotsee, Switzerland. In June 2011, 13C-labelling experiments were carried out at six depth intervals in the water column under ambient light as well as dark conditions to evaluate the relative importance of (chemo)autotrophic, mixotrophic and heterotrophic production. Label incorporation rates of phospholipid-derived fatty acid (PLFA) biomarkers allowed us to differentiate between microbial producers and calculate group-specific production. We conclude that at 6 m, net primary production (NPP) rates were highest, dominated by algal photoautotrophic production. At 10 m —the base of the oxycline— a distinct low-light community was able to fix inorganic carbon, while in the hypolimnion, heterotrophic production prevailed. At 2 m depth, high label incorporation into POC could only be traced to nonspecific PLFA, which prevented definite identification, but suggests cyanobacteria as dominating organisms. There was also depth zonation in extracellular carbon release and heterotrophic bacterial growth on recently fixed carbon. Large differences were observed between concentrations and label incorporation of POC and biomarkers, with large pools of inactive biomass settling in the hypolimnion, suggesting late-/post-bloom conditions. Net primary production (115 mmol C m−2 d−1) reached highest values in the epilimnion and was higher than glucose-based production (3.3 mmol C m−2 d−1, highest rates in the hypolimnion) and respiration (5.9 mmol C m−2 d−1, highest rates in the epilimnion). Hence, eutrophic Lake Rotsee was net autotrophic during our experiments, potentially storing large amounts of carbon.
Highlights
Within the global carbon cycle the cycling of organic matter in fresh-water systems has only recently been recognized as a significant component, potentially with global impacts (Battin et al 2008, 2009; Cole et al 2007; Raymond et al 2013; Richey et al 2002)
At 8.5 m water depth, production rates of phospholipid-derived fatty acid (PLFA) showed that multiple processes occurred simultaneously
3000 6000 pmol C L d with different PLFAs produced at different depths (Fig. 5)
Summary
Within the global carbon cycle the cycling of organic matter in fresh-water systems has only recently been recognized as a significant component, potentially with global impacts (Battin et al 2008, 2009; Cole et al 2007; Raymond et al 2013; Richey et al 2002). Carbon cycling is determined by in situ primary and secondary production, depending on nutrient availability and light, and by inputs of terrestrial organic carbon and methane (either locally produced or derived via groundwater from surrounding soil). The availability of these different carbon sources has a major impact on food-web functioning and structure by affecting the relative amounts of primary and secondary production (Cole et al 2006; Pace et al 2004). The microbial segment of food-webs (phytoplankton and bacteria) plays a crucial role in food-web functioning and transfer of organic matter through ecosystems to higher trophic levels (e.g. zooplankton, fish)
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