Despite of the importance for tracking sources and fates of organics in determining carbon cycling and assessing the overall health of lacustrine ecosystems, little is known of organics behaviors in shallow eutrophic lakes. In this study, for relating the benthic organics to pelagic sources, an ecosystem-level investigation was performed in hypereutrophic Lake Taihu, China, by using source-specific fatty acid biomarkers on the molecular level. Results exhibited the unexpected asymmetric phenomena of organics between surface water and sediments. In the abundance, the dominant organics shifted from cyanobacteria in surface water to terrestrial plants in sediments. In the spatial pattern, as opposed to terrestrial plants, cyanobacteria were found not site-to-site symmetrical, although both of their spatial distributions varied strongly. Essentially, these asymmetric phenomena ascribed to a more considerable loss of cyanobacteria compared to terrestrial plants during sinking, caused by aquatic animal consumption and microbial utilization with different degree. Combined dual stable isotopes (δ13C and δ15N) and fatty acid biomarkers revealed that there were only subtle differences in the diets of benthic and pelagic animals and cyanobacteria were their main food source. Concomitantly, results of δ13C of bacteria-specific fatty acids demonstrated that bacteria equally and profoundly affected organics accumulation or preservation in the sediments, because they preferentially utilized labile cyanobacteria as their carbon source instead of terrestrial plants (>95% within these two sources). Consequently, these novel findings clarify that not only in deep lakes, but also shallow eutrophic ones, the extensive losses of autochthonous organic matter can be expected during sedimentation coupling with the dramatical modifications of biogeochemical processes.
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