Autochthonous origin of semi-labile dissolved organic carbon in a large monomictic lake (Lake Biwa): carbon stable isotopic evidence |
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Authors: | Koh Maki Chulgoo Kim Chikage Yoshimizu Ichiro Tayasu Toshihiro Miyajima Toshi Nagata |
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Institution: | (1) Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano Tokyo, 164-8639, Japan;(2) Center for Ecological Research, Kyoto University, 2-509-3 Hirano, Otsu Shiga, 520-2113, Japan;(3) Present address: Center for Aquatic Ecosystem Restoration, Kangwon National University, Chun-Cheon, Kangwon, 200-701, Korea; |
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Abstract: | Semi-labile dissolved organic carbon (DOC) plays an important role in the transport and hypolimnetic remineralization of carbon
in large freshwater lakes. However, sources of semi-labile DOC in lakes remain unclear. This study used a carbon stable isotope
approach to examine relative contributions of autochthonous and allochthonous sources to semi-labile DOC. Vertical and seasonal
variations in the concentration and carbon stable isotope ratio (δ13C) of DOC were determined in large (surface area 674 km2; maximum depth 104 m), monomictic Lake Biwa. A sharp vertical gradient of δ13C of DOC (δ13C-DOC) during the stratification period mean ± standard error (SE) −25.5 ± 0.1 and −26.0 ± 0.0‰ in the epi- and hypolimnion,
respectively] indicated the accumulation of 13C-rich DOC in the epilimnion. Vertical mixing explained the intermediate values of δ13C-DOC (−25.7 ± 0.0‰) measured throughout the water column during the overturn period. Both DOC concentration and δ13C-DOC decreased in the hypolimnion during stratification, indicating selective remineralization of 13C-rich DOC. Using a two-component mixing model, we estimated the δ13C value of semi-labile DOC to be −22.2 ± 0.3‰, which was close to the δ13C of particulate organic carbon collected in the epilimnion during productive seasons (−22.7 ± 0.7‰) but much higher than
the δ13C-DOC in river waters (−26.5 ± 0.1‰). Semi-labile DOC appeared to be mainly autochthonous in origin, produced by planktonic
communities during productive seasons. The spatiotemporal uncoupling between production and remineralization of semi-labile
DOC implies that hypolimnetic oxygen consumption may be affected by pelagic primary production during productive seasons of
the preceding year. |
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