Autochthonous origin of semi-labile dissolved organic carbon in a large monomictic lake (Lake Biwa): carbon stable isotopic evidence

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 (delta C-13) of DOC were determined in large (surface area 674 km(2); maximum depth 104 m), monomictic Lake Biwa. A sharp vertical gradient of delta C-13 of DOC (delta C-13-DOC) during the stratification period [mean +/- A standard error (SE) -25.5 +/- A 0.1 and -26.0 +/- A 0.0aEuro degrees in the epi- and hypolimnion, respectively] indicated the accumulation of C-13-rich DOC in the epilimnion. Vertical mixing explained the intermediate values of delta C-13-DOC (-25.7 +/- A 0.0aEuro degrees) measured throughout the water column during the overturn period. Both DOC concentration and delta C-13-DOC decreased in the hypolimnion during stratification, indicating selective remineralization of C-13-rich DOC. Using a two-component mixing model, we estimated the delta C-13 value of semi-labile DOC to be -22.2 +/- A 0.3aEuro degrees, which was close to the delta C-13 of particulate organic carbon collected in the epilimnion during productive seasons (-22.7 +/- A 0.7aEuro degrees) but much higher than the delta C-13-DOC in river waters (-26.5 +/- A 0.1aEuro degrees). 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.