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

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 (d 13 C) of DOC were determined in large (surface area 674 km 2 ; maximum depth 104 m), monomictic Lake Biwa. A sharp vertical gradient of d 13 C of DOC (d 13 C-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 13 C-rich DOC in the epilimnion. Vertical mixing explained the intermediate values of d 13 C-DOC (-25.7 ± 0.0%) measured throughout the water column during the overturn period. Both DOC concentration and d 13 C-DOC decreased in the hypolimnion during stratification, indicating selective remineralization of 13 C-rich DOC. Using a two-component mixing model, we estimated the d 13 C value of semi-labile DOC to be -22.2 ± 0.3%, which was close to the d 13 C of particulate organic carbon collected in the epilimnion during productive seasons (-22.7 ± 0.7%) but much higher than the d 13 C-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.Keywords Semi-labile dissolved organic carbon Á Autochthony and allochthony Á Mixing model Á Carbon stable isotope ratio Á Large lake

Biogeochemical control on fluorescent dissolved organic matter dynamics in a large freshwater lake (Lake Biwa, Japan)

“…This contradiction could be due to the difference in the origin of the dominant DOM pool (allochthonous vs. autochthonous) in these systems. The low concentrations of DOC and FDOM T in the inflowing rivers (Mostafa et al 2005) and the dominant planktonic contribution of accumulated DOM (d 13 C signature identical to that of phytoplankton; Maki et al 2010) could indicate that accumulated DOM in Lake Biwa is of autochthonous origin. Experimental studies have shown that a reduction in allochthonous input and bacterial activity can lead to a shift in net consumption to net accumulation of protein-like FDOM in freshwater lakes (Guillemette and del Giorgio 2012).…”

“…However, the influence of terrestrial organic matter input decreases with the increase in lake size (Kelly et al 2001), and consequently the DOM concentration and composition in large lakes is expected to be driven by in situ processes. The vertical and seasonal distribution of the chemical composition (C : N : P ratio) and stable isotopic (d 13 C) signature of DOM in large freshwater lakes suggests that autochthonously produced semi-labile DOM in surface layers during productive seasons is redistributed to deeper layers under winter mixing (McManus et al 2003;Kim et al 2006;Maki et al 2010). These observations indicate that DOM cycling in large lakes has more resemblance to cycling in the ocean than to the small lakes.…”

“…Further, the stable isotopic (d 13 C) signature of TCO 2 accumulated in the hypolimnion indicated that most of the organic matter mineralized in the hypolimnion originated in situ during the mineralization of phytoplankton-derived organic matter (Miyajima et al 1997). The rivers draining to Lake Biwa carry relatively low DOC (75.6 6 52.0 mmol L 21 ; Mostafa et al 2005;Maki et al 2010), while the sewage treatment plants (which serve 85% of the population) surrounding Lake Biwa reduce the anthropogenic input of nitrogen (by 70%) and phosphorus (by 95%) in addition to organic matter (Okubo 2012). Despite the fact that sewage discharges are important sources of protein-like FDOM (Hudson et al 2007), the low concentration of protein-like FDOM in the inflowing waters relative to the lake (Mostafa et al 2005) is supported by the d 13 C ratio of DOC, indicating that DOM variations in the lake are mainly driven by in situ processes (Maki et al 2010).…”

“…The rivers draining to Lake Biwa carry relatively low DOC (75.6 6 52.0 mmol L 21 ; Mostafa et al 2005;Maki et al 2010), while the sewage treatment plants (which serve 85% of the population) surrounding Lake Biwa reduce the anthropogenic input of nitrogen (by 70%) and phosphorus (by 95%) in addition to organic matter (Okubo 2012). Despite the fact that sewage discharges are important sources of protein-like FDOM (Hudson et al 2007), the low concentration of protein-like FDOM in the inflowing waters relative to the lake (Mostafa et al 2005) is supported by the d 13 C ratio of DOC, indicating that DOM variations in the lake are mainly driven by in situ processes (Maki et al 2010). As protein-like FDOM is a useful indictor of autochthonous production and mineralization of semi-labile DOM (Yamashita and Tanoue 2003;NietoCid et al 2006;Romera-Castillo et al 2013), this optical technique may be useful in tracing the surface production and entrainment of semi-labile DOM through seasonal vertical mixing and its mineralization in the hypolimnion of large freshwater lakes.…”

Characterization of Protein-like Fluorophores Released from Lake Phytoplankton on the Basis of Fractionation and Electrophoresis

“…10,11 This approach was used to examine relative contributions of autochthonous and allochthonous sources to dissolved organic matter (DOM) in Lake Biwa. 12,13 In our previous study, DOM in Lake Biwa and rivers was investigated so as to elucidate its origins and behavior. The main origin of hydrophobic acids in Lake Biwa may be humic substances from soils around rivers that flow into Lake Biwa, while hydrophilic DOM may be due to inner production by phytoplankton.…”