6. IMPLICATIONS OF LAKE WATER DIC ISOTOPIC COMPOSITION ON CARBON
CYCLE
Lake ecosystems are active components of the global carbon cycle as they
continually fix and release carbon through various biological processes,
including photosynthesis, food web activity, and bacterial degradation
(Tranvik et al., 2009; Holgerson & Raymond, 2016). The lake carbon
cycle is mainly composed of carbon input via runoff and atmospheric
CO2 diffusion, different forms of carbon transformation
in the lake (including organic matter decomposition,
photosynthesis/respiration of aquatic plants, and carbonate
precipitation), and carbon output via runoff and gaseous
CO2 emissions through the water surface (Hu et al.,
2014).
The DIC and its isotopes are an important tool to elucidate the carbon
cycle of lake ecosystems (McKenzie, 1985; Quay et al., 1986; Herczeg &
Fairbanks, 1987; Wachniew & Różański, 1997; Herczeg et al., 2003; Lei
et al., 2012; Mu et al., 2016). Striegl et al. (2001) found that during
the ice melting period, the average
δ13CDIC-L value in 132 freshwater
lakes in temperate and cold regions was –14 ‰. Similarly, Bade et al.
(2004) reported that the average
δ13CDIC-L value in 108 freshwater
lakes in different regions was –15 ‰. In freshwater lakes, the
photosynthesis of aquatic plants and organic matter decomposition in
sediments are active components of the carbon cycle (Lei et al., 2012).
In freshwater lakes on the Qiangtang Plateau, variations in
δ13CDIC-L also showed that organic
matter decomposition significantly contributed to the carbon cycle of
the lake ecosystems (Lei et al., 2012). However, Lei et al. (2012) found
that the δ13CDIC-L values of endorheic
lakes on the Qiangtang Plateau were relatively high, approaching the
δ13CDIC-L values of equilibrated
CO2 in water to atmospheric exchange.Therefore, water surface to atmospheric CO2 exchange
drives the carbon cycle in endorheic lakes on the Qiangtang Plateau (Lei
et al., 2012).
The δ13CDIC-L values of the Genggahai
Lake significantly exceeded the
δ13CDIC values of freshwater lakes
(> 8 ‰), but were more negative than those of lakes on the
Qiangtang Plateau (> –5.71 ‰) (Lei et al., 2012). This
indicates that the carbon cycle of the Genggahai Lake significantly
differs from those of freshwater lakes and lakes on the Qiangtang
Plateau. Variations in the δ13CDIC-Lvalues of the Genggahai Lake indicate that organic matter decomposition
and water to atmospheric CO2 exchange are not likely the
main components of its carbon cycle (section 5.1). Carbon input from
inflowing groundwater and the photosynthesis of aquatic plants may be
the main components of its carbon cycle.
Recent vegetation surveys have shown that the main terrestrial plants in
the watershed of the Genggahai Lake are Artemisia desertorum ,Oxytropis aciphylla , Achnatherum splendens , Orinus
kokonorica , and Agropyron cristatum . This is identical to that
in the Qinghai Lake watershed (Duan & Xu, 2012; Liu et al., 2013). The
δ13Corg values of 3C
plants in the Qinghai Lake watershed varied from –27.7 to –24.5 ‰. The
δ13Corg values of soil in the Qinghai
Lake watershed varied from –26.9 to –24.8 ‰ (Liu et al., 2013). When
the DIC in groundwater only originates from soil respiration and organic
matter decomposition, if we neglect the effect of carbon isotope
fractionation due to organic matter decomposition, the
δ13CDIC of groundwater ranges from
–27.7 to –24.5 ‰. Considering the isotope fractionation between
CO2 (aq) and HCO3–(approximately –10 ‰) (Zhang et al., 1995), the
δ13CDIC of groundwater ranges from
–17.7 to –14.5 ‰. As previously mentioned, most
δ13CDIC values of groundwater in the
Genggahai Lake watershed were more positive than –14.5 ‰. This
indicates that soil respiration and organic matter decomposition may not
be the only carbon source for the groundwater DIC in the Genggahai
watershed.
Significantly higher δ13CDIC(approximately –3 to +3 ‰) values in groundwater can occur in karstic
regions where a proportion of the carbon atoms derive from the
dissolution of catchment limestones (Andrews et al., 1997). For example,
the δ13CDIC of groundwater from the
Donggi Cona catchment on the north-eastern Qinghai-Tibetan Plateau
varies from 0.9 to 2.0 ‰ (Weynell et al., 2016). Paleolake sediments
that formed in the early and middle Pleistocene are widely distributed
throughout the Gonghe Basin (Dong, 1993). Here,
HCO3– originates from the weathering
of paleolake sediments and subsequent mineral dissolution, which is
enriched in 13C. Water and groundwater flows transport
the HCO3–, thus yielding more
positive δ13CDIC values. We suggest
that HCO3– originates from paleolake
sediment weathering, which affects the groundwater DIC pool in the
Genggahai watershed and yields relatively positive groundwater
δ13CDIC values. Thus, variations in
the δ13CDIC values of groundwater may
reflect the relative contributions of two carbon sources to the
groundwater DIC pool. One carbon source is CO2 that
derives from soil respiration and organic matter decomposition while the
other is HCO3– from paleolake
carbonate sediments. Besides, we elucidated that carbonate weathering
and soil respiration related to vegetation succession control the
variations in δ13CDIC-I. Therefore,
variations in the δ13CDIC-L values of
the Genggahai Lake may reflect the lake productivity and carbon cycle of
the watershed. Clearly, more data are required to constrain the
environmental significance of the
δ13CDIC-L values of the Genggahai
Lake.