5. DISCUSSION
5.1 Factors affecting
δ13CDIC in the Genggahai Basin
waterbodies
Several major processes affect the stable carbon isotope compositions of
the lake water DIC, such as in-lake processes (including lake
metabolism, organic matter decomposition, calcite precipitation, and
exchange with atmospheric CO2) and the climatic and
geographical environment of the catchment (including carbonate rock
weathering and dissolution and soil respiration) (Bade et al., 2004).
The climatic and geographical environment in the catchment can alter the
carbon isotope composition of the lake water DIC by influencing the
aqueous CO2 and alkalinity of inflowing water.
Generally, the five following factors affect the isotope composition of
the DIC.
1) Lake inflow carbon isotope composition. The isotopic composition of
the lake inflow directly affects the isotopic composition of the lake.
This is especially significant in exorheic lakes or lakes with a short
water retention time. In the Chara spp. community, the
δ13CDIC-L and
δ13CDIC-I values were positively
correlated (r 2 = 0.68, p <
0.01), whereas in the M. spicatum community, these values were
uncorrelated (Fig. 5). This is because the M. spicatum community
was located far from the lake inflow in the south-eastern region, where
exchange with spring water was weak (Fig. 1b). This reveals that the
δ13CDIC-I values were key factors
influencing the δ13CDIC-L values.
{Figure 5}
2) Exchange with atmospheric CO2. The DIC pool in the
lake tended to be in isotopic equilibrium with the atmosphere via
CO2 exchange. During the CO2 exchange
process, 12C-rich CO2 is
preferentially released from the lake surface to the atmosphere,
yielding a DIC pool enriched in 13C. This exchange
process is slow; therefore, its effect on the
δ13CDIC-L values is more notable in
endorheic lakes characterised by long retention time, whereas it is not
as observable in lakes with short retention times or rapid circulation
(Shen et al., 2010). When lake water to atmospheric CO2exchange reaches an equilibrium, the
δ13CDIC-L values range from 1–3 ‰
(Deuser and Degens, 1967; Leng & Marshall, 2004).
The exchange between the DIC of lake water and atmospheric
CO2 is continuous in exorheic lakes. At equilibrium,
isotope fractionation occurs between atmospheric CO2 and
dissolved carbonate species, i.e., CO2 (aq),
HCO3–, and
CO32– (Zhang et al., 1995), as
follows:
ε aq-g = –(0.0049 ± 0.003) × T (℃) –
(1.31 ± 0.06), (1)
ε HCO3-g = –(0.141 ± 0.003) × T (℃) +
(10.78 ± 0.05), and (2)
ε CO3-g = –(0.052 ± 0.03) × T (℃) + (7.22
± 0.46) (3)
Temperatures recorded using a water level data logger showed that from
May to September of 2013–2015, the mean water surface temperature of
the Genggahai Lake was 17.0 ℃. Based on Eqs. (1)–(3), the carbon
isotope fractionation factors between
H2CO3,
HCO3–, and
CO32– and atmospheric
CO2 were –1.5, 8.38, and 5.89 ‰, respectively. The
isotopic composition of global atmospheric CO2 is
approximately –8.1 ‰ (Das et al., 2005). Therefore, at equilibrium, the
isotopic values of H2CO3,
HCO3–, and
CO32– in lake water are –9.6, 0.28,
and –2.21 ‰, respectively. Different forms of DIC have varying
δ13C values at isotopic equilibrium; the magnitude of
the δ13CDIC value depends on the
proportions of the different DIC forms in lake water, which is related
to its pH value (Stumm & Morgan, 1970). When the pH value was 5.5, 80
% of the DIC in a water body has an aqueous CO2 form
(aq). When the pH is 8.5, CO2 (aq) accounts for
< 1 % of the DIC, which predominantly takes
HCO3– and
CO32– forms. When the pH reaches 10,
HCO3– accounts for < 50 %
of the DIC, whereas CO32– dominates
the DIC (Stumm & Morgan, 2012). The pH value of the Genggahai Lake
varied from 8.1 to 10.6, which indicates that
HCO3– and
CO32– were its dominant forms of DIC.
Notably, the actual δ13CDIC values of
the Genggahai Lake were generally more negative than the
atmosphere-equilibrated δ13CDIC.
3) Organic matter decomposition in lake sediments. Sedimentary organic
matter in lakes includes native aquatic plants and terrestrial organic
debris transported into the lake from the surrounding watershed. Once
degraded, this organic matter increases the12C-enriched DIC composition of lake water (Myrbo &
Shapely, 2006). Organic matter decomposition in Qingmuke lake (i.e., a
freshwater lake located on the Qiangtang Plateau) resulted in a DIC
isotope value equal to or even lower than that of river water (Lei et
al., 2012). In contrast, the isotopic composition of the DIC in the
Genggahai Lake was significantly more positive than that of the Shazhuyu
River, indicating that organic matter decomposition may have had a
relatively small effect on the DIC composition of the lake.
Additionally, methane produced by organic matter decomposition resulted
in a more negative δ13CDIC value.
Organic matter decomposition can cause a decrease in the
δ13CDIC values to –50 ‰ (Sun et al.,
2013). This value is significantly lower than the mean
δ13CDIC-L value of the Genggahai Lake,
which indicates that the CO2 or methane produced via
decomposition did not have a significant effect on seasonal or
interannual changes in the δ13CDIC-Lvalues.
During the observation period, the mean carbon isotopic composition of
organic matter (δ13Corg) in theChara spp. community was –16.0 ‰, whereas the mean values of
δ13Corg in the P. pectinatusand M. spicatum communities were –12.7 and –11.4 ‰,
respectively. If we neglect the effect of carbon isotope fractionation
owing to organic matter decomposition, the δ13C of the
CO2 released via organic matter decomposition is then
equal to δ13Corg. According to Eq.
(2), if HCO3– is the dominant form of
DIC in the lake, then the equilibrium isotopic value of
HCO3− in the lake water is 0.28 ‰. In this case, the
mean δ13CDIC-L value of theChara spp., P. pectinatus , and M. spicatumcommunities would be –15.72, –12.42, and –11.12 ‰, respectively.
However, the observed mean δ13CDIC-Lvalues for these three communities were –5.4, –7.4, and –7.9 ‰,
confirming that organic matter decomposition has a limited effect on the
δ13CDIC-L of the Genggahai Lake.
4) Lake photosynthetic activity. In highly productive lakes,
photosynthesis is a key factor that affects the
δ13CDIC values of the lake water
(McKenzie, 1982, 1985). During photosynthesis, plants preferentially
uptake 12C, which yields more negative
δ13C values for plants and the
δ13CDIC of the water body becomes more
positive (Andrews et al., 2004). Charaphytes are an important submerged
aquatic macrophyte. Compared with vascular plants, charaphytes have a
higher photosynthetic rate and lower respiration rate. The preferential
uptake of 12CO2 for photosynthetic
purposes could have led to the 13C-enrichment of the
DIC in the lake water (Pełechaty et al., 2010). During intense
photosynthesis, dissolved CO2 in lake water is limited
(Herczeg & Fairbanks 1987). When this occurs, charaphytes use
HCO3 for photosynthetic activity. Compared with vascular
plants, charaphytes can use HCO3– for
photosynthetic activity more effectively (van den Berg et al., 1999).
According to Eqs. (1)–(3), the δ13C values of
HCO3– were more positive than those
of H2CO3 and
CO32– in the lake water. In contrast,
the photosynthetic activity of charaphytes results in carbonate
precipitation in the surrounding waters, forming thick
CaCO3 encrustations (Pełechaty et al., 2010). This also
led to 13C-enriched water in the charaphyte growth
area.
We found that the seasonal bias in the
δ13CDIC-L values of the Charaspp. community were more positive in July (2012–2014) (Fig. 4a).
Additionally, at the beginning of Chara spp. growth (in May), the
δ13CDIC-L value of the Charaspp. community was equal to the value at the end of Chara spp.
growth (in September), especially in 2012. In contrast, the
δ13CDIC-L values in the P.
pectinatus and M. spicatum communities showed no seasonality
(Fig. 4a). This phenomenon may have occurred because Chara spp.
has a higher photosynthetic rate and a lower respiration rate than that
of the other communities (Van den Berg et al., 2002). Pentecost et al.
(2006) also observed seasonal variations in the
δ13CDIC-L values of the Charaspp. community in the UK. Pełechaty et al. (2010) suggested that an
increase in δ13CDIC results from the
intense photosynthetic activity of Chara rudis during the early
summer. Moreover, we found that the differences in the
δ13CDIC-L values between Charaspp. and vascular plants were smaller at the beginning and end of the
growing season in the Genggahai Lake, but larger during the mid-growth
season (July) (Fig. 4a). There may have been a limited impact from
photosynthesis on the δ13CDIC-L values
in areas with submerged vascular plants. This trend was not evident
during certain months, e.g., in July 2015. Due to data limitations, we
cannot provide an explanation for this phenomenon. Nevertheless, we can
reasonably conclude that the variations in the
δ13CDIC-L values of the lake water
were related to the intensity of photosynthetic activity in different
aquatic plants.
5) Water retention time. In arid regions, with extended lake water
residence times, strong evaporation leads to the preferential loss of
the lighter 12CO2 and16O2 isotopes, yielding more positive
δ13CDIC-L and oxygen
(δ18O) isotopic lake water compositions; furthermore,
there was a significant positive correlation between
δ13CDIC-L and
δ18OL (Li & Ku, 1997). Monitoring
results revealed that the δ18OL values
of the Genggahai Lake deviated significantly from the global meteoric
water line, but were consistent with the local evaporation line (LEL),
indicating that evaporation affected the
δ18OL composition of the lake water
(Jin et al., 2015; Qiang et al., 2016). However, this study found that
the δ13CDIC-L and
δ18OL values of the Genggahai Lake
were not correlated (Fig. 6), which indicates that evaporation may have
had only a minimal effect on the
δ13CDIC-L value of the lake.
{Figure 6}
The preceding analysis demonstrated that the lake surface to atmospheric
exchange of CO2 and evaporation had a relatively minimal
effect on δ13CDIC-L;
δ13CDIC-I primarily influenced the
changes in δ13CDIC-L. The high
photosynthetic efficiency of Chara spp. indicates that the its
corresponding δ13CDIC-L values showed
a seasonal trend of more positive values, which were more positive than
the δ13CDIC-L values of areas with
vascular plants. This shows that the photosynthetic activity of vascular
plants has a negligible effect on the
δ13CDIC-L of lake water.
5.2 Isotopic composition of DIC in the Genggahai Basin
groundwater and Shazhuyu River
The δ13CDIC-I and
δ13CDIC-R values were significantly
more negative than δ13CDIC-L during
the same period, which suggests that isotopic fractionation owing to
atmospheric exchange or the photosynthesis of aquatic plants occurred
after groundwater inflow into the lake (Leng & Marshall, 2004).
Compared with the δ13CDIC-I value,
δ13CDIC-R was significantly more
positive, possibly because the Shazhuyu River essentially forms via
surface runoff, as well as the more frequent exchange of atmospheric
CO2 with surface water than groundwater.
We found a significant correlation between
δ13CDIC-I and
δ13CDIC-R (n = 17,r2 = 0.46, and p < 0.01).
Additionally, given that groundwater is the main water source of the
Genggahai Lake, δ13CDIC-I was the main
factor affecting the δ13CDIC-L values
of the lake water (see section 5.1). Therefore, we further analysed the
influencing factors of δ13CDIC-I.
Three main species compose the DIC in water bodies: CO2,
CO32–, and
HCO3–. In this study, the titration
method was used to determine the DIC composition of groundwater in the
Genggahai Basin. We found that HCO3–was the dominant form of DIC. Additional research has shown that the
primary source of HCO3– in the
groundwater of the Genggahai Basin and the Shazhuyu River is the
chemical weathering of rocks, especially carbonate rocks (Jin et al.,
2019). Based on the reaction equation for chemical weathering,
CO2 is an essential component of this process. Dissolved
CO2 in groundwater originates from the atmospheric flux,
watershed soil respiration, and organic matter decomposition (Gu, 2011).
Atmospheric CO2 normally has δ13C
values of approximately –8 ‰ (Leng & Marshall, 2004), which is
relatively stable. The δ13C values of soil
CO2 from areas with C3 and
C4 plants range from –32 to –20 ‰ (Brunet et al.,
2005) and –17 to –19 ‰, respectively. The δ13C value
of HCO3– from carbonate dissolution
during subsurface weathering is approximately 0 ‰ (Weynell et al.,
2016). The isotopic equilibrium between CO2 and
HCO3– from different sources can be
attained through the following exchange reaction:
13CO2(g) +
H212CO3(aq) =12CO2(g) +
H213CO3(aq). (4)
The fractionation factor of CO2 and
HCO3– in the
CO2–HCO3– system in
soil is approximately 10 ‰ (Leng & Marshall, 2004). After soil
CO2 is dissolved in water, its δ13C
value becomes more negative than that of
HCO3–. However,
carbonate rock dissolution increases the δ13C value of
HCO3–. Evidently, the carbon source
that affects the isotopic composition of DIC in groundwater,
δ13CDIC-I, consists of two components:
(1) 13C from the weathering and dissolution of
carbonate rocks, which possesses a more positive isotope ratio and (2)12C from CO2 generated through soil
respiration, which has a more negative isotope ratio. Therefore, the
relative contribution of these two carbon sources to the groundwater DIC
determines the composition of
δ13CDIC-I in the Genggahai Basin.