4. RESULTS
4.1 Physical and chemical parameters of lake water
Together with the previously published physical and chemical parameters of water from the Genggahai Lake water from 2012 to 2013 (Jin et al., 2015), the overall trends for the temperature and pH of the lake water were relatively consistent in all three plant communities (Fig. 2a and 2b). More specifically, the overall temperature remained relatively constant throughout the entire monitoring period, but its annual fluctuations were more notable, with higher temperatures in June, July, and August, and relatively low temperatures in May and September (Fig. 2a). From 2012 to 2015, the lake water pH exhibited an overall gradually increasing trend from May to September, with only a few exceptions (e.g., June and July 2015; Fig. 2b). Beginning in May, both the number of aquatic plants and water temperature of the lake increased, which led to the increased consumption of CO2 and HCO3 via plant photosynthesis. Moreover, the variation trend of dissolved oxygen (DO) was not notable, with all three plant communities showing inconsistencies, which may have been related to the high sensitivity of the DO to temperature changes (Fig. 2c).
{Figure 2}
A comparison of the three plant communities showed that, except for 2012, the lake water temperature and pH were slightly higher in theChara spp. community than those in the P. pectinatus andM. spicatum communities throughout the testing period; however, these differences were not notable (Fig. 2a and b). In contrast, differences in the DO values of lake water in the three communities were more notable: the Chara spp. community had relatively higher DO values in June and July (Fig. 2c).
4.2 Spatial variations in DIC isotopic composition in the Genggahai Basin waterbodies
From 2012 to 2015, the δ13CDIC values of water from the Genggahai Lake (δ13CDIC-L) ranged from –17.3 to 1.6 ‰, with a mean value of –6.91 ‰. The δ13CDIC of water from the Shazhuyu River (δ13CDIC-R) ranged from –15.9 to –5.6 ‰, with a mean value of –10.8 ‰. The δ13CDIC for the groundwater spring, i.e., a lake water source, ranged from –17.3 to –1.1 ‰, with a mean value of –11.1 ‰. Overall, the δ13CDIC-L values were the most positive, followed by the δ13CDIC-Rvalues, whereas the δ13CDIC values of the inflowing spring water (δ13CDIC-I) were the most negative (Fig. 3). Jin et al. (2015) first reported the δ13CDIC values in the Charaspp. growth area and δ13CDIC-R values from 2012 to 2013.
{Figure 3}
The δ13CDIC-L values in theChara spp. community were more positive than those in theM. spicatum and P. pectinatus communities. From 2012 to 2015, the δ13CDIC-L values in theChara spp. community ranged from –9.9 to 1.6 ‰, with a mean value of –5.4 ‰. In the P. pectinatus community, the δ13CDIC-L values ranged from –15.2 to –3.9 ‰, with a mean value of –7.4 ‰. In the M. spicatumcommunity, the δ13CDIC-L values ranged from –17.3 to –2.9 ‰, with a mean of –7.9 ‰. With the exception of one month (September 2012), the P. pectinatus and M. spicatum communities had similar δ13CDIC-L values: both communities had more negative values than the Chara spp. community (Fig. 3a).
4.3 Temporal variations in DIC isotopic compositions in the Genggahai Basin waterbodies
In the Chara spp. community, the δ13CDIC-L values were relatively positive in July from 2012 to 2014; in 2015, there was a gradual decrease in the δ13CDIC-L values. In contrast, the δ13CDIC-L values of theP. pectinatus and M. spicatum communities did not show a seasonal bias of more positive values (Fig. 3a). Figure 4a shows the interannual variations in the δ13CDIC-L values. From 2012 to 2015, the mean δ13CDIC-L values of theChara spp. community were all more positive (–4.0 ‰), whereas in 2013 and 2014, these values were more negative (–6.8 ‰). From 2012 to 2015, the δ13CDIC-L values in theP. pectinatus and M. spicatum communities showed a gradual positive trend. In comparison, the δ13CDIC-I values from 2012 to 2015 showed an overall increase; however, in 2013 and 2014, these values became increasingly negative (Fig. 4b). During the same period, the δ13CDIC-R values showed a gradually increasing trend (Fig. 4b).
{Figure 4}