3. MATERIALS AND METHODS
Between 2012 and 2015, from May to September of each year, lake water samples were collected at a depth of 20 cm in three different areas characterised by Chara spp., M. spicatum , and P. pectinatus communities. At the same time, watershed groundwater samples and surface water samples from the Shazhuyu River were also collected. The collected samples were stored in 500 mL polyethylene plastic bottles; after adding 1 mL of HgCl2 solution, each bottle was sealed with a sealing membrane. A total of 53 lake water samples, 19 groundwater samples, and 17 river water samples were collected. The physical and chemical parameters (i.e., the temperature, dissolved oxygen, and pH) were measured in situ using a portable water quality analyser (AquaRead-1000).
The plant samples, including Chara spp., M. spicatum , and P. pectinatus were sampled monthly adjacent to the lake water sampling point. Besides, Gyraulus sibiricus samples adhered toChara spp. were collected. The fresh plant samples and shells were stored in polyethylene plastic bags and numbered. These samples were then transported to lab for further pre-treatment and analysis.
The water samples were subjected to the following laboratory pre-treatment procedure before DIC isotope determination. First, each sample was filtered through a 0.45-μm glass fibre filter and 4 mL of a saturated BaCl2 solution was added to induce BaCO3 precipitation (Wachniew & Róžański, 1997). After the samples settled, the clear liquid was siphoned away; the remaining solids were placed in a drying oven to dry at a constant low temperature of 30 ℃. Finally, the dried samples were ground to a uniform size and appropriate amounts were weighed for the DIC isotope determination. The prepared DIC samples were tested using a MAT-253 isotope ratio mass spectrometer (Thermo-Finnigan), together with a micro-carbonate sampling device (Kiel IV). Tests were performed at the Key Laboratory of Western China’s Environmental Systems, MOE, Lanzhou University. Test results were reported relative to the PDB standard. Analytical precision values for δ13C and δ18O were ± 0.03 ‰ and ± 0.05 ‰, respectively.
To analyse the δ13C of the plant bulk organic matter (δ13Corg), samples were pre-treated based on the methods reported in Song et al. (2012). The δ13Corg was analysed using an on-line Conflo Ⅲ-DeltaPlus isotope ratio mass spectrometry, combined with a Flash EA1112 elemental analyser at Lanzhou University. The results were reported in ‰ relative to PDB. The standard samples were glycine, puge, and wheat, whose standard deviations were 0.04, 0.07, and 0.05 respectively. All values are reported and discussed in the following section. Temperature data from the Gonghe Station were obtained from the China Meteorological Data Service Centre: ‘Daily data from surface meteorological stations in China.’