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.’