Impact of high water level fluctuations on stable isotopic signature of POM and source identification in a floodplain lake—Bang Lake (Poyang Lake)

In order to recognize impact of water levels on the sources and migration of particulate organic matter in a floodplain lake—Bang Lake (Poyang Lake), suspended particulate matters (SPM), superficial sediment organic matters (SOM) and aquatic plants were collected in and around Bang Lake from June 2012 to May 2013, and their carbon and nitrogen stable isotopic compositions (δ13C, δ15N) and organic C/N ratio were determined. The results showed that the δ13C and δ15N values of SPM obviously changed with water levels. In the flowing period, the SPM showed the most negative δ13C (−35.4 to −30.8 ‰), δ15N (+4.2 to +7.6 ‰) and C/N > 8, suggesting that SPM in the period was mainly derived from 13C-depleted land C3 plants. The highest δ13C and C/N ratios of SPM appeared in the flooding period because the lake and river were connected and the soil organic matter and the river-derived SPM were input to the lake. In the lowering period, the relatively lower δ13C and δ15N values of SPM were almost consistent with those of the decayed aquatic plants, meaning that aquatic plants were the main components of the SPM. In the lentic period, the δ13C and C/N ratios of SPM were similar to those of SOM due to the re-suspended SOM in the shallow lake. The SOM in the lentic period (δ13C: −30.4 to −25.5 ‰) mainly came from the decline of aquatic plants. With water level rising, the δ13C of SOM turned to be higher (−26.1 to 24.3 ‰), while the C/N ratios changed small. It may be the result of that fast water flow scoured and suspended the surface of sediment. The δ13C and C/N ratios of aquatic plants changed obviously with water levels, which was mainly due to the seasonal variation of photosynthesis and respiration. The δ15N values of aquatic plants were dependent on that of water-soluble N at different water levels. These results indicated that the sources of the organic matter in the lake were obviously influenced by the water levels.