On The Stable Carbon And Nitrogen Isotope Ecology Of Aquatic Macrophytes In Lake Taihu Watershed

Aquatic Macrophytes are the primary producer in lake ecosystems, and have significant environmental effects and ecological effects. It has important significance to maintain the biodiversity and stability of aquatic ecosystem. Due to the isotopic composition is stable in the process of migration and transformation of pollutants, therefore stable isotope methods can have been widely applied to the study of ecosystem. To study the variation of isotopic compositions of aquatic macrophytes can not only indicate the changes of pollutant input load, but also indicate carbon absorption and utilization in photosynthesis.In our investigation, there were 17 species belonging to 10 families and 14 genera in Taihu catchment. Large number distributions of aquatic plant species were Potamogeton wrightii, Phragmites australis, Potamogeton crispus, Zizania latifolia, Myriophyllum spicatum, Nymphoides peltatum and so on. We measured the δ13C values of aquatic plants, it is concluded that different life forms of aquatic plants had significant differences in terms of carbon source with different isotopic compositions. Submerged macrophytes are often capable of utilizing HCO3- for photosynthesis at times of CO2(aq) shortage. However, emergent macrophytes can only utilizing CO2 in air. The δ13CHCO3-will be 7-11‰ less negative than δ13CCO2. Fixation of bicarbonate-derived HCO3- relative to assimilation of dissolved CO2 lead to less 13C-depleted tissues. There had a significantly negative correlation between the δ13C value of submerged macrophytes and dissolved inorganic carbon concentrations(DIC). However, there had a significantly positive correlation between the δ13C value of submerged macrophytes and p H values. The results are mainly related to plant carbon demand and carbon availability. When the p H values are lower and DIC concentrations are higher, plants discrimination against the heavier 13 C isotope, the plants have lower δ13C values. When the p H values are higher and DIC concentrations are lower, carbon demand exceeds its supply can hinder isotopic fractionation under the carbon limitation, the plants have higher δ13C values. Aquatic plants have different photosynthetic pathways, such as C3, C4 and CAM pathway. In our plant samples, the submerged plants of H. verticillata demonstrate C4-like metabolism. PEP carboxylase shows a limited fractionation against the heavy carbon isotope. Moreover, Ru BP carboxylase exhibits higher fractionation against naturally occurring 13CO2 than PEP carboxylase. Thus, 13 C is depleted in C3 plants compared with that in C4 plants.In recent years, the pollution of Taihu catchment and the degeneration of aquatic plants become more seriously, and the pollutant emissions are increasing year by year. Thus, more accurate assessments of the effect of anthropogenic nitrogen inputs to the aquatic ecosystem are needed. The nitrogen isotope signature of primary producers has become a useful biological index. We measured the δ15N values of aquatic plants, and concluded that life forms and photosynthetic pathway and seasonal variation didn’t affect them. We analyzed the relationships between the δ15N of aquatic plants and water nutrient concentrations with the analysis of the GAM. It is concluded that the δ15N and NH4+, PO43-, TN in the water column have significant correlation. The δ15N of aquatic plants is related to plant nitrogen demand and nitrogen availability. When the concentration of nitrogen is limited, the plant demands increase and plants have higher δ15N values consistent with constrained discrimination against the heavier 15 N isotope. When the concentration of nitrogen is higher, where nitrogen availability exceeds plant demand and plants discrimination against the heavier 15 N isotope, the plants have lower δ15N values.The Taihu catchment consists of Wuxi, Yixing, Suzhou, Wujiang, Changzhou and Huzhou regions.There is extremely significant difference in the δ15N of aquatic plants between Wuxi regions and Wujiang regions. The water pollution of lakes in Wuxi area was the highest so that the latter have lower δ15N values than the former. The results showed that the aquatic plant nitrogen isotope can indicate anthropogenic nitrogen inputs, the δ15N values of plants were higher with the increasing of pollutants. Thus, the nitrogen isotopes of aquatic plant can be used as an effective and simple biological indicator to evaluate the effect of anthropogenic nitrogen inputs into ecological systems.