流溪河水库颗粒有机物及浮游动物碳、氮稳定同位素特征

Carbon and nitrogen stable isotope characteristics of particulate organic matter and zooplankton in Liuxihe Reservoir

为了解影响流溪河水库颗粒有机物(POM)碳和氮稳定同位素(δ13C和δ15N)变化的主要因素,及其与浮游动物δ13C和δ15N之间的关系,于2008年5月至12月份对POM及浮游动物的δ13 C和δ15 N进行了研究。颗粒有机物碳稳定同位素(δ13CPOM)和氮稳定同位素(δ15NPOM)的季节性变化幅度分别为5.1‰和2.2‰,5月和7月份δ13CPOM较高,而在10月和12月份降低,这主要与降雨将大量外源有机物带入水库而引起的外源及内源有机物在POM组成上发生变化有关。δ15NPOM总体呈上升趋势,可能是由降雨引起的外源负荷、初级生产力、生物固氮等因素共同作用的结果。浮游动物的δ13C及δ15N总的变化趋势与POM的相似,也具有明显的季节性变化,食物来源的季节变化可能是造成其变化的主要原因。在5月份,浮游动物的食物来源为POM中δ13C较高的部分,也就是外源有机物,而在10月及12月份,其食物则可能主要为浮游植物。

Particulate organic matter （POM） in lakes originates from autochthonous （macrophytes and phytoplankton） and allochthonous （terrestrial detritus） inputs into the surface water of lakes. There are many factors that can influence the composition of POM, such as rainfall, temperature, solar radiation, water velocity, hydrological retention time. Because the carbon （δ13C） and nitrogen （δ15N） stable isotope ratios of the terrestrial detritus fraction in POM is often different from algal fraction, the isotopic compositions of POM reflect the relative proportions of terrestrial and aquatic contributions. In addition, many factors in lakes influence the values of δ13CPOM and δ15NPOM： these include primary productivity, trophic state, external loading, and isotopic composition of dissolved inorganic carbon and nitrogen. Zooplankton can graze on a wide range of food sources （i.e. phytoplankton, detritus, and bacteria）. As a result, phytoplankton- and terrestrial- derived C may be conveyed to zooplankton and hence to higher trophic levels, along several direct and indirect pathways. Because the isotope ratios of consumers reflect those of their diet, the δ13C and δ15N of zooplankton will be affected when isotopic composition of POM changes over time. In order to understand the factors influencing the seasonal variations in carbon and nitrogen stable isotopes of POM and zooplankton and to assess relationship between POM and zooplankton, especially the relative importance of autochthonous and allochthonous materials to zooplankton, δ13C and δ15N of POM and zooplankton were analyzed in Liuxihe Reservoir from May to December in 2008. As the results of relatively stable solar energy inputs and low trophic state, the seasonal amplitudes of δ13CPOM and δ15NPOM in Liuxihe Reservoir were small, displaying seasonal variations of 5.1‰ and 2.2‰ due to the monsoon climate and summer storms. δ13CPOM was high in May and July and then decreased dramatically. Precipitation had significant positive correlation with δ13CPOM, but there were no significant correlations between δ13CPOM and TN, TP and Chl a. Rainfall brought abundant allochthonous organic matters into the reservoir, resulting in high δ13CPOM in the wet period （May to July）. The low δ13CPOM in the dry period （October and December） was likely attributed to the low growth rate of phytoplankton in these cold months. Conversely, there was a seasonal increase in δ15NPOM in general. Precipitation, TN, TP and Chl a have insignificant correlations with δ15NPOM. We speculated that external loadings, primary productivity and nitrogen fixation by cyanobacteria were likely responsible for the seasonal variation in δ15NPOM in Liuxihe Reservoir. The variations of zooplankton δ13C and δ15N signatures were similar to POM, and there was a significant positive correlation between δ13C in zooplankton and δ13C in POM as well as between δ15N in zooplankton and δ15N in POM, which indicated that zooplankton used POM as their major food source. δ13C in zooplankton was 2.2‰ higher than δ13C in POM in May suggesting that zooplankton fed more 13C-enriched fraction （terrestrial detritus） in POM. However, δ13C in zooplankton was lower than δ13CPOM from July to December, implying that zooplankton selectively fed more on phytoplankton.