不同组织碳、氮元素含量和同位素分馏特征研究—以抚仙湖草鱼、鱇浪白鱼为例

结合抚仙湖浮游植物、水生植物等样品, 对野生植食性草鱼(Ctenopharyngodon idellus)31个组织和野生肉食性鱇浪白鱼(Anabarilius grahami)17个组织样品碳、氮元素含量和稳定同位素比值(δ¹³C、δ¹⁵N)进行了对比分析.结果显示, 鱼类不同组织的碳稳定同位素分馏效应显著, 草鱼不同组织δ¹³C分布范围为-20.66‰~-11.62‰, 极差为9.04‰, 鱇浪白鱼为-27.55‰~-19.71‰, 极差为7.84‰.而氮稳定同位素分馏较小, 草鱼δ¹⁵N分布范围为9.09‰~10.97‰, 极差为1.88‰, 鱇浪白鱼为7.91‰~12.51‰, 极差为4.60‰.同时, 草鱼和鱇浪白鱼不同组织δ¹³C值与对应C元素含量呈极显著的负相关关系, C含量每升高10%其δ¹³C值分别减少1.83‰和1.35‰, 反映了不同组织对C元素吸收及合成过程伴随着碳稳定同位素的明显分馏.不同组织在δ¹⁵N值与N元素含量的相关关系上不显著.进一步开展草鱼不同组织与肠含物的同位素对比分析显示, 草鱼背部肌肉组织与肠含物中苦草的碳、氮同位素分馏系数分别为0.40‰和2.66‰, 表明背部肌肉组织的碳、氮同位素组成对识别鱼类食性和确定营养级水平的可靠性.研究结果显示, 植食性草鱼和肉食性鱇浪白鱼具有较为稳定的食性特征, 可以用黏液的δ¹³C值和校正后的鳞片的δ¹⁵N值替代背部肌肉作为食性分析和确定营养级的类比指标.该研究表明, 鱼类组织碳、氮元素组成与同位素分馏特征可为识别云南高原湖泊的食物网结构和营养转移路径提供可靠的分析手段, 其中黏液、鳞片等非致命组织的替代性取样与同位素分析对于濒危鱼类的研究与保护具有较好的应用意义.

Carbon and nitrogen contents and isotopic fractionation in different tissues of Ctenopharyngodon idellus and Anabarilius grahami in Fuxian Lake

Stable isotope signal and element composition of carbon and nitrogen in lake organisms have been widely applied to identify food sources and trophic structure of lake food webs. This study was conducted to evaluate ecological processes of elemental assimilation and stable isotope fractionation among different tissues by determining carbon and nitrogen contents and stable isotopes of 31 tissues of Ctenopharyngodon idellus and 17 tissues of Anabarilius grahami collected from Fuxian Lake. The results show that there are significant carbon isotopic fractionations among tissues, ranging from -20.66‰ to -11.62‰ for Ctenopharyngodon idellus and from -27.55‰ to -19.71‰ for Anabarilius grahami. Meanwhile, the nitrogen isotope fractionations are relatively moderate for both kinds of fish, with a maximum range of 1.88‰ and 4.60‰, respectively. The δ¹³C values show a significantly negative correlation with carbon contents of tissues. A 10% increase in carbon content results in a depletion of δ¹³C value by -1.83‰ for Ctenopharyngodon idellus and -1.35‰ for Anabarilius grahami, indicating that the absorption and synthesis of carbon in tissues are accompanied by significant isotopic fractionation. However, the δ¹⁵N values of different tissues may be affected by essential amino acids but not associated with nitrogen content; because a high content of essential amino acids in tissues often leads to a depletion of ¹⁵N. The isotopic fractionation coefficients between the dorsal muscle of Ctenopharyngodon idellus and intestinal content (Vallisneria natans) are 0.40‰ for carbon isotope and 2.66‰ for nitrogen isotope, suggesting carbon and nitrogen isotopic compositions of dorsal muscle are a reliable indicator of food source and trophic level. For fish without obvious dietary change over life time, the δ¹³C value of mucus and the δ¹⁵N value of scales (after correction) can be used as alternative indices for dorsal muscles. Obviously, the carbon and nitrogen contents and isotope fractionations of fish tissues can be used to identify the trophic structure and the pathway of trophic flow in lakes of southwest China. The alternative sampling of non-lethal tissues such as mucus and scales can be of great potential for effectively conservation of endangered fish.