大宁河下游主要鱼类营养结构的时空变化

Temporal and spatial variations in the trophic structure of key species in downstream of the Daning River

为了解大宁河下游主要鱼类食物网营养结构的时空变化特征,应用稳定同位素技术分析了大宁河下游巫山和大昌2个采样点在不同水位时期鱼类的碳、氮稳定同位素比值,并计算了δ13C-δ15N同位素生态位中的6个营养结构的量化指标。结果表明,不同水域的颗粒有机物(POM)的δ13C值随与河口距离而减小,δ15N值与河口距离无显著相关性。同种鱼类的δ13C值均无显著性差异;除了5月,巫山站点与大昌站点同种鱼类的δ15N值无显著性差异外,其他不同站点、不同水位时期同种鱼类的δ15N值均存在显著性差异。对同种鱼类组成的鱼类群落同位素量化指标比较发现,巫山站点10月鱼类群落的营养多样性指标及营养位置高于5月群落;大昌站点,不同水位时期的各指标变化趋势与巫山站相反。出现这种变化的原因可能是由于目前大宁河静水水域存在丰富的饵料导致。各鱼类群落营养结构的时空变化并未显示出处明显的规律性,可能与采样时间、鱼类种类的选择和样本量的大小有关。基于各种同位素值量化的生态指标应用于大宁河水域的鱼类食物网结构变化特征的研究,能为大宁河渔业生态调控措施以及渔业生态系统重建与修复提供理论依据。

The use of stable isotope ratio data is increasingly important in deciphering the characteristics of community structure and niche width of community members. Because stable isotope ratio values measured in tissue are closely linked to those in the diet and habitat,thus providing both bionomic( resource) and scenopoetic( habitat) ecological information,the isotopic niche concept,defined as an area( in δ-space) with isotopic values( δ-values) as coordinates,has been recently formalized. Isotopic data,which are routinely presented as δ13C-δ15N bi-plots,delineates an organism' s isotopic niche and ultimately provides representation of an organism's trophic niche. Additionally,several quantitative metrics have also been proposed to quantitatively characterize community-wide aspects of trophic structure. In the present study,24 fish species from two sampling sites,Wushan and Dachang,downstream of the Daning River,were collected in May and October,representing the period of low- and high-water level of the Three Gorges Reservoir,respectively. Six quantitativemetrics were calculated for each community assembled by the same species in food-web space using mean δ13C and δ15N values of each species as the x-y coordinates in bi-plots. The results showed that δ13C values of particulate organic matter( POM) decreased with distance upstream from the outlet,yet δ15N values were not significantly correlated with distance.There were no significant differences in δ13C signatures among communities of the same species either for the different sampling sites or for the different water-level periods. However,δ15N exhibited significant differences between sites and water-level periods,except for across-sites in May. Fish communities of the same species at each water-level period varied markedly across sites. In May,range of δ13C( CR),total area( TA) and mean distance to centroid( CD) values of the Dachang communities were greater than those of Wushan,suggesting that isotope niche width and trophic diversity were greater in Dachang. Conversely,these values for the Dachang communities were lower in October,indicating a decrease in the trophic diversity of the Dachang communities. At each site,fish communities of the same species exhibited totally opposite variations in trophic structure over time. In Wushan,the increase in isotope niche width and trophic diversity from May to October was reflected by an increase in δ15N range( NR),CR,TA and CD values. Conversely,NR,CR,TA and CD values declined in Dachang over time,thereby decreasing the total extent of trophic diversity. Additionally,at Dachang,smaller nearest-neighbor distance( NND) and standard deviation of nearest-neighbor distance( SDNND) values of the fish communities in October suggested increased trophic redundancy and more even distribution of trophic niches compared with May. The above results show that temporal and spatial variations in the trophic structure in the Daning River exhibited distinctly different tendencies,and that food abundance could be the main explanation. Other possible reasons could be that quantitative metrics were influenced by sampling time,fish species and / or sample size. Although the study was limited by these factors,the metrics derived from stable isotope ratios provided a quantitative description of the food-web structure in the Daning River and could be applied to further study and understanding the range of structural variations in food webs in any future ecological restoration of the Daning River.