应用稳定同位素技术构建胶州湾食物网的连续营养谱

Construction of a continuous trophic spectrum for the food web in jiaozhou bay using stable isotope analyses

根据2011年春季和秋季在胶州湾进行的渔业资源综合调查,应用稳定同位素示踪技术,分析了胶州湾主要渔业生物的碳、氮稳定同位素比值(δ13C,δ15N),并计算其营养级,进而构建胶州湾食物网的连续营养谱。分析的生物种类包括浮游植物、浮游动物、大型无脊椎动物和鱼类,其生物量之和占总渔获量的95%。结果表明,胶州湾食物网的δ13C值范围是-25.63‰—-17.16‰,跨度为8.47‰,平均值为(-19.42±1.80)‰;δ15N值范围是4.15‰—14.11‰,跨度为9.96‰,平均值为(11.98±1.77)‰。胶州湾食物网中的主要生物种类可以划分为4个营养组群,即初级生产者、初级消费者、次级消费者以及顶级捕食者。δ15N值分析显示,胶州湾主要生物种类的营养级范围是1.10—4.03。与文献中基于胃含物分析计算的营养级相比较,37个种类中有29种的营养级分析结果基本一致(在0.5个营养级的误差范围之内)。因此,氮稳定同位素法是一种研究水生生态系统食物网营养位置的有效方法。其中,有8种鱼类的营养级与历史文献相比有所下降,分析方法的不同可能是原因之一,此外,这些鱼种摄食饵料生物营养级的下降也是导致其营养级降低的另一个主要原因。根据营养级计算的结果,构建了胶州湾食物网的连续营养谱,胶州湾食物网中,绝大多数生物种类都属于初级和中级肉食性种类。

Jiaozhou Bay is an important spawning and feeding ground for many commercially important marine species in China. However, its trophie function remains poorly understood. In recent years, stable isotope （e.g., carbon and nitrogen） analysis has become a powerful tool for studying food webs in rivers, flood plains, salt marshes, lakes, and marine ecosystems. In this study, stable isotope analyses were employed to explore the trophic spectrum of the food web in Jiaozhou Bay based on the data collected from two surveys in the spring and fall of 2011. The species included plankton （i.e., phytoplankton and four sizes of zooplankton）, 29 invertebrates （ i.e., decapods, brachyurans, cephalopods, bivalves, gastropods and polychaetes） and 34 fishes. All of these species accounted for 95% of the total biomass of the catches, and covered all the dominant invertebrates and fish species documented in the Jiaozhou Bay ecosystem. The carbon and nitrogen stable isotope ratios （δ^13C and δ^15N） of these species were measured by isotope ratio mass spectrometer （IRMS, Isoprime; GV, Manchester, UK）. The results showed that the δ^13C values of these species ranged from -25.63%0 to -17.16%o, with the highest δ^13C-enriched values being exhibited by Portunus trituberculatus and the lowest values being exhibited by 300-- 500 μm sized zooplankton. The ^15N values ranged from 4.15%o to 14.11%o, with the highest 15N-enriched values being exhibited by Odontamblyopus rubicundus and the lowest values being exhibited by phytoplankton. The average δ^13C and δ^15N values were （ -19.42 ± 1.80）%o and （ 11.98±1.77）%o, respectively. A hierarchical cluster analysis was performed on the Normalized Euclidean distances of the δ^13C and δ^15N values. Cluster analysis showed that the major species in the food web of Jiaozhou Bay were classified into four trophic groups： primary producers （phytoplankton）, primary consumers （zooplankton）, secondary consumers （1 invertebrate Philine Kinglippini and two fishes, Ammodytes personatus and Syngnathus acus）, and top predators （other invertebrates and fishes）. The trophic levels for these species were estimated from the^15N enrichment per trophic level （△δ^15N）. Almost all of the species in this study belonged to trophic levels between 1.0 and 4.0, with phytoplankton occurring in the lowest trophic level （1.10） and O. rubicundus occurring in the highest trophic level （4.03）. Trophic levels estimated from nitrogen stable isotope ratios （TL~） were compared with those estimated by stomach content analysis （TLo） from the published literatures. In all 37 species for which the TL~ was available, the difference between TLn and TLD was less than 0.5 trophic levels in 29 of the species. Thus, nitrogen stable isotope analysis represents an effective method for studying the trophic position of organisms in the aquatic ecosystem. However, the TLn was lower than the TLD for 8 fish species, including Sebastes schlegelii, Pholis fangi, and Conger myriaster. In addition to the difference between stable isotope and stomach content analysis, the decline in the number of prey items in each trophic level might explain the reduction in the trophic level of these fish species. The continuous trophic spectrum of the food web in Jiaozhou Bay was established from the trophic levels of the species present in this system. The trophic levels of most species （59 of 63） were between 3.0 and 4.0, which indicated that the food web of Jiaozhou Bay mostly contains lower and mid- level carnivorous species. We recommend the use of both stable isotope and stomach content analyses to improve our understanding about the food web characteristics of aquatic ecosystems.