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

Abstract

Jiaozhou Bay is an important spawning and feeding ground for many commercially important marine species in China. However, its trophic 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 (δ13C and δ15N) of these species were measured by isotope ratio mass spectrometer (IRMS, Isoprime; GV, Manchester, UK). The results showed that the δ13C values of these species ranged from -25.63 % to -17.16%, with the highest13C-enriched values being exhibited by Portunus trituberculatus and the lowest values being exhibited by 300—500 μm sized zooplankton. The δ15 N values ranged from 4.15 δ to 14.11 δ, with the highest 15N-enriched values being exhibited by Odontamblyopus rubicundus and the lowest values being exhibited by phytoplankton. The average δ13C and δ 15N values were (-19.42 δ 1.80) δ and (11.98 δ 1.77) δ, 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 (Δδ15N). 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 (TLN) were compared with those estimated by stomach content analysis (TLD) from the published literatures. In all 37 species for which the TLD 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.