Trophic relationships in a polyculture pond based on carbon and nitrogen stable isotope analyses: A case study in Jinghai Bay, China

Trophic relationships between various species such as primary producers, benthic macro-invertebrate and fishes in a polyculture pond were investigated based on carbon and nitrogen stable isotope analyses. The results showed that no significant difference in the δ13C values was found between particulate organic matter (POM) and sediment organic matter (SOM), which constituted the pool of organic matter (OM). In addition, such similarity in the δ13C values between POM and SOM indicated the existence of benthic-pelagic coupling in the pond. A positive relationship was found between δ13C values of POM and water Chl a concentration (r2 = 0.75, P < 0.01) suggesting the effect of primary productivity intensity on the absorption efficiency of dissolved inorganic carbon (DIC) by planktonic microalgae. The macroalgae exhibited significantly higher δ13C value than POM and SOM. Consequently, the food sources could be divided into 2 groups based on the δ13C values, i.e., the OM pool including POM & SOM and macroalgae, representing the 2 principal carbon sources utilized by consumers. Assuming a 15N-enrichment factor of 2.75‰ between consumer and prey for each trophic level, the structure of the food web in the pond was composed of 4.14 trophic levels. Among the consumers, filter feeding bivalves and jellyfish belong to the lowest trophic levels ranging from 1.92 to 2.19 while crustaceans and fishes occupied the highest trophic level between 2.58 and 4.14. The trophic levels of echinoderm and polychaete with the average value of 2.34 were intermediate.