The relationship between carbon and nitrogen stable isotopes of zooplankton and select environmental variables in low-latitude reservoirs

Abstract

Cyclopoids were collected from 18 reservoirs in southern China during August (a wet month) and December (a dry month) of 2010 for the analysis of carbon and nitrogen stable isotopes (δ13CZoo and δ15NZoo). The objectives of this study were to examine whether δ13CZoo and δ15NZoo can be better indicators of primary productivity and trophic state than the stable isotope composition of suspended particulate organic matter (POM), and to evaluate the relationship between δ13CZoo and δ15NZoo and select environmental variables. The δ13CZoo in these reservoirs was enriched in August and depleted in December, and varied significantly along the continuum of trophic levels. By contrast, δ15NZoo was depleted in August and enriched in December, and did not increase significantly with an increase in trophic state. Both δ13CZoo and δ15NZoo were more strongly correlated with environmental factors than δ13CPOM and δ15NPOM were. In addition, more environmental factors were significantly correlated with δ13CZoo and the δ15NZoo than with δ13CPOM and δ15NPOM. When data from two seasons were pooled, δ13CZoo was strongly correlated with dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP) and the DIN:SRP ratio, while δ15NZoo was weakly correlated with nutrient concentrations. This study indicates that, compared to the stable isotope composition of POM, δ13CZoo is a better indicator of primary productivity and trophic state, while δ15NZoo may be used as a proxy for nitrogen sources in aquatic ecosystems.