Seasonality of stable carbon and nitrogen isotopes within the pelagic food web of Taihu Lake

¹ State Key laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, the Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China.
² Graduate School of the Chinese Academy of Sciences, Beijing 100039, China.

Corresponding author: fxkong@niglas.ac.cn

Abstract

Particulate organic matter (POM), dissolved organic matter (DOM), bacteria and cladoceran were sampled seasonally at Zhihugang Estuary and Lake Center in Taihu Lake. The δ¹³C of the four organic matter fractions showed consistent temporal variation, with heaviest values in summer and lower at other times of the year. The cladoceran δ¹³C showed a significant correlation with that of POM, reflecting a heavy dietary dependence on POM during the study period. The bacteria became enriched in ¹³C compared with that of DOM throughout the sampling dates, although no significant relationship was found between the two fractions. δ¹³C values of POM, cladoceran and bacteria were all negative significantly correlated with oxidation and reduction potential (ORP), and specific conductivity (SpCond). As for δ¹⁵N, the seasonal pattern of food web components was variable. The POM δ¹⁵N signature exhibited the most enriched isotope ratios during the summer months when dissolved inorganic nitrogen (DIN) nutrients were at their lowest concentrations. The consumption of DIN in summer can explain in part the progressive accumulation of heavy nitrogen isotopes during this period. Spatially, δ¹³C and δ¹⁵N of the food web components were all slightly depleted at Estuary than that at Lake Center during the study period, possibly due to large allochthonous inputs at Zhihugang Estuary. Relatively wide ranges of stable isotopic values from both sites suggest that seasonality should be considered when attempting to establish food web structures in a eutrophic lake.