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

Abstract

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