Stable Isotope Analyses of Size-Based Microplankton Trophic Structure in Taihu Lake during a Cyanobacterial Bloom

ABSTRACT

Variations of δ¹³C and δ¹⁵N in dissolved organic matter and size-fractionated plankton, ranging from 0.7 to 200 μm, were studied in Taihu Lake on two occasions during a cyanobacterial bloom. Paired-sample t-tests indicated that stable isotope ratios of samples collected did not significantly differ from each other on the two sampling dates. The overall correlation between δ¹³C and δ¹⁵ of forty taxa of the plankton community was weak (r²=0.227, N=40), indicating a non-trophic enrichment effect in δ¹³C. The δ¹⁵N values increased along with plankton size. We used δ¹⁵ as an index of the trophic level and demonstrated that logarithmic organism size was positively and significantly related to trophic level, indicating that a size-dependent approach is preferable in the analysis of trophic isotope enrichment in microplankton food webs.