Zooplankton were not manually removed from the net tow samples during the summer 2006 sampling event, thus summer d34S values are not available. Zooplankton d34S values can be estimated by subtracting the trophic fractionation factor for sulphur assimilation from the d34S value of the obligate planktivore threadfin shad Dorosoma petenense (G ¨unther) (Miller, 1967), which were available as a by-catch from the D. cepedianum sampling effort. The d34S values of D. petenense were mean S.D. 10 2 05% (n ¼ 11, size range 22–114 mmL T). The trophic fractionation factor (mean S.D. þ08 04%) was calculated from the difference between spring D. cepedianum larvae and zooplankton, their presumed food source (Dettmers & Stein, 1992). Correcting for this trophic fractionation factor provided a corresponding zooplankton d34S value of 93 06%. This value could be slightly overestimated if Microcystis sp. are part of the diet of D. petenense, as this species feeds mostly on zooplankton and some phytoplankton (Miller, 1967).