Microcystis sp. occasionally had low d34S values (3–4%), such as in the upper water column during winter 2007 and in the lower water column during summer 2007. Such low values are probably exceptional since sediments, which consist largely of Microcystis sp. (pers. obs.), generally have high d34S values (74–134%). Moreover, fresh Microcystis sp. was rarely observed in D. cepedianum stomachs (M. H. Schaus, unpubl. data), suggesting that Microcystis sp. with low d34S value are an unlikely food source for young D. cepedianum.