Dorosoma cepedianum showed a gradual increase in d34S with size suggesting a second ontogenetic diet shift. Food sources enriched in 34S gradually increased in importance in the diet of the fish. The potential food sources with high d34S values were sediment organic matter, some macrophytes such as Typha sp., Microcystis sp. and zooplankton (Fig. 3 and Table I). Phytoplankton (other than Microcystis sp.) was occasionally observed in the foreguts, but the d34S value of this potential food source was not determined. The phytoplankton d34S composition is, however, probably similar to that of Microcystis sp. and floating macrophytes in Lake Dora (range 75–14%) because they all use dissolved sulphate ðSO2 4 Þ as a sulphur source. It is unlikely that an increase in sulphur assimilated from sediment organic matter (and not the associated microflora) would cause such a substantial increase in d34S because D. cepedianum probably assimilate sulphur mostly from the associated microflora (see above). Indeed, D. cepedianum selectively ingest the more nutritious components of the detritus (Mundahl & Wissing, 1988). In addition, adult fish are equally efficient as young D. cepedianum in digesting the detritus components (Mundahl & Wissing, 1988).