d 13 C and d 15 N were used to study food sources of zooplankton in the Scheldt Estuary. The SPOM carried by the river is a mixture of antropogenic and terrestrial detritus with associated bacteria and locally produced autotrophic organic matter. Samples of SPOM and zooplankton were taken monthly from June 1999 to April 2000 at four stations located along a salinity gradient (0-14 ppt). We investigated the relative importance of detritus, heterotrophic and autotrophic organic matter in the diet of zooplankton.Both d 15 N SPOM and d 13 C SPOM varied seasonally, paralleling the change in biomass of autotrophic organisms. Generally, phytoplankton bloom periods were characterised by high d 15 N SPOM (maximum +12.9‰) and low d 13 C SPOM values (minimum -31.1‰). Winter d 15 N SPOM and d 13 C SPOM values were characteristic for pure antropogenic/terrestrial detritus (d 15 N = +2.5‰; d 13 C = -26.8‰). The summer increase of d 15 N SPOM was attributed to autotrophic consumption of NH 4 + strongly enriched in 15 N due to intense nitrification. However, the seasonal pattern between the stations differed reflecting differences in the timing, duration and intensity of the phytoplankton bloom. d 13 C zooplankton and d 15 N zooplankton co-varied with d 13 C SPOM and d 15 N SPOM . The high deviation from the original d 13 C SPOM and d 15 N SPOM suggested selective feeding on specific components of the SPOM.