We investigated the trophic level of larvae of two fish species (lesser sandeel. Ammodytes marinus, and dab, Limanda limanda) in spring 2004 by means of stable isotope signature at the Helgoland Roads Station (54 degrees 11.18'N and 07 degrees 54.00' E). The signatures were contrasted with the spring Succession of phytoplankton and zooplankton. Phytoplankton biomass remained low until the middle of April, when a bloom developed. The delta(15)N signature of the seston increased until the bloom started then decreased during the bloom. The delta(15)N of the larvae of both fish species generally followed the development of the baseline, but the decrease in the fishes' trophic level (expressed as the Delta delta(15)N) was larger than that of the seston, suggesting that larval fish switched their diet to lower trophic levels. For larval sandeel we found that the switch to feeding on lower trophic levels was accompanied by a decrease in nutritional condition, while this pattern was not apparent in larval dab. Hence, larval sandeel were notable to substitute the lack of high trophic level zooplankton prey with prey originating from lower trophic levels; however, at least the smaller size classes of larval dab could successfully switch diets.