We investigated how fast the δ15N and δ13C stable isotope signature of copepods changes and how fast the isotope signals in microzooplankton and copepods change in the field. In experiments C1, copepodites of the copepod Acartia tonsa raised on Rhodomonas baltica were switched to a diet of Oxyrrhis marina and the δ15N and δ13C signature followed for 8 days. The δ15N increased by 0.18 ± 0.03‰ day−1 in animals fed O. marina while it decreased by 0.14 ± 0.02‰ day−1 with a R. baltica diet and equilibrium was reached after 5 days. Enrichment of nitrogen 15N (Δ15N) was 2.4 ± 0.26‰ with R. baltica and 4.1 ± 0.38‰ with O. marina as food, and corresponding factors for carbon (Δ13C), 1.7 ± 0.38‰ and 2.3 ± 0.30‰. Δ15N in the field, using seston 0-10 μm as the baseline, ranged from 2.4 to 3.1 for copepods and 1.0 to 1.6 for the microzooplankton (10-90 and 90-200 μm). Nitrate input resulted in low Δ15N for copepods (2.4‰) and high for microzooplankton (1.4-1.6‰), indicating that copepods were feeding indiscriminately on phyto- and microzooplankton after the event. The second period showed higher Δ15N (3.1‰) and a higher trophic level of copepods, but lower of microzooplankton (1.0-1.1‰).