Abstract
We investigated how fast the dN and dC 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 dN and dC signature followed for 8 days. The dN increased by 0.18+ 0.03‰ day in animals fed O. marina while it decreased by 0.14+ 0.02‰ day with a R. baltica diet and equilibrium was reached after 5 days. Enrichment of nitrogen N (DN) was 2.4+ 0.26‰ with R. baltica and 4.1+ 0.38‰ with O. marina as food, and corresponding factors for carbon (DC), 1.7+ 0.38‰ and 2.3+ 0.30‰. DN in the field, using seston 0–10 mm 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 mm). Nitrate input resulted in low DN for copepods (2.4‰) and high for microzooplankton (1.4–1.6‰), indicating that copepods were feeding indiscriminately on phytoand microzooplankton after the event. The second period showed higher DN (3.1‰) and a higher trophic level of copepods, but lower of microzooplankton (1.0–1.1‰).
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