Daily changes in δ15N and δ13C stable isotopes in copepods: equilibrium dynamics and variations of trophic level in the field

We investigated how fast the delta N-15 and delta C-13 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 delta N-15 and delta C-13 signature followed for 8 days. The delta N-15 increased by 0.18 +/- 0.03 parts per thousand day 21 in animals fed O. marina while it decreased by 0.14 +/- 0.02 parts per thousand day 21 with a R. baltica diet and equilibrium was reached after 5 days. Enrichment of nitrogen N-15 (Delta N-15) was 2.4 +/- 0.26 parts per thousand with R. baltica and 4.1 +/- 0.38 parts per thousand with O. marina as food, and corresponding factors for carbon (Delta C-13), 1.7 +/- 0.38 parts per thousand and 2.3 +/- 0.30 parts per thousand. Delta N-15 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 mu m). Nitrate input resulted in low Delta N-15 for copepods (2.4 parts per thousand) and high for microzooplankton (1.4-1.6 parts per thousand), indicating that copepods were feeding indiscriminately on phyto-and microzooplankton after the event. The second period showed higher Delta N-15 (3.1 parts per thousand) and a higher trophic level of copepods, but lower of microzooplankton (1.0-1.1 parts per thousand).