Abstract
Eurytemora affinis (Copepoda) were fed 15 N‐labeled Rhodomonas salina (Cryptophyta) or 15 N‐labeled Nodularia spumigena (Cyanobacteria) in excess under controlled laboratory conditions. Zooplankton collected from the Baltic Sea were fed natural phytoplankton amended with 15 N‐labeled N. spumigena . We quantified the direct incorporation of 15 N tracer from N 2 ‐fixing N. spumigena (diazotroph nitrogen) and ammonium‐utilizing R. salina into the amino acid nitrogen (AA‐N) of zooplankton using complementary gas chromatography‐combustion‐isotope ratio mass spectrometry, gas chromatography‐mass spectrometry, and elemental analysis‐isotope ratio mass spectrometry approaches. Specific and mass‐specific TN and AA‐N incorporation rates of the 15 N tracers were calculated for zooplankton. Highest incorporation of 15 N was found in field zooplankton relying on N. spumigena and in E. affinis relying on R. salina . Lowest incorporation was found in E. affinis relying on N. spumigena . Decreasing specific and mass‐specific rates during field experiments possibly were due to food shortage, whereas decreasing rates in E. affinis grazing on R. salina were more likely due to satiation. Specific and mass‐specific rates were consistently low in E. affinis when exposed to N. spumigena , suggesting that these animals were reluctant to feed on N. spumigena . Essential isoleucine received most of the diazotroph nitrogen in field zooplankton, while nonessential amino acids received most 15 N tracer in E. affinis . N. spumigena was clearly an important amino acid nitrogen source for Baltic Sea zooplankton.