Abstract
We used amino acid (AA) compound‐specific isotope analysis ( δ 15 N AA and δ 13 C AA values) of midwater zooplankton and suspended particles to examine their dynamics in the mesopelagic zone. Suspended particle δ 15 N AA values increased by up to 14‰ with depth, whereas particle trophic status (measured as trophic position, TP) remained constant at 1.6 ± 0.07. Applying a Rayleigh distillation model to these results gave an observed kinetic isotope fractionation of 5.7 ± 0.4‰, similar to that previously measured for protein hydrolysis. AA‐based degradation index values also decreased with depth on the particles, whereas a measure of heterotrophic resynthesis (ΣV) remained constant at 1.2 ± 0.3. The main mechanism driving 15 N enrichment of suspended particles appears to be isotope fractionation associated with heterotrophic degradation, rather than a change in trophic status or N source with depth. In zooplankton the “source” AA phenylalanine (Phe) became 15 N enriched by up to 3.5‰ with depth, whereas zooplankton TP increased by up to 0.65 between the surface ocean and midwaters. Both changes in the δ 15 N values of food resources at the base of the zooplankton food web and changes in zooplankton TP drive observed zooplankton 15 N enrichment with depth. Midwater zooplankton δ 15 N Phe values were lower by 5–8‰ compared with suspended particles, indicating this organic matter pool is not a significant zooplankton food resource at depth. Instead, 62–88% of the N sustaining midwater zooplankton is surface derived, obtained through consumption of sinking particles, carnivory of vertical migrants, or direct feeding in surface waters at night.