Midwater zooplankton and suspended particle dynamics in the North Pacific Subtropical Gyre: a stable isotope perspective

We used amino acid (AA) compound-specific isotope analysis (δ15NAA and δ13CAA values) of midwater zooplankton and suspended particles to examine their dynamics in the mesopelagic zone. Suspended particle δ15NAA 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 (&Sgr;V) remained constant at 1.2 ± 0.3. The main mechanism driving 15N 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 15N enriched by up to 3.5‰ with dep...