Data from: 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
depth, whereas zooplankton TP increased by up to 0.65 between the surface
ocean and midwaters. Both changes in the δ15N values of food resources at
the base of the zooplankton food web and changes in zooplankton TP drive
observed zooplankton 15N enrichment with depth. Midwater zooplankton
δ15NPhe 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.