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.

我们采用中层浮游动物与悬浮颗粒物的氨基酸（amino acid, AA）单体化合物同位素分析（δ¹⁵N_AA与δ¹³C_AA值），探究其在海洋中层带的动态变化。悬浮颗粒物的δ¹⁵N_AA值随水深增加最高可升高14‰，而颗粒物的营养状态（以营养位trophic position, TP表征）始终维持在1.6±0.07。对该结果应用瑞利分馏模型（Rayleigh distillation model）后，测得的动力学同位素分馏（kinetic isotope fractionation）为5.7±0.4‰，与此前针对蛋白质水解（protein hydrolysis）测得的分馏值相近。颗粒物上基于氨基酸的降解指数值同样随水深降低，而表征异养再合成（heterotrophic resynthesis）的指标ΣV则稳定维持在1.2±0.3。驱动悬浮颗粒物氮-15富集的主要机制似乎是与异养降解相关的同位素分馏，而非营养状态或氮源随水深发生的变化。对于浮游动物而言，作为"源"氨基酸的苯丙氨酸（phenylalanine, Phe）的δ¹⁵N值随水深增加最高可富集3.5‰，而从表层海洋到中层水域，浮游动物的TP最高提升0.65。浮游动物食物网基部的食物资源δ¹⁵N值变化，以及浮游动物自身营养位的改变，共同导致了观测到的浮游动物δ¹⁵N值随水深富集。中层浮游动物的δ¹⁵N_Phe值较悬浮颗粒物低5~8‰，表明该有机质库并非深层水域中浮游动物的重要食物来源。相反，支撑中层浮游动物生存的氮中有62%~88%源自表层，这些氮通过摄食沉降颗粒物、捕食垂直洄游生物，或夜间在表层水域直接摄食获得。