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...本研究采用海洋中层浮游动物与悬浮颗粒物的氨基酸（amino acid, AA）化合物特异性同位素分析（δ¹⁵N_AA与δ¹³C_AA值），探究二者在海洋中层带的动态变化规律。悬浮颗粒物的δ¹⁵N_AA值随深度增加最高可升高14‰，但其营养状态（以营养级TP表征）始终维持在1.6 ± 0.07的水平。将瑞利分馏模型（Rayleigh distillation model）应用于上述结果后，测得的动力学同位素分馏值为5.7 ± 0.4‰，与此前针对蛋白质水解（protein hydrolysis）测得的分馏结果相近。基于氨基酸的降解指数值同样随颗粒物所在深度增加而降低，而异养再合成（heterotrophic resynthesis）的表征参数ΣV则维持在1.2 ± 0.3的恒定水平。驱动悬浮颗粒物δ¹⁵N富集的核心机制似乎是与异养降解相关的同位素分馏，而非随深度变化的营养状态或氮源差异。对于浮游动物而言，作为"源"氨基酸的苯丙氨酸（phenylalanine, Phe）的δ¹⁵N值随深度增加最高可富集3.5‰……