Stable isotope composition of zooplankton, fishes and sea birds from the northern Humboldt Current System during Maria S. Merian cruise MSM80

Stable isotopes of carbon and nitrogen (δ13C, δ15N) were analyzed using a mass spectrometer with IAEA-VPDB (IAEA-C1) and atmospheric air (IAEA-N1) as standards, respectively. Two different approaches were used to determine a δ15N baseline for the food web of the northern Humboldt Current System. i) Particulate organic matter (POM) samples representing trophic position 1 and ii) Filter-feeding salps (Iasis cylindrica) as primary consumers representing trophic position 2. In general, low isotope ratios at the surface occurred in Thaliacea (I. cylindrica and Pyrosoma sp.). δ15N values of copepods inhabiting waters <50 m depth ranged from 3.9 ± 0.2‰ in Nannocalanus minor to 6.7 ± 0.6‰ in Calanus chilensis. Deeper-living copepods (>200 m depth) within the oxygen minimum zone had about 4 to 8‰ higher δ15N values (~11-15‰) than surface living ones. For the diel vertical migrating euphausiids, mean δ15N and δ13C' values were in a similar range as for copepods from the upper water layers. The squat lobster Pleuroncodes monodon had extraordinarily high δ13C'. Among the key fish species, the eastern Pacific bonito Sarda chilensis had a slightly higher δ15N value compared to the anchovy Engraulis ringens. In some species a strong increase of the δ15N ratio of up to ~5‰ was determined from North (8.5°S) to South (16°S). These pronounced differences in d15N values highlight the importance of adequate baseline values to avoid misinterpretations of a species' regional feeding behaviour. In order to compensate for the reginal shift, using a filter-feeding salp as reference for herbivorous consumers seemed advantageous. In fact, calculated trophic positions for C. chilensis and other species did not differ between regions, when shifts in the baseline were taken into account.

本研究采用质谱仪（mass spectrometer）分别以国际原子能机构维也纳佩德布克碳酸盐标准（IAEA-VPDB，IAEA-C1）与大气氮标准（IAEA-N1）作为参比物，对碳、氮稳定同位素比值（δ¹³C、δ¹⁵N）开展分析。针对北洪堡洋流系统（Northern Humboldt Current System）的食物网，本研究采用两种不同方法确定δ¹⁵N基线：其一为代表营养级1的颗粒有机物（Particulate organic matter, POM）样品，其二为代表营养级2的初级消费者——滤食性海樽（Iasis cylindrica）。总体而言，表层水体中的被囊动物（Thaliacea，尾索动物纲）（包括圆柱形海樽I. cylindrica与火体虫属Pyrosoma sp.）的同位素比值较低。栖息于水深<50 m水域的桡足类，其δ¹⁵N值范围为小哲水蚤（Nannocalanus minor）的3.9±0.2‰至智利哲水蚤（Calanus chilensis）的6.7±0.6‰。栖息于氧最低层（oxygen minimum zone, OMZ）、水深>200 m的深层桡足类，其δ¹⁵N值较表层栖息的同类高约4~8‰（范围约11~15‰）。对于昼夜垂直迁移的磷虾类（euphausiids），其平均δ¹⁵N与δ¹³C'值与上层水体桡足类的数值范围相近。智利蹲龙虾（Pleuroncodes monodon）的δ¹³C'值异常偏高。在关键鱼类物种中，东太平洋智利狐鲣（Sarda chilensis）的δ¹⁵N值略高于秘鲁鳀（Engraulis ringens）。部分物种的δ¹⁵N比值从北部海域（8.5°S）至南部海域（16°S）出现最高可达约5‰的显著升高。δ¹⁵N值的这类显著差异凸显了采用合适基线值的重要性，可避免对物种的区域摄食行为产生误判。为补偿区域同位素基线偏移，采用滤食性海樽作为植食性消费者的参考物似乎更为有利。事实上，当纳入基线偏移因素后，智利哲水蚤（C. chilensis）与其他物种的计算营养级在不同区域间并无显著差异。