Abstract Size-based approaches are paramount tools for the study of marine food webs. Here, we investigated the relationship between zooplankton body size, stable isotope composition and trophic level (TL) along a large-scale onshore-offshore gradient in the western tropical Atlantic. Samples were obtained on the Brazilian continental shelf, slope and in oceanic waters (off Fernando de Noronha archipelago and Rocas Atoll) in September and October 2015. Zooplankton was sieved into five size fractions. Zooplankton was dominated by copepods, except for the largest (>2000 μm) size fraction, that showed a high biovolume of chaetognaths, decapods, and fish larvae. Maximum zooplankton abundance and biovolume was found at the continental slope. POM showed consistently lower δ13C than zooplankton, indicating a selective use of 13C-rich primary food sources by zooplankton. Particulate organic matter (POM) was more 13C-enriched in shelf areas (average: −22.8, −23.6 and − 24.3‰ at the shelf, slope and oceanic islands, respectively), probably due to the higher abundance of diatoms nearshore. POM had δ15N values between 2.5 and 6.9‰ (average: 4.0, 4.9 and 4.2‰ at the shelf, slope and oceanic islands, respectively). Zooplankton δ15N and TL increased with body size. The δ15N of the 200–500 μm size fraction was used as baseline for TL estimation. Oceanic areas (average baseline δ15N = 5.8‰ ± 0.52, n = 14) showed a higher baseline δ15N than the shelf (average = 3.9‰ ± 0.69, n = 9) and the slope areas (average = 3.1‰ ± 0.93, n = 9). In spite of differing baselines, the δ15N data produced a consistent pattern of log-linear increase in TL with increasing size, in all areas. The choice of input trophic enrichment factor (TEF) values only slightly changed the log10 (body size) vs TL slopes, but this choice had a considerable effect on the estimates of predator/prey size ratio (PPSR) and predator/prey mass ratio (PPMR). Using a TEF above 2.3 leads to unrealistic PPSR and PPMR estimates. Overall average slope was 0.59 ± 0.08 TL μm−1 with TEF = 2.3 and 0.42 ± 0.07 TL μm−1 with TEF = 3.2.

中文翻译：

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热带大西洋西部浮游动物的体型和稳定同位素组成

摘要 基于大小的方法是研究海洋食物网的重要工具。在这里，我们沿着热带大西洋西部的大规模陆上-海上梯度研究了浮游动物体型、稳定同位素组成和营养级 (TL) 之间的关系。样品于 2015 年 9 月和 10 月在巴西大陆架、斜坡和海洋水域（费尔南多·迪诺罗尼亚群岛和罗卡斯环礁附近）采集。浮游动物被筛分为五个大小部分。浮游动物以桡足类为主，除了最大的 (>2000 μm) 大小部分，显示出高生物量的毛颚类、十足类动物和鱼类幼虫。在大陆坡发现了最大的浮游动物丰度和生物量。POM 的 δ13C 始终低于浮游动物，表明浮游动物选择性使用富含 13C 的主要食物来源。陆架区域的颗粒有机质 (POM) 更富含 13C（平均：-22.8、-23.6 和-24.3‰，分别在陆架、斜坡和海洋岛屿），可能是由于近岸硅藻丰度较高。POM 的 δ15N 值介于 2.5 和 6.9‰之间（大陆架、斜坡和海洋岛屿的平均值分别为 4.0、4.9 和 4.2‰）。浮游动物 δ15N 和 TL 随体型增加而增加。200-500 μm 尺寸部分的 δ15N 用作 TL 估计的基线。大洋区（平均基线 δ15N = 5.8‰±0.52，n = 14）的基线 δ15N 高于陆架（平均 = 3.9‰±0.69，n = 9）和斜坡区（平均 = 3.1‰±0.93，n = 9). 尽管基线不同，δ15N 数据在所有区域都产生了一致的 TL 对数线性增加模式，随着大小的增加。输入营养富集因子 (TEF) 值的选择仅略微改变 log10（体型）与 TL 斜率，但这种选择对捕食者/猎物大小比（PPSR）和捕食者/猎物质量比的估计有相当大的影响（ PPMR）。使用高于 2.3 的 TEF 会导致不切实际的 PPSR 和 PPMR 估计。总体平均斜率是 0.59 ± 0.08 TL μm-1，TEF = 2.3，0.42 ± 0.07 TL μm-1，TEF = 3.2。3 导致不切实际的 PPSR 和 PPMR 估计。总体平均斜率是 0.59 ± 0.08 TL μm-1，TEF = 2.3，0.42 ± 0.07 TL μm-1，TEF = 3.2。3 导致不切实际的 PPSR 和 PPMR 估计。总体平均斜率是 0.59 ± 0.08 TL μm-1，TEF = 2.3，0.42 ± 0.07 TL μm-1，TEF = 3.2。