Table_2_Influence of seasonal variability on the trophic structure of pelagic communities.docx

Seasonal variations in hydrodynamic conditions play a critical role in prey availability at the base of the food web and thus have implications for trophic interactions at higher trophic levels. Here we use the combination of stomach content analysis (SCA) and stable isotope analysis (SIA) to investigate predator-prey interactions and trophic structure of the pelagic fish community of the Bay of Biscay (BoB) in spring and late summer of 2020 and 2021. We found that trophic guilds were more influenced by intra-specific affinities rather than seasonal variability. Main prey for the pelagic fish comprised a set of species belonging to different trophic positions (TPs), from low TP class Thaliacea (salps) to mid TP Engraulis encrasicolus. Vertically migrating meso- and macrozooplankton (mainly copepods and krill) also played an important role as food source for the pelagic fish community showing high trophic overlap among most of species. No differences were detected between the trophic structure in spring and late summer with fish, squids and chaetognaths at the top TPs, whereas krill, meso-zooplankton and salps had the lowest values. However, the TPs estimated using both SCA and SIA approaches were consistently lower in spring, likely due to higher densities and, thus, low TP prey availability in spring. When analyzing ontogenic changes in TPs with predator’s size, significant and positive relationships were found when TP was estimated using SIA but not when using SCA. Overall, our results reveal new insights into the seasonal trophic dynamics of the pelagic fish community of the BoB.

水动力条件的季节波动对食物网基础层级的猎物可获得性具有关键调控作用，进而对更高营养级的营养交互关系产生深远影响。本研究结合胃含物分析（stomach content analysis, SCA）与稳定同位素分析（stable isotope analysis, SIA），于2020年与2021年的春季及夏末，对比斯开湾（Bay of Biscay, BoB）的中上层鱼类群落的捕食者-猎物交互关系及营养结构展开调查。研究结果表明，鱼类的营养类群更多受种内亲和性驱动，而非季节变异。中上层鱼类的主要猎物类群涵盖多个不同营养级（trophic positions, TPs）的物种，从低营养级的尾海鞘纲（樽海鞘），到中营养级的欧洲鳀（Engraulis encrasicolus）均有分布。垂直迁移的中型及大型浮游动物（主要为桡足类与磷虾）同样作为中上层鱼类群落的重要食物来源，在多数鱼类物种间呈现出较高的营养重叠度。春季与夏末的鱼类营养结构并无显著差异，鱼类、头足类及毛颚动物处于最高营养级，而磷虾、中型浮游动物及樽海鞘的营养级最低。不过，通过胃含物分析与稳定同位素分析两种方法估算得到的营养级，在春季均显著偏低，这可能是由于春季低营养级猎物的密度更高，因而其可获得性更强所致。当结合捕食者个体体型分析营养级的个体发育变化时，通过稳定同位素分析估算的营养级与捕食者体型间存在显著的正相关关系，而胃含物分析则未检测到此类关联。总体而言，本研究结果为比斯开湾中上层鱼类群落的季节营养动态提供了全新的认知视角。