Many paleoenvironmental studies based on benthic foraminiferal assemblages use different protocols for sample analysis. A standardized protocol has been recently established for biomonitoring applications, but for paleostudies, the influence of size fraction on benthic foraminiferal composition and biodiversity is poorly documented. We studied fossil foraminiferal assemblages along two paleorecords (BOBGEO-CS05 and SU81–44) from the Bay of Biscay covering the last ∼35 ka cal BP. We investigated diversity and community composition to compare the impact of each size fraction (63-150 μm, >150 μm, >63 μm) on environmental interpretations. Foraminiferal diversity was affected by the accumulation of small opportunistic species. In terms of faunal composition, both paleorecords displayed a different pattern depending on the size fraction selected. While in both cores, the 63-150 μm fraction blurred the signal of some rare indicator species, our results show that i) in BOBGEO-CS05, it yielded no extra ecological information compared to the large fraction whereas ii) in SU81–44, it contained small opportunistic species that were not present in the >150 μm, impacting therefore paleoenvironmental interpretations. According to these findings, we recommend: i) to focus on the large fraction for a thorough taxonomic determination and a detailed analysis of benthic assemblages, and ii) to analyse the small fraction separately after a taxonomical identification of major species and strategic selection of studied samples. Although the 125 μm size limit was not tackled in this study, we recommend to use it for the limit between the small and large fractions instead of 150 μm for harmonization with the previously published standardized protocol for living faunas.

许多基于底栖有孔虫组合的古环境研究使用不同的样本分析方案。最近为生物监测应用建立了标准化协议，但对于古研究，大小分数对底栖有孔虫组成和生物多样性的影响记录很少。我们研究了来自比斯开湾的两个古记录（BOBGEO-CS05 和 SU81-44）的有孔虫化石组合，涵盖了最后的 ~35 ka cal BP。我们调查了多样性和群落组成，以比较每个大小部分（63-150 μm、>150 μm、>63 μm）对环境解释的影响。有孔虫多样性受到小型机会性物种积累的影响。就动物群组成而言，两种古记录都显示出不同的模式，具体取决于所选的大小比例。虽然在两个核心中，63-150 μm 部分模糊了一些稀有指示物种的信号，但我们的结果表明 i) 在 BOBGEO-CS05 中，与大部分相比，它没有产生额外的生态信息，而 ii) 在 SU81-44 中，它包含不存在于 >150 μm 中的小型机会物种，因此影响古环境解释。根据这些发现，我们建议：i) 将重点放在大部分上，以进行彻底的分类测定和底栖生物组合的详细分析，以及 ii) 在对主要物种进行分类鉴定和研究的战略选择之后，单独分析小部分样品。尽管本研究未解决 125 μm 的尺寸限制，