A methodological proposal for the analysis of Cladocera (Crustacea, Branchiopoda) subfossils from sediments of temporary aquatic ecosystems

Abstract The main objective of the present study was to promote some adaptations to the standard methodology used in the analysis of Cladocera subfossils. The samples used in this study were collected at Lagoa dos Coutos, a temporary lake located on an outcrop with iron-rich duricrust in the Serra do Gandarela (Minas Gerais, Brazil). The sediment from this pond, typical of temporary ecosystems in the region, is characterized by rigid concretions that are difficult to dissolve in aqueous media, making it difficult to visualize and analyze subfossils taxa. Sediment samples were prepared according to the standard methodology used in paleolimological studies and the methodological adaptations proposed in the present study, which included the addition of 1 mg of Kochia scoparia, a palynological marker used to estimate subfossil density, as well as the addition of hydrofluoric acid (HF) to oxidize silicate materials. The samples prepared according to the new methodological procedure improved Cladocera subfossil visualization, facilitating the taxonomic analysis of the species. The samples submitted to the standard methodological procedure had a significantly higher mean of sediment particles (mean of 84.2 sediment particles) and 70% more residues compared to the samples prepared with the newly proposed methodology (mean of 22.1 sediment particles). Absolute data on the temporal variation of the density of Cladocera subfossils was obtained by using the palinological marker K. scoparia, which showed higher densities of subfossils in more recent, humid periods (112,000 subfossils/cm3 at 5 cm, equivalent to 3,500 years BP). The results indicate that the new methodology can contribute to advances in paleolimnological studies of temporary aquatic ecosystems, whose lake histories are rarely investigated, despite their ecological relevance as ecosystems that indicate environmental and climatic changes.

摘要 本研究的主要目标是对枝角类（Cladocera）亚化石分析的标准实验方法开展适配性优化。本研究所用样品采自巴西米纳斯吉拉斯州甘达拉拉山脉（Serra do Gandarela）一处发育富铁硬壳岩层的露头之上的临时湖泊——科托斯湖（Lagoa dos Coutos）。该湖泊沉积物属于该区域典型的临时水生生态系统沉积物，其特征为存在难以在水介质中溶解的硬质结核，这会给亚化石类群的观测与分析带来极大阻碍。研究人员分别采用古湖沼学研究的标准方法，以及本研究提出的方法改良方案对沉积物样品进行前处理：后者包括添加1毫克的地肤（Kochia scoparia）——一种用于估算亚化石密度的孢粉标记物——以及加入氢氟酸（HF）以氧化硅酸盐类物质。相较于采用新方法制备的样品（平均沉积物颗粒数为22.1），采用标准方法制备的样品其沉积物颗粒平均数量显著更高（平均为84.2），残留杂质更是多出70%。借助孢粉标记物地肤（K. scoparia），研究人员获取了枝角类亚化石密度随时间变化的绝对量化数据：在较新近的湿润沉积时期，5cm深度处的亚化石密度达到112000个/立方厘米，对应距今3500年BP。研究结果表明，该新型方法可为临时水生生态系统的古湖沼学研究提供助力——尽管这类临时水生生态系统在指示环境与气候变化方面具备重要生态价值，但其湖泊生态历史却鲜有研究。