Diatom community samples

1- Diatom sampling and analysesDiatoms were sampled as part of the national biomonitoringprogram for rivers. The sampling procedure followed the French standard (Afnor, 2007). Diatoms were collected once per year, during the low-flow period. Benthic diatoms were collected from at least five stones from the lotic parts of the sampling sites. The upper surfaces of the stones were scrubbed with a toothbrush to collect the biofilms in which diatoms live. Then the sample was fixed in 4% formaldehyde. In laboratory, the diatom valves were cleanedusing 40% H2O2 and HCl. Clean valves were mounted in a resin (Naphrax©). At least 400 valves from each sample were counted and identified using a light microscope (1000× magnification) according to European (European Committee for Standardization, 2004) and French (Afnor, 2007) standards. The abundances of all observed taxa were expressed as relative counts. Identifications were carried out using Krammer and Lange-Bertalot (1986–1991) to species and sub-species level.<br>2- Physical and chemical sample analysesPhysical and chemical analyses were carried-out at the same sampling sites every month. Dissolved oxygen and conductivity were measured in the field. For NO3−, NH4 +, Kjeldahl nitrogen (TKN), PO43−, Na+, Ca2+, Cl−, K+, Mg2+, SO42−, Biological-Oxygen-Demand (BOD), Chemical-Oxygen-Demand (COD), water samples were collected and analysed in the laboratory according to standard procedures (APHA, 1995).

1. 硅藻采样与分析
硅藻采样作为全国河流生物监测计划的一部分实施。采样流程遵循法国标准（Afnor, 2007）。每年枯水期开展一次硅藻采样：在采样点的流水区段，至少采集5块石块以获取底栖硅藻；用牙刷刷洗石块上表面，收集硅藻栖息的生物膜，随后将样品以4%甲醛固定。实验室阶段，使用40%过氧化氢（H₂O₂）与盐酸（HCl）清洗硅藻壳瓣，再将洁净的壳瓣封片于树脂介质（Naphrax©）中。按照欧洲标准（欧洲标准化委员会, 2004）与法国标准（Afnor, 2007），使用光学显微镜（放大倍数1000×）对每个样品的至少400个壳瓣进行计数与物种鉴定，所有观测到的分类单元的丰度以相对计数形式表征。物种鉴定参考Krammer与Lange-Bertalot（1986–1991）的权威资料，鉴定精度可达种与亚种水平。
2. 理化样品分析
理化分析每月在同一采样点开展：溶解氧与电导率在野外现场测定；针对硝酸盐（NO₃⁻）、铵盐（NH₄⁺）、凯氏氮（TKN）、磷酸盐（PO₄³⁻）、钠离子（Na⁺）、钙离子（Ca²⁺）、氯离子（Cl⁻）、钾离子（K⁺）、镁离子（Mg²⁺）、硫酸盐（SO₄²⁻）、生化需氧量（BOD）与化学需氧量（COD），采集水样并按照标准流程（APHA, 1995）在实验室完成分析。