Over the last decades, the ongoing global climate change, combined with consequent increasing water demand for human needs, is causing recurrent droughts in previously perennial streams. These phenomena have been dramatically increasing their extent, with significant repercussions on the entire food web. Consequences of water scarcity are particularly remarkable in mountain streams, where the frequency of droughts is increasing at a rate that does not allow species to adapt. In the present research, we monitored benthic diatom communities within an intermittent Alpine river (Pellice River; North-Western Italy) during the three key phases of its hydrological cycle: i) stable flow ii) lentification iii) rewetting of the riverbed after a supraseasonal drought lasting five months. We tested the response of diatom communities in terms of compositional, structural and functional metrics (primary production, species composition, ecological guilds, life forms and eco-morphological groups) hypothesising both taxonomic and functional changes during the different steps of this hydrological cycle. Significant changes in benthic chlorophyll a occurred in the three hydrological phases. In particular, the relative proportion of the chlorophyll a of the three main autotrophic groups inhabiting the periphyton (namely diatoms, cyanobacteria and green algae) resulted as a reliable metric for the evaluation of the hydrological disturbance. Diatom chlorophyll a significantly decreased during both lentification and drought. The three phases were significantly characterized by different species and functional groups. During the stable flow the low profile (i.e., species of short stature, adapted to high current velocities and low nutrients concentrations) was the most representative guild and Achnanthidium pyrenaicum was the most abundant species; this phase was also characterized by the presence of stalked taxa. We observed a significant decrease of high profile species (i.e., species of tall stature, adapted to high nutrients concentrations and low current velocities) during the lentification phase, which was characterized by taxa belonging to the genera Navicula, Nitzschia and Ulnaria. During the rewetting, small and medium sized high profile diatoms as well as motile ones (i.e., fast moving species) characterized the assemblages. Our results showed that the complete recovery of diatom communities took at least 70 days after water return. The rapid and widespread extension of droughts in the Alpine area will have severe consequences on the river biota, also favouring the spread of invasive taxa. For this reason, outlining patterns of diatom response to droughts and detecting reliable metrics for the evaluation of this specific impact is very urgent and important. Corresponding author: elisa.falasco@unito.it

中文翻译：

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高山河流超季节性干旱：对底栖初级生产和硅藻群落的影响

在过去的几十年里，持续的全球气候变化，加上随之而来的人类需求不断增加的水需求，正在导致以前常年河流反复发生干旱。这些现象的范围急剧扩大，对整个食物网产生了重大影响。缺水的后果在山间溪流中尤为显着，那里的干旱频率正在以不允许物种适应的速度增加。在目前的研究中，我们监测了间歇性高山河流（佩利斯河；意大利西北部）在其水文循环的三个关键阶段内的底栖硅藻群落：i）稳定流 ii）缓慢化 iii）超季节性后河床的再润湿干旱持续五个月。我们测试了硅藻群落在成分方面的反应，结构和功能指标（初级生产、物种组成、生态行会、生命形式和生态形态群体）假设在这个水文循环的不同阶段发生分类和功能变化。底栖叶绿素a在三个水文阶段发生了显着变化。特别是，居住在附生生物（即硅藻、蓝藻和绿藻）的三个主要自养类群的叶绿素 a 的相对比例是评估水文干扰的可靠指标。硅藻叶绿素 a 在低温和干旱期间均显着降低。这三个阶段的显着特征在于不同的物种和官能团。在稳定流动期间，低剖面（即身材矮小，适应高流速和低营养物质浓度）是最具代表性的类群，而蕲菜是最丰富的物种；这一阶段的特征还在于存在被跟踪的分类群。我们观察到在 lentification 阶段高姿态物种（即身材高大，适应高营养浓度和低流速的物种）显着减少，其特征是属于 Navicula、Nitzschia 和 Ulnaria 属的分类群。在再润湿过程中，小型和中型高位硅藻以及能动的硅藻（即快速移动的物种）表征了组合。我们的结果表明，硅藻群落的完全恢复在回水后至少需要 70 天。高山地区干旱的迅速和广泛蔓延将对河流生物群产生严重后果，也有利于入侵类群的传播。出于这个原因，概述硅藻对干旱的反应模式并检测用于评估这种特定影响的可靠指标是非常紧迫和重要的。通讯作者: elisa.falasco@unito.it