Biogeochemical hot moments upon flow resumption and coupled sediment transport: A comprehensive approach for temporary and perennial stream ecosystems (FlowReSeT)

该项目将阐明在有和没有泥沙输送的情况下水流恢复的不同年表中调节河床生物地球化学（微生物群落的代谢和氮吸收）的机制。干燥后的恢复流动被认为是一个生物地球化学热时刻，代谢和氮吸收的高速率受到先前干燥频率的影响。调节这一热时刻的机制鲜为人知。迄今为止的研究大多是对临时溪流和河流生态系统的单因素研究。然而，在多年生河流生态系统中，水流的间歇和恢复也越来越多地发生，地表水流往往意味着泥沙的搬运（例如，迁移的波纹，上层平面床），特别是在砂质河流中。此外，水流恢复可以遵循不同的时间顺序，如降雨瞬间恢复或地下水上升缓慢恢复，并且水流恢复时浸出的营养物质和碳浓度也会影响生物地球化学响应。我提出了一个新的一般概念，即“间歇溪流栖息地”，用于河床的所有区域，尽管各种因素相互作用，但在某个时间点干涸（即缺乏地表水）。本文提出的对不同水流恢复年代学、耦合泥沙输运、临时生境和多年生生境机制的研究，将证明这种普遍和综合的观点是否适用。采用多年生和临时河流间歇生境泥沙群落的微观实验，研究了径流恢复、泥沙输运、养分和碳浓度与干燥频率的相互作用。需要关注的响应变量有：通过暗、光条件下氧浓度变化测量的碳代谢，通过添加稳定同位素15N （15NH4Cl）测量的净氮吸吸量，以及微生物群落的结构和结构（即生物膜）（仅用于耦合输沙）。研究结果将有助于全面了解易发生严重流动振荡和干燥的生态系统的碳氮动力学机制。这些发现将使水流恢复纳入目前的溪流生物地球化学概念。这种概念框架是管理地中海和越来越多的温带河流生态系统的关键，这些生态系统由于采水量增加和气候变化而经历干燥。

The project shall clarify the mechanisms that modulate streambed biogeochemistry (metabolism and nitrogen uptake of the microbial community) at various chronologies of flow resumption with and without sediment transport. The resumption of flow after drying is considered as a biogeochemical hot moment, with high rates of metabolism and nitrogen uptake influenced by the frequency of previous drying. The mechanisms modulating this hot moment are little known. Research so far were mostly single factor studies in temporary stream and river ecosystems. However intermittency and resumption of flow occurs increasingly also in perennial stream ecosystems and surface flow often implies sediment transport (e.g. migrating ripples, upper stage plane bed) especially in sandy streams. In addition, flow resumption can follow different chronologies such as instant by rain or slow by rising groundwater, and the concentrations of nutrient and carbon leached upon flow resumption can also influence the biogeochemical response. I suggest a new general concept of “intermittent stream habitats” for all areas of a streambed that dry (i.e. lack of surface water) at some point in time despite variable interactions of factors. The here proposed investigation of the mechanisms at various flow resumption chronologies, coupled sediment transport, and in temporary and perennial habitats will show if such general and integrative view can be applied. The interactions of flow resumption, sediment transport, nutrient and carbon concentrations and drying frequency will be studied in microcosm experiments with sediment communities of intermittent habitats from perennial and temporary streams. The response variables under attention are: Carbon metabolism, measured by means of changes in oxygen concentration in darkness and in light, net Nitrogen uptake by means of the addition of the stable isotope 15N (15NH4Cl) and the structure and architecture (i.e. biofilm) of the microbial community (just for coupled sediment transport). The results will contribute to a complete mechanistic picture of the Carbon and Nitrogen dynamics of lotic ecosystems prone to severe flow oscillations and drying. The findings will enable to incorporate flow resumption in the current concepts of stream biogeochemistry. Such conceptual framework is key for management of Mediterranean and more and more temperate stream ecosystems that experience drying due to increased water abstraction and climate change.