Overlooked bed shift? Modulation of the stream microbial food web and metabolism by patches of migrating sand ripples

该项目的总体目标是了解沙纹迁移过程中的沉积物扰动对微生物食物网结构以及溪流和河流新陈代谢的重要意义。河流生态学中常见的泥沙扰动概念集中在引起泥沙深冲和远下游迁移的“灾难性”大洪水事件。然而，低排放已经可以使细小的沉积物重新定位。沙质沉积物占很大比例的溪流经历了沙粒在成片的迁移涟漪(MRP)中的移动。MRP发生在广泛的水流中，最大程度上可以覆盖整个河床。鉴于世界各地大多数耕作流域的沙子输入增加，MRP即使不是大多数河流的主要特征，也是许多河流的主要特征。尽管这种现象无处不在，但迁徙涟漪对微生物食物链和新陈代谢的影响在很大程度上被忽视了。现有的少数几项关于流沙重要性的观察性研究的数据报告了与发现相矛盾的结果。拟议项目的目的是阐明通过周期性移动MRP中的沙子对微生物食物网(群落结构和功能摄食模式)、流向无脊椎动物的碳流(微生物和无脊椎动物的PLFA中纳入13C)和河床新陈代谢(群落呼吸和初级生产总值)的影响的重要性。根据以前的研究，我们假设MRPs会由于群体或物种的特定反应而改变稳定底泥斑块(SBPs)中的局部(波纹斑块内)和相连的微生物食物网，进而影响溪流的整体新陈代谢和营养状态。我们提出：i)MRP中的沉积物扰动使新陈代谢、微生物食物网和碳流向异养方向移动，而与光照强度和营养物质浓度无关；ii)由于MRP增加了对潮下稳定沉积物的总体垂直水通量，整个河流的新陈代谢与MRP覆盖率呈非线性关系；iii)在波纹迁移的周期性沉积物扰动后，代谢和微生物群落结构的恢复取决于MRP的持续时间和光的可获得性。该项目在实验室微观世界装置上进行了一系列实验，模拟了迁移涟漪中的微环境，并在16个不同比例的MRPs和SBPs的室外实验溪流中进行了一系列实验，以接近其目标。这些实验将通过实地测量新陈代谢和垂直水通量以及观察小溪和河流迁移涟漪中的微生物食物网和碳流来完成和验证。我们希望对迁移波纹斑块(MRPs)对整个溪流代谢的影响有一个机械性的理解。该项目的调查结果将大大有助于更好地评估溪流和河流中的流沙，并对生态系统功能和服务的管理产生影响。

The overall objective of the project is to understand the significance of sediment disturbance in migrating sand ripples for the structure of the microbial food web and the metabolism in streams and rivers. The common concept of sediment disturbance in stream ecology focusses on 'catastrophic' singular big flood events that cause deep scour and far downstream relocation of sediments. However, low discharges can already relocate fine sediments. Streams with a large proportion of sandy sediments experience movement of sand in patches of migrating ripples (MRPs). MRPs occur over a wide range of flows and can cover the entire stream bed in the extreme. Given the increased input of sand in most cultivated catchments around the world, MRPs are a predominant feature in many, if not the majority of streams. Despite the phenomenon’s ubiquity, implications of migrating ripples on the microbial food web and metabolism have been largely neglected. Data from the few existing observational studies on the significance of shifting sands report contradicting findings. The aim of the proposed project is to clarify the significance of sediment disturbance by periodically shifting sands in MRPs for the microbial food web (community structure and functional feeding mode), carbon flow towards invertebrates (13C incorporation into PLFA of microorganisms and invertebrates) and stream bed metabolism (community respiration and gross primary production). Derived from previous studies we assume MRPs to change local (within ripple patch) and linked microbial food webs in stable bed sediment patches (SBPs) due to group or species-specific responses, and in turn affect the overall metabolism and trophic state of the streams. We propose i) sediment disturbance in MRPs to shift metabolism, microbial food web and carbon flow towards heterotrophy disregarding light intensity and nutrient concentrations, ii) that whole stream metabolism changes non-linearly in relation to MRP coverage due to increased overall vertical water flux to hyporheic stable sediments by MRPs and iii) that return of metabolism and microbial community structure after periodic sediment disturbance by ripple migration depends on duration in MRPs and light availability. The project approaches its objectives in a series of experiments in a laboratory microcosm device simulating the micro-environment in migrating ripples and in 16 outdoor experimental streams with variable proportion of MRPs and SBPs. The experiments will be completed and verified by field work measuring in situ metabolism and vertical water flux and observing the microbial food web and carbon flow in migrating ripples of streams and rivers. We want to achieve a mechanistic understanding for the whole stream metabolic effects of migrating ripple patches (MRPs). The project findings will significantly contribute to better valuate migrating sand in streams and rivers with implications for the management of ecosystem functions and services.