Early nineties of previous century aquatic biodiversity was low in many West-European countries as a consequence of pollution, hydromorphological degradation and a lack of sound environmental legislation. Since the early 2000s and the implementation of the European Water Framework Directive a big step forward has been made to achieve a good ecological status of surface waters in Europe. However, to date little is known about the response of biological life in small lowland streams on improving conditions for both water quality and habitat diversity. In this study, we assessed possible relationships between chemical water quality and aquatic biodiversity of the Zwalm River basin (Belgium) using a dataset of long-term measurements of the chemical and biological water quality. Specifically, we wanted to know 1) the evolution in chemical and biological water quality over the last three decades and 2) if an improvement in chemical water quality was reflected in the macroinvertebrate and fish diversity. We found that there was indeed a significant improvement in the chemical water quality thanks to the installation of wastewater treatment plants and river basin management plans. Shortly after this improvement in chemical water quality, an increase in the number and abundance of pollution sensitive macroinvertebrate taxa was observed, which was consequently also seen in the biotic index score, based on macroinvertebrates. Despite a limited change in community composition observed for fish, no significant increase in diversity, or in the biotic index score of fish could be found. Currently, a stagnation of the improving water quality is observed. Hydromorphological restoration and free fish migration remain important key factors to improve diversity of macroinvertebrates and fish species. Based on the results of this study, we provide guidelines (for example restoration of spawning beds, decrease diffuse pollution from agriculture) to optimize the investments made in river restoration and to achieve the good ecological status by 2027 as defined by the European Water Framework Directive.

由于污染、水文形态退化和缺乏健全的环境立法，上世纪 90 年代初，许多西欧国家的水生生物多样性很低。自 2000 年代初和欧洲水框架指令的实施以来，欧洲在实现良好的地表水生态状况方面向前迈进了一大步。然而，迄今为止，人们对低地小溪流中的生物生命对改善水质和栖息地多样性条件的反应知之甚少。在这项研究中，我们使用化学和生物水质的长期测量数据集评估了兹瓦尔姆河流域（比利时）的化学水质与水生生物多样性之间可能的关系。具体来说，我们想知道 1) 过去三十年化学和生物水质的演变，以及 2) 化学水质的改善是否反映在大型无脊椎动物和鱼类的多样性中。我们发现，由于安装了污水处理厂和流域管理计划，化学水质确实有了显着改善。在化学水质改善后不久，观察到污染敏感的大型无脊椎动物类群的数量和丰度增加，因此也可以在基于大型无脊椎动物的生物指数得分中看到。尽管观察到的鱼类群落组成变化有限，但没有发现鱼类的多样性或生物指数得分有显着增加。现在，观察到改善水质的停滞。水文形态恢复和鱼类自由洄游仍然是改善大型无脊椎动物和鱼类多样性的重要关键因素。根据这项研究的结果，我们提供指导方针（例如恢复产卵床，减少农业扩散污染）以优化河流恢复投资，并在 2027 年之前实现欧洲水框架指令所定义的良好生态状况.