Availability and representativeness of freshwater macroinvertebrate data to enable Europe-wide assessment of biodiversity trends.

Freshwater ecosystems, though covering less than 1% of Earth's surface, harbor around 10% of all species. Pressures like pollution, water extraction, and habitat loss can exhibit complex interactions in their effects on freshwater biodiversity. Understanding the effects of these pressures on ecosystem services and functions of freshwater systems is crucial for effective environmental policies, and asks for availability of species data. We therefore compiled European monitoring data on macroinvertebrates from European, national, and regional biomonitoring and analysed their representativeness with respect to river size, land use as well as their suitability to derive biodiversity trends. The dataset consists of over 2.3 million macroinvertebrate abundance records from 2010 to 2015, spanning 28 European countries. Geographically, data are concentrated in central Europe and England and are not evenly representative across space or river types. They vary regarding reporting units, general sampling methods, and taxonomic levels (mainly species level). These differences hamper European-scale biodiversity comparisons. The WFD primarily focuses on assessing the "ecological status" of water bodies, which can be achieved using a menu of methods and many biodiversity research questions require improvements in harmonization of taxonomic resolutions and in method standardization to enable accurate transnational comparisons. This study emphasizes the need for improved European coordination and support in collecting, validating, exchanging, and sharing freshwater biodiversity data to enable meaningful comparisons among countries. It also delivers an open-access database that enables analyses of trends and ecological scenario development, offering further opportunities to enhance freshwater biodiversity conservation and management.