Seasonal and spatial variation of stream macroinvertebrate taxonomic and functional diversity across three boreal regions

1. The exploration of biodiversity has predominantly been based on taxonomic measures, whereas functional diversity, a key component of biodiversity, is comparatively understudied. Therefore, studies simultaneously investigating patterns of taxonomic and functional diversity change in biological communities are of increasing interest. 2. We collated high-resolution macroinvertebrate and environmental data from 70 boreal headwater stream sites across three European countries (Germany, Finland, Sweden) to (1) investigate seasonal variation in taxonomic diversity, functional diversity, and redundancy, and (2) identify their potential drivers of spatial and seasonal variation. 3. Seasonal changes in boreal macroinvertebrate taxonomic diversity were decoupled from changes in functional diversity. Seasonal shifts in environmental conditions, including acidity and nutrient variability, drove fluctuations in taxonomic diversity which were far more pronounced than those of functional diversity. 4. Seasonal shifts in environmental conditions including variation in the quantity, quality, and state of organic carbon (dissolved vs particulate) facilitate an exchange of taxa, leading to taxonomically unique communities that exploit the pool of available seasonal resources. Thus, similar levels of functional diversity across seasons—even as taxonomic diversity changes—suggest limited differences in interspecific changes in community function, potentially indicating functional resistance rooted in redundancy. 5. We highlight the spatial and seasonal discrepancies of freshwater communities, emphasising the need for both taxonomic and functional diversity patterns to be assessed in future biodiversity monitoring programmes.