How streams change over time? 30 years of community changes and seasonal dynamics

Despite covering less than 3 % of Earth’s surface, freshwater ecosystems are highly biodiverse (they host nearly 9.5 % of all described animal species) and provide crucial services like drinking water, food and recreation. However, a long history of anthropogenic exploitation has made freshwater ecosystems among the most heavily impacted during the ongoing sixth mass extinction. In this context, high-resolution time series are essential to provide evidence-based management, infer trends in taxonomic and functional diversity, and ultimately understand how biological communities respond to stressors over time. In this study, we analyse long-term data gathered by the Water Agency of the Basque Country from over 170 sampling sites corresponding to 21 river catchments in Northern Spain, including biological (e.g., macroinvertebrate and fish abundance), hydromorphological (e.g., flow and habitat quality indexes), and more than 21 abiotic variables. These variables were sampled twice a year across 30 years, thereby providing both interannual and intraannual (i.e., seasonality) information. Here we present trends in abundance, taxonomic and functional diversity in relation to the physicochemical properties of the habitat, and explore changes in seasonality over the past decades. Our results reveal a significant decline in fish abundance and diversity, and a slight increase in both the abundance and diversity of macroinvertebrates. River catchments show a trend towards environmental homogenization as they become more oxygenated and less polluted. Since river physicochemical status improved over the past three decades, the observed differences among macroinvertebrate and fish may be driven by hydromorphological alterations. Intraannual differences in abundance between samplings before and after the summer period have diminished and, is some cases, reversed, suggesting a shift in the phenology of the organisms studied. Altogether, our study highlights the potential value of exploring manager-gathered data from water quality monitoring to reveal long-term and seasonal patterns in freshwater communities essential for tackling the challenges posed by global change.