Rising, falling, stalling: complex biodiversity trajectories in English rivers

Freshwater ecosystems face intense anthropogenic pressures. Yet in parts of Europe, including the UK, studies have documented widespread increases in macroinvertebrate diversity following investments to improve water quality. More recent analyses, however, suggest that these gains have slowed, plateaued, or even reversed. Despite increasing recognition that biodiversity trajectories vary substantially among locations, the majority of analyses are still based on spatially aggregated data which mask important local heterogeneity, complicating interpretation of national‐scale trends. Consequently, the mechanisms of local biodiversity dynamics remain poorly resolved. We address this gap using macroinvertebrate time‐series data from 803 sites in England's Environment Agency monitoring network. We used an unsupervised clustering approach to group sites with similar temporal trajectories for three community‐level metrics: pollution sensitivity (average score per taxon, ASPT), richness, and abundance. We then compared the environmental characteristics and community composition of these groups to identify the factors most strongly associated with different trajectories. Our analyses reveal substantial heterogeneity in temporal trajectories: more than half of the sites (~ 53%) exhibited declining or non‐directional trajectories in at least one of the three metrics. Sites that exhibited increases in ASPT and/or richness were strongly associated with a lower distance from source, higher oxygen availability, and lower nutrient concentrations. In contrast, declines were more common in downstream sites with finer sediments and elevated nutrient and pollutant loads. Further, trends varied far more among individual sites than among larger aggregated regions such as catchment or water bodies. Collectively, our findings show that macroinvertebrate community dynamics are a mosaic of non‐linear, site‐specific trajectories. This reflects the combined influence of multiple processes, likely including species interactions and dispersal dynamics, operating across scales. A stronger mechanistic understanding of these processes is therefore essential for improving river management and conservation strategies, complementing existing catchment‐ and waterbody‐scale management with greater recognition of site‐level ecological heterogeneity.