Context matters: how river typology shapes biotic responses to fine sediment pressure

Context Excess fine sediment is a global stressor affecting freshwater biodiversity. However, little consideration has been given to how large-scale landscape controls and temporal variability may influence the effect of fine sediment deposition and storage on biological communities. Objectives We assess if ecological responses to deposited fine sediment are spatially and temporally consistent through the application of the river typology approach. Methods We used 2,940 records from 391 wadable streams across England and Wales to identify taxonomic and functional community composition change points, in addition to individual family responses along the fine sediment gradient. We also examined the association of taxonomic and functional community diversity metrics and biomonitoring metrics with deposited fine sediment coverage. Results Mid-altitude rivers displayed a lower community threshold (~ <10% fine sediment cover) of deposited fine sediment before the majority of community change occurred, whilst lowland rivers were more tolerant (20-25%). Critically, we found that both mid-altitude river types demonstrated no association with two fine sediment stressor-specific metrics and that some community metrics displayed a positive association with increasing fine sediment cover. Conclusions Community and family level responses to deposited fine sediment are non-linear, which can be characterized effectively by river typologies and most notably altitude groupings. Low levels of deposited fine sediment may not act as a stressor in mid-altitude catchments as these may be resource limited. Our research underlines the need to consider context-specific effects of fine-grained sedimentation rather than seeking to generalise stressor effects.