What do macroinvertebrate indices measure? Stressor‐specific stream macroinvertebrate indices can be confounded by other stressors

Monitoring programmes worldwide use biota to assess the “health” of water bodies. Indices based on are used describe change in status sites over time, identify progress against management targets and diagnose causes biological degradation. A variety numerical stressor-specific biotic indices have been developed response differences stressors among sites. Yet, it is not clear how variation pressures within sites, what time period, combination has greatest impact different groups. An understanding temporal influences assessment would assist setting achievable help focus catchment-scale mitigation strategies ensure that they deliver desired improvements condition. Hydrochemical data provided by a network high-frequency (15 or 30 min) automated monitoring stations 3 years were matched replicated understand influence spatio-temporal pollution indices. summarised various ways reflect central tendency, peaks, troughs 1–90 days before collection each sample. objective model selection procedure was determine which hydrochemical determinand, best explained Stressor-specific derived from macroinvertebrates purportedly stress low flows, excess fine sediment, nutrient enrichment, pesticides organic significantly inter-correlated reflected periods oxygen concentration, even though only one index (ASPTWHPT, average score per taxon) designed for this purpose. Changes community composition resulting stressor frequently lead confounding effects Variation ASPTWHPT described dissolved calculated as Q5 10 days, suggesting events had most period. Longer-term apparent, but masked recovery. Macroinvertebrate abundance Q95 stream velocity 60 slower recovery numbers than trait ASPTWHPT. Although supported, we recommend additional independent evidence should be corroborate any conclusions regarding degradation drawn other The such alone risks mistargeting if putative stressor-index approach taken more reliable results herein suggest.