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

Abstract

1. Monitoring programmes worldwide use biota to assess the "health" of water bodies. Indices based on biota are used to describe the change in status of sites over time, to identify progress against management targets and to diagnose the causes of biological degradation. A variety of numerical stressor-specific biotic indices have been developed based on the response of biota to differences in stressors among sites. Yet, it is not clear how variation in pressures within sites, over what time period, and in what combination has the greatest impact on different biotic groups. An understanding of how temporal variation in pressures influences biological assessment indices would assist in setting achievable targets and help focus catchment-scale mitigation strategies to ensure that they deliver the desired improvements in biological condition. 2. Hydrochemical data provided by a network of high-frequency (15 or 30 min) automated monitoring stations over 3 years were matched to replicated biological data to understand the influence of spatio-temporal variation in pollution pressures on biological indices. Hydrochemical data were summarised in various ways to reflect central tendency, peaks, troughs and variation over 1-90 days before the collection of each biological sample. An objective model selection procedure was used to determine which hydrochemical determinand, and over what time period, best explained variation in the biological indices. 3. Stressor-specific indices derived from macroinvertebrates which purportedly assess stress from low flows, excess fine sediment, nutrient enrichment, pesticides and organic pollution were significantly inter-correlated and reflected periods of low oxygen concentration, even though only one index (ASPT WHPT , average score per taxon) was designed for this purpose. Changes in community composition resulting from one stressor frequently lead to confounding effects on stressorspecific indices.

Assessing the use of RIVPACS-derived invertebrate taxonomic predictions for river management

“…Species that are less sensitive to pollution, such as those that can live in eutrophic waters, have lower scores than more sensitive taxa. There is functional overlap both between individual monitoring metrics, as well as between the scores each individual taxa represents (Jones et al, 2023). Improvements in WHPT ASPT O/E can be seen whilst a site might be impacted by stressors affecting LIFE O/E or the proportion of sediment-sensitive invertebrates (PSI O/E), measuring flow and sediment associations, respectively.…”

“…However, these metrics provide only narrow and incomplete assessments of ecosystem health and are not capable of providing insights into underlying community dynamics. Additionally, some metrics can co-vary, and thus it can be difficult to distinguish between underlying stressors (Jones et al, 2023). These metrics have also been shown as insensitive to larger trends such as the impact of climate change, or invasive species (Mathers et al, 2016;Thompson et al, 2018).…”