Re-evaluating expectations for river phytobenthos assessment and understanding the relationship with macrophytes

•When human pressures are low alkalinity has a big effect on diatom assemblages.•UK has too few pristine high alkalinity sites for assessment of baseline conditions.•Non-linear quantile regression was used to estimate “best available” conditions.•This doubled the strength of the diatom-nutrient relationship, over that used now.•Averaging with macrophytes is recommended rather than using either in isolation. The reference model underlying the UK phytobenthos (diatom) tool for Water Framework Directive assessments is revisited and a new approach is proposed which uses quantile regression to predict the lowest values of the Trophic Diatom Index (equating to the best available condition) at any level of alkalinity . Whilst a reference model based on least disturbed or minimally impacted conditions would be preferable in theory, in practice the absence of lowland high alkalinity streams in a minimally impacted condition in the UK precludes the use of these approaches. Having proposed a revised reference model for phytobenthos, we then go on to examine the relationship between phytobenthos and macrophytes. These two groups respond to nutrients and other stressors in different ways with phytobenthos being more sensitive to nutrients whilst macrophytes better reflect the extent to which secondary effects are likely. We argue that averaging the two sub-elements of the “macrophytes and phytobenthos” biological quality element is a more realistic option than the current approach of taking the lower of the two assessments. It is, however, possible, to predict the value of the combined quality element from either sub-element, though we recognize that this also risks misclassifications.