Reach hydromorphology: a crucial environmental variable for the occurrence of riverine macrophytes

Abstract

AbstractManagement measures targeting riverine macrophyte species often address dissolved nitrogen and phosphorus compounds. With recent reductions of nutrient pollution, effects of other variables have become more apparent. This study aims to shed light on the hierarchy of variables determining riverine macrophyte distribution, covering both surface water nutrients and a range of other variables, in a Central European landscape. We used a dataset of 1,371 monitoring samples to predict the presence and absence of 30 common macrophyte species with multiple random forest models. Besides surface water chemical variables (P-PO4, TN and pH), five reach hydromorphological variables (substrate, flow velocity, shading, width and depth) and a variable related to river size (distance from the river source) were use as predictors. The most important variables for the prediction of the species’ presence and absence were substrate, distance from the river source and flow velocity. Additionally, P-PO4 and shade were also important, whilst TN, pH, width and depth played only a minor role. River hydromorphology was more important than surface water chemistry for the occurrences of individual riverine macrophyte species, in particular under less extreme eutrophic conditions. Species occurrences were related to multiple interacting variables, amongst which hydromorphology often prevailed. Presence/absence of species and composition of assemblages cannot be attributed to surface water nutrients alone, even when considering river typologies. Therefore, the results support the conjecture that a broader approach considering a range of stressors is necessary to design restoration measures for management and enhancement of macrophyte diversity.