Assessment of the efficiency of river macrophytes to detect water-column nutrient levels and other environmental conditions in Irish rivers

Macrophytes are an essential biological element of freshwater aquatic ecosystems and are well known to reflect prevailing ecological conditions in rivers. Their use as bioindicators of nutrient status in river systems is widespread yet their reliability is hotly debated. The aim of this investigation was to assess whether macrophytes are reliable indicators of nutrient levels or better applied as indicators of other non-nutrient environmental conditions. The importance of two water-column nutrients; nitrates (N) and orthophosphates (P) were assessed in terms of their influence on macrophyte species richness and diversity in relation to other non-nutrient environmental factors using 395 river plots from Ireland. Then, in this context, the efficiency of macrophytes to detect nutrient levels was assessed by selecting two macrophyte-based water quality assessment tools; the Mean Trophic Rank (MTR) and the Predictions And Classification System for river macrophytes (LEAFPACS2). Finally, the ability of phytosociological communities within these rivers to reflect trophic levels was examined using the same two water quality assessment tools. It was shown that water-column nutrients N and P have a minor influence on macrophyte richness and no significant influence on macrophyte diversity and that MTR and LEAFPACS2 were only weakly correlated with N and P levels. It was concluded that macrophytes are sensitive to environmental changes but respond to a combination of ecological factors rather than N or P alone. Therefore, this study suggests that macrophytes are not the most efficient taxon group to apply when assessing trophic changes in isolation of other non-nutrient factors.