Macroinvertebrate habitat requirements in rivers: overestimation of environmental flow calculations in incised rivers

Flow variability determines the conditions of river ecosystems and river ecological functioning. The variability of ecological processes in river ecosystems gradually decreases due to river channelization and incision. Prediction of the environmental flow allows us to keep biological diversity and river health developed as a response to the degradation of aquatic ecosystems overexploited by humans. The goal of the study was to test the influence of river incision on environmental flow estimation based on the Biological Monitoring Working Party (BMWP) macroinvertebrate index. A total of 240 macroinvertebrate assemblages of 12 waterbodies differing in bed substrate and amplitude of discharge were surveyed in southern Poland. Variations in the distribution of 151 466 macroinvertebrates belonging to 92 families were analysed. The similarity of benthic macroinvertebrates reflects the typological division of the rivers into three classes: Tatra mountain streams, mountain flysch rivers, and upland carbonate and silicate rivers. As a response variable reflecting the macroinvertebrate distribution in the river, the BMWP_PL index was chosen. The river incision significantly increased the values of e-flow calculations in relation to redeposited channels. The area of optimal habitat for macroinvertebrates decreased with the bed incision intensity. In highly incised rivers, the environmental flow values are close to the mean annual flow, suggesting that a high volume of water is needed to obtain good macroinvertebrate conditions. As a consequence, river downcutting processes and impoverishment of optimal habitats will proceed.