Long-term effects of hydromorphological stream restoration on changes in microhabitats of Ephemera danica (Ephemeroptera) and its population

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Relationships between various techniques of stream restoration and selected characteristics of larvae of Ephemera danica (Müller 1764), were investigated along a small lowland watercourse, the Kwacza (N Poland). Effects of stream restoration were assessed on the basis of abundance, biomass, and body length of mayfly larvae as well as their biological condition (i.e. relation of mean body weight and length) within a decade (2007–2017) at each of 10 various hydrotechnical constructions along a 2.5-km stretch of the watercourse. Our results show that hydromorphological changes in the stream bed were strong stress factors for the mayfly population, especially in the first year of monitoring. In the following years, the applied restoration techniques created windows of opportunity for the population of E. danica which were successfully used by this species, as it increased in abundance and body length and colonized the whole restored stretch of the stream. Finally, 10 years after the river bed transformation, mayfly density increased 4-fold, while larval body length and biomass increased about 40-fold, as compared with the pre-restoration period. The improved habitat conditions of E. danica were observed in the sections where stream banks were strengthened with tree trunks, double groynes, and double semi-palisades to protect the banks against erosion. Surprisingly, the highest increase in larval abundance was recorded in a by-pass zone, an isolated part of the stream bed, which played the role of a refuge. Using redundancy analysis (RDA), we found that the mayfly population development after the stream restoration was primarily due to modified water velocities in the transformed sections and changes in water quality (e.g. higher pH, lower concentrations of N-NO3−). We inferred that the modification of the stream bed has contributed to favourable changes in the studied mayfly population, which constitutes an important food base for the fish fauna.

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