Early recolonization of a dredged lowland river by dragonflies (Insecta: Odonata)

Abstract

The influence of dredging on the dragonfly assemblages of the small regulated lowland River Krąpiel (north-western Poland) was analyzed a short time after the dredging. Dragonfly assemblages were destroyed, but they began to recover rapidly. Many biocoenotic indices reached high values at just six months after the dredging. The recolonization first occurred as a result of larval drift, and then, via dispersion of adult dragonflies. This process took place in conditions different from the prevailing conditions in the period before dredging, in terms of microhabitat availability and physico-chemical conditions. Compared to the previous assemblage, the emerging assemblage was more typical of assemblages found in small, natural running waters. Therefore, dredging (carried out for economic reasons) could be regarded as a process that unintentionally had a positive influence on odonate assemblages. Currently, when most small watercourses are regulated, dredging that is properly planned and controlled has proven to protect the natural fauna. It is worthwhile to apply lessons learned from examples of active fauna protection to what is currently known as "the rotational model" for dredging.