From isolation to connectivity: the effect of floodplain lake restoration on sediments as habitats for macroinvertebrate communities

The present study aimed to identify potential macrozoobenthic habitat indicators of the ecological success of restoration projects. As a part of the complex restoration project in the Słupia River floodplain (N Poland), the connectivity between three oxbow lakes and the river channel was re-established to improve biodiversity of the floodplain area, including bottom fauna. Following restoration, changes in the dynamics of flowing water and water levels induced the transformation of oxbows from plesiopotamal (lentic) to eupotamal (lotic) and subsequently to parapotamal (semi-lotic) habitats. The restored sites underwent a rapid depletion of benthic coarse particulate organic matter, with direct changes in most of the investigated parameters of bottom sediments, including conductivity, total organic carbon, soluble reactive phosphorus and total nitrogen. Redundancy analysis revealed that the changes in benthic fauna structure resulted from the increased connectivity with the river (flow rate) and changes in the chemical properties of sediments. The restored oxbow lakes were colonized by six new macroinvertebrate species whose density increased substantially. The assessment of the overall river-floodplain system restoration project indicated positive implications for improving the qualitative and quantitative structure of benthic fauna. However, to confirm ecologically successful restoration, it is necessary to evaluate appropriate sets of indicators based on a complex food web structure and more efficient or enhanced ecosystem functions. This study contributes to the discussion of sustainable management of floodplains to provide benefits to macroinvertebrates as indicators of aquatic ecosystem health under different restoration activities.