A study on the response of macroinvertebrate assemblages to the restoration of hydrological connectivity of an oxbow lake through the channel excavation is presented. The study included a five-year environmental monitoring (hydrological, hydro-chemical measurements and invertebrate sampling) carried out in the years 2008–2009 and 2011–2012 in the floodplain of the Słupia River (N Poland). The results allowed for assessing ecological effects of the hydrotechnical treatments (re-opening of an old river bed, declogging and installation of wooden deflectors) applied in the oxbow restoration. The results confirmed the preliminary hypothesis that the level of hydrological connectivity determines the dynamics of invertebrate fauna communities in river-floodplain systems. Analysis of the data revealed that such reconnection considerably influenced the structure of hydrobionts by altering abiotic habitat conditions. Effects of radical changes in the habitat morphology and hydrodynamic conditions and monitoring of macroinvertebrate assemblages preformed in the restored lake indicated a significant instability of the ecosystem soon after the treatment, what was confirmed by the results of canonical analysis, in which 50% of the total variance remained unexplained. Among factors analysed, water quality parameters explained 21.4% of the total variance in macroinvertebrate communities. However, the hydrobionts showed a significant instability with respect to variable hydrological conditions (flow through the lake) what contributed to a low share of water flow along the oxbow in explaining the total variance. The analysis of long-term changes that occurred in the studied ecosystem showed that the restoration of full hydrological connectivity brought only a short-term increase in benthofauna abundance. The most distinct reorganization in the structure of macroinvertebrate communities was observed in the first year after the reconnection while the consecutive four years brought only insignificant changes, mainly the appearance of a few species, mainly molluscs. Our investigation suggests that the taxonomic composition of macroinvertebrates in the studied oxbow lake having the only one-arm opening, before the improvement of hydrological connectivity, seemed to be more stable and optimal for this kind of habitat, than after the hydrotechnical works, when the ecosystem became passable due to both-arms connections to the river channel. Therefore, semi-lotic oxbows, connected to parent rivers only with one arm, can be properly functioning aquatic ecosystems in river floodplains.
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