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Extreme floods of the Danube in 2013 – track changes of the ecological state of the river applying the phytoplankton assemblage index

Extreme hydrological events have become more frequent and intensive in the Danube River Basin. The highest ever recorded discharge of the Danube at the entrance to Croatia (river 1424.85 km) was in June of 2013. The aim of this study was to evaluate changes in the ecological status of the Danube according to the phytoplankton functional approach and applying the phytoplankton assemblage index (Q-index). The lowest phytoplankton abundance and biomass were found during extremely high water discharge. A total of 119 phytoplankton species sorted into 20 FGs were found. Codon TB, represented by benthic diatom species, was the most successful during the whole investigated period and dominant in the conditions of extreme floods. The highest value of the Q-index (4.49) was established in the condition of extreme floods, indicating the excellent ecological state. The worst ecological state was in April, in the conditions of high nitrogen concentrations in riverine water and the low value of the Q index (2.48) indicating poor ecological status. Our results have shown that the assemblage index is a reliable instrument to assess the ecological status of large lowland rivers influenced by extreme hydrological events.

phytoplankton, extreme hydrological events, Danube River Basin

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