The Effect of Restoration Measures on the Benthic Invertebrates of a Danube Backwater (Alte Donau)

Abstract

Benthic invertebrates were used as bioindicators to document the effect of restoration measures in the backwater Alte Donau in Vienna, a former side-arm of the Danube. The study covers four periods of lake management: (1): the mesotrophic year before eutrophication (1987), (2): the 2 years of chemical iron chloride treatment aimed at the phosphate precipitation in the water column and the oxidization of nitrate-treated sediment surface layers (1995–1996), (3): further 3 years of other lake management measures during the restoration period (1995–1999), and (4): an early stage of the re-establishment of underwater vegetation (2000, 2003). Over eight survey years from 1987 to 2003, about 330 benthic invertebrate taxa with three most abundant systematic groups were identified: 37 species of oligochaetes, 23 species of molluscs (18 gastropods and 5 bivalves), and 190 species of the chironomids and other dipterans. The trophic classification index that refers to a habitat quality score by chironomids indicates the year 1987 as mesotrophic (3.46, the index range for mesotrophic conditions is 2.50–3.49). In this year the chironomids and oligochaet species inhabited a variety of diverse habitats ranging from soft sediments (clay and mud), sand, gravel, pebbles and stones to dense stands of macrophytes. The biomonitoring record of 15 mollusc species was significantly higher in this mesotrophic reference year than in any other following survey year. The trophic classification index denotes 1995 as the most eutrophied year. With the restoration and the re-establishment of macrophytes the values of this index decreased but remained higher than during the mesotrophic condition. In the years 1995 and 1996, when the chemical treatment with iron chloride, slaked lime and calcium nitrate was applied and the macrophytes were rare, the biomass of chironomids and oligochaets was particularly low. Among the three important taxonomic groups, i.e. the chironomids, bivalves, and oligochaets, only the first two groups achieved relative biomass dominance. The relative importance of chironomids over oligochaets during these both years of the Riplox-treatment might rather mirror the losses of oligochaets being affected by nitrate exposure than the re-colonization by ‘new’ chironomid species in the sediment. In addition, in particular active filter feeders such as bivalves seemed to adjust well to the muddy sediment environment after chemical treatment. In the following years of the restoration and the early stage of macrophyte re-establishment, oligochaets and bivalves became the dominant groups mainly contributing to the macrozoobenthic biomass, while the biomass of chironomids remained relatively low. Different from the indication by enhanced water transparency and low phytoplankton biomass achieved by restoration measures in 2004, the shift towards a mesotrophic environment seemed to be retarded when assessing Alte Donau by the chironomid habitat quality score index. The main reason why the chironomid assemblage did not follow the other indicators of mesotrophic conditions in 2004 can be seen in the disruption of the sediment (e.g. by sedimentation of precipitation chemicals) and associated loss of underwater habitat structure due to still relatively low macrophyte biomass during the last invertebrate survey. The phytophilic chironomid species still had a low abundance when only about 50% of the macrophyte biomass was recorded if compared with the mesotrophic situation in 1987. Despite the wax and wane of benthic invertebrates described during the eight-year survey, invertebrate biomass is relatively high in the long-term average and thus characterises Alte Donau as a lowland environment along habitats from low- to high altitudes.