Phytoplankton in Alte Donau: Response to Trophic Change from Hypertrophic to Mesotrophic Over 22 Years

The long-term phytoplankton study in groundwater-seepage lake Alte Donau, a former side-arm of the Danube River in Vienna, covers four main lake treatment periods (1–4) from 1993 to 2014. During hypertrophic conditions with annual total phosphorus (TP) concentrations of 50–70 μg L−1 and mean summer phytoplankton biovolume of 18–24 mm3 L−1 before restoration (1), the filamentous cyanobacterium Cylindrospermopsis raciborskii was the main taxon in association with Limnothrix redekei. The drastic phosphorus reduction by chemical RIPLOX-precipitation was repeated twice (2a/b, 1995 and 1996) and resulted in a prompt drop of summer phytoplankton to 4.6 mm3 L−1 in 1995 and 1.7 mm3 L−1 in 1996. Non-filamentous cyanobacteria contributed here only moderately while relative high peak contributions of chlorophytes occurred. After years of re-establishment of macrophytes (3), the summer phytoplankton biovolume remained low during the period of sustained ‘stable conditions’ (4) with values between 0.5 and 1.5 mm3 L−1. In the long-term, phytoplankton was responding to low annual total phosphorus (10–11 μg L−1) which finally indicated a mesotrophic state close to oligotrophic conditions according to the lake classification scheme. The long-term median of chlorophyll-a (chl-a) content was 0.50% of wet weight phytoplankton biomass. As the phytoplankton composition shifted from a cyanobacteria dominated assemblage to a phytoplankton assemblage that was composed of taxa of various taxonomic affiliations, the chl-a content varied considerably. Chl-a content reached its lowest median value of 0.19% when cyanobacteria formed blooms contributing 77% to total phytoplankton (period 1) and was highest with 0.83% during the peak development of chlorophytes which contributed 18% to total biovolume (period 2b). The relationship between phytoplankton chl-a and TP is more robust than between phytoplankton biovolume and TP for indicating the lake’s trophic state, although both response curves are statistically significant and provide roughly the same main picture of an ecosystem shift from hypertrophic in 1993 to mesotrophic in 2000 and the persistence of mesotrophic conditions for the 15 recent years. Trophic shifts were also indicated by the phytoplankton assemblage metric when comparing phytoplankton species composition between the lake treatment periods. The main picture of seasonal development of phytoplankton taxa and functional phytoplankton groups indicated that assemblages either prevailed in winter to spring or summer to autumn. Annual phytoplankton development thus seems primarily distinctive between the two half-year-cycles, namely the winter-spring and the summer-autumn period, rather than between the four seasons. While the seasonal development of phytoplankton follows the lake phenology commonly observed in temperate lakes, long-term compositional shifts of phytoplankton especially responded to the sustained reduction of TP forced by lake treatment measures in Alte Donau.