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Why do phytoplankton species composition and “traditional” water quality parameters indicate different ecological status of a large shallow lake?


Long-term data on phytoplankton species composition in large and shallow Lake Võrtsjärv indicated a sharp deterioration of the ecological status at the end of the 1970s. The more traditional water quality indicators, such as the concentrations of nutrients and chlorophyll a, phytoplankton biomass, and Secchi depth, failed to capture this tipping point or even showed an improvement of the status at that time. As the shift coincided with a large increase of the lake’s water level (WL), we hypothesized that direct effect of the changing WL on traditional water quality indicators might have blurred the picture. We removed statistically the direct effect of the WL and the seasonality from the traditional water quality indicators in order to minimize the effects of natural variability. The average of the standardised water quality indicators, used as a proxy for the ecological status, distinguished a period of fast eutrophication in the first half of the 1970s (not captured by the phytoplankton species index), a fast improvement at the end of the 1970s (when the species index showed deterioration) followed by a continuous deterioration trend (when the species index remained rather constant). The causes of this inconsistency are discussed in the light of the alternative stable states theory and the priority of biotic indicators stipulated by the EU Water Framework Directive.

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The study was supported by Estonian target funding project SF 0170011508, by grant 7600 from Estonian Science Foundation, and RE 201—the Estonian Environmental Monitoring Programme.

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Correspondence to Lea Tuvikene.

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Guest editors: T. Blenckner, T. Nöges, L. Tranvik, K. Pettersson, R. Naddafi / European Large Lakes II. Vulnerability of large lake ecosystems - Monitoring, management and measures

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Tuvikene, L., Nõges, T. & Nõges, P. Why do phytoplankton species composition and “traditional” water quality parameters indicate different ecological status of a large shallow lake?. Hydrobiologia 660, 3–15 (2011).

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  • Water Framework Directive
  • Phytoplankton taxonomic index
  • Trophic state indicators
  • Long-term data
  • High natural variability
  • Alternative stable states