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 total 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, we hypothesized that direct effect of the changing water level on traditional water quality indicators blurred the picture. We constructed a new multimetric index based on the traditional water quality indicators in which we statistically removed the direct effect of the water level and the seasonality in order to minimize the effects of natural variability. The new index 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 advantages and disadvantages of both methods are discussed in the light of the alternative stable states theory and the priority of biotic indicators stipulated by the EU Water Framework Directive.

TUVIKENE Lea;  NOGES Tiina;  NOGES Peeter; 

2010-12-13

SPRINGER

JRC54741

0018-8158,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC54741,   

10.1007/s10750-010-0414-5,