ESTONIAN ACADEMY
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Estonian Journal of Ecology

Modelling the hydrochemical status of small stratified Lake Verevi (Estonia); 87–105

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Authors
Henno Starast, Tõnu Möls, Katrin Ott, Aini Lindpere, Marina Haldna

Abstract
A survey is given of the vertical distribution and seasonal changes of the chemical composition of water in Lake Verevi. The majority of data consist of 166 to 1011 measurements per parameter from 1984–2001. Statistical analysis was performed with the SAS system, Release 8.1, using a 64-parameter linear model with independent factors ‘sampling year’, ‘sampling day within the year’, and ‘sampling depth’. The depth intervals where the chemical composition of water changed significantly, confidence limits, and predicted mean values were estimated using specific contrasts tailored for the General Linear Models (GLM) procedure of the SAS program package. The hydrochemical state of the lake in summer, autumn, and winter is characterized on the basis of 18 parameters. Stratification is the strongest in summer when water temperature, dissolved oxygen, oxygen saturation, and pH decrease towards the bottom. At the same time, alkalinity, the sum of Ca and Mg, conductivity, Si, CODCr and CODMn increase in the same direction. The most significant increase towards the bottom was shown by total P, PO3-4 total N, and NH+4 The concentrations of SO2-4 NO-2 and NO-3 show relatively constant values from surface to bottom. In autumn, at the time of convective mixing, differences in the water composition from surface to bottom persist but become less pronounced. According to chemical composition, the water in L. Verevi is stratified all the year round.
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