Shifts in water quality in a drinking water reservoir during and after the removal of cyprinids |
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Authors: | Tiia Pedusaar Ilkka Sammalkorpi Arto Hautala Jaana Salujõe Ain Järvalt Margus Pihlak |
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Institution: | 1.Department of Environmental Engineering,Tallinn University of Technology,Tallinn,Estonia;2.Finnish Environment Institute,Helsinki,Finland;3.Department of Biological and Environmental Sciences,University of Jyv?skyl?,Jyv?skyl?,Finland;4.Centre for Limnology,Estonian University of Life Sciences,Tartu,Estonia;5.Department of Mathematics,Tallinn University of Technology,Tallinn,Estonia |
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Abstract: | Lake ülemiste, the drinking water reservoir of Estonia’s capital city Tallinn, was biomanipulated by manual removal of cyprinids
in 2004–2006 and its impact on water quality in the vegetation period was studied. A total biomass of 156 tonnes corresponding
to 160 kg ha−1 of fish, predominantly cyprinids, were removed. A decline in the unit catches of fishing was observed. The removed fish biomass
versus phosphorus concentration of the lake was considered sufficient to reduce the impact of cyprinids on water quality.
The phosphorus removed within fish biomass corresponded to 38 μg l−1 and 21% of the external phosphorus load of the fishing period. The mean total phosphorus concentration dropped from >50 to
≤36 μg l−1. However, the densities of planktivorous young-of-the-year percids remained high and the role of zooplankton grazing in improving
water quality was found non-significant or transient. The cladocerans biomass decreased and the small-sized Daphnia cucullata remained almost the only daphnid in Lake ülemiste during and after the manipulation. Predomination of filamentous cyanobacteria
was replaced by a more diverse phytoplankton composition and co-domination of micro- and pico-sized colonial cyanobacteria
during summer. Mean phytoplankton biomass decreased from 15 to 6 mg l−1 primarily as a result of decreased in-lake TP availability. The Secchi disc transparency increased only in May 2005–2007.
The effects of coincidental events, a decline of external loading of phosphorus and a simultaneous flushing induced by heavy
rainfall, on lake water quality are discussed with some implications to the future management of the reservoir. |
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