Biomanipulating the drinking water reservoir of Estonia's capital city: Prospects for success

Abstract

The possibility of utilizing biomanipulation to improve the water quality of Tallinn's drinking water reservoir (Lake Ülemiste) was analysed on the basis of water quality data, test fishing by different methods, and earlier studies on aquatic plants, light climate and sediments. Eutrophic, polymictic Lake Ülemiste is characterized by a prevalence of high filamentous cyanobacteria biomass, rotifers in the zooplankton community, mature bream in the fish biomass, and a high density of planktivores (YOY perch). Several prerequisites for being a potential target for biomanipulation were identified, including (i) decreased external total phosphorus (TP) loading; and (ii) a sufficient stock of piscivorous fish in the lake. Prohibition of fishing should favour biomanipulation efforts. The potential for recolonization of macrophytes and large Daphnia species in Lake Ülemiste could be judged from historical data. The in-lake TP concentrations are within the limits that facilitate successful restoration. As phytoplankton biomass was significantly limited by phosphorus in Lake Ülemiste, its water quality improvement will be driven primarily by reduced nutrient concentrations, and then by zooplankton grazing. Internal TP loading and a new phytoplankton species community were assessed as major challenges for successful implementation of biomanipulation as a means of improving the water quality of Lake Ülemiste.

Shifts in water quality in a drinking water reservoir during and after the removal of cyprinids

“…Since the fish removal was selective for cyprinids, the high densities of youngof-the-year (YOY) percids observed in 2003 (\10,000 ind. ha -1 , Pedusaar et al, 2008) were not affected. Median catch per unit effort (CPUEn) of littoral YOY perch in Nordic multimesh gillnets varied from 14 to 83 ind.…”

“…During MayOctober 2005-2007, univariate regression analyses showed mainly significant though weaker effect of TP on Chl a and phytoplankton biomass explaining again better changes in Chl a than in phytoplankton biomass ( Table 3). The slopes of the regressions were smaller than before fish removal (Table 3; Pedusaar et al, 2008). The year 2005 was an exception as despite the significant TP increase, phytoplankton biomass and Chl a decreased, reflecting the importance of factors other than TP.…”

“…Slow growing Limnothrix redekei, the main dominant in the phytoplankton community of Lake Ü lemiste (Trei, 2002;Pedusaar et al, 2008) and the indicator species of hypertrophy in Estonian lakes (Laugaste et al, 2001) nearly collapsed suggesting changes in the lake's ecosystem. Limnothrix redekei is a successful competitor in nutrient rich and lowlight conditions (e.g.…”

“…At present, the water level is controlled by the Water Treatment Plant (WTP) and the mean water residence time (WRT) is approximately 1 year. More about the lake, its biota and catchment area can be found in Erm et al (2001), Faulkner et al (2003 and Pedusaar et al (2008).…”

“…according to the Baltic system) 32 mm 3 ] is situated on the southernmost border of Tallinn. A map of the lake is shown in Pedusaar et al (2008). Lake Ü lemiste is a shallow, polymictic hardwater lake.…”