Optical and biological properties of Lake Ülemiste, a water reservoir of the city of Tallinn I: Water transparency and optically active substances in the water

Abstract

Optical and biological measurements were performed in Lake Ülemiste in the summer of 1997 (four measurement days), and from May to October in 1998 (12 measurement days). This kind of data, describing the type and amount of optically active substances in the water, phytoplankton characteristics, the underwater light field, and temperature–oxygen situation in the lake are necessary when estimating the ecological state of the lake. Lake Ülemiste is the main drinking water reservoir of Tallinn, the capital of Estonia. Phytoplankton abundance and biomass, chlorophyll a and suspended matter were determined from collected Lake Ülemiste water samples in the laboratory. Spectrophotometrical processing of the filtered and unfiltered water was carried out to describe the beam attenuation coefficient spectra and optical influence of yellow substance in the water. Vertical profiles of downwelling irradiance of the PAR (400–700 m) region of the spectrum (and from these data the averaged over depth diffuse attenuation coefficient) were determined. The relative transparency of the water was estimated by using a Secchi disk. Passive optical remote measurements were episodically made from aboard a boat. Results obtained confirmed that Lake Ülemiste is turbid (almost hypertrophic), comparable with most turbid lakes in Estonia (e.g. Lake Võrtsjärv) and Finland (Lake Tuusulanjärvi). Its chlorophyll a content varied within the range 13–121 mg m–3, phytoplankton biomass 3–107 mg L–1, phytoplankton abundance 65 000–999 000 mL–1, suspended matter 8–34 mg L–1, effective concentration of the yellow substance 6–30 mg L–1, diffuse attenuation coefficient of light in the PAR region 1.0–3.3 m–1 and a Secchi disk depth of 0.5–1.75 m. The temporal variation of the spatial averages of these parameters during 1998 was analysed. Almost all characteristics showed an increase from May to midsummer with a maximum in late July or in August (correspondingly the Secchi depth values were minimal in late summer). The amount of yellow substance was an exception, which was nearly constant during the observation period. The maximum level of chlorophyll a content in July and August 1998 markedly exceeded that in 1997, despite the fact that the summer of 1997 was sunny, but the summer of 1998 was cold and rainy. The correlative relationships between the different parameters were investigated together with the respective data for other lakes. They show that the data of Lake Ülemiste supplemented the correlation graph in the region of turbid lakes, whereby in all cases the correlation coefficient increased following the addition of Lake Ülemiste data. The highest correlation coefficient was obtained when light attenuation coefficient values were correlated with a sum of weighted concentrations of chlorophyll a, yellow substance and suspended matter (multiple regression analysis).

Relations of phytoplankton in situ primary production, chlorophyll concentration and underwater irradiance in turbid lakes

“…During last years our group of marine optics performed a number of studies on turbid lakes in Estonia (Arst et al, , 2006Herlevi, , 2002Erm et al, 2001Erm et al, , 2002Arst, 2003). Turbid lakes deserve more attention because several water parameters like concentrations of optically active substances, plankton biomass and primary production can gain extreme values in such lakes, which have to be taken into account when calibrating of different functional models describing water ecosystems.…”

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

“…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).…”

Reconstruction of the Time Series of the Underwater Light Climate in a Shallow Turbid Lake

“…The methods applied in this study for measuring OAS are fully described in Erm et al 2001. Here, only a short description is given.…”

“…Hydrological regime and description of Lake Ü lemiste Lake Ü lemiste (59°24¢1¢¢ and 24°45¢48¢¢) is a shallow (in 2004, mean depth: 3.4 m; maximum depth: 5.2 m) eutrophic lake that has served as a water reservoir for the city of Tallinn since the 14th century (details in Erm et al 2001). The total volume of the Lake Ü lemiste is 32 Mm 3 ; its usable storage as a water reservoir is 16 Mm 3 , the corresponding area is 9.75 km 2 and the water level (staff gauge reading) is 190 cm (36.62 m above the sea level).…”