Optical and biological properties of Lake Ülemiste, a water reservoir of the city of Tallinn II: Light climate in Lake Ülemiste

Abstract

Abstract Underwater light climate in strongly eutrophic Lake Ülemiste was studied by applying in situ measurements and model calculations. The attenuation of light in the waters of Lake Ülemiste is rather high, with a diffuse attenuation coefficient of photosynthetically active radiation, spectral region 400–700 nm (PAR) being 1.0–3.3 m –1 . From the subsurface irradiance, only 3–13% reaches the depth of 1 m. However, in most cases, 1% from the irradiance just under the surface of water is available at the average depth (2.5 m) of the lake. A simple model is used for an approximate description of the temporal variation of underwater PAR at different depths. The initial data are the episodically determined values of the diffuse attenuation coefficient of PAR, K d,PAR and the values of continuously recorded incident irradiance. Light is limiting phytoplankton growth in Lake Ülemiste only for a very short time each year: in early spring and late summer. The study should be continued to investigate the effect of artificially regulated influx and outflow of water and nutrients to toxic cyanobacteria blooms in this lake.