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

Abstract

Lake Ü lemiste is a shallow, eutrophic lake which has served the city of Tallinn as a water reservoir for many centuries. Its light climate was studied by combining a routinely measured data set with a modelling approach. For 26 years , data was collected on such optically active substances (OAS) and water parameters as water colour, turbidity and phytoplankton biomass. Simple modelling enabled the quantification of long-term time-series data and the subsequent calculation of the diffuse attenuation coefficient, euphotic depth and average light of the mixed layer. Several changes in the hydrological cycle have taken place during the period under study, among which are an increase in the water level of about 0.5 m and a decrease in the external water load from 108 million m 3 year -1 to about 25 million m 3 year -1 . At the same time euphotic depth has shown a distinct trend towards increasing since the early 1990s. The euphotic depth also showed an increase (from 1.1 to 1.4 m) due to an improvement in underwater light conditions -mainly in the spring (April and May) and autumn (October and November) because of the lower amount of dissolved organic matter in the lake. The average light availability in the mixed layer has increased, but this has not affected the phytoplankton biomass as the latter is not light-limited during the summer period.

The influence of city water consumption on the water balance and quality of drinking water supply with implications for altered operating rules

“…Also, Reinart & Pedusaar (2008) showed that water colour is a good indicator of the amount of CDOC in the lake.…”

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

“…The euphotic depth, calculated by equation provided by Reinart & Pedusaar (2008) exceeded 2 m, reaching its highest level since 1978, suggested effect of the fish removal. Diatoms, cryptophytes and chrysophytes dominated, restoring some of the seasonality in phytoplankton that used to occur in early 1960s in the lake .…”

“…The phosphorus (PO 4 -P) load to the lake decreased from 0.38 g m -2 year -1 in 1991 to 0.18 g m -2 year -1 in 2003. Although the external loading had steadily decreased, there was no constant downward trend in water turbidity (Reinart & Pedusaar, 2008), and cyanobacteria continued to dominate the lake's phytoplankton community (Trei, 2002). The water treatment technology of the plant is adjusted to the water quality in the lake, therefore, better raw water quality, especially in the summer months, would allow more cost-efficient drinking water production.…”

Reconstruction of long-term changes of the underwater light field in large shallow lakes Peipsi and Võrtsjärv, North-East Europe

“…E z= − In lakes Võrtsjärv and Peipsi, water is typically mixed down to the bottom, therefore mix z was taken equal to the average water depth, avg , Z of the lake (Table 1). For calculating mix E we converted energy units (MJ m -2 day -1 ) into quantum units (mol m -2 day -1 ) according to the relationship 1 J m -2 s -1 = 4.61 µmol m -2 s -1 in air (Reinart et al, 1998;Reinart and Pedusaar, 2008).…”

“…In large shallow lakes the influence of resuspended sediment region (PAR, 400-700 nm) is of primary interest. In many limnological studies the diffuse attenuation coefficient over the PAR band d,PAR ( ) K is used to describe the vertical decrease of irradiance in the water column (Paavel et al, 2006(Paavel et al, , 2008Zhang et al, 2007;Reinart and Pedusaar, 2008). If underwater PAR irradiance at different depths is measured, the average d,PAR K for a water column can be estimated using the least square fit of the irradiance vs. depth d,PAR (K is the slope of this exponential regression).…”