Water Quality in Slovenia - Agencija RS za okolje

Lake Bled
S U R F A C E W A T E R S
The establishment of monitoring at Lake Bled and its inflows was related to remedial action for
the improvement of the lake status that was in the ‘70s already eutrophic, occasionally even
hypereutrophic. The status of Lake Bled has therefore been regularly monitored since 1975. The
results of monitoring show that the artificial inflow the Radovna (1964), the siphon that functions
as a depth outflow from the lake (1980/81) and the partial restoration of the sewage system in
Bled (1982–1985) contributed to the improvement of the status of Lake Bled. Since 1983, Lake Bled
has been classified as a mesotrophic lake according to OECD criteria. A reduction of the nutrients
concentration and a general improvement of the Lake Bled status are apparent in the years 2003
to 2006 in comparison with the years 1979 to 1981. But nevertheless, an occasionally increased
production of phytoplankton, expressed by a relatively high average concentration of chlorophyll-a,
represents a warning about pressures from the lake basin’s which cannot be entirely amended by
the beneficial effects of restoration measures, the Radovna and by the siphon. In addition, the results
of the monitoring of inflows show that the concentration of phosphorus, which is the essential
biogenic element, has been recently increasing in Mišca. On the basis of phosphorus, Table 7 shows
the effectiveness of the siphon in removing the load from the lake and pollution load of Mišca in the
years 2003 to 2006.
Table 7: The annual input and output of phosphorus by Mišca and the siphon in the years 2003 to 2006
Year
2003
(kg P)
2004
(kg P)
2005
(kg P)
2006
(kg P)
Siphon 308 266 217 260
Mišca 231 372 335 402
Lake Bohinj
According to criteria set by the Group for Alpine Lakes (34) in the process of intercalibration of biological
elements, Lake Bohinj belongs to the category of alpine lakes where the impacts of human activity
on the aquatic environment are either not present or are insignificant. Additionally, according to
OECD criteria, Lake Bohinj was classified as a clear oligotrophic lake with low productivity in the years
2003 to 2006, although the average concentration of nitrogen compounds exceeded the limit value
for oligotrophic lakes. A high concentration of nitrogen compounds has been present in Lake Bohinj
since the beginning of monitoring of the lake’s status (1993) and is not increasing, which indicates
that it is the natural state. With regards to phytoplankton, some species occasionally appear that
were not present in previous years and are characteristic of more productive lakes. An assessment of
the ecological status of Lake Bohinj will be made when the assessment methods for other biological
elements are ready and will also include other pressures and not just the eutrophication.
Coccal green algae Nephrocytiun agardhianum, characteristic of more productive lakes, which also appeared in Lake
Bohinj in the years 2003 to 2006.
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