miljötillstånd och utveckling - Mälarens vattenvårdsförbund

Lake Mälaren, the third largest lake in Sweden, has been
monitored since the mid 1960’s. Large improvements in
water chemical composition and biological status occurred
during the late 1960’s and the 1970’s, chiefly due to the introduction
of chemical precipitation in municipal sewage
treatment plants. This resulted in a 60% reduction of total
phosphorus input to the lake. Presently, the main nutrient
input originates from diffuse sources in the catchment area.
After the initial nutrient reductions, there are no obvious
trends in water chemical composition, except for alkalinity,
which has increased steadily during the period. The reduced
phosphorus input decreased phosphorus concentrations
in lake water, and lowered phytoplankton biomass. In
particular, the maximum phytoplankton biomass has declined.
The largest biomass reductions occurred in the Galten
and Ekoln basins (60% reduction). The decline in other
parts was generally between 15 and 35%. Occasionally,
cyanobacteria blooms are noted in the more eutrophic
basins, but, so far, only the bloom in Galten in August and
September 1997 was classified as a severe problem. On the
other hand, Galten has a comparatively high nitrogen-fixation
potential as the N/P-ratio indicates periods with nitrogen
deficit, and the cyanobacteria problem might have
more adverse effects in the future. The reduced phosphorus
input has also reduced the abundance of benthic fauna, but
with a four-year time lag. In contrast, the communities of
common reed, Phragmites australis, have generally become
denser. The abundances of floating-leaved macrophytes
have also increased. The general expansion of macrophytes
is probably caused by increasing regulation of water levels,
resulting in more stabile conditions for macrophytes.
Long- and short-term environmental guidelines were
Summary
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established for Lake Mälaren in 1993. These guidelines declare
that within 10 years the phosphorus concentrations,
depending on the basin, shall be 10–25% lower than the
levels in 1981-1985. On a long-term basis the concentrations
shall not exceed twice the “natural” background-levels. The
existing background-level estimates are questioned, as recent
paleolimnological sediment investigations indicate
that the natural levels might be much higher that has previously
been estimated in some basins. Simulations by a new
dynamic phosphorus model for the different basins show
that an 80% phosphorus reduction would be required to
accomplish the long-term environmental guidelines in Galten.
Hence, there is an urgent need to confirm the background-levels
with more comprehensive sediment investigations.
This will help resolve the question of how far realistic
nutrient input reductions will go towards natural nutrient
loadings.
According to the guidelines of the Swedish Environmental
Protection Agency, drainage areas with very large
annual nitrogen (> 32 kg N/ha) and phosphorus (> 0.64 kg
P/ha) losses, shall be given the highest priority for restoration.
Such very large areal phosphorus losses occur only in
Sagån and Märstaån. The nitrogen outflow from Lake
Mälaren is subject to very large inter-annual variation,
caused primarily by variations in water flow. Consequently,
conclusions are uncertain about eventual trends in nitrogen
losses, and whether the losses are approaching the guidelines
(twice the background-levels).
In conclusion, Lake Mälaren responded well to the nutrient
reductions in the late 1960’s and 1970’s, and has now
reached a new, less eutrophic status. To further improve the
conditions there is a strong need for better information on