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Daugava basin
Lielupe basin
Venta basin
Gauja basin
Salaca basin
Daugava basin
The total nitrogen and phosphorus discharges from
point sources in the Daugava basin show a decline. In comparison
to discharges in 1991, the total nitrogen amounts have decreased
by six times, and the total phosphorus amounts by three times.
However, a significant part of the impact on the water basin is
due to diffuse pollution sources.
Long-term changes in the concentrations of
nutrients show that the concentrations of nitrogen and phosphorus
in Daugava waters declined during the 1990's. The decrease in
phosphorus concentrations is more evident than for nitrogen, which
may be explained by differences in supply and binding processes
between the two elements. In contrast to the phosphorus
concentrations, phosphorus loads to the Baltic Sea have changed
minimally, because the water discharge from the Daugava has
increased causing more intensive release of accumulated phosphorus.
The saprobiological quality of small rivers in the
Daugava basin indicates a significant effect from past
anthropogenic impact: only 20% of the small rivers are classified
as clean to slightly polluted, and more than half (65%) are
slightly polluted.
As in the country as a whole, the proportion of
treated (according to standards) waste waters has increased by two
times since 1991.
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Lielupe basin
The trend of decreasing discharges from point
sources in the Lielupe basis is less evident than in other river
basins of Latvia.
The total nitrogen discharge from point sources in
2000 decreased by 44%, in comparison to that in 1991. A decline
was seen also for total phosphorus - by 19%. At the same time, it
is evident that the decrease in loads is less than in the Daugava
basin.
Long-term changes in nutrient concentrations
indicate a decrease in the concentrations of nitrogen compound.
Similar to the Daugava basin, the saprobiological quality of
the Lielupe basin indicates the effects of past anthropogenic
impact: only 11% of small rivers are clean to slightly polluted,
70% are slightly polluted, and several are polluted and very
polluted.
In general, the anthropogenic loads in the Lielupe basin are
relatively high and the impacts on water quality are significant.
Although the proportion of treated waste waters flowing into
the Lielupe basin increased by 4.5 times during the past ten years,
there was a decrease in 1999-2000, when only slightly more than
half of the cleaned waste waters corresponded to standards.
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Venta basin
In contrast to other river basins in Latvia, the
nutrient discharge in the Venta basin has increased. In 2000, the
total nitrogen discharge from point sources was 8 times higher
than in 1991, and the total phosphorus discharge was 1.4 times
higher.
During 1997-2000, the mean total nitrogen
concentrations tended to decrease, while phosphorus concentrations
slightly increased.
In comparison with the Lielupe and Daugava basins,
the saprobiological quality of small rivers in the Venta basin
indicates that they have been less polluted: a greater proportion
of the rivers are classified as clean to slightly polluted waters.
Already in the early 1990's, a relatively large
amount of waste water discharged into the Venta basin was treated
at levels meeting standards (an exception was in 1993), which
makes the increasing trend less obvious.
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Gauja basin
The decline of nutrient discharge from point
sources in the Gauja basin is less evident than for other river
basins in Latvia. The total nitrogen discharge from point sources
in 2000 decreased by 1.4 times, in comparison with 1991, and total
phosphorus discharge decreased by 2 times.
The mean total nitrogen concentration had a
tendency to decrease, but phosphorus concentrations, after a
decrease between 1992 and 1997, again started to increase.
According to saprobiological quality, the rivers
in the Gauja basin are relatively clean: 96% of rivers are
classified as clean to slightly polluted.
Similar to the country trend, the proportion of
treated waste waters (meeting standards) grew by two times since
1991.
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Salaca basin
The nutrient discharges from point sources in the
Salaca basin show less change than in other river basins in Latvia.
Since the early 1990's, the nitrogen and phosphorus discharges
from point sources have been rather stable.
The mean total nitrogen concentration had a
tendency to decrease, while the phosphorus concentration tended to
increase in recent years.
In general, the water quality of the Salaca River
is determined by Lake Burtnieks, which is considered to be
eutrophic (although insufficiently studied), and by runoff from
natural sources. The water basin has suffered from relatively low
human impact, in comparison with the other river basins in Latvia.
Small rivers in the Salaca basin can be considered
to be minimally polluted according to saprobiological quality. All
of the rivers can be classified as clean to slightly polluted, and
hence there are no moderately polluted to very polluted small
rivers in the Salaca basin.
Already at the beginning of the 1990's, a large
proportion of the waste waters released in the Salaca basin were
treated (meeting standards). An exception was in 1992, but
otherwise the proportion has been stable at over 90% during the
past 10 years.
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