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Nutrient
discharge from rivers
Nutrient
concentrations in water
Chlorophyll a
concentrations
Macrozoobenthic
organisms
Nutrient discharge from
rivers
The total nutrient discharge to the sea depends on
the discharge from rivers. Although the observed nitrogen
discharge from rivers may indicate a decreasing trend, the time
period of data collection is too short to be conclusive.
In contrast to nitrogen, the discharge of total
phosphorus has been stable or slightly increasing. One of the
reasons for increased phosphorus discharge may be intensive forest
harvest during recent years.
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Nutrient concentrations
in water
In the Baltic Sea and the Gulf of Rîga, nitrogen
and phosphorus concentrations in winter are the most suitable
indicators for monitoring of long-term trends, since the
concentrations at this time are stable.
Nitrate nitrogen concentrations in winter showed an increasing
trend during the 1980's, reaching a maximum in 1991. Subsequently
the nitrate concentration decreased and stabilized, but at a
higher level than in the 1970's. The dynamics of nitrate
concentrations probably reflect agricultural development:
intensification during the 1980's and a fall in production in the
1990's.
Trends in the winter phosphate concentration
differ from the long-term trends for nitrate. The phosphate
concentrations significantly rose at the end of the 1980's,
lasting into the 1990's when the nitrate concentration began to
decline. The intensification of eutrophication has apparently
caused changes in benthic processes. Eutrophication causes
increased sedimentation of organic material, which in turn
promotes microbiological decomposition. This increases oxygen
consumption in the bottom layer. When deficit oxygen
concentrations are reached, bound phosphorus is released,
increasing the concentrations in the water column (phosphorus pump).
A side effect of this process is an increased possibility of
blooming of toxic blue-green algae, which typically occurs when
phosphorus is abundant, utilizing nitrogen from the atmosphere.
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Chlorophyll a
concentrations
The chlorophyll a concentration in summer months
in the upper water layer represents the amount of nutrients
accumulated in living tissue. In general, a significant increasing
trend in chlorophyll a concentration in the Gulf of Rîga is
evident. The concentration stabilized and even began to decrease
in the 1990's, but the highest chlorophyll a concentration was
observed in 2000.
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Macrozoobenthic
organisms
Eutrophication has resulted in changes in the
distribution of macrozoobenthic organisms. At the beginning of the
1990's, the numbers of macrozoobethic organisms rapidly declined
and the species composition changed. These changes in the central
part of the Gulf of Rîga were probably caused by oxygen depletion
and hydrogen-sulphide accumulation in the upper sediment layers.
Along the coast, where the bottom oxygen concentrations are more
suitable, changes in the species composition have also occurred,
but numbers of individuals have not changed much, and even a
slight increase was observed during 1990's.
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