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TAGUS ESTUARY |
Category I – Degree of Nutrient Enrichment |
|
Actual Situation
(2004) |
Criteria of Classification |
Partial
Classification |
Notes |
|
1. Riverine Total N and/or Total P inputs and direct
discharges (RID) |
Nitrogen
14300
(ton N/year) |
Description:
Based on 2004 monthly data field registers in
stations near the rivers discharge: |
 |
- |
- |
|
Tagus
River
8000 ton N/year |
|
Sorraia
River
1300 ton N/year |
|
Trancăo
River
1500 ton N/year |
|
Domestic
Load
3500 ton N/year |
|
2. Winter DIN and/or DIP Concentrations |
43 µmol N/l
(average)
60 µmol N/l
(Percentile90) |
Description:
Based on 2004 monthly data field registers in 24
stations located almost uniformly in all the estuary
(212 data field points) |
Background Value |
Elevated Value |
- |
Look for
complementary information in Additional
Information table. |
|
34
µmol
N/l |
51
µmol
N/l |
|
3. Increased
winter N/P ratio |
10
(average)
16.3
(Percentile90) |
Description:
Based on historical registers (658
field data points) |
 |
- |
. |
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|
TAGUS ESTUARY |
Category II –
Direct Effects |
|
Actual Situation
(2004) |
Criteria
of Classification |
Partial
Classification |
Notes |
|
1.
Maximum and Mean
Chlorophyll-a Concentration |
6.1 µg/l
(average)
7.2 µg/l
(percentile 90) |
Description:
Summer
of 2004
(170 data points) |
Background Value |
Elevated Value |
- |
- |
Look for complementary
information in Additional Information table. |
|
9
µg/l |
14
µg/l |
|
2.
Region/Area
Specific phytoplankton indicator species |
Diatoms are
the most important phytoplankton group with indicators
species such as Skeletonema costatum, Pseudonitzschia
spp. and Chaetocerus subtilis. |
- |
Not
available information on phytoplankton indicator species
shifts. |
|
3.
Macrophytes
including macroalgae (region specific) |
The study of macrophyte algae in the
estuary was carried out between 1985 and 1998. The main
substrates colonized by algae in the Tagus estuary were
old oyesterbeds located in the intertidal zones within
the mixing and seawater salinity zones. The brown alga
Fucus vesiculosus is the most abundant species
while fast growing species such as Ulva lactuta reached maximum values two times lower than
the Fucus maximum. A comparison of maximum algal
biomass for fast growing species in the Tagus estuary
with that obtained in typical euthrophic systems shows
that the maximum biomass for Ulva lactuta in the
Tagus estuary can be considered low.[2] |
- |
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OVERALL
CLASSIFICATION |
|
Category I
Degree of Nutrient
Enrichment |
Category II
Direct Effects |
Category III and IV
Indirect Effects/Other
possible effects |
Initial
Classification |
Appraisal of all
relevant information |
Final Classification |
|
- |
- |
- |
Non
Problem Area |
Modelling confirms that
the Tagus is a non problem area, because it is a
well-mixed estuary with a high dilution potential and
production is light limited. Consult Additional
Information and Discussion tables. |
Non
Problem Area |
|
| |
|
DISCUSSION |
|
In the Tagus
estuary the trophic level is limited by light
penetration due to the turbidity in the water
column, which is associated to the resuspension of
the fine sediments deposited in the intertidal
areas, by tidal currents and surface waves generated
by the long fetch of local wind. As a consequence, a
reduction of the nutrient loads discharged by the
rivers or by the Urban Waste Water Treatment Plants
(UWWTP) has no consequences for the trophic activity
in the estuary.[3]In conclusion, the Tagus estuary is a well-mixed estuary
with a high dilution potential and a moderate freshwater
inflow.
Nutrient inputs to the estuary are considered low with a
tendency to be even lower in the future.[2]
|
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ADDITIONAL INFORMATION |
|
DIN
and Salinity Distribution |
|
Figure 1 and 2 represent
spatial distribution of the properties computed by MOHID
Modelling System for the Tagus estuary. Figure 1
establishes the areas in which DIN concentrations are
below the background value, between the background and
the elevated level and the areas where the concentration
is above the elevated. In Figure 2 is shown the salinity
distribution in each area. The figures show clearly the
existence of three different zones in the Tagus Estuary:
seawater zone, mixing zone and tidal fresh zone. The
actual situation is however characterized by a large
range of values distributed in the estuary, with high
values of DIN where the salinity values are lower near
the river boundary and low values of DIN where the
salinity values are higher near the ocean boundary. This
interpretation leads to the conclusion that the
nutrients distribution depends essentially from the
rivers contribution.
Despite the average value
considered to apply the assessment criteria, it is
important to note that the model results evidence an
important gradient of DIN concentrations in the estuary,
characterize the actual situation with a large range of
values, between 0 and 100 µmol N/L.
Figure 3 shows DIN vs.
salinity curve based on the field data points measure
between 1994 and 1998 (historical data) and field data
points measure in 2004. The figure evidence a
linear relation tendency
between the two properties, which can be explained by the fact that the major DIN source (Tagus)
is also the major fresh water source. The discrepancies
from the major trend are associated to local DIN
discharges (UWWTP and other rivers). |
Figure 1 
|
|
Figure 2
 |
Figure 3
 |
|
Chlorophyll_a Distribution |
|
Figure 4 shows a time
series of average Chlorophyll-a between 1980 and 1999
and also 2004. The figure shows a high interannual
variability which can be explained by the number of
samples. The background concentration is defined as the
average of the oldest years (1980, 1981 and 1982), but
it is important to notice the existence of a high
variability between different years. This variability
can also be related with climatologic factors affecting
the river flows (Tagus river mostly) that will affect
directly the nutrients offer in the estuary and
indirectly the light limitation factor. Together with
these effects, variability can be also due to
variability of production together with tidal
oscillating transport and their influence on sampling.
Figure 5 represent the Chlorophyll-a distribution in the
estuary, according to the MOHID results. |
Figure 4
 |
|
Figure
5  |
|
Oxygen Distribution |
|
Figure 6 represents the
average distribution of Oxygen in the Tagus estuary,
showing concentrations above the limit for deficient
conditions, 6 mg/L. |
Figure 6  |
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[1]
2004 is considered as the actual situation because it is
the most recent complete year, having monthly measures.
[2]
Assumed
as 1980, 1981 and 1982 averages (the oldest years with
available data)
[3] Assumed
as 50% above the background concentration |
|
