SADO ESTUARY

Category I – Degree of Nutrient Enrichment

Actual Situation

Criteria of Classification

Partial

Classification

Notes

1. Riverine Total N and/or Total P inputs and direct discharges (RID)

Nitrogen

3800

(ton N/year)

Description:

-

- 

Sado River and Marateca Channel

230 ton N/year

Industrial Loads

1300 ton N/year

Domestic Load

200 ton N/year

2. Winter DIN and/or DIP Concentrations

24 µmol N/l

(average)

52 µmol N/l

(Percentile90)

Description:

Calculated with available data since 1978 (206 data points in the winter)

Background Value[1]

Elevated Value[1]

-

Look for complementary information in Additional Information

21 µmol N/l

32 µmol N/l

3. Increased winter N/P ratio

5

 (average)

6

(Percentile90)

Description:

Based on historical registers (658 field data points)

-

.

SADO ESTUARY

Category II – Direct Effects

Actual Situation [1]

Criteria of Classification

Partial Classification

Notes

1.

 Maximum and Mean Chlorophyll-a Concentration

5 µg/l

(average)

7.1 µg/l

(percentile 90)

Description:

Summer (228 data points)

Background Value[2]

Elevated Value[3]

-

-

Look for complementary information in Additional Information table.

6 µg/l

9 µg/l

2.

Region/Area Specific phytoplankton indicator species

Diatoms are the most important phytoplankton group with indicators species such as Skeletonema costatum, Thalassiosira excentrica, Pleurosigma angulatum, Odontella mobiliensis and Chaetocerus subtilis.[1][6]

-

No shifts were observed in phytoplankton indicator species until 2000 [6]

3.

Macrophytes including macroalgae (region specific)

Submerged aquatic vegetation (SAV) occurs in the Sado, essenciatially around the Tróia peninsula (seawater zone). [3]

-

SADO ESTUARY

Category III and IV – Indirect Effects and Other possible effects

Actual Situation [1]

Criteria of Classification

Partial Classification

Notes

1.Degree of oxygen deficiency

7.6 mgO₂/l

(average)

5.4 mgO₂/l

(percentile 10)

Description:

275 data points

-

-

Look for complementary information in Additional Information table.

2.Changes/kills in Zoobenthos and fish mortality

Description:

No changes/kills in Zoobenthos and fish mortality have been reported in the literature

-

3.Organic Carbon/Organic Matter

Data Not Available

4.Other possible effects: Algal Toxins (DSP/PSP mussel infection events

Description:

No nuisiance or toxic blooms have been reported in the literature

-

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 Sado is a non problem area, because it is a well-mixed estuary with a high dilution potential and a moderate flushing potential. Production is nutrient limited. Consult Additional Information and Discussion tables.

Non Problem Area

DISCUSSION

Sado estuary is a well-mixed estuary with interannual high mixing and a moderate flushing potential, behaving at low flows almost like a coastal lagoon. Nutrient inputs to the estuary are low, with a tendency to be lower in the future.

In Sado estuary, the primary production is limited by nutrients and by the interaction between the phytoplankton and zooplankton. The residence time of the water, inside each part of the estuary, is of the order of one week, resulting into strong mixing between zones of the estuary, with deposition and mineralization of the particulate organic matter in the shallow intertidal areas [4].

The increase of percentage of treated wastewater in the estuarine area, mainly in Setúbal, will decrease nutrient inputs from these sources, since no significant population and industrial development is expected. Consequently, due to decreased future nutrient pressures, an improvement in eutrophic conditions and nutrient related symptoms in the Sado estuary is expected. [3]

ADDITIONAL INFORMATION

DIN and Salinity Distribution

Figure 1 represents the evolution of winter DIN Concentration measure in the Sado estuary in different years. Table 1 shows the number of data points considered to calculate de average concentration for each year. The low frequency number establishes the relative soundness of this analysis and must be the main reason for interannual variability. The tendency of the moving average suggests an increase of DIN concentration during 1982, 83 and 84 but, in fact, in these years the number of data points is low. The average value of the gathered data is 24.3 µmol N/L which, according to Table2, is below the elevated value considered for this estuary.

Similarly to the other estuaries, the value representing percentile 90 concentrations, resulting from the statistical analysis, is above the elevated value which can be justified by the MOHID results. Figure 4 represents the areas where DIN concentration values are above the elevated. As expected, these areas are found in Sado river channel (Alcácer Channel), pointing it as the main source of nutrients. Since primary production is nutrient limited, phytoplankton concentrations are higher in this channel. Due to consumption and especially to intense mixture with poor ocean waters, DIN concentration decreases strongly downstream. It is important to note that most estuary is characterized by DIN concentrations below the background value, 21 µmol N/L (Figure 2). Figure 3 shows the small area were model results are between the background and elevated values.  

The next plot (Figure5) shows the DIN vs. salinity curve.  In the Sado estuary the linear relation between the DIN and Salinity values are not so clearly as in the Tagus estuary. For the high salinity ranges it’s possible to observe a wide range of DIN concentrations that are probably associated with local anthropogenic inputs. These DIN inputs have cessed gradually with the implementation of treatment facilities, so this plot shows an historic situation that presently doesn’t occur. From figure analysis it is also possible to conclude some irregularity distribution in samples, since most part of the points are localized in areas with salinity above 34 psu.

Figure5

Chlorophyll_a Distribution

Figure 6 shows a time series of Chlorophyll-a Concentration. The time doesn’t show any explicit tendency for Chlorophyll-a evolution. Table 3 shows the number of data points used in the annual average calculation, showing that the number of samples is always below 30. This fact, together with the natural variability, explains the essence of trend. Comparisons of these values with the model results show that they are representative of the average concentrations in the estuary.

The average value for actual situation (5 µg/L) is less then the background value, which means that there are no indications of eutrophication. In fact, according to Table 4, 90% of the data field register are below the elevated value.

The next collection of figures represents the spatial distribution of Chlorophyll-a according to MOHID model.

The highest values of Chlorophyll-a concentration are found in the upstream zone, Alcácer Channel (Figure 31), where nutrient availability is higher. Downstream, next to the boundary, the concentrations are below the background concentration, essentially as a consequence of nutrients limitation. Areas where Chlorophyll-a is between background and elevated value are localized in central zone of the estuary, Figure 30. Like in Tejo and Mondego estuaries, the model results for Sado indicate a wide range of Chlorophyll-a values (between 2 and 17 µg/L).

Figure5

Oxygen Distribution

Figure 6 represents annual average of oxygen spatial distribution, in the Sado estuary as a result of MOHID model simulation.

The concentrations in the interior of the estuary are always superior to the threshold value for deficient in oxygen (6 mg/L), which means that in terms of oxygen the Sado estuary do not show any symptom of eutrophication.

Figure6