Överkörd Natur

Lac de Tunis

Sven Björk

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Sven Björk Photo

Welcome to
The Lake of Tunis
Lac de Tunis

The project financed by SIDA
Swedish International Development Cooperation Agency.

Introduction

Experiences gained in the Swedish restoration projects - with sediment pumping, aeration and vegetation removal - were all useful in drawing up plans for the restoration of the Lake of Tunis (42.2 km2). The city of Tunis had long suffered from the bad breath of the polluted lake. Until the 1970s the stench was most repelling close to the population center, and it was there that the Tunisian authorities wanted to create an attractive environment. The planning of the Lake of Tunis restoration project was in 1972 organized and sponsored by the Tunisian Government in cooperation with the Swedish International Development Authority (SIDA) and the Institute of Limnology at Lund.

The City of Tunis is surrounded by very shallow, highly saline lakes. As the Lake of Tunis is connected with the Mediterranean, it does not dry up as the other lakes do. The water level, however, is dependent upon that in the sea.

1989

The Lake of Tunis - the al-Bahira lagoon - is divided into the North and South Lakes (Lac Nord and Lac Sud). Increasing volumes of raw sewage and increasing amounts of industrial waste (from canneries, chemical industries, metal factories etc.) water have been discharged into the North and South lakes, respecitively. Both lakes were polluted by oil and the North lake was influenced by discharge into its southeastern corner of coolig water from the STEG power plant.

The North lake is the object of the restoration project dealt with here. It has an area of 29.5 km2, but contains only 27.6 million m3 of water (mean depth ca 0.9 m). At the beginning of the 1970s the westernmost part of the lake was filled with sewage sludge up to the water surface. The gas production was intensive in these sediments.

A navigation canal running E-W divides the Lake of Tunis into Lac Nord and Lac Sud. The canal connects the Mediterranean Sea at the harbour of La Goulette with Tunis Marine, an artificial harbour excavated in the westernmost part of the lagoon. The North Lake is connected to the Mediterranean Sea through the Khereddine Canal and the Radets Canal is the corresponding connection of the South Lake. Between Lac Nord and the Navigation Canal there are 4 main openings with fisheries. The South Lake has altogether 7 such connections.

The following description of the structure and function of the ecosystem of the Lake of Tunis is based on investigations made by Björk and co-workers during the period 1972-1978. On the request of the Ministère de l'Agriculture in Tunis these studies were performed with the aim to elaborate a plan for the restoration of the lake. The plan for the North Lake was presented in 1973 (Björk 1973). During the initial phase of the project the technical realization of the plan was followed by Björk, who also delivered a plan for the restoration of the South Lake (Björk 1978).

In the 1970s the north lake was still receiving raw sewage to the westernmost part and effluent from a treatment plant to the northwestern part. In addition to the sewage discharge into the lake an uncontrolled disposal of solid and liquid waste took place around Lac de Tunis. Thanks to the high salinity of the water (specific conductivity ranges from about 45 mS20 in early June to about 65 mS20 in early August), the coagulation and precipitation of particles was good. The sewage sludge deposits were, therefore, concentrated to restricted areas.

Lac Nord the western end with sewage outfalls close to the Espalande. 1973
Pictures from the
 nortwest part of Lac Nord.
Accumulation of sewage sludge in the Esplanade basin. 1973
 
Unregulated dumping of solid waste along the shores of Lac Nord. 1973
 
Outlet of a drainage ditch at teh nortwest portion of Lac Nord. 1972

The self-purification capacity of the lake was extremely good. This was apparent from the compressed zonation and steep gradients of environmental conditions and organism communities east of the outlets. In the summer the zones of bacteria and phytoplankton were typically very narrow in the westernmost part, while the water in the rest of the lake was clear. In the clear area, luxurient meadows of Ulva (sea lettuce) covered the bottom from shore to shore (Enteromorpha was also common), with the exception of the eastern portion where brown algae were numerous. Here also open bottom areas that consisted of pure shell gravel appeared. Reefs of Mercierella enigmatica (tube worm) were common. These reefs were problematic from a hydrological point of view. However, the animals perform a filtration function, and thus play an important role in the lake's self-purification process.

The direct discharge of nutrients and the release of nutrients from the sewage sludge deposits speeded up the growth of Ulva and other algae in the warm, shallow lake, and conditions were similar to those prevailing in a very effective algal culture. Due to the efficient photosynthesis, gas bubbles formed in Ulva leaves, and large amounts were set afloat. Thus, green mats of loosened algae periodically covered vast areas of the water surface. When the weather was warm and the water stagnant, the crop of Ulva and other algae decomposed and the water became oxygen-deficient. As the consequences of primary and secondary pollution, a nasty stench of raw sewage and hydrogen sulfide was apparent, and fish kills occurred. A zone of no-man's land developed around the western portion of the lake. At times the waters of the Lake of Tunis became wine-red, and it looked as through the lake has been visited by one of the "seven plagues". This wine-red color was caused by the mass development of planktonic microorganisms, a common phenomenon in salines and highly saline lakes in North Africa.

 
The shore at the northeastern part of Lac Nord
 

The water

The supply of surface water and sewage from the area of the City of Tunis reduced the salinity in the westernmost part of Lac Nord. Heavy rains caused flooding in the city and an intensified supply of solid material which settled in the lake.

The massive discharge of sewage resulted in very high concentrations of nutrients in Lac Nord. However, the extremely good self purification capacity of the water body was illustrated in the steep nutrient gradients from east to west.

Wind induced water currents in alternating directions occur regularly in the connections with the Mediterranean Sea and the Navigation canal. Thereby sea water very poor in nutrients moved back and forth through the Khereddinde Canal as did heavily polluted water through the Tunis Marine connection between the harbour and the lake.

The lake shores were besmeared by oil emanating from the city area, the harbour and the power plant STEG.

The sediment

The natural, characteristic bottom substrate is clayey and sandy soils and shell fragments. Outside the outlets of sewage in the western part of Lac Nord concentrated deposits of sewage sludge had accumulated. Also in calm water areas, as among the reefs of Mercierella a top sediment rich in organic matter occured. However, in relation to the total bottom area the distribution of sewage sediments was small.

The concentrations of nitrogen and phosphorus were very high in the sewage sediment and the concentration of ammonia extremely high in the interstitial water. As to the organic top sediment layer, several substances showed steep concentration gradients from west to east.

The exchange of substances between sediment and water was investigated under aerobic and anaerobic conditions (laboratory experiments). The studies revealed that very large quantities of ammonia and phosphate were released from the top sediment to the water. Thus, it was estimated that 500-900 mg NH4-N and 25-30 mg PO4-P were added to the lake water per day from a sediment area of 1 m2. In laboratory experiments the concentration of ammonia nitrogen in Lac Nord water increased to 40-70 mg/l within two weeks. As to the oxygen concentration in the water, the heavy release from the sediment of phosphate took place under anaerobic as well as under aerobic conditions. Under prevailing high pH conditions and intensive formation of sulphide in the sediment, iron did not play any role for either the precipitation or the release of phosphate.

In the thick deposits of organic matter in the western part of Lac de Tunis there were, with respect to the release of phosphate, considerable differences between the top layer and layers deeper down. Thus, from the superficial layer twice as much was released as from the layers at 100-150 and 150-200 cm. It is characteristic for recipients which gradually become overladed that the concentrations of nutrients and other substances increase upwards in the sediment.

Through the intensive gas ebullition the concentrations of nutrients in the water increased still more as interstitial water was mixed with lake water.

Characteristic features in the development of organisms

The steep W-E gradients described above for the environmental conditions corresponded to an, at least seasonally, very obvious zonation of organism communities. In the western part a zone of some 100 m with bacterilogical metabolism dominated. Eastwards this was followed by an intermediate zone where fermenting organic sediments still ocurred but where the turbid grey water also became greenish in colour from phytoplankton. Moving further eastwards single specimens of Enteromorpha appeared. This and Ulva became more and more abundant and in the main portion of the lake especially Ulva covered the bottom nearly totally. In the easternmost part marine brown algae occurrred and fairly large areas of the minerogenic bottom (often shell deposits) were free of vegetation. In the zones with Ulva and brown algae the water was very clear with a transparency much greater than the water depth.

The young growing Ulva and Enteromorpha are fixed to solid objects on the bottom. Through the photosynthetic activity gas bubbles are formed in the thallus of both of these macro-algae. As they grow older they loosen from the bottom and start to flotate at the water surface. They are then easily transported by wind-induced currents and accumulate along exposed shores. Periodically very large areas were covered by floating masses of algae. The water beneath the algal cover became depleted of oxygen and the formation of hydrogen sulphide could occur.

In areas made very shallow by the deposition of sludge, the bottom was covered by mats of microscopic algae. Oxygen bubbles, formed through photosynthetic activity, caused the loosing of lumps of algae with attached sediment which then drifted at the surface of the water. In this way considerable amounts of organic matter and nutrients were also dispersed over the lake from the portions severely affected by pollution.

A most conspicuous structural element in the lagoon ecosystem was the polychaete worm Mercierella enigmatica, occuring in mushroom-like reefs. As very efficiant filters the worms played a significant role in making the selfpurification capacity of the lake so high during the greater part of the year.

The fish production in Lac de Tunis was high and fishing for eel was of considerable economic value. However, in the highly polluted lake massive fish-kills suddenly occurred, especially during the late summer-early autumn period. Calm weather, high temperature and copious amounts of degradable organic matter (algae etc.) were the prerequisites for sudden oxygen depletion.

Especially at the time of rapid degradation of Ulva, periods with red water (eau rouge) occurred. As in salines, the red colour in the lake was caused by red sulphur bacteria.

The problems

Although the self-purification capacity of Lac Nord was very high during most of the year, the ecosystem of the whole lake collapsed periodically (oxygen depletion, fish-kills). Furthermore, during nearly the whole year the odour of sewage and hydrogen sulphide from the permanently highly overloaded western part was a most unpleasant environmental embarrassment in the environs of the City of Tunis. The area bordering the lake was really a no-man's-land impossible to use for human habitation. Instead it was used for the unregulated deposition of rubbish and, generally speaking, was in a deplorable state of degradation.

The fish catches from the overloaded, periodically collapsing lagoon ecosystem were successively reduced. This had economic consequences and the smell after severe fish-kills aggrevated the odour problems. The bad smell from the lake could even be noticed at the Tunis international airport, Carthage, thus discouraging tourists from visiting the capital area.

In the early 1970s the situation was so bad that it was decided to make efforts immediately to alleviate the worst problems consequent on the pollution.

The restoration plan for Lac Nord

The first phase of the restoration had to be carried out during a period when the sewerage system and the existing primitive treatment plant were being put in order. Although the selfpurification capacity of the lagoon was very high, Lac de Tunis should not be used as a receiver of sewage and industrial waste water. Instead the capital, built on the shore of the lagoon, should have access to a water esthetically attractive and enjoyable. Lac de Tunis had alredy in the early 1970s been included in the CW-list, i.e. selected for protection as a lake/wetland of utmost value for waterfowl (CW is the abbreviation of Conservation of Wetlands).

The goals for the measures to be taken immediately in Lac Nord were mainly to:

  1. avoid the nasty smell,
  2. improve the lagoon's efficiency as a receiver of sewage during the years it still had to be utilized as such,
  3. avoid fish-kills,
  4. constitute phase one in a systematically realized plan for a definitive restoration of Lac de Tunis.Thereby prerequisites should be created partly for an attractive environment and close contact with the water in the westernmost part closest to the capital, partly to ensure the high value of the lagoon as a fish-producing water.

The methods recommended were:

  • a. suction dredging of sewage sludge sediments,
  • b. construction of settling ponds for temporary treatment of sewage,
  • c. removal of floating big algae,
  • d. aeration,
  • e. direction and control of water flow through the lagoon,
  • f. restoration of the littoral zone (deposition sites for solid waste etc).

I. Suction dredging

Sewage sludge sediments deposited outside Esplanade (sludge area 720 x 300 m, sludge volume 300.000 m3), Montplaisir-Borgel (15.000 m3) and Cherguia (45.000 m3) should be pumped to settling ponds on land, constructed close to the sections from which the sludge is to be removed. Nothing but suction dredging should be allowed. When pumping sludge from the lake especially two demands have to be met, viz.
        1) that no turbidty - a sign of addition of nutrient-rich interstitial water to the lake water - should be allowed, and
        2) that the intermixture with lake water should be as small as possible.

The run-off water from the settling ponds should be treated for nutrient reduction before it is allowed to return to the lagoon. Laboratory studies to clarify the best treatment method for the run-off water indicated that addition of calcium oxide (CaO) was the most appropriate in this case. The utilization of possibilities to strip ammonia in connection with the treatment (adjustment of pH) was recommended. Furthermore, the possibilities to use the dried and naturally desalinated sediment for different purposes should also be investigated locally.

Because the shores have been exploited for the deposition of solid waste, a contour with peninsulas and bays had been created. In connection with the dredging and the construction of settling ponds the peninsulas should be removed to create a smooth contour in order to avoid the accumulation of organic material in the bays, which are not allowed to be used for the deposition of dredged sludge.

II. Removal of algae

In cases when microscopic algae are responsible for the primary production of organic matter no practical methods were so far available for the removal of algal material in order to reduce secondary oxygen demand and recycled nutrients.

However, in this respect the Lake of Tunis offered possibilities for the regulation of biomass quantity, thereby also reducing the amount of recycling nutrients. It was however, necessary to design new equipment for harvesting floating macroalgae under the special conditions characterizing Lac de Tunis (very shallow water, sloping shores, frequent occurrence of Mercierella reefs nearly reaching the water surface and often with a diameter of the more or less fat top disc of some meters). It was also stressed that none of the important ecosystem elements (Ulva, Enteromorpha, Mercierella) should be allowed to be eliminated or reduced to such an extent that the function of the system was jeopardized. All risks of causing a shift from macro to microalgae as quantitatively dominating primary producers should be avoided.

The thickness of the layer of drifting algae, of course, depended on the force and duration of the wind. According to investigations on 29th July, 1972, at the Checkli fishery station, the weight - Ulva after 2 min. drainage of water - per m2 amounted to 3.5 kg and in areas with Ulva mixed with much Enteromorpha to 7 kg. In both cases the floating layer had a thickness of about 30 cm schematically illustrates the gain for the lake ecosystem that is made when one load of Ulva (ca. 2.000 kg, wet weight) is removed from the lagoon. According to this example the phosphorus content of one load (0.24 kg P) corresponds to the phosphate-phosphorus content in ca 4.700 m3 water at the time and site for sampling.

III. Aeration

In order to avoid anaerobic conditions bubble hoses should i.a. be installed at sites which are of special importance from a fishery point of view. These bubble systems should only be run during periods with critically low oxygen conditions.

IV. Direction and control of water flow through the lagoon

The flow of water through Lac Nord should be directed in such a way that nutrient-rich water be transported from the lagoon and replaced by nutrient-poor water from the Mediterranean Sea. When directing the flow advantage should be taken of the mainly wind-induced and the lake's very characterstic water-level fluctuations. Only in-flow should be allowed at Khereddine and only out-flow through other connections between Lac Nord and the harbour Tunis Marine by way of the navigation canal. The connections closest to La Goulette should be closed. The quality of the water flowing into Lac Nord through Canal Khereddine should be carefully checked and should not be allowed to degrade. Canal Khereddine should be widened from its present 24 m to at least 40 and preferably 45 m. The depth should be kept at 1,5 m. Considering the small depth of the lake no greater depth is necessary in the connection with the Mediterranean Sea.

The inflow at Khereddine and the outflow at Tunis Marine and the Checkli fishery should be regulated by means of automatic water gates. The sensitivity of the gates should be so high that even small differences in water level can be made use of for water flow-through.

V. Restoration of the littoral zone

Considerable sectors of the shores at Lac Nord as well as at Lac Sud were heavily exploited for the unregulated dumping of solid waste, latrine and other liquid waste. This type of waste deposition means lasting damage to the lake through the release and leakage of nutrients and the risk of the introduction to the ecosystem of poisonous substances. A plan including the whole lake and its shores should be worked out in order to clear the areas from solid waste and to stop all dumping with negative effects on the lake.

The industrialized area west of Lac Sud. 1972
Pictures from an ecological
West  -  east transelt
through Lac Nord
Wasre water outfall at the westernmost part of the lake to the right. 1972
Vuews of the shores of the western part of Lac Sud 1972 resp. 1974
 
 
 
Vuews of the shores of the western part of Lac Sud 1974

VI. Oil spill barriers

In order to stop the inflow into Lac Nord of polluting oil from the Tunis Marine harbour, from still existing waste water outlets etc, oil spill barriers, pneumatic and booms, should be installed. Strict control of all potential polluting industries, ships etc. should be enforced by law to prevent pollution.

VII. Ecological control of technical restoration measures

The very serious environmental problems concerning Lac de Tunis are caused by an unlimited exploitation in such a way that the balance in the ecosystem of the lagoon has been destroyed. It is possible to eliminate the problems only provided that careful attention is given to the ecological relationships. The technical restoration measures should be ecologically correct. A continuos ecological control is necessary within the frame of an ecological-technical cooperation including, among other things, negotiations concerning possible modifications of methods.

The sewage zone closest to Esplanade 1972.
An ecological west-east transekt through Lac Nord
Ebullation of methane from the sewage sludge east of Esplanade basin. Algae make the water greenish 1972.
Futher eastwords in Lac Nord the thickness of the sewage sludge sediment is reduced. The sludge is deposited on the original clayey marine bottom. Intensive water bloom 1972.
In the middle of Lac Nord sea lettuse (Ulva lactuca) dominates and also Enteromorpha appears 1972.
Initially the big algae are fixed to the clean minerogenic bottom free from organic sediment. As they grow bigger large amounts are set afloat 1972.
 
The filtration of the polychaete worm Mercierella enigmatica, occuring as mushroomlike reefs, increase the selfpurification capacity of the Lac Nord ecosystem.
 
In the eastern portion of Lac Nord the water mostly cleasr. MArine brown algae characterized the flora 1972.

Accomplishment, ecological aspects

The whole project was extended over a much longer time period than originally planned. Several measures have been taken and changes were made without ecological sanction and control.

1. Suction dredging

The settling ponds were built rapidly. However, without any forewarning, the construction of a highway started across part of the pond area. Arrangements were made to compensate lost capacity.

In the selection procedure for final acceptance of a contractor for suction dredging no ecological-technical negotiations were taken up. Among ecologically troublesome problems in connection with the dredging were the strong turbidity caused by the machines and the large amounts of pumped water. Furthermore, the treatment of the run-off water from the ponds to the lake was most unsatisfactory. This again caused an increase in the nutrient loading of the lake still in use as a receiver of sewage. From the westernmost part of Lac Nord (at Esplanade) sewage sludge was removed by means of a dragline which also caused severe turbidity.

The increased loading of Lac Nord by nutrients and organic matter implied the risk of increased primary production and periodically severe fish-kills following oxygen depletion. During at least one risky period, windy weather and fairly low temperatures prevented an ecosystem collapse.

2. Removal of algae

As no workshop for the maintenance of the prototype harvester was available, the machine degraded due to corrosion etc. caused by the aggressive saline water. Algae were to some extent removed manually and deposited along the shores.

3. Aeration

Among the suggested aeration sites to prevent fish-kills during critical periods, the one at the Checkli fishery was installed and used.

4. Direction and control of water flow through the lagoon

The widening of the Khereddine Canal was performed according to plan. It included, among other things, difficult undertakings concerning elongation of heavily trafficked road an railroad bridges.

Automatic water gates were installed in the inlet for sea water at Khereddine and at the two outlets for lagoon water at Tunis Marine and the Checkli fishery.

By these arrangements a very important renewal of nutrient-poor water will be secured. As the wind is the main driving force for the water level fluctuations made use of in this system, the renewal is brought about in the cheapest possible way. However, it is of utmost importance that the water gates are carefully maintaned in order to function properly also with minor changes in the water level.

The arrangement should not be looked upon as a simple hydromechanical measure. Instead it is a means of creating a suitabe nutrient budget in order to control the productivity of the lagoon ecosystem.

5. Restoration of the littoral zone

In 1980 District de Tunis worked out plans to stop and control unregulated deposition of solid waste along the shores.

6. Temporary treatment of sewage

The ecological plan included interim treatment - sedimentation, aeration, possibly ammonia stripping - of sewage in two basins constructed on either side of the Cherguia outlet in the lake. The basins were built but were not used.

7. Oil spill barriers

Barriers to protect the lake against oil pollution were installed in1976 at still existing sewage outlets at Esplanade. Collected oil was initially deposited along the shore.

8. Ecological control of technical restoration measures

Until 1980 a considerable number of serious changes in and deviations from the restoration plan were made without preceding negotiations among administrators, ecologists and technologists. It is quite clear that a project of this type needs continuous ecological supervision to avoid risks and to make co-ordinated efforts efficient for the environmental improvement. A frank and close cooperation of all administrative units in the treatment and utilization of the lake would also have been higly valuable for the project, not least with respect to co-ordinated planning and reduction of costs.


The periodical collaps of the ecosystem

The combination of huge masses of drifting Ulva and Enteromorpha, high temperature and calm weather resulted in oxygen deficiency within large areas of Lac Nord  1972.
Oxygen deficiency and oil pollution often caused severe fishkills Foto Wilh Ripl 1973.
Especially at the time of rapid degradation of Ulva and Enteromorpha, periods with red water (Eau rouge) occured in Lac Nord 1972.

Concluding remarks

In spite of all the delay in the accomplishment of the restoration plan, considerable environmental improvement was achieved. It is also of great value that the restoration project acted as an incentive for environmental protection in Tunisia. Big investments for improved sewage treatment, preparative work for sewage diversion from the lake etc. have now been made. It was also decided to construct a canal along the shore from Monplaisir via Esplanade to Tunis Marine to collect stormflow water and to pump it to the navigtion canal. Unfortunately the location of the canal hinders direct access to the lake from the downtown area.

In the earaly 1980s the achieved improvement of the environment initiated large-scale planning for the economical exploitation of the zone around Lac Nord which, due to the former degraded state of the lagoon, was a no-man's-land. This udertaking was a joint Tunisian-Saudiarabian project. The lake - included in the CW-list - will certainly be influenced by all the construction activities on its shores.

Already before these exploitation works started a dike was built across the lake from Cherguia via the island of Checkli to Tunis Marine. In order to prepare the low land close to the lake for the construction of buildings, minerogenic sediments were pumped from the lagoon to the shores. Furthermore, on different occasions digging machines have been used to create deeper portions, as well as islands at fishery stations, and Mercierella reefs have been cleared. These measures have not been ecologically planned or sanctioned and the effects on the ecosystem have not been recorded.

There is no doubt that proper ecological redevelopment and management should permanently have transformed the lagoon to a highly esteemed environmental asset. On the other hand, it is equally clear that clumsy technical encroachments and ecological carelessness would spoil its potential value for the capital of Tunis. A holistic view including ecological-technical synthesis is necessary to bring about environmental redevelopment.

Restoration methods

Methods for correcting the balance of the Lake of Tunis ecosystem, especially that of the nort lake

A. Suction dredging of sewage sludge from the basin closest to Tunis city; treatment of run-off water
B. Aeration of basin until the sewage has been diverted from the Lake.
C Suction dredging of sewage sludge outside the sewage outlets at Montplaisir and Cherguia, treatment of run-off water.
D Until the sewage has been diverted from the lake, the waste from the Cherguia treatment plant should be collected in two simple ponds constructed on both sides of the formeroutlet pier. These ponds should be areated (D1)
E. By means of lock gates - regulated by water level differences - water is taken in form the Mediterranean and led out to the Tunis harbour and the navigation canal.
F. G. Pontoonequipped and amphibious skimmers remove the floating masses of algae (Ulva and Enteromorpha) Oil barriars should be installed to prevent oil pollution from entering the lake from the harbour and the canal. The total lake area is 42.2 km2 

Pumping of sewage sludge from the Esplanade basin to settling ponds. 1972.
Wind-induced inflow of nutrient poor (ca 20 μg 1-1 PO4-P in July - August 1972) from the Mediterranien through the automatic lock gates installed at the Canal Khereddine  1979.
Wind-driven outflow of water from Lac Nord at Tunis Marine to navigation canal. In July-August 1972 thes water had a PO4-P concentration of 400-700 μg 1-1. 1979.

Further reading

Björk, Sven. 1972. Swedish lake restoration program gets results. - Ambio 1.
Björk, Sven. 1973. Projet de restauration du Lac de Tunis. Etude exécutée sur la demande du ministère de l'agriculture, Tunis. - Institution de limnologie, Université de Lund, Suède.
Björk, Sven. 1978. Projet de restauration du Lac de Tunis, Lac Sud. Rapport No. 2. Etude exécutée sur la demande du Ministère de l'agriculture, Tunis. - Institution de limnologie, Université de Lund, Suède.
Hermansson, Lars. 1980. Restaurering av Tunissjön. Historik och evaluering. (Restoration of the Lake of Tunis. History and evaluation). - Report for SIDA (Swedish International Development Authority.

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