Lake Trummen, Sweden
In Sweden, thanks to the intensive construction of sewage-treatment plants with phosphorus precipitation, the conditions in hundreds of eutrophicted lakes have improved. Nevertheless, some lakes had been so severely affected that they did not recover.
General description and type of disturbance of Lake Trummen
Lake Trummen (56o52' N, 14o50' E) at the town of Växjö in south-central Sweden was a good example of the large group of polluted lakes which had deteriorated within and around urbanised areas. The originally oligotrophic, brown-water Lake Trummen (drainage area 12 km2, lake area 1 km2 and depth, until 1970, 2 m) had been used for swimming and for water supply, at least to some degree, until the 1920s. Up to 1957-1958, it was exploited as a recipient of sewage from the town of Växjö and, from 1941 to 1957, of wastewater from a flax factory.
An extensive survey concerning the human impact on the lake during the nineteenth and twentieth centuries has been compiled by Lettevall (1969, 1977). The pollution became more and more severe and in the 1940s the hypertrophic ecosystem collapsed. After that there were regular fish-kills due to total oxygen depletion in winter and the summer transparency depressed to 15-20 cm by cyanobacteria ('blue-green algae'), especially Microcystis.
Despite that the sewage and industrial wastewater discharge to the lake was stopped in 1957-1958, the lake did not recover - a very interesting but not surprising fact to limnologists and a severe problem for the town authorities. About 10 years after the diversion of wastewater from the lake, the summer transparency was still only 15-20 cm. At this point, the town authorities definitely had to face the Trummen problem when planning the urbanisation of the surroundings. Of course, the simple, narrow-minded technological solution of filling the lake was considered, but the authorities decided to follow a proposal to restore the lake. Later, the restoration project developed a very successful coopertion between limnologists, palaeolimnologists and other ecologists, politicians, technologists and administrators. The scientific pre-project investigations carried out during 1968-1970 were designed as a typical ecosystem oriented study.
Originally Lake Trummen had the same ekological character as most forest lakes in Central South Sweden; i.e. oligotrophic (poor in nutrients and low production) with brownish water.
Palaeolimnological investigations have revealed that in the intact, oligotrophic Lake Trummen, the sediment growth rate was betweeen 0.2 and 0.4 mm per year, but due to the very high nutrient supply during the recipient period it increased to 8 mm (Digerfeldt 1972). In the 1960s the sediment of Lake Trummen showed a very characteristic stratification - a brown, well-consolidated gyttja overlaid by the 20-50 cm of black, loose deposits from the recipient period (Björk and Digerfeldt 1965). The irreversibility of the damage to Lake Trummen was due to the presence of that layer from which nutrients were being released (Bengtsson and Fleischer 1971, Bengtsson et al. 1975) and to which the plankton crop was being deposited after every vegetation period. Broad reedbeds, developed as a floating plaur zone, surrounded the lake.
Degradation of the shores of the heavily polluted Lake Trummen 1969
In 1970, the topmost half metre of sediment was removed by suction dredging, and in 1971, another half metre was dredged. Altogether about 400,000 m3 of sediment (gyttja) were removed during nine working summer months. Furthermore, at least 200,000 m3 of lake water were mixed with the pumped sediment. The limnlogists requested a nozzle for dredging which would make it possible to suck in the sediment without making the lake water turbid, and with a minimum intake of lake water.
The sediment was pumped to simple, settling ponds constructed in an abandoned farming area from which the topsoil had first been removed. The run-off water from the settling ponds - a mixture of lake and interstitial water - was treated with aluminium sulphate in a simple plant for precipitation of phosphate and suspended matter. Before restoration, the total phosphorus content of the lake water was about 600 μg l-1. The phosphorus concentration of the water from the settling ponds was of the order of milligrams per litre before the treatment. After the pricipitation, the total phosphorus content of the run-off water was about 30 μg/l.
The pre-project investigations also comprised the growth test with the sediment to prove its suitability for agricultural and horticultural purposes The pumped sediment was, after drying, sold by the town to be used in parks, gardens, roadsides etc. The income financed the preparation of green areas around the lake. A separate overgrown bay was left as a waterfowl reserve (an observation tower was constructed on the shore), and an artificial island was built for the birds.
Overall project evaluation
As was forseen, the changes in the lake itself were dramatic (Cronberg 1982). The phosphorus and nitrogen concentrations decreased and the transparency increased. The cyanobacterial blooms disappeared, and a plankton community with a higher diversity replaced the monocultures of Microcystis. The population of the freshwater mussel Anodonta that was wiped out in the hypertrophic lake, recolonised the bottom immediately after dredging. The reappearance of the mussels means that the ecosystem became enriched with their function of an efficient seston filtrator. The restoration caused such a change in the structure of the ecosystem that the lake recovered to a functioning unit, characterised by a balance between its production and decomposition.
The plancton community
became suitable for sport fishery, swimming, windsurfing and other forms of
recreation within the urbanised area. Only nine summer months were needed to
transform the lake from an environmental problem to an environmental asset. The
total cost was about US$ 500,000 (in 1971). The Lake Trummen restoration project
was used for the training of professional limnologists and other ecologists (about
20 have been involved). It also served as a demonstration project for converting
politicians and administrators to believers in ecology and nature conservation
offering sustainable, high quality environmental conditions.
Management and monitoring programmes
Following the highly significant nutrient reduction through the sediment removal, intensive fishing was carried out (during the period of 1976-1979) to reduce the populations of bream (Abramis brama) and roach (Rutilus rutilus). Altogether 16 ton, i.e. 190 kg ha-1 of bream and roach were removed, while predators, such as pike (Esox lucius) and perch (Perca flviatilis), were left in the lake (Andersson et al. 1978, Andersson 1985). There is hardly any doubt that the synchronous decrease in total phosphorus and nitrogen concentrations as well as in phytoplankton biomass was caused by this intensive selective fishing. It has, however, not been possible to obtain stability in these respects. Reduction of planctivorous fish took place also in 1994, 1996-1997 and 2000. In the case of Lake Trummen there is a considerable exchange of fish between the lake and its tributaries and outflow including the lakes located upstream and downstream. The occasional development of a food resource, such as large planktonic animals, is utilised by both invading and in-lake reproduced fish.
In a eutrophicated lake, after reduction of, for example, bream and roach, the eventual appearance of a valuable food resource (cladocerans, etc.) will not be left untouched and unexploited. It again opens up the possibities for the prosperous development of consumers (fish) that will reproduce successfully until their biomass becomes the food resource of a higher consumer or is reduced through laborious fishing. The oscillations might be great in disturbed systems, but the base of the trophic pyramid will always remain broad and the top narrow. If a snapshot of biomass distribution in the open water should happen to show upside-down relations, these are of limited duration only.
It should be stressed, that Lake Trummen was restored by suction dredging after sewage and industrial wastewater had been diverted from the lake. The external loading could, therefore, be considered as fairly normalised with respect to regional limnological conditions. However, since the restoration, the surroundings have been urbanised, i.e. the character of the catchment area has changed considerably.
In the limnological management plan for the restored Lake Trummen, it was suggested to make arrangements for continuous and careful control of the water quality in the tributaries. Among other things, construction of basins for the reduction of nutrients, and the protection against oil spill from industrialised sections of the catchment were recommended. The suggested arrangements have in course of time largely been realised.
Changes in the lake ecosystem following the restoration were studied by a research programme covering the period 1968-1980. Since then, the lake development is followed as part of the routine environmental control programme executed by the local authorities. Ten to 15 years after the restoration of Lake Trummen, there appeared indications of increased plankton turbidity caused by tiny cyanobacteria (among these Cyanodictyon imperfectum) decreasing the transparency. In order to revert to the restored stage, it is necessary to reduce the external loading from the recently urbanised part of the catchment area, including nutrient trapping at the mouths of the tributaries.
Removal of sediment by suction dredging
Experience gained during the 1960s and 1970s from sediment suction-dredging projects made it clear to limnlogists that the dredging technique must meet the following three demands:
These requirements were fulfilled by the introduction of the automatically controlled suction dredging technology, developed in a limnological-technical co-operation.
Automatically controlled suction dredging technology
In principle, an automatically controlled suction-dredger consists of three parts: the float, the nozzle, and the automatic control system.
1. The float is designed in the simplest way. Its size is determined by local conditions (size of the lake, waves, transport conditions, etc.). It is imperative that the size of the dredger corresponds to the size of the actual water body in order to optimise transport and installation costs.
2. The nozzles developed for soft organic sediment include pump and grid for protection against stones. (Primarily, the sediment [gyttja] to be removed does not include stones. However. stones, ammunition, bicycles. etc., have often been thrown into lakes, especially in urbanised regions). For other types of sediment, a special design and function of the nozzle is needed to fulfil the limnological demands.
3. The control system consists of measuring devices from which signals are continuously transmitted to the unit directing the position of the nozzle that pumps a predetermined concentration of sediment, the speed of movement of the dredger, its location on the lake, etc. The measuring devices of the automatically-controlled sediment pumping system include reading of water depth, density-, flow- and other meters for recording physical and chemical parameters of pumped sediment and sediment/water mixtures. Cf. Björk 1985, and Björk, Pokorný & Hauser 2010.
The restored Lake Trummen
Lake Trummen illustrating the possibilities of sientific research and ecological team-work, training of limnologists and development of lake restoration methods in connection with the implementation of a lake restoration project.
The transport of Phosphorus (indicated by arrows) from the catchment area for deposition in the sediment during four periods. The figures within the lake denote lake depth, rate of deposition ande the amount of phosphorus per annual deposition. Compiled on the basis of data from Gunnar Digerfeldt, doctorial thesis 1971
Improvment of suction dredging tecnology based on experiences from the Lake Trummen restoration project
After the normalization of the external and internal nutrient loading on the Lake Trummen ecosystem, Dr Andersson carried out a research project with the aim to demonstrate the influence of fish population on the character of the system.
Summary of the whole lake results from nutrient reducting and regulation of fish populations in Lake Trummen
International interest in research on restoration and management of lake ecosystems
Demonstration of sampler for studies of vertical distribution of animals in Lake Trummen sediment 1969
Andersson, G. 1985. The influence of fish on eutrophic lake ecosystems. -
Proceed. internat. congr. on lake pollution and recovery. Europ. Water Pollution
Control Ass. Rome.