What about the Swiss small lakes?
The trophic status of 163 small lakes in Switzerland was assessed according to the guidelines for watercourse assessment of the German Länderarbeitsgemeinschaft (LAWA) and presented for the first time in overview form. Despite successful remediation in many places, more than 50% of the small lakes studied at colline altitude are still in an inadequate trophic state.
What importance do you attach to small lakes? In Switzerland, there are about 1600 lakes with a minimum area of 0.5 hectares. About 1500 of these lakes are smaller than 50 hectares. These water bodies, referred to as small lakes in this report, show a high diversity both in terms of their formation and their use. Many small lakes, such as dead lakes, moraine reservoirs, and sinkholes, were formed naturally. Artificially created small lakes, such as peat ponds, impoundment ponds, and dredge lakes, are equally diverse. The small lakes are used by man in very different ways, e.g. for recreation, fishing, flood retention basins, power generation.
All these small lakes also form an important habitat for partly endangered animal and plant species. Although the water surface area of all small lakes is only about 3% of the water surface area of all Swiss lakes, they harbor about 50% of the species of aquatic snails, aquatic beetles, aquatic plants and dragonflies found in Switzerland and about 80% of the species of amphibians found in Switzerland [1] - they are an absolute hotspot for biodiversity.
Knowledge about the condition of small lakes is important in order to preserve these valuable habitats, to protect them in a targeted manner and to ensure the diverse use of these lakes in the long term. However, there is no nationwide overview of their condition yet, because uniform assessment methods are lacking and the data situation is very heterogeneous due to the different monitoring practices in the individual cantons.
In order to nevertheless provide an overview, an assessment of the small Swiss lakes studied was carried out in accordance with the guidelines "Gewässerbewertung - stehende Gewässer" (Water assessment - standing waters) of the German Länderarbeitsgemeinschaft (LAWA) [2-5]. For this purpose, all cantonally available data on the trophic status of the small lakes were compiled, summarized and, if sufficient data were available, evaluated using these guidelines [6].
Valuation record
The assessment of a standing water body according to the LAWA approach is basically based on the actual trophic level and the reference trophic level to be defined for the specific water body, in which the water body would be under natural or near-natural conditions. The reference trophic level takes into account the individual characteristics of the water body and its catchment area and can be determined individually for each lake from the morphometric conditions or the potential natural nutrient input from the catchment area. The measure of assessment is the deviation of the actual trophic level from the reference trophic level [2]. By trophicity is meant the intensity of organic photoautotrophic production, which is strongly dependent on the available nutrient supply.
With this approach, the achievement of the quality objectives listed in the GSchV (no unnaturally high production of biomass), which is the legal basis for monitoring small lakes, can be checked directly. It can be assumed that a deviation of the actual trophic level from the reference trophic level influences the occurrence of sensitive animals and plants and thus has far-reaching consequences for the functioning of the ecosystem. Therefore, this approach can also be used indirectly to check compliance with the requirements of the Water Protection Ordinance (no impairment of the reproduction, development and health of sensitive plants, animals and microorganisms; natural conditions).
Determination of the actual trophy
The determination of the trophic actual state is based on a data set to be collected according to defined specifications, which includes phosphorus concentrations, chlorophyll-a contents (Chla) as well as visual depths.
The actual trophic level was taken from existing studies and cantonal technical reports due to a lack of a uniform data set of the required parameters. The classification of the actual trophic level was done in 4 steps:
| oligotrophic | weakly productive |
| mesotrophic | Moderately productive |
| eutrophic | Highly productive |
| Highly eutrophic | Very high productive |
The reference trophicity can be determined on the one hand from the morphometric conditions of the lakes, such as basin shape and depth or the stratification behavior, and on the other hand from the potential natural phosphorus input from the catchment area. A comparison of the results from the two independent reference approaches allows a plausibility check of the determined reference trophies.
In the present evaluations, the reference trophic level was determined solely on the basis of morphometry due to the lack of the necessary data. The basis for this is a statistical relationship between trophic parameters such as Chla concentrations, visible depth and total phosphorus concentration and factors characterizing the basin shape of a lake such as volume, surface area, maximum depth, length and width. This statistical relationship is based on current measured data on 108 glacially formed lakes in Germany, which was calibrated with data on 51 glacial lakes in natural state in Belarus [2].
Evaluation
The measure for the assessment is the deviation of the actual state from the reference state. Based on the available data, the assessment is presented in 3 stages in deviation from the LAWA guideline:
| good | Actual trophic equal or better than reference trophy |
| moderate | Actual trophic level is at most one level worse than reference trophic level |
| unsatisfactory | Actual trophic level is more than one level worse than reference trophic level |
Small lakes for which the reference trophic level could not be determined due to insufficient morphometric data were not assessed. For these small lakes only a trophic classification is available. This concerns all artificial and some alpine and subalpine small lakes.
CONDITION OF THE SMALL LAKES
Information on the trophic situation is available for 163 small lakes. The studied small lakes are distributed over all altitudinal levels from colline to alpine. The proportion of studied lakes with information on the trophic situation ranges from 14 to 18% in the colline, montane and subalpine altitudinal zones, and from 6% in the alpine altitudinal zone. Alpine lakes are relatively underrepresented in the studied sample, but overall a good sample size is achieved at all altitudinal levels.
In order to provide an overview of the condition of the Swiss small lakes and to be able to discuss the results in context, the lakes were divided into different categories. The grouping took into account factors influencing the trophic situation, such as the origin, altitude and stratification behavior of the small lakes.
First of all, the lakes were grouped into natural and artificial lakes on the basis of their formation. This seems appropriate, since the assessment approach according to the LAWA Guideline is based on a comparison of the actual state with a near-natural reference state, which has yet to be defined for artificial small lakes. Therefore, only a trophic classification was carried out for the artificial lakes, but no actual assessment according to the LAWA Guideline. The status of the small lakes is therefore presented below separately according to natural and artificial small lakes.
As a second criterion, the altitude was used for grouping. On the one hand, this is considered an indication of the anthropogenic load from the catchment area of the lakes, since the settlement density and the share as well as the intensity of agriculture decrease with increasing altitude. On the other hand, this classification also takes into account the climatic conditions relevant for primary production. The altitude classification was made according to geobotanical criteria in four altitude levels. The division into altitude levels allows statements to be made about lakes with comparable stress situations.
| Altitude level | Altitude [m a.s.l.] |
Share Agricultural land % |
Share Settlement area % |
| collin | < 700 | 49 | 18 |
| montan | 700-1200 | 44 | 4 |
| subalpine | 1200-1900 | 18 | 1 |
| alpine | > 1900 | 1 | 0 |
The stratification behavior served as a third criterion for the grouping. A distinction was made between unstratified and stratified small lakes. In contrast to stratified lakes, in shallow unstratified lakes the nutrients released in the water or sediment after decomposition of the biomass are immediately available again for algal growth. The trophic situation of lakes is thus significantly influenced and is also an important parameter for determining the actual and reference trophicity according to the LAWA guideline.
Natural small lakes
Actual state
As a rule, small lakes at lower elevations are more nutrient-rich and more likely to be in insufficient trophic condition than small lakes at higher elevations. For example, 21 of 36 colline small lakes surveyed (60%) are in insufficient condition. At all elevations, unstratified small lakes are generally rated better than stratified ones because they tend to naturally have higher trophic levels than stratified ones due to their morphometry (polymixy, high self-fertilization).
The unsatisfactory condition of colline small lakes can be attributed to one or more causes. 80% of the small lakes in moderate and unsatisfactory condition are located in catchments with intensive agricultural use. Impairments due to this intensive agricultural use in the watershed likely play the major role in their unsatisfactory condition. Nutrients enter the water bodies on the one hand through fertilization of the agricultural areas, some of which are directly adjacent to the shore, and the resulting surface nutrient runoff, and on the other hand via drainage discharges. Many small lakes were lowered during World War II, and the surrounding boggy areas were drained and made usable for agriculture. The peat is decomposed by the influx of air. The released nutrients are also transported to the lakes via drainages.
In some small lakes, nutrients still enter via urban drainage, e.g. through discharges from flood spillways or meteoric water or, in individual cases, from households not connected to a wastewater treatment plant. Investigations of nine small colline lakes in the canton of Bern indicate both nutrient loads from agriculture and inputs from domestic sewage, which are cited as reasons for their inadequate condition [7].
However, these nutrient inputs from point sources are usually of minor importance. In most cases, they are diffuse inputs from agriculture. This could be shown by means of a substance flow analysis, for example, at Lake Inkwil (BE/SO) [8]. The diffuse inputs, with a share of 91 to 98% for phosphorus and nitrogen, respectively, were far above the inputs from point sources (stormwater discharges, meteoric discharges). Inputs from cropland were found to be the most significant sources of nutrients.
A dominance of nutrient inputs from agriculture is likely to occur in many small lakes of the Central Plateau, which have a high proportion of intensive agriculture. Significant nutrient inputs from point sources of urban drainage are usually of minor importance, according to the assessment of cantonal agencies.
The considerable share of 40% of the small lakes in the subalpine area, which are in insufficient condition, is striking. It is likely that alpine farming, which accounts for up to 60% of the catchment area use in some lakes, is of great importance for their insufficient trophic condition. The proximity of alpine pastures to the water body or use as a cattle watering place is also likely to cause significant eutrophication, particularly of small subalpine lakes. Furthermore, discharges from individual farms or businesses that are not treated or are insufficiently treated in terms of wastewater technology can impair the trophic conditions in the lake.
Virtually all of the studied subalpine small lakes in insufficient condition are located in karst areas. Due to the special hydrological conditions, even low nutrient loads can have a major negative impact on their trophic situation. The extent to which high lake level fluctuations of some subalpine lakes used for energy production affect the trophic level remains to be investigated. The exact causes of the impairments must be determined on a case-by-case and lake-specific basis.
The small alpine lakes assessed are in good condition.
State development
Long-term studies of 27 small lakes where remediation measures were carried out show that targeted measures outside the lake (remediation of urban drainage, extensification of agriculture, creation of buffer zones) and within the lake (deep water drainage, circulation support) have restored about one third of the lakes to their natural state and they are currently in good condition.
Successful off-lake remediation measures included the rehabilitation of urban drainage, the extensification of agricultural land, especially on areas close to the watercourse, the creation of buffer zones, especially on slopes, and adapted fertilization practices. In-lake measures such as deep water drainage and circulation support were particularly successful in combination with out-of-lake measures. It has been shown that the measures must be individually adapted and applied to each water body in order to achieve restoration success.
However, about two thirds of the small lakes are still in an insufficient condition despite remediation measures. Phosphorus re-dissolution from the sediments under oxygen-free conditions, which causes high self-fertilization from sediments polluted over decades, is one reason for this situation. This is especially important in unstratified small lakes.
Only one out of five unstratified small lakes has reached a good condition thanks to rehabilitation measures. This proportion is twice as high for the stratified small lakes.
The long-term studies also show that remediation measures must be implemented over a long period of time until equilibrium and a stable new improved condition are established. The amount of algae in a lake reacts with a delay to the external nutrient decline due to phosphorus re-dissolution from the sediments or internal lake fertilization.
Another reason for the still insufficient condition of the small lakes studied over many years is still the too high external nutrient load from the catchment area mainly due to agriculture and partly due to urban drainage.
To further improve the condition of small lakes in Switzerland, measures must be implemented primarily in the area of agriculture through extensification, reduction of livestock, creation of buffer strips and erosion control measures. In individual cases, measures in the area of urban drainage, e.g. keeping away flood spillways, are likely to be necessary. In addition, measures already taken must continue to be maintained even if improvements are not yet apparent.
Artificial small lakes
The 61 artificial small lakes studied show a great diversity of trophic levels, depending both on their history of formation and on the type and intensity of use. The small lakes at lower elevations are more frequently rich in nutrients compared to those at higher elevations, which is probably due to the greater anthropogenic load from intensive agriculture and urban drainage at low elevations [6].
Most of the 61 small artificial lakes studied could not be assessed according to the LAWA guidelines due to a reference trophic level that could not be determined, which is why this article does not discuss the condition of the small artificial lakes in more detail.
Conclusion and outlook
Of the approximately 1500 small lakes in Switzerland, 163 have so far been examined for their trophic condition.
The comparison of water body type-specific trophic actual condition and trophic reference condition was considered a suitable approach for a uniform assessment of natural small lakes throughout Switzerland, as it allows the achievement of the legal objectives and compliance with the requirements to be verified.
Half of the natural small lakes in the Central Plateau are in an inadequate condition. The cause of the inadequate condition of many lakes on the Central Plateau is the excessively high nutrient load from intensive agriculture. In the case of subalpine lakes, about one third of the lakes are in an unsatisfactory condition, whereby this can be explained by impairments from both alpine farming in the catchment area and discharges from individual farms and businesses. Small alpine lakes are generally in good condition.
Results from long-term studies of small natural lakes show that, with suitable measures outside the lake in the areas of agriculture and urban drainage, improvements can be achieved even in catchments that are heavily used for anthropogenic purposes, and that it is possible to achieve a near-natural status. The combination of lake-external and lake-internal measures to achieve good status has proven successful in practice. However, long-term studies also show that many small lakes are still in an inadequate condition despite the remediation measures that have been implemented.
Further measures to reduce nutrient pollution, particularly in the agricultural sector, are necessary to further improve the condition of the small lakes and thus ensure their long-term function as valuable habitats and recreational areas.
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Bibliography
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Federal Office for the Environment, Bern et Laboratoire d'Écologie et de Biologie Aquatique, Université de Genève
[2] LAWA (Länderarbeitsgemeinschaft Wasser) (1998): Gewässerbewertung - stehende Gewässer. Preliminary guideline for an initial assessment of naturally occurring lakes according to trophic criteria, Berlin.
[3] LAWA (Länderarbeitsgemeinschaft Wasser) (2001): Preliminary Guideline for the Trophic Classification of Dams, Berlin.
[4] LAWA (Länderarbeitsgemeinschaft Wasser) (2003): Preliminary guideline for an initial assessment of dredged lakes according to trophic criteria, Berlin.
[5] LAWA (Länderarbeitsgemeinschaft Wasser) (2014): Guideline for the determination of the trophic index according to LAWA for natural lakes, dredged lakes, dams and reservoirs, Berlin.
[6] Binderheim, E. (2018): Status report of Swiss small lakes. Report for the attention of the Federal Office for the Environment, Bern.
[7] Guthruf-Seiler, K. et al. (2015): State of the small lakes. Office for Water and Waste of the Canton of Bern, Bern.
[8] Fankhauser, R. (2018): Nutrient inputs across the catchment area of Lake Inkwil. Report for the attention of the Office for the Environment, Canton Solothurn.
