Aarhus Universitets segl

No. 417: Lakes 2019. NOVANA

Johansson, L.S., Søndergaard, M. & Andersen, P.M. 2021. Søer 2019. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 68 s. - Videnskabelig rapport nr. 417



The monitoring programme

The current monitoring programme for lakes (encompassed by NOVANA) includes monitoring in relation to the EU Water Framework Directive (European Union 2000) and the EU Habitats Directive (European Union 1992). According to the Water Framework Directive (WFD), there are two main types of monitoring – surveillance monitoring and operational monitoring.  According to the Habitats Directive, surveillance monitoring and mapping of lake habitats are required.  As to surveillance monitoring of lake habitats and mapping of small lakes and ponds <5 ha, a separate programme exists.  The location of the lakes in the surveillance monitoring referred to in this report is shown in Figure 1.

The surveillance monitoring of lakes according to the WFD is divided into two types: monitoring of the general state of lakes (represented by the so-called KT lakes, comprising 180 lakes >5 ha, where each lake is investigated every six years) and monitoring of the development of lakes (the so-called KU lakes, comprising 18 lakes >5 ha).  In the KU lakes, water chemistry and physical parameters are measured every second year, vegetation is studied twice in a six-year period, and phytoplankton, fish and sediment chemistry are recorded every six years. Within the operational monitoring geared towards lakes at risk of not complying with the goals for nature and the environment as far as their environmental state is concerned, a total of 413 lakes >5 ha were investigated (not including the KT and KU lakes also being monitored operationally) during the period 2016-2021.  The lakes in the operational programme are not included in this year’s report.

Table 1 provides an overview of the lakes (number and sampling year) represented in this report. For data from the KT lakes, it should be noted that the six-year period from 2014-2019, from which results are presented, does not follow the official monitoring period, which covers the years 2016-2021. For the KT lakes, the current status of selected physical/chemical parameters is shown besides results on phytoplankton, vegetation and fish studies. In addition, a general description is given of the development in the field of water chemical results and phytoplankton data in the KT lakes. For the KU lakes, a general status and the development in the biovolume and taxonomic composition of phytoplankton are presented for the period 2004-2018.

In connection with implementing the Water Framework Directive and preparing River Basin Management Plans, Denmark is working with 11 different lake types that are defined by water depth (deep, shallow), alkalinity (high alkalinity/low alkalinity), browning (brown water, non-brown water) and salinity (fresh, brackish). The presentation of the data in this report mainly follows this classification. 

The Danish Environmental Protection Agency (MST) is responsible for the standardised sample collection. All collected data are reported to the National Topic Centre for Freshwater, which prepares annual progress reports on the general environmental state and development in Danish lakes

Surveillance monitoring of lake environmental state

The 180 KT lakes that are included in the monitoring of lake environmental state were originally selected randomly, taking into account geographical stratification, and as such constitute a representative sample of the Danish lakes > 5 hectares. The majority of lakes are relatively small (the median value for lake area is 23 ha) and shallow (the median value for mean depth is 1.6 m). Furthermore, the lakes are generally nutrient-rich with a median value of total phosphorus of 0.094 mg/l and total nitrogen of 1.6 mg/l. Similarly, the lakes are generally turbid with a median value of Secchi depth of 1.1 m and a chlorophyll content with a median of 38 μg/l. All values are calculated on the basis of the summer average in the individual lakes.

Water chemistry

Water chemistry data are available from nine of the 11 lake types under the Water Framework Directive (see Table 1.1). Some of these types are only represented by very few lakes. The four most common lake types (9, 10, 11 and 13) constitute a total of 91% of all the KT lakes. Among these, lake type 11 (high alkalinity, non-brown water, shallow and saline) is the most nutrient-rich (median summer total phosphorus and nitrogen concentrations are 0.174 and 1.83 mg/l, respectively). The medians of chlorophyll concentration and Secchi depth are at the same level for lake type 9 and 11: 52-57 μg/l and 0.8-0.9 m. For lake type 15, some of the parameters indicate a poorer condition, but this type is only represented by two lakes.

Overall, there has been a positive development since 1989 in the individual KT lakes for which there is sufficient data material to assess the development. This trend was most evident in the first ten years of the period, while the development in the past 20 years has been less clear. If you look at all the KT lakes together and compare the last two six-year periods (2008-2013 and 2014-2019), a significant increase in the lakes’ content of total nitrogen, total phosphorus and chlorophyll is seen. The most significant changes have occurred for nutrients; there is no significant change in Secchi depth.


The biovolume and composition of phytoplankton in the KT lakes depend to a large extent on the lake type. Among the most common types of lakes (9, 10, 11), the median value of the total biovolume in the summer period (May-September) is, however, quite uniform, approx. 10 mm3/l for types 9, 10 and 11. For the remaining lake types, the summer biovolume varies notably, and the data base is poorer, but the smallest values are seen for the most nutrient-poor and low alkalinity lake types. On average, blue-green algae dominate the lakes in the summer, especially lake type 9, 11, 13 and 15. For lake type 10, the composition is more versatile and diatoms and dinoflagellates are at the same level as blue-green algae. As for the remaining lake types, different groups dominate, usually the heterogeneous group “others“. An analysis of the dominant taxa across all lakes shows, among other things, that the genera Cryptomas and Rhodomonas lacustris are the most frequently found taxa and that many of the most dominant species have a preference for specific lake types.

For 87 lakes examined during the period 2014-2019, it is possible to study and compare the development in the phytoplankton biovolume (measured in summer) with the previous six-year period, 2008-2013. For the total biovolume and for all lakes, there is no significant change. For the individual algae groups, there is a slight reduction in the biovolume of dinoflagellates across the lakes and for lake type 10. For the group “others”, there is also a slight reduction in biovolumes in lake type 9 and 10, while the group “green algae” exhibits an increase for lake type 11.

For comparing median values for phytoplankton biovolume along a nutrient gradient, the lakes are divided into three groups: 1: brackish/alkaline/shallow/ brown water and non-brown water 2: fresh water/alkaline/deep/ non-brown water and 3: fresh water/alkaline/shallow/ brown water and non-brown water. As expected, there is an increase in total biovolume with increasing concentrations of total phosphorus and total nitrogen. The summer biovolume is generally highest in the brackish lakes (1) and lowest in the fresh water/high alkalinity/shallow lakes (3).


The results for the submerged vegetation, represented by four parameters (plant coverage, plant-filled water volume, plant growth depth and species number), vary greatly within the individual lake types. Thus, coverage varies from 0 (lowest coverage in lakes with plants is 1.4%) to 88%. The plant-filled volume in lakes with vegetation varies between 0.04 and 60%, and the growth depteh in the most common deep lake type (lake type 10) varies between 0.8 and >20 metres in lakes with vegetation. The number of species (number of taxa) also varies markedly, the highest number is 39 species.


The presentation of the results of the fish investigations includes the parameters CPUE (catch per unit effort, i.e. catches per net per night), measured in terms of both number and weight, the percentage of predatory fish and cyprinids and the average individual size of the fish. The median values of CPUE, measured as both number and weight, are highest in lakes of type 9, where it constitutes, respectively, 137 fish per net and 4.7 kg per net. The median values of the percentage share of predatory fish and cyprinids are very close to each other in the two most common lake types (type 9 and type 10), accounting for 30-32% and 49-51%, respectively. The average individual biomass is higher in lake type 9 (median: 31 g) than in type 10 (median: 23 g).

Phytoplankton in the KU lakes 2004-2018

The biovolume of phytoplankton measured in the summer period and its composition in the 18 KU lakes, studied in the period 2017-2018, show great variations. The total summer biovolume varies between 0.55 and 51.84 mm3/l, and there is a clear trend towards higher values with increasing concentrations of total phosphorus. The proportions of the different algae groups also vary markedly, but the overall picture is that several groups are dominant in the most nutrient-poor lakes, while a few groups (blue-green algae, green algae and, sometimes, dinoflagellates) typically dominate in the more nutrient-rich lakes.

An analysis of the development in the KU lakes for the period 2004-2018 shows that the total summer biovolume has only changed significantly in Lake Arreskov, where there has been a reduction. Some of the lakes experienced a change in the volume of one-more individual algae groups, but there is no clear pattern.

Environmentally hazardous substances in the surveillance monitoring 2017-2019

In the period 2017-2019, sediment samples from 54 KT lakes and from all 18 KU lakes were collected for analysis of up to 54 environmentally hazardous substances (EHS). The pollutants are divided into six groups: metals, aromatic hydrocarbons, phenols, polyaromatic hydrocarbons (PAH), plasticisers and organotine compounds.

With a few exceptions, all the substances studied within the groups metals, aromatic hydrocarbons and PAH have been found in concentrations above the detection limit in the sediment in most of the 54 lakes studied, but in very varying concentrations. Organotine compounds have been detected in 8-82% of the lakes and plasticisers in 2-61%, while the presence of the individual substances in the phenols group varies between 0 and 8% among the lakes.

Concentrations higher than the environmental quality requirements are recorded for vanadium and methylnaphthalenes in more than half of the lakes studied. Lead and naphthalene are found in concentrations that are higher than the environmental quality standards in some of the lakes studied. Alkylphenols are not found in concentrations higher than the environmental quality requirements.

Mercury in fish 2017-2019

The mercury content in fish is studied by measuring the content of muscle tissue, primarily in perch with a length of 20-25 cm. The lakes studied are virtually the same lakes (54) that were studied for EHS in the sediment. The content of the mercury (measured in mg/kg) in the fish varies, but the fish caught in the low alkalinity lakes have the highest mercury content. 

For all samples, except for one, the mercury concentration exceeded the environmental quality requirements of the WFD. The food requirement was exceeded in a single sample.