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No. 207: Lakes 2015

Johansson, L.S., Søndergaard, M., Jeppesen, E., Landkildehus, F., Kjeldgaard, A., Sortkjær, L., Windolf, J. & Bøgestrand, J. 2016. Søer 2015. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 90 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 207. http://dce2.au.dk/pub/SR207.pdf



The monitoring programme 

The current monitoring programme for lakes (NOVANA) includes monitoring relative to the EU Water Framework Directive and the Habitats Directive. According to the Water Framework Directive (WFD), there are two main types of monitoring – control monitoring and operational monitoring. According to the Habitats Directive, control monitoring and mapping of lake habitats are required. As to control monitoring of lake habitats and mapping of small lakes and ponds <5 ha, a separate programme exists. 

Control monitoring of lakes is classified into two types – monitoring of the general environmental state of lakes (represented by the so-called KT-lakes, which in the period 2010-2015 included 180 lakes >5 ha) and monitoring the development in the lakes (the so-called KU-lakes, which included 18 lakes > 5 ha). In 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, during the period 2011-2015 a total of 362 lakes >5 ha were examined (not including the KT-lakes and KU-lakes already being monitored operationally). Control monitoring of lake habitats included a total of 350 small lakes and ponds, while in the period 2011-2015 mapping included approximately 1800 lakes. 

The Agency for Water and Nature Management (SVANA) is in charge of the standardized collection of samples. 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.  

A comprehensive overview of the environmental state in the investigated monitoring lakes for four key parameters is given in table 0.1. The environmental state in the lakes included in control monitoring is generally better than in the lakes that are operationally monitored. This can be due to the fact that the control monitored lakes are selected representatively, while the operationally monitored lakes, for the most part, have been selected because they not are expected to meet the target.  

In connection with implementing the WFD and preparing water plans, Denmark is working with 11 different lake types that are defined by water depth (deep, shallow), calcium content (calcareous, lime poor), browning (brown water, non-brown water) and salinity (fresh, brackish). The presentation of the data in this report mostly follows this classification.

Control monitoring of development 

In general 

The 18 KU-lakes included in the control monitoring of development cover a broad spectrum, both morphometrically (size and depth) and nutrient wise. For instance, the average total phosphorus concentration in the summer varies between 0.014 and 0.272 mg/l, and the total nitrogen concentration varies between 0.3 and 3.7 mg/l. The chlorophyll a concentration varies between 4 and 176 µg/l and the Secchi depth between 0.4 and 3.9 m. All values are from 2014.


The sediment in most of the KU-lakes has been examined five times since the beginning of the monitoring period in 1989 and has, typically, been examined at six depths, as deep as 50 cm. The sediment in the KU-lakes also varies a great deal, e.g. the dry matter content in surface sediment varies between 3 and 38%, the iron content between 3 and 137 mg Fe/g dw and the phosphorus content between 0.4 and 4 mg P/g dw. 

In general, the sediment has changed little during the monitoring period, and any changes are lost in the variation that also exists between the three sampling stations in each lake. This applies whether you are comparing surface sediment or depth profiles for shallow and deep lakes, respectively.


Submerged macrophytes have been studied since 1993/94 in 10 of the KU-lakes. In general, there has been a slightly positive development, where coverage, the plant filled water volume and the plant depth limit have increased. Among the individual KU-lakes, a statistically significant increase in coverage has been found in eight of 15 lakes that have been monitored for 6-18 years, while a reduction in coverage has been found in two lakes. The plant-filled water volume and the plant depth limit have also increased in the majority of the lakes.


Studies of fish in the KU-lakes have been conducted since 1989. Analyses indicate that the fish population is changing as a result of the generally decreasing nutrient supply. Thus, there is a tendency that cyprinids are decreasing and predatory fish increasing in some of the lakes, with the biggest changes occurring in bio-manipulated lakes and the lakes that were most nutrient-rich at the beginning of the monitoring period.

Contrary to the expectations from the general relationships in Danish lakes and the fish index used to determine the ecological quality based on fish, the average weight of fish has decreased significantly in many of the lakes, which, however, corresponds to results from the intensively studied Lake Søbygaard. The decrease is attributed to a warmer climate (better recruitment in spring and better survival during winter). As small fish more often are more planktivorous, this will have negative impacts on zooplankton and, consequently, on the grazing pressure on phytoplankton. This, therefore, counteracts the measures taken to reduce the nutrient input.

Control monitoring of environmental state 

In general 

The 180 KT-lakes studied in the period 2010-2015 were originally randomly selected taking geographic stratification into account, and, as such, they constitute a representative sample of Danish lakes >5 ha. The majority of the lakes are relatively small (median lake area is 22 ha) and shallow (median average depth is 1.7 m). Overall, the lakes are nutrient-rich with a median total phosphorus concentration of 0.075 mg/l and a mean total nitrogen concentration of 1.1 mg/l. Similarly, the lakes are generally turbid with a median Secchi depth (summer average) of 1.1 m and a chlorophyll a concentration of 32 µg/l (summer average).

Water chemistry in nine types of lakes 

Water chemistry data exists from nine of the 11 types of lakes under the WFD (see Table 2.1). Some of the types are only represented by very few lakes. The four most common types of lakes (lake types 9, 10, 11 and 13) together constitute 91% of all the KT-lakes. Of these, lake type 11 (calcareous, non-brown water, shallow and saline) is the most nutrient-rich (summer total phosphorus median is 0.140 mg/l), and also has the highest chlorophyll a concentration (summer average median is 56 µg/l) and lowest Secchi depth (median summer average is 0.8 m). 

Most KT-lakes have been studied in two periods (2004-2009 and 2010-2015) and comparisons show that, in general, only few changes have taken place between these two periods. However, overall, the most nutrient-rich lakes tend to have become less nutrient-rich. This is also evident if you look at the various lake types; for some of the common lake types, the nutrient content has declined statistically significantly from 2004-2009 to 2010-2015.


For the KT-lakes, sediment data from 140 lakes were examined in the period 2010-2015. For all lake types, the four measured variables vary greatly (dry matter content, loss on ignition, total phosphorus, total iron), and often a factor of 10 or more can be seen between the highest and lowest measured values. Particularly, the iron content varies greatly in all lake types. The general picture for all common lake types is that the dry matter content increases with depth, while the loss on ignition (an expression of organic content) and the phosphorus content decrease with depth. 

Only a few of the KT-lakes were also examined in the period 2004-2009, but there are no obvious changes compared with the period 2010-2015. For the most common lake types, empirical relationships can be established, which can be explained by the content of total phosphorus and iron content and/or LOI.


Submerged macrophytes have been studied in 180 KT-lakes covering a total of nine types of lakes. The four parameters (plant coverage, plant-filled water volume, plant depth limit and the number of species) vary greatly within each lake type. Both the coverage and the plant-filled volume vary from 0 to more than 50% (minimum coverage in lakes with plants is 0.14%), and the depth limit in the most common lake type (lake type 10) varies between 0,3 and >20 m in lakes with plants). The number of species also varies greatly in lakes, from one to 46 species. 

Compared with the period 2004-2009, no major changes have taken place up to the period 2010-2015. However, in the common shallow lake type (lake type 9) there is a statistically significant increase in coverage, depth limit and the number of species.


The fact that the fish stocks are changing as a result of the generally decreasing nutrient input can also be seen in the KT-lakes. The proportion of cyprinids (here defined as roach+bream+hybrids) of total fish biomass has declined from 2004-2009 to 2010-2015 in both shallow and deep lakes (6 % and 7 %, respectively), and, at the same time, the proportion of predatory fish (here defined as walleye and pike, all>10 cm) has increased. The average increase was 2.3% in shallow lakes and 6% in deep lakes. Particularly roach has decreased in lakes with a high roach biomass and, vice versa, perch biomass has increased in lakes with a low biomass. In other words, there is a positive development in many of the lakes.

Hazardous substances and metals in the sediment 

From 2011-2015, sediment samples were collected from 82 lakes for analysis of up to 53 environmentally hazardous substances and metals (including seven groups: metals, pesticides, aromatic hydrocarbons, phenols, polyaromatic hydrocarbons (PAHs), plasticizers and organotin compounds).


Nearly all the studied substances in metals, aromatic hydrocarbons, PAHs and organotin compounds were found in concentrations above the detection limits in most of the lakes, but in greatly varying concentrations. Phenols were detected in 1-53% and plasticizers in 6-62% of the lakes. Pesticides were only found in a limited number of lakes (0-8%).


For a few of the investigated substances environmental quality requirements are determined. In some lakes lead, cadmium and naphthalene were found in quantities exceeding the requirements, while the concentration of methylnaphthalenes exceeded the requirements in 79 % of the lakes.

Mercury in fish 

Mercury contents were studied in fish by measuring the contents in the muscle tissue, mainly in perch measuring 20-25 cm. The studied lakes overall included the same lakes that were studied for hazardous substances and metals in the sediment. 

The mercury content in fish varies and there appears to be no clear correlation between mercury content and lake type. At the same time, fish from some of the most nutrient-poor and/or lime-poor lakes have the highest mercury concentrations. The mercury content (per dry weight unit) increases with increasing length of the fish. 

For the major part of the fish, the mercury concentrations exceeded the environmental quality requirements of the Water Framework Directive, whereas the general food requirements were only exceeded in a few cases.

Operational monitoring of the lakes’ environmental state 

In the period 2011-2015, 362 lakes were studied in the operational monitoring (excl. the control monitored lakes that are also included in the operational monitoring). These lakes were selected in order to generate action plans to meet the goals and are not representative of the environmental state in the Danish lakes. The study included nine types of lakes, of which lake types 9, 10, 11 and 13 are the ones most commonly studied. 

The most nutrient-rich lake type, both in regards to total phosphorus and total nitrogen, is lake type 15 (calcareous, brown water, saline, shallow), and it is also here that the highest chlorophyll a concentrations and the lowest Secchi depths are seen. The two remaining brown water lake types (lake type 5 and 13) are also relatively nutrient-rich. The most nutrient-poor lake types are type 1 and 2 (lime poor, non-brown water, fresh). 

In most of the operationally monitored lakes, submerged macrophytes were also studied. They also show great variation in the studied variables for each lake type.

Climate and runoff 

Climatically speaking, 2015 stood out by being a bit warmer – the annual mean temperature for all of Denmark was ½ degree above the average for 1990-2014, and the amount of precipitation was also high and close to the record of 902 mm in 1999. In seven months, the precipitation was above the norm. Similarly, runoff was 421 mm, which is one of the highest values in the entire period from 1990 to 2015. Wind conditions were normal.