Johansson, L.S., Søndergaard, M., Landkildehus, F., Kjeldgaard, A., Sortkjær, L. & Windolf, J. 2018. Søer 2016. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 84 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 259. http://dce2.au.dk/pub/SR259.pdf
The current monitoring programme for lakes (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 – 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. The location covered by the various monitoring types according to the WFD is shown in Figure 1.1. The location of ponds and small lakes monitored according to the Habitats Directive is shown in Figure 9.1.
Control monitoring of lakes according to the WFD is classified into two types – monitoring of the general environmental state of lakes, represented by the so-called KT lakes, which in the period 2010-2016 included 180 lakes >5 ha, each investigated every second year, and for which a new round of monitoring of 35 of the 180 lakes was initiated in 2016, and monitoring of the development in lakes, the so-called KU lakes, including 18 lakes >5 ha. In the operational monitoring (the so-called OP lakes) 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 426 lakes >5 ha were investigated (not including the KT lakes and KU lakes already being monitored operationally) during the period 2011-2016. Control monitoring of habitat types in lakes <5 ha according to the Habitats Directive includes a total of 411 lakes. In the period 2011-2016 mapping included 3023 lakes of which most are located within Natura 2000 habitats. Thus, almost 1/3 of the approx. 10,000 ponds and lakes found in Natura 2000 areas in Denmark have been mapped. Table 1.1 shows an overview of lakes represented in this report and the year the monitoring was conducted.
The Danish Environmental Protection Agency (MST) is in charge of the standardised 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.
An overview of two key parameters in the investigated monitoring lakes is given in table 1.2. The median value of chlorophyll is for all three monitoring types around 40 μg/l. Secchi depth of the lakes in the control monitoring is around 1.3 m and around 0.8 m in the operationally monitored lakes.
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 mainly follows this classification.
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 summer chlorophyll a concentration varies between 3 and 180 µg/l and the Secchi depth between 0.3 and 4.3 m. All values are from 2015-2016.
The summer average concentration of dissolved phosphorus (orthophosphate) has decreased in 11 lakes since 1989 and has remained unchanged in four of the 15 lakes investigated during the entire monitoring period. Only few changes have occurred during the past 10 years. The summer average content of inorganic nitrogen (nitrite+nitrate) has declined significantly in nine lakes since 1989 and has remained unchanged in the other lakes. During the past 10 years, there has been no significant development in any of the lakes.
Since 1989, there has generally been a trend towards a decrease in the concentration of chlorophyll a and an increase in Secchi depth. Thus, summer average chlorophyll has decreased in six lakes and increased in three lakes, while summer average Secchi depth has increased in eight lakes and declined in one lake. The greatest changes occurred during the first part of the monitoring period, but during the past 10 years changes have been only modest.
The 35 KT lakes represent five different lake types of which lake type 9 (shallow) and lake type 10 (deep) are the two most common. Most of the lakes have a high chlorophyll a concentration (median of summer average is 38 µg/l) and a relatively low Secchi depth (median of summer average is 1.3 m).
The 35 KT lakes have been investigated during three periods (2004-2009, 2010-2015 and 2016). For lake type 10, and to a lesser extent lake type 9, there is a tendency to a reduction of the concentration of chlorophyll a and an increase in Secchi depth. For 17 of the KT lakes, measurements before 2004 exist, and the development since 1989 shows a significant reduction of chlorophyll concentrations in seven lakes, while Secchi depth has increased significantly in seven lakes and declined in one lake. The cover and species number of submerged macrophytes have shown a minor increase over the total period since 2004-2009 in lake type 9, while changes in lake type 10 have been more modest. The fish community has changed only little during the periods, excepting an increase in the catch in number per net in lake type 10.
From 2011-2016, sediment samples were collected from 101 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).
During the period 2011-2016 the substances were found in greatly varying concentrations. For example, metals were found in all lakes, phenols were detected in 4-50% and plasticizers in 4-65% of the lakes, while pesticides were only found in a limited number of lakes (0-4%).
Sediment samples were taken twice in 25 of the investigated lakes during the period 2011-2016. For the last sampling year, a comparison of concentrations shows significantly higher concentrations of the aromatic hydrocarbons naphthalene and trimethylnaphtalenes, of the PAHs phenathrene, benxghiperylene, benz(a)anthracene, indeno(1,2,3-cd)pyrene and fluorene and of the organotin compound monobutyltin. Moreover, the concentrations of nonylphenols were significantly lower in the second year of sampling. It should be noted, though, that the development tendency is founded on only two measurements of the substances in question and that the data represent no more than 25 lakes.
For some of the investigated substances, environmental quality standards (EQS) for sediment exist. Lead and cadmium were found in concentrations higher than the EQS in, respectively, 5% and 7% of the investigated lakes. EQS have also been established for nonlyphenols and octylphenol, but for these concentrations higher than the EQS were not recorded.
Mercury contents in fish have been studied by measuring the contents in the muscle tissue, mainly in perch sized 20-25 cm. The studied lakes overall included the same lakes that were investigated 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 standards of the WFD, whereas the general food safety thresholds were only exceeded in a few cases.
In the period 2011-2016, 426 lakes that are in risk of not fulfilling the goals set for nature and environment were studied in the operational monitoring. These lakes do not include the selection of control monitored lakes that are also part of the operational monitoring. The lakes were chosen in order to determine whether the lakes meet the goals or whether intervention is needed, and they are thus not representative of the environmental state in the Danish lakes. The study included ten types of lakes, of which lake types 9 and 10 are the ones most commonly investigated and together represent 56% of all the lakes included in the operational monitoring.
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), which also have the lowest concentrations of chlorophyll.
In most of the operationally monitored lakes, submerged macrophytes were also studied. They also show great variation in abundance for the individual lake types.
Climatically speaking, 2015 stood out by being a bit warmer than the average for the last 26 years – the annual mean temperature for all of Denmark was 9oC compared with 8,6 oC during the period 1990-2016. Especially in September and December, temperatures were markedly higher than normal. Compared with the period 1961-1990, the temperature was 1.3 oC higher in 2016.
In 2016, the amount of precipitation was a little lower than usual, 701 mm compared with an average of 714 for the period 1961-1990 and 763 mm for the period 1009-2016. Particularly April, June and July were rich in precipitation. In 2016, the area-specific freshwater runoff was 253 mm, which is somewhat above the norm for 1990-2016.
Mapping of habitat types in the period 2011-2016 included a total of 3023 ponds and small lakes <5 ha. Lakes designated as habitat type 3150 (nutrient-rich lakes) and type 1150 (coastal lagoons and beach lakes) were the most common types, constituting 33% and 26% of the lakes, respectively. Lobelia lakes (habitat type 3110) were the most rare (2%), while 3130 (relatively nutrient-poor with small amphibian plants along the shore), type 3160 (brown lakes) and type 3140 (Chara lakes) represented, respectively, 5-11% of the investigated lakes. 9% of the lakes could not be classified to a habitat type.
In most of the investigated lakes, the biological state index was above 0.6, which corresponds to "good" or "high" state. The majority of lakes of types 3110 and 3160 had a biological state index of more than 0.8, corresponding to the "high" state.
The control monitoring of lake habitat types included a total of 411 lakes. The distribution of habitats was roughly the same as in the total mapping, although there were slightly more type 3130 and correspondingly fewer type 1150 lakes. Here too, the biological state of most of the lakes was "good" or above.
The water chemical analyses included in the control monitoring of lake habitat types generally show that lakes of type 1150 and unclassified lakes are the most nutrient-rich.
A comparison of environmental state was made between two studies conducted in the same lake and with identical classification of habitat types. The available data are, however, sparse and do not give any indication of the development in the period 2007-2016. Correspondingly, a comparison was made of coverage across lake types, and this shows that a general improvement has occurred in more lakes within the size range 1-5 ha compared with the lakes with an area <1 ha.
There were errors in the analysis of total nitrogen and total phosphorus in 2016. Therefore, the two parameters are not included in the reporting of data from 2016. Analyses of the inorganic fractions (e.g. nitrate+nitrite, ammonium and phosphate) are not affected by this error.
