Johansson, L.S., Søndergaard, M. & Landkildehus, F. 2019. Søer 2017. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 42 s. - Videnskabelig rapport nr. 307. http://dce2.au.dk/pub/SR307.pdf
The current monitoring programme for lakes (part of the National Monitoring Programme for Water and Nature - 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 habitat types are required. As to control monitoring and mapping of lake habitat types in small lakes and ponds <5 ha, a separate programme exists. The location covered by the various monitoring types according to the WFD and mentioned in this report is shown in Figure 1.1.
The control 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, which includes 180 lakes > 5 ha, where each lake is investigated every six years), as well as monitoring of the development of lakes (the so-called KU-Lakes, comprising 18 lakes > 5 ha). 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 447 lakes >5 ha were investigated (not including the KT lakes and KU lakes also being monitored operationally) during the period 2011-2017. Table 1.1 provides an overview of the lakes (number and sampling year) represented in this report.
In connection with implementing the WFD and preparing water basin managing plans, Denmark is working with 11 different lake types that are defined by water depth (deep, shallow), calcium content (calcareous, lime poor), colour (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.
An overview of four key parameters (total phosphorus, total nitrogen, chlorophyll a and Secchi depth) in the investigated lakes in the monitoring programme is given in table 1.2. Generally, the lakes investigated in 2016 and 2017 included in the two monitoring programs are very similar. The median value of chlorophyll a for both monitoring types is around 40 μg/l. However, the median value of total P is slightly higher in the OP lakes then in the KT lakes, and the Secchi depth of the lakes in the control monitoring is around 1.29 m and around 0.85 m in the operationally monitored lakes.
The 64 KT lakes monitored in 2016-2017 represent seven different lake types, of which type 9 (shallow) and type 10 (deep) are the two most common. Most of the lakes have a high content of chlorophyll (median of summer averages is 40 µg/l) and a relatively low Secchi depth (median of summer averages 1.29 m).
The vast majority of the 64 KT lakes have now been investigated during three periods (2004-2009, 2010-2015 and 2016/2017). There are considerable variations between lakes, though generally small changes through time. In the type 9 lakes, there is a tendency to increased vegetation cover and Secchi depth, while in the type 10 lakes there is a tendency to a larger number of fish.
Part of the KT lakes were also investigated before 2004, and for 29 of these lakes at least eight years of data are available. This makes it possible to analyse the development of the individual lakes in more detail, over the entire period since 1989. This analysis shows that the overall level of eutrophication in the lakes has decreased. Secchi depth has increased, while the levels of chlorophyll a, total phosphorus and total nitrogen have declined. In about half of the lakes improvements have occurred, while the other half remains unchanged. The biggest changes occurred during the 1990s, but for some parameters, the development continued into the 2000s.
In the period 2011-2017, 447 lakes in risk of not complying with the goals for nature and the environment as far as their environmental state is concerned, were investigated. 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 programme included in 2011-2017 ten types of lakes, of which lake types 9 and 10 are the ones most commonly investigated and together they 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 most nutrient-poor lake types are type 1 (lime poor, non-brown water, fresh), type 2 (as type 1, but deep) and 10 (calcareous, non-brown water, fresh), which also have the lowest concentrations of chlorophyll and the highest Secchi depths.
The depth limit and coverage are largest in the three nutrient-poor lake types (type 1, 2 and 10) that also have the largest Secchi depth. The average species number of submerged macrophytes is highest in lake type 10 and lowest in lake type 15.
Climatically, 2017 was slightly warmer than the average for the past 26 years - the annual mean temperature for the whole of Denmark was 8.9 °C in 2017 against 8.6 °C for the period 1990-2017. Especially in the months of May and October, the temperature was higher than normal. Compared with the period 1961 to 1990, the temperature was 1.2 °C higher in 2017.
The amount of precipitation in 2017 was higher than usual, 847 mm relative to an average of 714 mm for the period 1961-1990 and 761 mm for the period 1990-2017. In particular the period June-October was rich in precipitation. The area-specific freshwater run-off in 2017 was 357 mm, which is slightly higher than the average for 1990-2017.
The analyses of total nitrogen and total phosphorus were flawed in 2016. Therefore, the two parameters are not included in the reporting of data from 2016. There may also be errors in the analyses of total phosphorus and total nitrogen since 2008, and interpretation of these data must so far be treated with caution.