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No. 354: Lakes 2018. NOVANA

Johansson, L.S., Søndergaard, M., Sørensen, P.B., Nielsen, A., Jeppesen, E., Wiberg-Larsen, P. & Landkildehus, F. 2019. Søer 2018. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 98 s. - Videnskabelig rapport nr. 354.  http://dce2.au.dk/pub/SR354.pdf

Summary

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 – 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 of the lakes in the control monitoring referred to in this report is shown in Figure 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) and 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 413 lakes >5 ha were investigated (not including the KT and KU lakes already 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.

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), colour (coloured water, non-coloured 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

Development of key parameters

An overview of the four key parameters (total phosphorus, total nitrogen, chlorophyll a and Secchi depth) in the investigated monitoring lakes is given in table 2. Overall, the KU lakes are generally slightly less nutrient-rich and have a lower content of chlorophyll and a greater Secchi depth than the KT lakes. For example, the median value of the summer average chlorophyll a is 41 μg/L in the KT lakes versus 30 μg/L in the KU lakes.

Monitoring of development (KU)

The 18 KU Lakes included in the control monitoring of development cover a wide spectrum of both morphometric (size and depth) and nutrient factors. The summer average content of chlorophyll a varies, for example, between 4 and 202 μg/L and the Secchi depth between 0.3 and 4.0 m. All values are from 2017-2018.

The total phosphorus content has declined as a summer average in 12 of the 15 KU lakes that have been followed since 1989 and is unchanged in three lakes. There have been few changes in the last 10 years, and these are only significant in two lakes. The total nitrogen content as a summer average has declined significantly in 13 out of the 15 lakes since 1989 and only one is unchanged. As for total phosphorus, most changes occurred early in the monitoring period and in the last 10 years the total nitrogen concentration as a summer average has only changed significantly in three lakes.

Since 1989, the development of the KU lakes has generally been in the direction of a reduced content of chlorophyll a and increased Secchi depth. Thus, the summer average of chlorophyll for the entire monitoring period has declined in six lakes and increased in two lakes, while the summer average Secchi depth has increased in eight lakes and decreased in one lake. The main changes occurred in the first part of the period, while the changes in the last 10 years have been modest, with only a few lakes having changed statistically significantly.

Submerged vegetation has been studied in 10 KU lakes since 1993/94 for at least 10 years. There are significant variations from year to year in plant coverage, plant-filled volume and the depth limit of plants. In the 16 KT lakes, which have been statistically tested (at least seven years of data), there was a significant improvement in the plant-covered area in nine lakes during the period 1993-2018 and a decline in two lakes. Focusing solely on the development since 2003, there are fewer lakes where significant changes have occurred, but in these cases most have been in the direction of greater coverage.

Macroinvertebrates in the littoral zone and their biodiversity were investigated for the first time in 16 KU lakes in 2017-2018. The 16 lakes comprise four different types of lakes in which the composition of the macroinvertebrates, especially in the two brackish lakes, differed significantly from the 14 freshwater lakes. Among the freshwater lakes, the difference was relatively small. In total, approximately 240 different taxa were recorded based on more than 100,000 individuals. A flatworm species (Girardia tigrina) was officially observed in Denmark for the first time.

The fish investigations in the KU lakes have been carried out since 1989. The analyses indicate that fish stocks are changing as a result of the generally decreasing nutrient input. There is thus a tendency towards a declining proportion of cyprinids and an increased share of predatory fish in several of the lakes, with the largest changes occurring in biomanipulated lakes and the lakes that were most nutrient-rich at the beginning of the monitoring period.

Contrary to the expectation from the general relations in Danish lakes and the fish index used to determine the ecological quality on the basis of fish, the average weight of fish has diminished markedly in many of the lakes. Since small fish are more planktivorous, this will have negative effects on the zooplankton and thus on the grazing pressure on phytoplankton. This will therefore partly counteract the effects of measures taken to reduce the nutrient input.

Nutrient balances and output sources have been calculated for 10 KU lakes for which well-defined water balances exist. Methods for the calculations have been updated relative to the latest reporting in 2014. In general, the water flow-weighted concentrations of nitrogen and phosphorus flows to and from the lakes have declined. Especially for phosphorus, the major changes occurred at the beginning of the monitoring period and for nitrogen there is a tendency to a slight increase from 2014. The relative nutrient retention shows great variations and generally no significant changes.

Control monitoring of state

The 93 KT lakes examined in 2016-2018 represent seven different types of lakes where lake type 9 (shallow) and lake type 10 (deep) are the ones on which most data are available. Most of the lakes have a high content of chlorophyll (median summer average 41 µg/l) and a relatively low Secchi depth (median summer average 1.14 m).

The vast majority of the 93 KT lakes have now been studied through three periods (2004-2009, 2010-2015 and 2016-2018), and this allows for an assessment of their development. In general, there are large variations between the lakes, but these are only small over the time periods examined. 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 40 of these lakes at least eight years of data are available. Thus, the database is large enough to be able to carry out statistical analyses of the development in each individual lake in more detail over the whole period since 1989. This analysis shows that the overall level of eutrophication in the lakes has decreased. The content of total phosphorus has declined in 20 lakes and only increased in one lake, while the total nitrogen content has decreased in 25 lakes, but not increased in any. Similarly, the chlorophyll content has declined in 19 lakes and not increased in any, while Secchi depth has increased in 19 lakes and decreased in one lake. The analysis also shows that, if contemplating only the last 20 years, the proportion of lakes where significant changes have occurred is pronouncedly lower than when considering the whole period since 1989.

For the KT lakes, fish investigations have only been carried out in about half of the 180 lakes in the last 6-year cycle and an overall assessment of development has therefore not been made in this report. For a number of lakes, data are now available from a longer period if also data from before 2004 are included. Overall, these lakes show an improvement, with decreasing biomass of cyprinids and an increased share of predatory fish. There are examples of lakes, where changes point in the same direction and, as expected, particularly significant changes in lakes undergoing restoration during the period by biomanipulation (removal of cyprinids).

Climate and runoff

Climatically, 2018 was slightly warmer than the average for the past 27 years - the annual mean temperature for the whole of Denmark was 9.4 °C in 2018 against 8.6 °C for the period 1990-2018. Especially in the period May-July, the temperature was markedly higher than normal. Compared with the period 1961-1990, the temperature as an annual average was 1.7 °C higher in 2018.

The amount of precipitation in 2018 was higher than usual, 594 mm relative to an average of 714 mm for the period 1961-1990 and 755 mm for the period 1990-2018. In particular, the period May-July was poor in precipitation. The area-specific freshwater runoff in 2018 was 294 mm, which is slightly higher than the average for 1990-2018 (325 mm).

Error analyses

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 2007, and interpretation of these data must so far be done with caution. At the time of writing, a clarification is being sought on how to deal with the errors.