Johansson, L.S., Søndergaard, M., Andersen, P.M. & Sørensen, P.B. 2023. Søer 2021. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 114 s. - Videnskabelig rapport nr. 528
http://dce2.au.dk/pub/SR528.pdf
About the monitoring programme
The monitoring programme for the Danish lakes has continuously been adjusted since its implementation in 1989 after the adoption of the first National Aquatic Monitoring Progamme in 1988. The most important requirements for lake monitoring are now being undertaken based on the two EU directives - The Water Framework Directive (European Union 2000) and the Habitats Directive (European Union 1992). This means that the monitoring as from 2010 consists of surveillance monitoring and operational monitoring of lakes.
According to the Water Framework Directive, the purpose of the surveillance monitoring is to “document the state of the lakes and the general development, including climatic-related and anthropogenic changes”. This part of the surveillance monitoring includes monitoring of the general state of lakes >5 hectares (KT lakes, a total of 180 lakes), monitoring of the development in lakes >5 hectares (KU lakes, a total of 18 lakes), assessed on the basis of long time series. In total, the 198 lakes > 5 hectares constitute approx. 1/3 of all Danish lakes in this size group. In addition, surveillance monitoring is carried out in accordance with the Habitats Directive, which is intended to provide a general picture of the state and development of the individual protected lake habitats.
The operational monitoring forms the basis for the establishment of the ecological state, the chemical state and the effort required in lakes where you are in doubt about the state. In addition, the operational monitoring provides data documenting the effects of national aquatic environment plans, river basin management plans, nitrate action programmes and other administrative measures. In the period 2016-2021, investigations of water chemical conditions were on average made every year in 95 lakes, of vegetation in 50 lakes and of fish in 11 lakes.
The content of the various monitoring programmes is adapted to the individual descriptions of objectives, both in terms of parameters, frequencies and the number of lakes. A number of chemical, physical and biological parameters are monitored to describe the state and development of the lakes, but not all parameters are monitored in all lakes, and the frequency varies between the individual types of monitoring.
The physical-chemical parameters include measurements of the concentrations of phosphorus and nitrogen, chlorophyll a, alkalinity, pH, oxygen and temperature profiles, conductivity, colour and Secchi depth. Different groups of environmentally hazardous substances are measured in the sediment from selected lakes, in muscle tissue from fish and to a limited extent in the water phase of the lakes. The biological parameters include studies of submerged macrophytes, fish and phytoplankton as well as benthic invertebrates and phytobenthos.
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), 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.
Surveillance monitoring of lake environmental state
The surveillance monitoring of lake environmental state covers a total of 180 KT lakes monitored during the period 2016-2021. The majority of the lakes are relatively small (median lake area 23 ha) and shallow (median depth 1.6 m).
Water chemical conditions
The KT lakes are generally nutrient-rich with a median total phosphorus value (summer mean) of 0.085 mg/l and a median total nitrogen value of 1.5 mg/L. The lakes are generally turbid with a median value of Secchi depth (summer mean) of 1.1 m and a chlorophyll content with a median of 35 μg/L (summer mean). Among the four most common lake types (lake type 9, 10, 11 and 13), lake type 11 (high alkalinity, non-brown water, shallow and saline) is the most nutrient-rich with a median phosphorus concentration of 0.169 mg/L and a median nitrogen concentration of 1.76 mg/L. It is also lake type 11 that has the highest median values of chlorophyll a (51 μg/L). The most nutrient-poor among the four most frequently occurring lake types is lake type 10 (high alkalinity, non-brown water, fresh and deep), where the median phosphorus concentration is 0.050 mg/l, and the median nitrogen concentration is 1.04 mg/L.
Since the monitoring programme was initiated, sufficiently long time series have been obtained for many of the KT lakes to allow assessment and testing of the development trends for the individual lakes. Among the 71 KT lakes with long time series, statistically significant changes of 0.1-10% significance level for the period 1989-2021 can be traced in nutrient concentrations, chlorophyll and Secchi depth in around half of the lakes. In the vast majority of cases, the changes during the whole period are positive in that the nutrient and chlorophyll concentrations have declined, while the Secchi depth has increased. The total phosphorus concentrations have thus decreased in 38 lakes and only increased in three lakes, while the chlorophyll concentrations have decreased in 37 lakes and increased in three lakes.
Considering only the development in the past 20 years, there are far fewer lakes with significant changes. This is primarily due to the fact that there are far fewer lakes, from which data are available from a sufficient number of years to test the development. Among the 13 lakes on which there are at least eight years of data, the development is mainly positive, which means that nutrient and chlorophyll concentrations have been reduced, while Secchi depth has increased. A comparison of the last two monitoring periods (2010-2015 and 2016-2021) shows that the concentrations of total phosphorus, total nitrogen and chlorophyll have generally increased significantly. The development is most significant for total nitrogen and chlorophyll, where most lakes exhibited higher values in the period 2016-2021 than in the period 2010-2015. This indicates a general negative development in the lakes during the last 12 years.
Phytoplankton
For the 180 KT lakes as a whole, the phytoplankton is dominated by blue-green algae (cyanobacteria), which average 52% of the total summer average biovolume. The largest amount of phytoplankton (expressed as the median value of biovolume) among the four lake types with data from at least 10 lakes is generally seen in lake type 11 (high alkalinity, non-brown water, saline, shallow). The average biovolume increases with increasing phosphorus concentrations, while the nitrogen concentrations mainly affect the brackish lake types.
Considering the development of the biovolume and composition of phytoplankton in the KT lakes using data from 143 lakes that were studied in both 2010-2015 and 2016-2021, the changes are generally modest. In the statistical test, and if contemplating all the lakes as a whole, the total biovolume has increased significantly, but for some of the lakes there are major differences between the two study periods. As for blue-green algae, there are both lakes with a reduced biovolume and some with a markedly increased biovolume from the first to the second period.
Vegetation
The distribution of submerged macrophytes in the 180 KT lakes varies greatly – from turbid lakes without plants to shallow and clear-water lakes where most of the bottom is covered. In the most common shallow lake type (lake type 9), the median value for the macrophyte-covered area is 16.7%, and in the most common deep lake type (lake type 10) the median value for the maximum depth limit of the macrophytes is 3.7 metres.
A comparison of the distribution of submerged macrophytes from the period 2004-2009 with the period 2016-2021 shows a big difference between the two periods. For lake types 9 and 10, the plant-covered area has increased, for type 9 the plant-filled volume has increased, while for type 9 and 10 and the lakes as a whole there has been an increase in the depth boundary. Between the last two six-year periods, no changes have occurred for any of the parameters used.
Fish
With regard to fish, the largest number and the largest biomass are generally found in the shallow, fresh, high alkaline lake type (lake type 9), where the median value was 151 fish caught per net and 4.4 kg per net. This is somewhat higher than for the corresponding deep lake type (lake type 10), which presumably reflects the lower nutrient level in the deep lakes.
There are no clear development trends in the fish communities between the two most recent study periods (2010-2015 and 2016-2021). If the two periods 2004-2009 and 2016-2021 are compared, the changes are slightly more significant. In lake type 9, there has been a significant reduction of both the weight of fish caught in the survey nets, the proportion of cyprinids and the average individual biomass. The latter two also apply to the lakes as a whole.
Surveillance monitoring of lake development
The 18 KU lakes included in the monitoring of development, most of which have been followed since 1989, cover a broad spectrum, both in terms of size and depth and the concentrations of nutrients.
Phytoplankton
It is now possible to assess the development since 1989. In general, both the total biovolume and phytoplankton composition vary considerably from year to year in many of the lakes – it is not unusual to have a difference of a factor of 5 or more in biovolume within a few years. In some lakes, also a change in biovolume seems to have occurred in the monitoring period.
Statistical tests for the 16 lakes, on which sufficient data are available, show that the total biovolume has decreased significantly in five of the 16 lakes and increased in one. If looking only at the past 20 years, there is a significant reduction in three lakes. In the majority of the lakes, no significant changes have occurred either throughout the entire monitoring period (1989-2021) or in the past 20 years. The phytoplankton groups, which have changed in most lakes, are dinoflagellates, which have increased significantly in six lakes and decreased significantly in three lakes, and blue-green algae, which have increased in three lakes and decreased in four lakes.
Mass balances
In ten of the KU lakes with well-defined water balances, it is possible to quantify nutrient balances and source apportionment. Compared to previously, an improved calculation of the water balance has been used as well as updated estimates for scattered dwellings. This update generally led to a reduction of the contribution from scattered dwellings, which decreases the flow-weighted concentrations of nitrogen and phosphorus in the water flowing to the lakes from point sources.
Especially for phosphorus, the most pronounced changes in the nutrient input to the lakes were found in the beginning of the monitoring period. The relative nutrient retention in the lakes demonstrates great variations and generally no significant changes. However, there is a minor phosphorus retention in the beginning of the period (1990-1994), which is attributed to a response to the release of a historical pool of phosphorus. There is an inverse trend for nitrogen, where the retention is greatest in the start of the period.
Environmentally hazardous substances
Sediment
In the period 2010-2021, studies were conducted of environmentally hazardous substances in lake sediment within the following groups: metals, aromatic hydrocarbons, phenols, polyaromatic hydrocarbons (PAH), plasticizers, organotine compounds and pesticides. Samples were taken in 85 lakes in the surveillance monitoring and 240 lakes in the operational monitoring. In most lakes, all metals, aromatic hydrocarbons and PAH were found above the detection limit, which also applies to some of the plasticizers (DEHP and diisononylphthalate) and monobutyltin. The rest of the substances were only found in a minority of the lakes. Among the substances for which environmental quality standards have been set (MKK) for sediment, concentrations higher than MKK were detected for the following: lead (12 lakes), vanadium (22 lakes), the sum of the methylnaphtalenes (49 lakes), naphtalene (13 lakes), nonylphenols (1 lake) and anthracene (115 lakes). There is generally no difference in concentration levels between the results from the surveillance monitoring and the operational monitoring.
When comparing the substance concentrations between 2010-2015 and 2016-2021, a significant difference was seen for zinc, monobutyltin and acenaphtene, where the values were highest in the last period, while the opposite was the case for the substances 4-tert-octylphenol, nonylphenol-mono-ethoxylate and acenaphtylene.
Water
In the years 2012 and 2020, monthly samples were taken of 21 pesticides in the water of eight lakes in the operational monitoring. Nine of the substances were not found above the detection limit in any of the samples. Glyphosate and its degradation product AMPA, 2.6-dichlorbenzamide (BAM) and trichloracetic ACID (TCA) were the most frequently occurring substances, being found in 22-61% of the samples. Environmental quality standards have been set for nine of the pesticides studied, and no cases of concentrations higher than the environmental quality standards were recorded.
For Denmark as a whole, the average temperature of 2021 was 8.7 º C, which was the same temperature as the average for the period 1990-2020. January and February were colder in 2021 than the average, while June and July were warmer. Global radiation in 2021 was also the same as the average for the period 1990-2020. The average annual wind speed for Denmark as a whole in 2021 was 4.3 m/s, which is lower than the average for the period 1990-2020 (4.9 m/s).
In 2021, precipitation was 744 mm, which is also at the same level as the average for the period 1990-2020 (762 mm). There were major variations during the year, and in May 2021 precipitation was almost twice as high (107 mm) than the average for the period 1990-2020. February, April and June 2021 were, on the other hand, rather dry compared to the average for 1990-2020. The rest of the year, precipitation was more normal, apart from a relatively wet October and a dry November. The area-specific freshwater run-off in 2021 was 304 mm, which is 6% lower than the average for the period 1990-2020 (324 mm).
