4.3. Monitoring of lakes
Norway, Finland and Sweden have numerous lakes accounting for approximately 5-10 per cent of the surface area. A large number of lakes can also be found in Denmark, the northern part of Germany, Ireland and the northern and western parts of the United Kingdom. In central Europe most natural lakes are situated in mountain regions, the ones at high altitude being relatively small and the one in the valleys being the largest, examples being Lake Geneva, Lake Constance, Lake Garda and Lake Maggiore in the Alps. In addition, several lakes can be found in Austria and the northern parts of Greece. In Portugal, Spain, France, Belgium, The Netherlands, southern England, and the central parts of Germany there are generaly few natural lakes. In these areas man-made lakes such as reservoirs and ponds occur more frequently than natural lakes. In Spain, for instance, there are more than 1,000 large reservoirs.
Chemical and physical assessment of lake water quality
Only few countries in the EEA area have national monitoring programmes for assessment of the chemical and physical water quality of lakes. Some countries, however, undertake local monitoring of lakes, the German Federal States (Länder), for instance, monitor the environmental state of lakes in their respective areas. Local lake monitoring activities are generally not standardized at a national level, and the variables and sampling frequency vary. During the last 10-15 years some countries have made national lake inventories and collected data and elaborated reports on the general environmental state of lakes based on locally gathered information.
In the Nordic countries, in which there are many natural lakes, monitoring programmes cover a vast number of lakes. Finland, for instance, has been monitoring a wide range of lakes since the early 1960s. During the 1980s Norway and Swden have made lake surveys to assess the extent of acidification and eutrophication. Some countries have a long tradition for monitoring large nationally important lakes, Austria has by way of example monitored Lake Constance (AU-L1) and Neusiedler See since 1961 and 1972, respectively, and the Norwegian Lake Mjösa has been studied since 1971 (NO-L3). Several countries, for example the Netherlands and Portugal, do not have a specific lake monitoring programme, but include their lakes in river or inland water programmes.
In Table 4.3.1 13 general monitoring programmes aimed at chemical and physical assessment of lake water quality are listed. The main national lake monitoring programme of each country is listed first.
Network set-up and sampling frequency
The general national monitoring programmes can be divided into two categories. The "Survey-type" programme typically covers a great number of lakes that are sampled at long intervals. Examples of this type are NO-L2 and SE-L1, including around 100 lakes each. In Ireland a national lake survey (IE-L1) was performed in the period 1987-1990, including a total of 170 large lakes and some representative smaller lakes. Additionally, a remote sensing survey including 360 Irish lakes was performed in 1989-1990. More intensive monitoring programmes with a sampling frequency of several times a year (eg. DK-L1, SE-L3) typically cover a smaller number of lakes. Survey-type lake monitoring programmes provide a general description of the environmental state of a wide range of lakes, whereas more frequent monitoring provides information on dynamics and seasonal variation that may be used to detect trends.
Table 4.3.1: National lake monitoring programmes.
|Country||Code||Lakes no.||Sites no.||Samp. freq.||No of determinands|
|Austria||no national lake monitoring programme1|
|France||no national lake monitoring programme2|
|Germany||no national lake monitoring programme3|
|Greece||no national lake monitoring programme4|
|The Netherlands||no national lake monitoring programme6|
|Portugal||no national lake monitoring programme7|
|United Kingdom||no national lake monitoring programme9|
1AU: Local monitoring of lakes. A national monitoring programme is planned. 2FR: Local monitoring of lakes.
3GE: Local monitoring of lakes by Länder.
4GR: The inland surface monitoring programmes GR-R1 and GR-R2 include also sampling of lakes, but the number of lakes and the analysis programme is unknown.
5IE: Past programmes. A new programme is in preparation.
6NL: The inland water monitoring programme R1 includes several large lakes.
7PT: The inland water monitoring programme R1 includes several reservoirs.
8ES: The R1 river monitoring programme includes a few reservoirs.
9UK: Local monitoring of lakes.
10 The purpose of the programmes is monitoring of toxic blue-green algae.
A total of 85 variables are measured in the lake monitoring programmes, but only 42 are represented in more than two programmes. In the following we will mainly consider variables present in at least three monitoring programmes. A brief overview of the main variables according to categories measured in lake monitoring programmes is presented in Table 4.3.2.
Table 4.3.2: Chemical and physical water quality variables measured in at least two lake monitoring programmes. Mnemonic codes are given in parenthesis.
|Organic matter & oxygen conditions||Nutrients/ Eutrophication||Acidification||Specific ions|
water temperature (TEMPW);
dissolved oxygen (OX);
Biological Oxygen Demand (BOD);
Chemical Oxygen Demand (COD);
Total Organic Carbon (TOC)
dissolved oxygen (OX);
ammoniacal nitrogen (NH4N)
dissolved reactive phosphate (PO4P);
total or Kjeldahl nitrogen (NTOT);
oxidized nitrogen (NO23N);
ammoniacal nitrogen (NH4N);
chlorophyll a (CHLA);
Secchi disc transparency (SDT)
|See section 4.4||chloride
|Metals||Org. micropollutants||Microbiological||Biological organism|
|see section 4.6||total coliform bacteria (COLITOT); faecal coliform bacteria (COLIFAEC); faecal streptococci bacteria (STR_FAEC)||macroinvertebrates
Variables describing the basic chemical and physical properties of lake water are included in most programmes (Table 4.3.3). Generally, the analysis programmes include measurement of water temperature (TEMPW), pH (PH), dissolved oxygen (OX) and conductivity (COND), whereas colour (CNR), turbidity (TURB) and absorbance (ABS) are only measured in a minority of programmes. Basic variables are missing in FI-L2/L3. These programmes are specifically concerned with biological monitoring(-L2) and monitoring of bioaccumulating compounds(-L3). Also NO-L1 (eutrophication) lacks the most essential basic variables.
Table 4.3.3: Basic variables measured in national lake monitoring programmes. For each monitoring programme the number of annual samples are listed.
Organic matter and oxygen conditions
Five summary variables as indicators for the concentration of organic matter are measured in the various lake monitoring programmes: Total Organic Carbon (TOC), Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD), suspended particulate solids (SM) and Turbidity (TURB).
Table 4.3.4: Summary variables for organic matter, dissolved oxygen and ammonium measured in national lake monitoring programmes. For each monitoring programme the number of annual samples are listed.
As trophic state is an important criterion for evaluating the ecological condition of lakes, variables related hereto are commonly measured in the various lake monitoring programmes. The variables can be divided into two groups: nutrients (phosphorus and nitrogen), and variables describing the response to the trophic state (Secchi disc transparency and chlorophyll a). Nearly all the lake monitoring programmes include measurement of total phosphorus (PTOT), total nitrogen (NTOT), oxidized nitrogen (NO23N) and chlorophyll a (CHLA). Measurement of dissolved reactive phosphate (PO4P), ammonium nitrogen (NH4N) and Secchi disc transparency (SDT) is included in most of the monitoring programmes. In the lake monitoring programmes with more than one annual sample the sampling frequency of eutrophicatoin indicators varies from 3-4 annual samples in the FI-L1, NO-L1 and SE-L2 monitoring programmes to 19 annual samples in the Danish lake monitoring programme (DK-L1).
Table 4.3.5. Indicators of eutrophication measured in national lake monitoring programmes. For each monitoring programme the number of annual samples are listed.
Specific major ions
Many of the lake monitoring programmes include measurement of the major cations: calcium (CA), magnesium (MG), sodium (NA) and potassium (K), and the major anions: sulphate (SO4) and chloride (CL). Silica (SiO2) is also measured in several lake monitoring programmes.
Table 4.3.6: Major specific ions measured in national lake monitoring programmes. For each monitoring programme the number of annual samples are listed.
Up to ten metals are measured in the various national lake monitoring programmes. Some monitoring programmes include only determination of very few metals such as, for instance, iron (FE) in the Danish lake monitoring programme (DK-L1), while the Swedish and Finnish lake monitoring programmes include measurement of several metals: iron, manganese (MN), aluminium (AL), cadmium (CD), chromium (CR), copper (CU), mercury (HG), lead (PB) and zinc (ZN).
Table 4.3.7: Metals measured in national lake monitoring programmes. For each monitoring programme and variable the number of annual samples are listed.
Biological assessment of lake water quality
Biological variables are direct indicators of environmental quality. While chemical and physical assessment measures variables decisive for environmental quality, biological assessment gives a direct description of the ecological impact. For instance, biological variables are important in the assessment of water quality with respect to eutrophication and acidification.
Biological variables are part of the sampling routine of many general lake monitoring programmes (DK-L1, NO-L1/L2, SE-L1/L2/L3) as well as programmes concerning specific localities (AU-L1/L2, NO-L3). The Finnish FI-L2 programme only includes biological variables. There are several ways of expressing biological variables. A phytoplankton community, for instance, may be expressed as a species list (qualitatively), or as biomass, biovolumes and cell counts (quantitatively). Furthermore, there may be differences in taxonomic resolution (species level, family level). Sampling and investigation of phytoplankton (PHYTPL) and zooplankton (ZOOPL) are components of several monitoring programmes. Apart from a general evaluation of the phytoplankton community, the objectives of some programmes are more specific such as assessment of the occurrence of potentially toxic blue-green algae in waterbodies used for bathing or drinking water supply (eg. UK-L1). Bottom fauna (INVERT), macrophytes (MAPHYT) and fish fauna (FISH) are also objects of study in some of the lake monitoring programmes.
Table 4.3.8: Biological organisms studied in national lake monitoring programmes. For each monitoring programme and variable the number of annual samples are listed.
Measurement of specific constituents of sediment and biota
In addition to monitoring of lake water quality and ecological state it may be relevant to examine the concentrations of various elements in sediment or living organisms, especially when dealing with pollution variables such as heavy metals and organic micropollutants, but also when dealing with eutrophication problems data on sediment nutrient contents is important. The most commonly measured metals are mercury, cadmium, lead, chromium, copper, nickel and zinc.
The following monitoring programmes include analyses of substances in sediment or biota:
DK-L1: Nutrients in sediment (every 5th year).
FI-L3: Metals and organic micropollutants in sediment, fish and clams.
NO-L4: Metals in sediment (yearly).
NO-L5: Survey of metals in sediment and mercury in fish (1986-1991).
SE-L2: Metals and nutrients in sediment (every 10th yr).
Mercury in fish (every 3rd yr).
For references, please go to www.eea.europa.eu/soer or scan the QR code.
This briefing is part of the EEA's report The European Environment - State and Outlook 2015. The EEA is an official agency of the EU, tasked with providing information on Europe's environment.
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