Chlorophyll-a in lakes
Summary
The amount of chlorophyll-a indicates the quantity of phytoplankton in a water body, which in turn is closely related to the nutrient content and eutrophication of the water bodies. The concentrations of the chlorophyll-a indicator in both humic and low-humic lakes are generally at a good level on average, according to the threshold values set for chlorophyll in national water management. However, it should be noted that the chlorophyll-a indicator alone does not reflect the ecological status of Finnish lakes, as the assessment of ecological status is based on the simultaneous examination of several biological indicators.
Status
Chlorophyll-a concentrations in both low-humic and humic lakes are generally at a level indicating good status. This assessment of the status indicator is based on the good status concentration values assigned to chlorophyll-a in national water management. The concentration threshold for the chlorophyll in water management is less than 11 µg/l in small, medium, and large humic lakes, and less than 7 µg/l in medium and large low-humic lakes (Aroviita et al. 2019).
The indicator value represents the average concentration of chlorophyll-a in a sample of various lakes. The condition of an individual lake relative to the chlorophyll concentration threshold may differ significantly from the average value provided by the indicator. The lakes included in the indicator (see “Data used”) may also have experienced periods where the amount of chlorophyll-a was higher or lower than the good status concentration thresholds assigned to the chlorophyll-a variable in national water management. These deviations can be influenced by factors such as the intensity of monitoring or weather conditions.
Trend
The trend in the indicator value has been slightly increasing during the 2000s which suggests that this aspect of water quality has been slightly declining (see the “Information about the service” page). Since the early 2000s, the concentration of chlorophyll-a has, on average, increased in both clear low-humic lakes and darker humic lakes at a rate of at least 0.1 percent per year with 90 percent certainty.
However, the overall very slight deterioration in lakes indicated by the chlorophyll-a trend during the 2000s is the result of two divergent trends: the amount of chlorophyll-a increased by about one percent per year until the early 2010s, but has since decreased at a similar rate. Therefore, the recent trend for this aspect is improving. If this trend continues, the overall development of the indicator for the 2000s is likely to stabilize or improve in the coming years.
Significance
The indicator reflects one aspect of the condition of lakes —the concentration of chlorophyll-a, which more specifically represents the total amount of phytoplankton present. The amount of phytoplankton (biomass) is closely linked to the trophic status of lakes and the nutrient load caused by human activity. Therefore, an increase in the indicator’s value signifies harmful eutrophication trends in the water body.
The chlorophyll concentrations in lakes primarily differ due to variations in nutrient levels. Additionally, the availability and penetration of light in the water column affect the chlorophyll concentrations in both clear and dark-water lakes, which also explains the differences in their phytoplankton communities. For example, the slime algae Gonyostomum semen, which thrives in dark, humic forest lakes and contains high levels of chlorophyll, may significantly increase the chlorophyll concentrations in some small humic lakes, although this does not generally indicate harmful eutrophication.
Chlorophyll-a concentration is one of the variables of the lake phytoplankton quality factor used in assessing the ecological status of surface waters under the Water Framework Directive (Aroviita et al. 2019, European Community 2000). In addition to phytoplankton, the assessment of ecological status also considers other biological quality factors (fish, benthic invertebrates, diatoms, and aquatic plants). The status indicator discussed here generalizes the overall development of phytoplankton in Finnish lakes and cannot be used to assess the condition of any individual lake in the sample. It is also important to note that chlorophyll-a is only one of the factors determining ecological status, and no single variable alone defines the ecological state of a water body. The assessment of ecological status is based on a collection of different biological quality factors and supporting physicochemical factors.
References
Data used
The monitoring of surface water quality forms one of Finland’s most extensive datasets describing the state of nature and the environment. Systematic monitoring of surface water quality began in the 1960s, and this monitoring has increased especially since 2009, as required by the European Union’s Water Framework Directive. Currently, the water quality registry has accumulated data from over 75,000 observation sites, and it is possible to create long-term time series from the data because observations have been collected from multiple sampling sites on many lakes at least annually. The amount of chlorophyll-a has been monitored in Finland’s largest lakes using comparable methods since the early 1980s.
Long-term monitoring also enables the classification of lakes based on their characteristics into naturally low-humic, humic, and highly humic lakes, allowing for the separate examination of water quality and biological trends in different types of water bodies. Naturally highly humic lakes are few in number and have also accumulated less monitoring data, which is why they have been excluded from the analysis of this indicator.
The 28 low-humic lakes included in the indicator (with the region where each lake is located in parentheses):
Höytiäinen (North Karelia)
Inarijärvi (Lapland)
Juojärvi (North Savo, North Karelia)
Keitele (Central Finland, North Savo)
Kermajärvi (North Karelia)
Kivijärvi (Central Finland)
Kivijärvi (South Karelia)
Kolima (Central Finland)
Konnevesi (Central Finland, North Savo)
Kukkia (Pirkanmaa, Kanta-Häme)
Kuolimo (South Savo, South Karelia)
Leppävesi (Central Finland)
Mallasvesi (Pirkanmaa)
Muojärvi (North Ostrobothnia)
Puruvesi (South Savo, North Karelia)
Puula (South Savo, Central Finland)
Pyhäjärvi (Central Finland)
Pyhäjärvi (South and North Karelia)
Pyhäjärvi (Satakunta and Southwest Finland)
Pyhäjärvi (North Ostrobothnia)
Pyhäjärvi (Kymenlaakso)
Päijänne (Central Finland, Pirkanmaa, Päijät-Häme)
Roine (Pirkanmaa)
Saimaa (South Karelia, South Savo)
Suvasvesi (North Savo, North Karelia)
Viinijärvi (North Karelia)
Vuohijärvi (Kymenlaakso, South Savo)
Yli-Kitka (Lapland, North Ostrobothnia)
The 25 humic lakes included in the indicator (with the region where each lake is located in parentheses):
Haukivesi (South and North Savo)
Kallavesi (North Savo)
Kemijärvi (Lapland)
Keurusselkä (Central Finland, Pirkanmaa)
Kiantajärvi (Kainuu)
Koitere (North Karelia)
Kyyvesi (South Savo)
Lappajärvi (South Ostrobothnia)
Lentua (Kainuu)
Lestijärvi (Central Ostrobothnia)
Näsijärvi (Pirkanmaa)
Ontojärvi (Kainuu)
Oulujärvi (North Ostrobothnia, Kainuu)
Orivesi (North Karelia, South Savo)
Pielinen (North Karelia)
Pihlajavesi (South Savo)
Punelia (Kanta-Häme)
Pyhäjärvi (Pirkanmaa)
Pyhäselkä (North Karelia)
Simojärvi (Lapland)
Sääksjärvi (Satakunta)
Unnukka (North Savo)
Vanajavesi (Pirkanmaa, Kanta-Häme)
Vuotjärvi (North Savo)
Ähtärinjärvi (South Ostrobothnia)
Additional information regarding the data:
Indicator calculation
The indicator includes chlorophyll samples taken since 1980, which were collected during the open-water season (June, July, August, or September) from a depth of 0–2 meters, and from which the concentration of chlorophyll-a was determined spectrophotometrically.
Since the objective of the indicator is to generalize the status and trends in clear low-humic lakes as well as in dark humic lakes based on chlorophyll concentrations, efforts have been made in the calculations to account for differences between lake types, individual lakes, different basins within lakes, and even individual sampling sites. In practice, this is done using so-called hierarchical generalized statistical mixed models, which can account for the structure of the data as accurately as possible, preventing individual extreme observations or sampling sites from significantly distorting the generalizations produced by the model. Such statistical models have been used to determine the status and trend direction of the indicator and to produce the indicator graph.
For the assessment of the indicator’s status, the average chlorophyll-a level over the last five years has been compared to the threshold values set for chlorophyll-a in humic and low-humic lakes, with the latter based on data collected from reference lakes (Aroviita et al. 2019).
The trend direction of chlorophyll concentration has been evaluated based on the average trend from the early 2000s to the present. A more detailed description of how the trend direction of the indicator is classified can be found in the “Information about the service” section. To provide an understanding of the longer-term trend over decades, the trend has also been calculated from the beginning of the time series up to the year 2000. Since changes in the trend direction of the indicator have been observed even in the 2000s, trends have also been calculated for the periods 2000–2010 and 2010–2023 to illustrate these changes.
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Sari Mitikka
Senior research scientist (Syke), Surface water quality monitoring
Marko Järvinen
Senior research scientist, group manager (Syke), Water ecology