State of the Environment

State of Lake Inarijärvi

The large and oligotrophic Lake Inarijärvi is situated upstream and upwind from the pollution sources.

Water level regulation has a significant effect on the ecological condition of the lake. Regulation-caused detrimental effects on the littoral biota have decreased in the 2000’s due to the use of a new regulation policy which is better for the aquatic environment. The clearest effects can be seen as lowering of the highest water levels and prospects of recreational use becoming more common. There are also positive changes in fish stock. Changes in the hydrological conditions of Lake Inarijärvi are related to global warming. Average temperatures of surface water and water column in the summer were increasing during the monitoring period in 1961-2009.

Water quality

The water quality of Lake Inarijärvi is mostly excellent. No clear signs of eutrophication have been observed. Sediment material


In the studies on water, sediment means the layered earth that has drifted and fallen out to the bottom of a water body.

Sediments are usually formed on the bottoms of seas, lakes and rivers. By analysing sediment layers, information may be obtained on the loading on the waterways and its fluctuations in the long-term.

For instance, sediment studies are used to identify changes in the state of waterways, as well as climate trends during different periods.

indicated slight eutrophication and the proliferation of some phytoplankton species over the past few decades may be a consequence of exceptionally warm summers. A few small regional differences are evident in water quality fluctuations, which are mainly due to the organic humic matter brought by the rivers and the nutrients it contains.

Over 70 % of nitrogen and phosphorus entering the lake comes from natural leaching of the soil. 20-25 % of nutrients come from airborne particles as fall-out of industrial emissions. The remaining approximately 5 % of loading is diffuse and point loading caused by humans. Its impact is minimal with respect to the large size of the lake, so it only has local significance for water quality.

The water is clear and has low humic content. The acidification trend that posed a threat in the 1980’s has now ceased, and recently some slight return of buffering capacity has been noticed. The water of the lake is potable throughout the lake, with the exception of sections located in the immediate vicinity of settlements.

Phytoplankton and zooplankton

Lake Inarijärvi may be regarded as nutrient-poor based on the amount of phytoplankton, and even as very nutrient-poor in its northern sections. There is almost 50 % more phytoplankton in the southern sections of the lake than in the northern sections, which correlates with the fluctuations in phosphorus content between different sections of the lake. The differences are not that evident in the composition of species, but merely in the abundance. Based on phytoplankton, Lake Inarijärvi is in excellent condition.

In lakes with low nutrient levels phytoplankton species normally comprise small-sized species and the diversity of chrysophytes is high. The most abundant phytoplankton groups are chrysophytes, cryptophytes, diatoms and dinoflagellates. Cyanobacterial community is typical of nutrient-poor lakes and comprises small-celled species and species forming small communities.

The zooplankton of Lake Inarijärvi comprises species typical of cool and clean lakes. Rotifers and cladocerans (water fleas) have the highest species abundance. In addition to these, the species composition also included protozoa and copepods. Although there are more species of rotifers than other zooplankters, their proportion of the biomass is small because they are small-sized.

The average zooplankton biomass is significantly small compared to reference lakes in Northern Finland, which is due to the lake being nutrient-poor and the short open-water period. In particular the numbers of water fleas are comparatively low. As with the phytoplankton, there is more zooplankton in the more nutrient-rich southern parts of the lake than in the northern sections. Their numbers also increase when moving from open parts of the lake to sheltered bays. Total biomass and abundance ratios of species fluctuate somewhat from one year to another.

Over the past decades the numbers of zooplankton may have been affected by fluctuations in fish populations, in particular, vendace. The populations of plaktivorous fish are greater in regions that have a large population of water fleas.


Macrophytes are aquatic plants that live completely or partly in water. Water lever regulation is the main stressor on the natural macrophyte community of Lake Inarijärvi because the anthropogenic nutrient load is estimated to be small. This can also be seen from the present plant species, because most of the detected species are sensitive to eutrophication and thus would not be found in Lake Inarijärvi if it were eutrophied due to nutrient loading.

The effects of water lever regulation on macrophytes are nevertheless small, and the vegetation has adapted to them well. The ecological state of Lake Inarijärvi’s macrophyte community is estimated as high. However, regulation has induced some changes in Lake Inarijärvi, for example, spring-flood dependent vegetation has a smaller distribution than in unregulated lakes.


Zoobenthos of Lake Inarijärvi has been studied since the 1960’s. Already then a lesser amount of zoobenthos was found in the littoral zone of Lake Inarijärvi than in nearby lakes Muddusjärvi and Nitsijärvi which are not regulated. Also, species that are known to be sensitive to regulation were observed in these reference lakes but not in Lake Inarijärvi. Despite the new regulation program which has been in use since beginning of the 2000’s, the littoral zoobenthos of Lake Inarijärvi still slightly suffers from regulation. The noticed anomalies in the littoral biota of Lake Inarijärvi are similar to regulation-related effects in other regulated lakes.

Condition of zoobenthos was measured with five different parameters: taxon composition, abundance ratios, important taxonomic groups, ratio between vulnerable and non-vulnerable taxons, and overall diversity. Condition of the rocky shores of Lake Inarijärvi has been classified as “good”, based on the last three monitoring years (2003, 2008, and 2012). Correspondingly, the condition of soft bottoms has been classified as “moderate” during the same monitoring period. The average condition of zoobenthos is good.

Fish populations and fish stocking

There are ten indigenous species of fish living in Lake Inarijärvi; perch, pike, burbot, minnow, three-spined stickleback, nine-spined stickleback, lake brown trout, Arctic char, grayling, and whitefish in its many forms. Many parameters indicated that the development of fish stock and catch started to grow in the beginning of the 2000’s.

In addition, Lake Inari has been stocked with vendace, lake trout and freshwater trout. Vendace has been stocked into the River Kemijoki watercourse region upstream from Lake Inari in the 1950’s and 1960’s, after which it has spread throughout the lake and established a natural, occasionally very prolific, breeding population. The populations of vendace increased rapidly in the early 1980’s, only to fall dramatically a decade later. After that population has stabilized again to normal levels. Freshwater trout from the Vuoksi watercourse has been stocked to Lake Inari since 1971, and the last stocking was made to River Ivalojoki in 2001. Lake trout from North America was stocked to Lake Inarijärvi for the first time in 1972, but stocking was ceased in 2012. So far there hasn’t been any indication of the lake trout reproducing in Lake Inarijärvi. The state of both species is followed with regular sampling and researching.

Stocking has been conducted as part of the so-called obligatory stocking since 1976 and it is used to compensate local fish stock for the harm caused by regulation of the watercourse. Obligatory stocking takes place in Lake Inari and its tributary waters. According to this obligation, each year fry of whitefish, lake brown trout or freshwater trout and arctic char are to be stocked into the lake. Lake brown trout or river whitefish fry are stocked into tributary waterways. Occasionally, mainly in the 1990’s, the lake has been stocked with more fish fry than is sensible considering the nutrient-poorness of the lake. Both whitefish and lake brown trout stockings have been diminished nowadays. There will be no large changes in the stocking amounts in the 2010’s.

Summary report ”Development of the status of Lake Inari between 1960 and 2009” (in Finnish, Norwegian, and Russian) provides a comprehensive overview of the status of Lake Inarijärvi and its development over the past decades.

Images: Pekka Räinä, Lapin ELY