The effect of climate change on water quality
The majority of Finland's water bodies have been rated as either high or good in terms of ecological status. Water quality issues are down to rising temperatures and high nutrient loads.
Water quality is affected by many factors
The majority of Finland's lakes, rivers, brooks, and artificial water bodies have been rated as either high or good in terms of ecological status. Water quality issues are down to high nutrient loads, low oxygen levels, increasing salinity, acidification, rising temperatures, erosion, poor microbiological quality, and various toxins and pesticides, for example.  Water bodies are sensitive to contamination . Industrial emissions cause water pollution and acidification, and nutrient leaching from agricultural land causes eutrophication in water. 
Global warming is a risk to water quality
The effects of climate change on water bodies are mostly attributable to temperature variations. Temperatures are believed to increase by between two and six degrees in Finland by the end of the current century . According to estimates produced as part of the Finnish Research Programme on Climate Change, water surface temperatures may increase by between one and two degrees by 2050. Temperatures are set to rise especially in Southern Finland.  Temperature variations affect oxygen levels in water, reduction potential, lake stratification and mixing, and biological growth. Rising temperatures are likely to increase algal blooms as well as the prevalence of bacteria and fungi in water bodies. High temperatures may also increase the release of harmful volatile and semi-volatile compounds from water into the atmosphere. Examples of these kinds of compounds and substances include ammonia, mercury, dioxins, and different kinds of pesticides. 
Rising temperatures may also accelerate eutrophication. The primary production of microalgae increases in eutrophic water, causing turbidity. This prevents light from penetrating into the water and increases oxygen demand. Oxygen levels may decrease especially near the base of the water column as a result of higher volumes of organic matter sinking to the bottom. Climate change has both direct and indirect effects on water quality. Indirect effects include, among others, higher levels of nutrient leaching especially from land used for agriculture and forestry during mild winters. The relative volume of surface runoff also increases when the soil is frozen. 
Will heavy rainstorms become more common?
Rainfall may increase in Finland as a result of climate change by up to 20 percent in the summer and by between 10 and 40 percent in the winter by the end of the current century. . Depending on the rainfall scenario, the increasing rainfall is likely to increase annual surface runoff by up to 60 percent by the end of the century . Increasing summer rainfall may not necessarily lead to higher flow rates, because evaporation will also increase in the higher temperatures . Annual surface runoff may actually decrease slightly around the great lakes of Southern and Central Finland due to lake evaporation . Increasing rainfall is nevertheless a risk to water quality. Higher rainfall increases the leaching of nutrients, pathogens, and harmful substances to water. Increasing levels of harmful substances lower water quality. 
In addition to the volume of rainfall, climate change also affects the intensity of rain. Winter rains in particular will become more common, and summer rainstorms will become heavier. Even if total summer rainfall remains relatively unchanged, future summers may be characterised by droughts on one hand and rainstorms on the other. 
Increased rainfall together with the higher intensity of rain accelerates water erosion. These factors also have implications on the stability of coasts and stream banks and on sediment transport. Steep stream banks are vulnerable to erosion and landslides caused by climate change.  Heavy rainstorms may also increase the likelihood of storm surge flooding .
Floods lower water quality
Climate change may increase the likelihood and change the temporal distribution of floods. Flooding presents a number of challenges to water supply and water quality. The capacity of combined sewers and pump stations may be exceeded due to rising water levels, which may result in wastewater having to be discharged directly to water bodies. Trees, ice, and other material carried by flood water may also cause system blockages and therefore increase the need to discharge untreated wastewater to water bodies.
Water quality issues also arise from flood water mixing with drinking water. Surface water may end up in wells both during floods and during the spring melt. Flood water poses a risk to water supply especially in sparsely populated areas. 
Rising sea levels are a threat to coastal freshwater reserves
Sea levels in Finland are affected by post-glacial rebound along the coast as well as the global sea level rise . Sea levels are rising as a result of thermal expansion, the melting of glaciers, and the retreat of the snowline. In addition to changes in the world's oceans, water levels in the Baltic Sea are also affected by the flow rates of rivers that discharge their waters to the Baltic Sea and changes in the seasonal distribution of inflows. Water levels in the Baltic Sea are estimated to be rising at a similar rate to those of the world's oceans.  There are nevertheless many uncertainties associated with these estimates . Rising sea levels may damage the quality of groundwater especially in coastal areas. Higher water levels in the sea may allow saltwater to intrude into aquifer systems, damaging groundwater quality.  
Droughts threaten water quality
In addition to floods, droughts may also damage water quality. Global warming may increase the number of dry days in vast areas of Finland. Lower water levels together with higher levels of nutrients and sediments lower the quality of groundwater and surface water.  Droughts lower flow rates, which causes water quality to deteriorate and oxygen levels to decrease as a result of internal loading. When groundwater levels drop, the levels of iron and manganese may increase as oxygen levels change. Hypoxia is also associated with the production of ammonium, organic matter, methane, and hydrogen sulphide gas, which may make water taste and smell unpleasant.  Moreover, droughts may limit the availability of high-quality drinking water.