IW4 Acidification

>> Background information

Development

Acidification refers to the decreasing ability of water bodies to neutralize acid compounds. Finnish lakes have received acid deposition since the 1950s, when the burning of fossil fuels increased and as a result the oxides of sulphur and nitrogen were released into the atmosphere. At the end of the 1970s, the acidifying effects of the oxides of sulphur and nitrogen became obvious, and international restrictions for their emissions were set. In late 1980s the Finnish lakes begun to recover as their buffering capacity or alkalinity started to increase again. The recovery has continued throughout the 1990s.

Fish kills in rivers are related to acidification caused by leaching of acid compounds from acid sulphate soils or the AS soils. AS soil sites are found in coastal areas, west coast in particular. In short term the amount of acidifying discharge into rivers from AS soils depends on the amount of run-off. Ever since the beginning of the 20th century, the main cause for increased leaching of acid compounds has however been drainage. Drainage exposes sulphide-bearing marine sediments in coastal regions to oxidation and promotes the development of AS soils.

Subsurface drainage became common on fields in 1950s and 1960s. It caused ground water level to drop down and the depth of oxidation to increase. According to some estimates the discharge from AS soils has as much as doubled during that time. This was followed by extensive fish kills in the rivers of the Ostrobothnia region in the early 1970s. Since the end of the 1970s the amount of acid discharge from AS soils has remained approximately the same. Another period with massive fish kills occurred just in 2006. The available record is however suggestive, although it does reveal the general trend.

Impact on biodiversity

Small forest lakes are affected by acidification from atmospheric deposition the most. Finnish lakes are typically sensitive to acidification because their buffering capacity is naturally low. Many lakes are also relatively acid, since they are affected by humic acids which leach from forests and mires. Acid deposition has been the greatest in southern Finland but the clear water lakes in the north have usually been the most sensitive ones.

Acidification damages fish populations, mainly perch and roach in small lakes. It also affects for example zoobenthos and algae, some of which are very sensitive to changes in pH. In addition, potentially toxic compounds are released in acid conditions from the soil into water bodies. These include for example certain metals (see also IW3). The organisms in acidified lakes have been observed to recover after the chemical conditions have become more favourable. Complete recovery may however take time, even decades in case of the most sensitive species.

Rivers of the western coast remain affected by AS soils, as drainage on these sites continues. The impact on river ecosystem depends on the proportion of AS soils in the catchment area as well as weather conditions and hydrology. They determine the severity, duration and frequency of low pH periods. Thus the phenomenon is not straightforward, but follows a complex rhythm with very different periods following each other. In addition, some smaller rivers have become chronically acid, which is why new fish kills have not been observed. The seriousness of the phenomena should nevertheless be appreciated, since fish kills are an extreme reaction to changes in the ecosystem.