Recovery responses of acidified Finnish lakes under declining acid deposition

The present work provides a regional-scale assessment of the changes in acidifying deposition in Finland over the past 30 years and the current pattern in the recovery of acid-sensitive lakes from acidification in relation to changes in sulphate deposition. This information is needed for documenting the ecosystem benefits of costly emission reduction policies and further actions in air pollution policy. The development of sulphate deposition in Finland reflects that of European SO2 emissions. Before the 1990s, reductions in sulphur emissions in Europe had been relatively small and sulphate deposition showed no consistent trends. Due to emission reduction measures that were then taken, sulphate deposition started to clearly decline from the late 1980s. The bulk deposition of sulphate has declined 40-60% in most parts of the country during 1990-2003. The decline in sulphate deposition exceeded the decline of base cation deposition, which resulted in a decrease in acidity and acidifying potential of deposition over the 1990s. Nitrogen deposition also decreased since the late 1980s, but less than that of sulphate, and levelling off during the 1990s. Sulphate concentrations in all types of lakes throughout Finland have declined from the early 1990s. The relative decrease in lake sulphate concentrations (average 40-50%) during 1990-2003 was rather similar to the decline in sulphate deposition, indicating a direct response to the reduction in deposition. There are presently no indications of elevated nitrate concentrations in forested headwater lakes. Base cation concentrations are still declining in many lakes, especially in south Finland, but to a lesser extent than sulphate allowing buffering capacity (alkalinity) to increase. The recovery has been strongest in lakes in which sulphate has been the major acidifying agent, and recovery has been the strongest and most consistent in lakes in south Finland. The recovery of lakes in central Finland and north Finland is not as widespread and strong as observed in south. Many catchments, particularly in central Finland, have a high proportion of peatlands and therefore high TOC concentrations, and runoff-induced surges of organic acids have been an important confounding factor suppressing the recovery of pH and alkalinity in these lakes. Chemical recovery is progressing even in the most acidified lakes, but the buffering capacity of many lakes is still low and still sensitive to acidic input. Chemical recovery is resulting in biological recovery with populations of acid-sensitive fish species increasing. Increasing TOC concentrations are indicated in small forest lakes in Finland, which appear to be related to decreasing sulphate deposition and improved acid-base status of the soil. A new challenge is climate change with potential trends in temperature, precipitation and runoff, which are expected to affect future chemical and biological recovery from acidification. The potential impact of mobilization and leaching of organic acids may become particularly important in Finnish conditions. Long-term environmental monitoring has evidently shown the success of international emission abatement strategies. The importance and value of integrated monitoring approach including physical, chemical and biological variables is clearly indicated, and continuous environmental monitoring is needed as a scientific basis for further actions in air pollution policy. The effect of climate change will increase data requirements, and should be taken into account when assessing long-term surface water quality and developing future monitoring networks, due to more complex processes involved

Preface to the Special Issue “Integrated Monitoring in the Valkea-Kotinen Catchment during 1990–2009 : Abiotic and Biotic Responses to Changes in Air Pollution and Climate”

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Water quality of a small headwater lake reflects long-term variations in deposition, climate and in-lake processes

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Carbon dioxide and energy fluxes over a northern boreal lake

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Responses of zooplankton to long-term environmental changes in a small boreal lake

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Recovery of acidified lakes in Finland and subsequent responses of perch and roach populations

Finnish-lake and fish-status surveys indicated that 4900 small headwater lakes suffered from acidic deposition and 1600–3200 roach (Rutilus rutilus) and perch (Perca fluviatilis) populations were affected or extinct by the end of 1980s. Since the late 1980s, successful sulphur emission reductions in Europe have induced a chemical recovery of acidified lakes. This resulted in decreases in sulphate and labile aluminium concentrations and increases in pH and alkalinity during the 1990s. The first signs of recovery in affected perch populations were observed in the early 1990s. New strong year-classes appeared and the population structure returned to normal. Little if any recovery of the affected populations of the more acid-sensitive species, roach, was recorded. This may have been due to still critical water quality conditions for successful reproduction of sensitive roach and/or organic acid episodes in the 2000s, suppressing the recovery of buffering capacity.Peer reviewe