Open access publications in the SLU publication database

Large emission reductions in recent decades have resulted in the same level of acidifying sulphur deposition as occurred 100 years ago. These emission reductions
were followed by less acidity in lakes and streams and signs of biological recovery.
However, many surface waters are still acidified and thus fail to achieve EU Water Framework Directive (WFD) mandated Good Ecological Status (GES). The magnitude
of the ongoing acidification problem highlights the need for "Natural Acidification Only" as one of Sweden's 16 environmental objectives.
This report is an assessment of the scope of surface water acidification in Sweden.
Specifically, it reports the degree to which acid deposition exceeds the critical load
for surface water acidification mandated by the “Natural Acidification Only” environmental
objective and evaluates whether or not lakes and streams achieve at least GES under the WFD acidification-related criteria. Trends in water chemistry are
presented from acid-sensitive lakes and rivers within the national and regional environmental monitoring programmes. The assessment of the proportion of acidified
lakes and exceedence of critical load is based on data from the national lake survey.
The evaluation has been done for four regions with different acidification pressures following county boundaries and the highest coastline. The assessment of biological recovery is based on published literature and a special study of historical and contemporary presence of roach as an indicator of recovery from acidification.
Water chemistry trends were analysed using data from acid sensitive lakes with NC <0.3 mEq / L (ANC = acid neutralizing capacity). The selection resulted in 95
lakes with time series from 1988 to 2013. Most of the lakes were originally selected as references for the liming program and are therefore well suited for the study of
recovery from acidification. For watercourses, data was delimited to the time period 1998-2013 in order to get enough stations for a regional analysis (64 streams). Watercourses have a heterogeneous composition ranging from small forest streams to the inland stations in large rivers.
Sulphate concentrations and ANC declined in lakes across the country from 1988 to 2013. A slight flattening of the trends occurred, but declines have continued in recent
years even though deposition has levelled off. In the southern half of Sweden base cation concentrations also declined which is consistent with decreased acidification
pressure. Concentrations of naturally occurring organic carbon (TOC) increased in lakes throughout the country except in the far north. In the most acidified
southwestern part of the country, alkalinity and pH increased. A significant pH increase
was also observed in lakes in the northern interior of Sweden. This increase
probably had natural causes.
Analysis of the 57 most acidic lakes in the data set showed that the occurrence of
pH values below 5.6 has declined over time. A pH value of 5.6 is usually stated as a
threshold for the presence of roach. Thus, the water chemistry results suggest that
there are chemical conditions conducive to biological recovery in many lakes.
Sulphate concentrations decreased in watercourses across the entire country. Significant
decreases in base cation concentrations were noted in southwest Sweden, as
well as increasing TOC concentrations in southwest, eastern and central Sweden. In
the northern interior, alkalinity increased significantly, probably due to natural cliInstitutionen
mate-driven variation. Three acid streams showed increasing frequency of pH values
above 5.6. Few significant trends were noted for the other parameters, in part
due to the 10 year shorter time series compared to lakes.
A review of trend analyses of benthic fauna in lakes and streams in Europe and
North America showed increased biodiversity was linked to decreasing sulphate
levels, with particularly clear trends in Sweden. However, deeper analysis emphasised
the complexity of controls on biodiversity which make it difficult to unequivocally
link observed trends in benthic fauna to recovery from acidification.
An investigation of trends in the presence of roach during the last ca. 100 years in
19 lakes where roach disappeared during the acidification period showed a slight
recovery from acidification, partly due to liming. In 11 of the lakes, roach had returned.
Of these 10 were limed. Roach had only reappeared in 1 of 3 unlimed lakes.
In 2010, 10 % of Sweden's lakes > 1 ha were classified as acidified. The assessment
is based on the official Ecological Quality Criteria and involves an estimated decrease
in pH of more than 0.4 units since 1860 according to geochemical modelling
with MAGIC. For limed lakes, chemistry without liming was estimated before the
assessment was made. The state means a recovery from 17 % acidified lakes in
1990. Projections for 2020 indicate no further recovery. The impacts are greatest in
southwest Sweden where 47% of the lakes have been acidified and lowest in the
northern interior where only 2% were acidified. A critical review of the assessment
shows that lack of knowledge about the dynamics of sulphur and carbon led to an
overestimation of the degree of acidification. The risk of overestimation is greatest
in northern Sweden, where acidifying deposition has returned to levels close to
background levels and where small errors can thus be of great importance.
In 2010, 17 % of Sweden's land received excessive acidifying deposition for surface
water. This is a significant reduction since 1980, when 58 % of the area had an
exceedance in deposition. In 2020, the expected exceedance will be down 10 % if
all decisions on reductions of emissions are carried out in Europe and for international
shipping.
Trends in both estimates of acidification and exceedance of critical loads for surface
waters show a comprehensive improvement in environmental conditions.
Meanwhile, forecasts suggest no further improvements. That means that with today's
environmental targets and current land management practices, Swedish Environmental
Objectives and EU WFD Good Ecological Status will not be achieved for all lakes and watercourses.