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Report2021Open access

Acidified or not? : a comparison of Nordic systems for classification of physicochemical acidification status and suggestions towards a harmonised system

Fölster, Jens; Garmo, Öyvind; Carlson, Peter; Johnson, Richard; Velle, Gaute; Austnes, Kari; Hallstan, Simon; Holmgren, Kerstin; Schartau, Ann Kristin; Moldan, Filip; Aroviita, Jukka


Acidification of lakes and streams from long-range transboundary air pollution is one of the most severe and spatially extensive environmental problems in northern Europe and North America. The Nordic countries, with acid sensitive soils and located downwind of the industrial areas of western and central Europe were particularly affected, with local extinctions of fish populations and other harmful effects on the aquatic ecosystems. Although the deposition of acidic pollutants today is tenfold lower than during peak years in the 1980s, acidification is still a major problem due to legacy acidification of the soils in the catchments of lakes and streams.

The Nordic countries have developed different criteria to classify acidification from chemical parameters and to distinguish anthropogenically acidified waters from naturally acidic waters. In brief, the different systems reflect dissimilarities in geology and climate and different forms of management. This has resulted in acidification assessments that are not directly comparable. In international reporting, for example to the UN-ECE Air convention and the EU Water Framework Directive, discrepancies among the Nordic countries reflect more the different classification systems used rather than environmental conditions. To address this issue, the Swedish Agency of Marine and Water Management and the Norwegian Environment Agency initiated a project to assess the possibility of harmonising classifications of acidification across the Nordic countries, as well as to lay a foundation for improved and harmonised systems and reporting. The project focused on analyses of a joint database, comprising data on water chemistry and biology, which was compiled by representatives from Norway, Sweden and Finland.

Comparisons of the national classification systems showed marked differences. The Finnish system focuses only on rivers, with primary attention given to acidification caused by the draining of sulphide soils. Both the Norwegian and the Swedish systems focus more on anthropogenic-induced acidification by deposition and both are based on reference values calculated using the MAGIC model. However, while the Norwegian system, like the Finnish, is based on water body types and type-specific class boundaries, the Swedish system is object specific. Furthermore, the Swedish system is based on changes in the whole macroinvertebrate community (i.e. including species with varying degrees of sensitivity/tolerance to acidification), while the Norwegian system is based on empirically derived critical levels of a single species (brown trout). A comparison of the different systems showed that classification using the Swedish system was much stricter: 74 of 373 water bodies (20 %) were considered acidified (moderate status or worse) according to the Swedish system, compared to 34 of 205 streams (17 %) using Finnish system and only 10 of 470 waters (2 %) using the Norwegian system.

The Nordic dataset with chemistry and biology included 165 lakes with data on littoral invertebrates, 114 lakes with data on fish, 99 streams with data on invertebrates and 80 streams with data on fish. The first objective of our study was to determine and quantify acidification indicator(s) that are robust predictors of biological change. Gradient forest and generalised additive modelling showed that the acid neutralising capacity (ANC), calculated as the difference between base cations (calcium, magnesium, sodium and potassium) and strong acid anions (sulphate, chloride and nitrate), was the strongest predictor. Our analyses also revealed that pH was a relatively poor predictor, a finding that contrasted with earlier studies on national datasets. This discrepancy might be explained by our use of a larger dataset, covering broader environmental gradients in ion concentrations and natural organic acids, compared to the earlier studies. The advantage of using ANC was further supported by analysis of interactions between environmental variables, e.g., responses between pH and biology were confounded by interactions with other environmental parameters, to a much higher degree than ANC.

For lake invertebrates and fish gradient forest revealed pronounced upper thresholds at around 150 µeq/l ANC with one or two peaks between 90 and 140 µeq/l ANC. The upper threshold in the most important community changes for both stream invertebrates and fish occurred at around 200 µeq/l. The higher threshold in rivers is likely due to the higher temporal variability of acidic conditions in streams, with the biotic responses reflecting the most acidic conditions. In our analysis we used mean values since the sampling frequency was highly variable and therefore it was unlikely that acidic episodes were captured in the chemical sampling of most streams. Mean values can then be interpreted as the risk of ANC levels below the critical levels during extreme events.

Here we propose an approach for Nordic classifications and exemplify this approach using the Swedish acidification index for macroinvertebrates in lakes (the MILA index). Similarly, this approach could also be applied to other indices, to streams and for fish. If decided that the approach should be developed further, we suggest that new indices are developed for ANC for both lakes and rivers using the Nordic dataset. A common Nordic classification for macroinvertebrates in lakes and rivers could then underpin classifications using ANC.

For sites with circumneutral and alkaline reference conditions, the class boundaries for ANC can be set in relation to the biological classification. For naturally acidic sites, we recommend an approach where the class boundaries are expressed as an EQR instead of a fixed ANC value. The EQR-derived class boundaries should be based on a biological classification system but should be adapted to reflect sensitivities across different ANC-ranges. For example, for lakes a smaller change in ANC is accepted for good status in the range of 90-150 µeq/l ANC where most of the change in species composition for both invertebrates and fish occur.

The MAGIC model, currently used for estimating reference values in both Norway and Sweden, cannot be applied to all water bodies requiring status classification. Results from our study showed that a simple regression model for reference ANC, as a function of BC, SO4 and Cl, could be calibrated using data from MAGIC-modelled lakes and rivers distributed across all of Sweden. Hence, following validation, it is expected this simple regression model could be used for Norway and Finland as well.

Our approach can potentially be developed into a harmonised Nordic classification system for acidification. However, the benefits of a revised system have to be weighed against other aspects that are important for society and decision makers. For example, should thresholds be based on the environmental requirements of single, relatively sensitive, species deemed important by society, or as a gradual change in species composition from a reference condition (sensu EU Water Framework Directive) as suggested in this report? Should ANC be used as single indicator for acidification as suggested here, or is pH preferred since it is well-known and widely used, or should inorganic aluminium be used since it is more directly related to toxicity? Should an object-specific system be chosen since it results in lower classification errors, or is a type-specific system preferred due to its simplicity? Even if the different countries decide differently to these and other questions, we hope that this report provides a good foundation for continued dialog in order to ultimately achieve a more harmonised classification of acidification between countries and between chemical and biological quality elements.

Published in

Rapport / Sveriges lantbruksuniversitet, Institutionen för vatten och miljö
2021, number: 2021:1
Publisher: Institutionen för vatten och miljö, Sveriges lantbruksuniversitet