A comparison of MAGIC and paleolimnological predictions of preindustrial pH for 55 Swedish lakes.

Two fundamentally different approaches to define reference conditions for acidification assessments are hydrogeochemical modeling and paleolimnological reconstructions. Both methods have been applied to calculate the preindustrial chemistry for 55 Swedish lakes in two independent studies. This paper investigates whether these methods give similar reconstructions of the preindustrial pH for these lakes. Special focus has been attached to the importance of total organic carbon concentrations and CO2 partial pressure (pCO2) in the conversion from ANC to pH in the hydrogeochemical modeling. With a uniform pCO2 value for all the lakes of 0.63 matm, the mean absolute difference between pH from the hydrogeochemical model and the paleolimnological pH was +0.23 units (mean absolute difference 0.36 units). If instead a lake specific preindustrial pCO2 is assumed, equal to contemporary pCO2, the mean difference in the predicted preindustrial pH between the two methods was reduced to +0.03 units (mean absolute difference 0.22 units). Statistical analyses indicated that with a lake specific pCO2, the difference between the reconstructions is smaller than 0.13 pH-units at a 95% level of significance. The results of this study build confidence in the reliability of both methods, providing that lake-specific estimates of pC02 are used.

Modeling recovery of Swedish ecosystems from acidification.

Dynamic models complement existing time series of observations and static critical load calculations by simulating past and future development of chemistry in forest and lake ecosystems. They are used for dynamic assessment of the acidification and to produce target load functions, that describe what combinations of nitrogen and sulfur emission reductions are needed to achieve a chemical or biological criterion in a given target year. The Swedish approach has been to apply the dynamic acidification models MAGIC, to 133 lakes unaffected by agriculture and SAFE, to 645 productive forest sites. While the long-term goal is to protect 95% of the area, implementation of the Gothenburg protocol will protect approximately 75% of forest soils in the long term. After 2030, recovery will be very slow and involve only a limited geographical area. If there had been no emission reductions after 1980, 87% of the forest area would have unwanted soil status in the long term. In 1990, approximately 17% of all Swedish lakes unaffected by agriculture received an acidifying deposition above critical load. This fraction will decrease to 10% in 2010 after implementation of the Gothenburg protocol. The acidified lakes of Sweden will recover faster than the soils. According to the MAGIC model the median pre-industrial ANC of 107 microeq L(-1) in acid sensitive lakes decreased to about 60 microeq L(-1) at the peak of the acidification (1975-1990) and increases to 80 microeq L(-1) by 2010. Further increases were small, only 2 microeq L(-1) between 2010 and 2040. Protecting 95% of the lakes will require further emission reductions below the Gothenburg protocol levels. More than 7000 lakes are limed regularly in Sweden and it is unlikely that this practice can be discontinued in the near future without adverse effects on lake chemistry and biology.