Sulphate derived from the sulphur oxide emissions was identified as the main
acidifying agent (Neary and Dillon 1988; Sullivan et al. 1988). As a consequence of
the overwhelming evidence, serious efforts were made to control the sulphur
emissions. Laws aiming to the reduction of emissions came into force in Europe
and North America (UNECE 2016a). Recovery of acidified ecosystems did not
follow the reduced emissions immediately. There is a hysteresis in the reversal of
acidification. However, there is nowadays growing evidence that recovery is in
progress, although there are also some uncertainties about how it will be in the next
future (Wright et al. 2005). There are for instance confounding factors related to
climate change that can mask or counteract the recovery, such as increase of pCO 2
and organic acids in soil and runoff and increased sea salt atmospheric inputs.
(Wright et al. 2006). Furthermore, the global emissions of sulphur may rise again as
emerging economic powers develop their industry (Streets and Waldhoff 2000;
Smith et al. 2011).
As mentioned above, high mountain environments and, in particular, surface
waters lying on crystalline bedrocks are sensitive to acidification. The sensitivity of
lakes, for instance, can be assessed according to the acid neutralising capacity
(ANC) of its waters. ANC is a chemical parameter that measures the difference
between dissolved base cations and acid anions. ANC is expressed in terms of
equivalents (the usual unit for diluted waters is µeq L
−1 ). ANC is closely related to
pH. Negative ANC values indicate acidic lakes, with pH below 5.5. Organisms able
to survive below that pH are an exception. In the Pyrenees, c. 2% of lakes have
negative ANC. Lakes with ANC between 0 and 50 µeq L
−1 have a pH in the range
5.5–6.5. Many organisms are affected in some degree by acidity in this range of pH.
Furthermore, the buffering capacity is so low that small additions of acid may lead
to negative ANC values. Approximately 15% of Pyrenean lakes lie in this range of
ANC and pH. pH above 6.5 do not cause particular damage to organisms.
Nevertheless, lakes with ANC < 200 µeq L
−1 are susceptible to reach seasonally
low pH values as, for instance, during snowmelt runoff in spring. For this reason,
lakes below 200 µeq L
−1 are considered potentially sensitive to acidification. In the
Pyrenees, c. 44% of lakes falls in the range of ANC between 50 and 200 µeq L
−1 .
Altogether, about 60% of Pyrenean lakes are, in higher or lower degree, sensitive to
acidification.
How has acidification evolved in the Pyrenean lakes? Modelling on the basis of
reconstructed acid deposition and the current chemical features of lakes indicates
that the effects of acidification have been moderate in the Pyrenees (Camarero and
Catalan 1998). The modelling results indicate that Pyrenean lakes have experienced
an ANC loss of c. 35 µeq L
−1 on average since 1850. This has caused only 2% of
lakes to cross the boundary of ANC = 20 µeq L
−1 (the lower limit for salmonids
survival), low in comparison with the 25% in many areas of Central and Northern
Europe and the 90% in the most sensitive areas of Scandinavia. The reasons for this
are, on the one hand, that the atmospheric acid load has been lower on the Pyrenees
than on the more polluted Central and Northern Europe and, on the other hand, that
dust inputs from the Iberian Peninsula and Northern Africa are important here. Dust
is a large source of base cations able to neutralise acidity in precipitation and to
328
L. Camarero
acidifying agent (Neary and Dillon 1988; Sullivan et al. 1988). As a consequence of
the overwhelming evidence, serious efforts were made to control the sulphur
emissions. Laws aiming to the reduction of emissions came into force in Europe
and North America (UNECE 2016a). Recovery of acidified ecosystems did not
follow the reduced emissions immediately. There is a hysteresis in the reversal of
acidification. However, there is nowadays growing evidence that recovery is in
progress, although there are also some uncertainties about how it will be in the next
future (Wright et al. 2005). There are for instance confounding factors related to
climate change that can mask or counteract the recovery, such as increase of pCO 2
and organic acids in soil and runoff and increased sea salt atmospheric inputs.
(Wright et al. 2006). Furthermore, the global emissions of sulphur may rise again as
emerging economic powers develop their industry (Streets and Waldhoff 2000;
Smith et al. 2011).
As mentioned above, high mountain environments and, in particular, surface
waters lying on crystalline bedrocks are sensitive to acidification. The sensitivity of
lakes, for instance, can be assessed according to the acid neutralising capacity
(ANC) of its waters. ANC is a chemical parameter that measures the difference
between dissolved base cations and acid anions. ANC is expressed in terms of
equivalents (the usual unit for diluted waters is µeq L
−1 ). ANC is closely related to
pH. Negative ANC values indicate acidic lakes, with pH below 5.5. Organisms able
to survive below that pH are an exception. In the Pyrenees, c. 2% of lakes have
negative ANC. Lakes with ANC between 0 and 50 µeq L
−1 have a pH in the range
5.5–6.5. Many organisms are affected in some degree by acidity in this range of pH.
Furthermore, the buffering capacity is so low that small additions of acid may lead
to negative ANC values. Approximately 15% of Pyrenean lakes lie in this range of
ANC and pH. pH above 6.5 do not cause particular damage to organisms.
Nevertheless, lakes with ANC < 200 µeq L
−1 are susceptible to reach seasonally
low pH values as, for instance, during snowmelt runoff in spring. For this reason,
lakes below 200 µeq L
−1 are considered potentially sensitive to acidification. In the
Pyrenees, c. 44% of lakes falls in the range of ANC between 50 and 200 µeq L
−1 .
Altogether, about 60% of Pyrenean lakes are, in higher or lower degree, sensitive to
acidification.
How has acidification evolved in the Pyrenean lakes? Modelling on the basis of
reconstructed acid deposition and the current chemical features of lakes indicates
that the effects of acidification have been moderate in the Pyrenees (Camarero and
Catalan 1998). The modelling results indicate that Pyrenean lakes have experienced
an ANC loss of c. 35 µeq L
−1 on average since 1850. This has caused only 2% of
lakes to cross the boundary of ANC = 20 µeq L
−1 (the lower limit for salmonids
survival), low in comparison with the 25% in many areas of Central and Northern
Europe and the 90% in the most sensitive areas of Scandinavia. The reasons for this
are, on the one hand, that the atmospheric acid load has been lower on the Pyrenees
than on the more polluted Central and Northern Europe and, on the other hand, that
dust inputs from the Iberian Peninsula and Northern Africa are important here. Dust
is a large source of base cations able to neutralise acidity in precipitation and to
328
L. Camarero