Interaction of Climate Change and Acid Deposition - Climate Change Impacts on Freshwater Ecosystems - page 165

(a)

8

ANC
m eq I -1

6

< -50
< -75
< -100

4

2

0

1975-79 1980-84 1985-89 1990-94 1995-99 2000-04

(b)

8

ANC
m

6

eq I -1

4

< -50
< -75
< -100

2

0

1975-79 1980-84 1985-89 1990-94 1995-99 2000-04

(c)

15

ANC
m eq I -1

10

< -50
< -75
< -100

5

0

1975-79 1980-84 1985-89 1990-94 1995-99 2000-04

Figure 7.11 Summary of episodes with threshold ANC < −50 μ eq l −1 at Birkenes
1975-2004 by 5-year period. (a) Mean frequency of episodes; (b) mean duration of
episode; (c) mean number of weeks per year below threshold. (From Wright 2008.)

Snowmelt is typically the period of year with lowest pH and highest toxicity
of streamwater to fish and other aquatic organisms. In streams in northern
Sweden, for example, the pH can be well above 5 most of the year, but below 5
and toxic during a few weeks each spring (Laudon & Bishop 1999). The severity
of these snowmelt-induced acid episodes has decreased in recent years owing to
reduced S and N deposition (Laudon & Bishop 2002b). Also, combined MAGIC
and pBDM ('one point' Boreal Dilution Model) models for the Lysina catchment,
Czech Republic, suggested that the recent recovery of pH, ANC and inorganic
monomeric Al during spring snowmelt floods has been faster than the recovery
of annual average chemistry (Laudon et al . 2005).
Acid pulses can come with the first runoff following droughts. The accompanying
anion is usually SO 4 . The mechanism is oxidation of reduced sulphur compounds