Interaction of Climate Change and Acid Deposition - Climate Change Impacts on Freshwater Ecosystems - page 163

7.0

1979 - 84

6.5 1985 - 89

1990 - 94

1995 - 99

6.0

2000 - 04

5.5

5.0

1995-99

4.5

2000-04

1990-94

4.0

1985-89

1979-84

3.5

0 1000 2000

Discharge (l/s)

3000 4000 5000

Figure 7.9 Stream pH versus mean discharge on the day of sampling, for five
approximately equal time intervals. Lines show logarithmic least squares regression fit
for pH versus discharge for each time interval. (From Evans et al . 2008.)

Acid episodes can cause acute toxicity to aquatic organisms, such as the fish kill
in 1993 in Norway following a sea-salt episode (Hindar et al . 1994). Scenarios
of future climate change entail increased frequency and severity of such climatic
extremes. The implication is that these may delay biological recovery in response
to decreased S and N deposition.
Evans et al . (2008) developed a procedure by which each acid episode could
be assigned to one of four main drivers: hydrology (i.e. direct influence of
precipitation), summer drought, snowmelt, and sea-salt deposition. The procedure
entails quantifying the relative magnitudes of changes in base cations and the
three major strong acid anions, SO 4 , NO 3 and Cl, in explaining the observed
decrease in ANC. At Afon Gwy, a small moorland stream in mid-Wales, Evans
et al . (2008) found that the severity of episodes had decreased during the past 20
years, due to the decreased deposition of S and N. For example, high flow
episodes in recent years have much higher pH levels than comparable flows in
the early 1980s (Fig. 7.9).
A similar decrease in the severity of acid episodes has occurred at the nearby,
and partially afforested, Afon Hafren catchment (M. Hutchins unpublished data).
Here, the weekly data were split into four time-periods, stratified and summarized
according to discharge conditions, and then the major driver was identified by
means of the method of Evans et al . (2008). There was a strong relationship