Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 58

Organic horizon

90
80
70
60
50
40
30
20
10
0

Control
Drought

Mineral horizon

35

Control
Drought

30

25

20

15

10

5

0

Figure 3.15 DOC concentrations over 4 years of experimentally induced summer
drought on a heathland at Clocaenog, North Wales, the United Kingdom. Shaded areas
indicate experimental drought periods.

to much lower levels. In some years (e.g. in 2004), it appears that this drought-
induced reduction was followed by increased concentrations on re-wetting,
although this was not observed in all years. Similar patterns of reduced DOC
concentrations during droughts, and raised concentrations on re-wetting, have
been noted in several previous studies of peatland soil solutions and surface
waters (e.g. Watts et al . 2001; Clark et al . 2005). Other work undertaken at the
Clocaenog site suggests that reduced DOC losses in summer are linked to
decreased activity of the phenol oxidase enzyme, which has an important role in
litter decomposition and DOC production (Toberman et al . 2008). In the mineral
soil solutions at Clocaenog, on the other hand, changes to the seasonal cycle are
less evident; instead, there is some indication that repeated droughts are leading
to progressive, year-round DOC increases (Fig. 3.15).