Page 379 - Soil and water contamination, 2nd edition

366 Soil and Water Contamination

18.4.4 Temporal variation
As already touched upon in the previous sections, lake water composition varies considerably
during the seasons. Figure 18.20 shows the seasonal variations in the concentrations of
-
chlorophyll , soluble reactive P (RP (< 0.45)), total P, and NO in the near-surface water
3
(epilimnion ) of Lake Windermere, Cumbria, UK, in 1973 and 1974. Lake Windermere is 17
km long and 0.4 m to 1.5 km wide and reaches a maximum depth of about 65 m. During
spring and summer, the temperature rises and algae start to grow, as can be seen from the
chlorophyll-α concentrations. The growth of algae increases the rate of uptake of N and P
-
and reduces the concentration of RP (< 0.45) and NO in the water. Total P concentrations
3
remain more stable than soluble reactive P fraction (RP (< 0.45)). Total P includes soluble P
and the P in living organisms and detritus suspended in the lake water. Primary production
by algae occurs from April to late September. The rapid decline of the RP (< 0.45) in April
suggests that algal growth is P-limited. In autumn, primary production is followed by death,
settling of the detritus to the bed sediment , and decomposition . The decomposition of
detritus causes the N and P to be recycled back into the dissolved phase .
In shallow lakes , the dynamics of P are different from those in deep lakes such as
Lake Windermere discussed above. Figure 18.21 shows the seasonal variation of total

Chlorophyll-a
2.0

μg l -1 1.0

0
RP (<0.45)
24

16
μg l -1 8

0
Total P
40

μg l -1 20

0
NO 3 -N
600
400
μg l -1
200
6642 6642 6642 0
J F M A M J J A S O N D J F M A M J J A S O N D
1973 1974

Figure 18.20 Seasonal variation of chlorophyll -α and nutrients in the surface waters of Lake Windermere, UK.
Adapted from Thomas et al. (996.).

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