Lake Washington in the United States of America is a large fresh water lake. Between 1941 and 1963 an increasing amount of untreated sewage polluted the lake.

1. (a)Suggest and explain the effect of this untreated sewage on the lake ecosystem.[5]
2. (b)Between 1963 and 1968 new sewage treatment facilities were constructed. By 1968 the amount of untreated sewage entering Lake Washington was reduced to zero. Fig. 4.1 shows how the transparency of water in the lake has changed over time. Water transparency is a measure of how far light travels through water. Calculate the percentage increase in water transparency between 1957 and 1968.[2]
3. (c)(i)One species of fish that lives in Lake Washington is the Three Spined Stickleback. Fig. 4.2 shows three different distribution patterns of armour plates on the skin of these fish. [Figure 4.2] Table 4.1 shows how the distribution patterns of armour plates on the skin of Three Spined Sticklebacks have changed over time. [Table 4.1] Use data from Table 4.1 to describe how the distribution patterns of armour plates have changed over time.[4]
4. (c)(ii)Armour plates on the skin of a Three Spined Stickleback protect it from predators. Describe how the process of natural selection may have caused these changes in the distribution of plates over time.[5]

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