40
30
E
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"6
E
20
8
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8
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O
1900
10
20
30
40
50
60
1970
Year
From 1900 to 1970, there has been a precipitaus drop in dissolved oxygen in the
North Central Basin of the Baltic Sea, which has been used as a dump by bordering
nations. Natural barriers between the sea and the Atlantic Ocean tend to make the
Baltic stagnant and therefore a highly inappropriate receiving water for waste.
enough for light to penetrate to the bottom, allowing large aquatic weeds to
grow. As these plants die, sediments may accumulate very rapidly.
,
Limiting Factors
In studying the causes of eutrophication, it is necessary to learn not only how
nutrients are distributed in the aquatic environment, but which of the many
nutrients needed for plant growth are most important. Plants do not need
,
identical quantiües of all essential nutrients. The most critical nutrients are
usually required in a ratio of 106 units of carbon to ten of nitrogen and one of
ph05phorus. If only one of these elements is missing, the others become use.
less to a plant; if one is found in very small quanties, even though the other
nutrients are available in excess, the limited element will determine how
Limiting Factors
133