Page 235 - Materials Chemistry, Second Edition

10 Life Cycle Impact Assessment 221

compounds from flue gases resulting from energy conversion based on fossil
resources. Calcium, potassium and magnesium occur in lime and clay, respectively,
which exist in large quantities in soils.
In practice, one of the two last macronutrients, nitrogen and phosphorus, is
therefore almost always the limiting element for the growth of primary producers,
and it is therefore reasonable to regard only the elements nitrogen and phosphorus
as contributors to nutrient enrichment. In many lakes, phosphorus deficiency, or a
combination of nitrogen and phosphorus deficiencies, is typically limiting growth,
and their addition promotes algal growth. In coastal waters and seas, nitrogen is
often the limiting nutrient. Substances which contain nitrogen or phosphorus in a
biologically available form are therefore classified as potential contributors to
nutrient enrichment. As is evident from the formula for the average composition of
aquatic organisms, the ratio of nitrogen to phosphorus is of the order of 16. If the
concentration of bioavailable nitrogen is significantly more than 16 times the
concentration of bioavailable phosphorus in an ecosystem, it is thus reasonable to
assume that phosphorus is the limiting nutrient, and vice versa. Since most of the
atmosphere consists of free molecular nitrogen, N 2 , further addition of N 2 will not
have any effect, and it is also not directly bioavailable. N 2 is therefore not classified
as contributing to nutrient enrichment.
For aquatic eutrophication, the starting point of the cause–effect chain is the
emission of a compound containing either nitrogen (N) or phosphorus (P).
Increased availability of nutrients will primarily increase the growth of algae and
plants, especially in summer with abundant sunlight. This algae growth is visible as
rivers, lakes or coastal waters turn turbid in summer. Eventually, the algae will sink
to the bottom where they are decomposed by degraders like bacteria under con-
sumption of oxygen in the bottom layer. With the sunlight being increasingly
blocked from reaching deeper water layers, the build-up of a temperature gradient
causes stratification in deep lakes and some coastal waters in the summer months. In
the marine environment, stratification is determined by density differences between
salt water flowing in from the sea and brackish water flowing out from river deltas
and fjords. Such stratification prevents effective mixing of the water column. If
fresh oxygen-rich water from the surface does not find its way to the bottom layers,
the oxygen concentration near the bottom will gradually be reduced until the
bottom-dwelling organisms move away or die. As the oxygen concentration
approaches zero, poisonous substances such as hydrogen sulphide, H 2 S, are formed
in the sediments, where they accumulate in gas pockets which, when released again,
kill those organisms exposed to them.

The main cause–effect chain as shown in Fig. 10.14 can be summarised as:
• Emission of N or P containing substances
• Growth and blooming of algae and higher plants increases
• Sunlight no longer reaches lower water layers, which creates a temperature
gradient with increasing depth
• This supports a stable stratification of water layers reducing the transport of fresh
oxygen-rich surface water to deeper layers