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Michigan Environmental Education Curriculum
Water Quality - Pollutant Sources and Impacts

The sun is the primary source of energy for all life on earth. The sun's energy is stored in the bonds of carbon-based chemicals (organic matter) produced by plants through the process of photosynthesis:

CO2 + H2O Changes via Suns Energy C(H2O) + O2

The energy is stored in chemical bonds of the organic matter, C(H20), and can later released for use by microbes and higher plants and animals through the process of respiration.

Plants are called autotrophs (or self-feeders) because they produce their own organic matter. They also require inorganic nutrients such as nitrogen and phosphorus, the fertilizing chemicals that we apply to farm fields, gardens and lawns. There is often one special nutrient that is needed for growth--the limiting nutrient. In freshwater ponds, lakes and rivers, that nutrient is phosphorus.

Consider this animation: in order to build an ‘item’ (an algal cell or a higher aquatic plant), we might need 4 units of oxygen, 3 units of carbon and 2 units of phosphorus. The ‘supplies’ available in a lake are shown on the left. We can build 2 ‘items’ from the ‘supplies’ and then we run out of phosphorus. Even though we have some carbon and oxygen left, growth stops because it is limited by the availability of phosphorus.

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