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Name: Deborah
Status: other
Grade: other
Location: NH
Country: USA
Date: Fall 2012


Question:
I live by Great Bay in NH. Our bay contains excess nitrogen, which is considered to be a pollutant. If I understand correctly, this excess nitrogen acts as a fertilizer, causing the over-growth of algae (just algae?) This algae, in turn, will eventually remove oxygen from the water, which will render the water unsuitable for other marine species that rely on the bay to reproduce and feed. How does excess algae remove the oxygen from the water?



Replies:
Deborah,

The excess nitrogen that you mentioned is usually in the form of fertilizers that stimulate plant growth (so not just algae). While the presence of plant life can cause the return of oxygen into water when the plant goes through a process of photosynthesis (converting carbon dioxide and water into sugars and oxygen (as a byproduct). However, plants also respire (converting the same sugars into energy and using up oxygen). The "compensation point" - the point at which the balance between oxygen use (in respiration) and oxygen production (in photosynthesis) depend on many factors: respiration is more or less constant, but photosynthesis depend on the amount of sunlight and time of day. On the whole, there is more oxygen production than oxygen use - however, when there is an overabundance of plant life, another factor becomes important: decomposition. During decomposition of dead plants, oxygen is used as the main "decomposer" (oxidizing agent). So, decomposition uses up oxygen, and if this process (of which there is no oxygen return unlike photosynthesis) then it is possible for a body of water to start losing dissolved oxygen.

Greg (Roberto Gregorius) Canisius College



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