Nitrogen Pollution and Alga
Date: Fall 2012
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?
Be careful. Your statement, “Our bay contains excess nitrogen, …” contains some openings that can be misinterpreted. It does not identify the chemical form of the nitrogen. In current usage, “nitrogen” refers to the atom (N), diatomic nitrogen (N2) refers to the diatomic molecule. This form of nitrogen refers to the diatomic molecule that present at about 80% of the atmosphere. It is not reactive and does undergoes few chemical reactions. In common usage pertaining to reactions in soil, the term “nitrogen” refers to ammonia and its reaction products with water (NH3, NH4(OH), NH4[+1]). So your comment, “Our bay contains excess nitrogen, which is considered to be a pollutant.” must refer to those compounds of nitrogen. Some other more esoteric nitrogen compounds such as hydrazine (N2H4) and oxides of nitrogen (N2O, NO, NO2, NO3 [-1], N2O3, N2O4, N2O5), NO2NH2, HNO3) are often loosely referred to as “nitrogen”.
The growth of algae usually means nitrate (NO3[-1]). This venue is too short to address the nitrogen cycle. You can find that discussed in any of a number of websites, just do a “Google” search on nitrogen + algae or any sort of combination of those terms. You will find more info than you need or want!!
The topics will include more discussion than you can use, and will take more space and time than we can cover here.
The process you describe is eutrophocation. The simple answer is that in euthrophication, the algae are so numerous they utilize all the available oxygen so that nothing else can live.
A little more in-depth answer: Algae are ever present in most lakes, but usually are in a dynamic equilibrium of aquatic species feeding on them and their reproductive rate. That equilibrium is pushed toward an extreme reproduction rate with excess nitrate and phosphate.
Water soluble nitrogen(nitrate) and phosphate from fertilizers and waste leach into runoff water. The water finds it way into streams, ponds and lakes. Alga species are capable of assimilating the excess nitrates and phosphates readily. They reproduce rapidly producing a "bloom". Those alga require oxygen to live and quickly usurp available oxygen faster than higher plant species and animals.
The water quickly becomes anaerobic, the algae die and take the nutrients to the bottom where bacteria further decompose the algae, utilizing more oxygen. One can feel the sludge on the bottom of a eutrophying lake. The water becomes stagnant and unable to support life. Higher plants get choked out and fish move out of the area.
A compounding problem is that the cloudiness of the bloom and subsequent death of algae prevents sunlight from reaching plants at the bottom of the lake. Blooms cause cloudy water, typically colored a shade of green, yellow, brown, or red.
In most states, certain phosphate and nitrate fertilizers are not allowed for widespread use. The death of a lake can occur very quickly. Hopefully, NH Natural Resources may act quickly to help avert catastrophic algal takeover in your bay.
Peter E. Hughes, Ph.D. Milford, NH
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)
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Update: November 2011