What’s in the Water: Why Lakes exhibit decreased dissolved oxygen concentrations due to Eutrophication.

Kelsie Dixon, Christian Ayisi, Chathurika Henpita



Eutrophication can be best described as the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen, phosphorus, or both (Correll 1995). When talking about this process, the question that is being asked is, “Why is there less oxygen in some streams than in others?” There are many factors that come in to play when talking about the dissolved oxygen concentration. In this lab we worked with three different types, or levels, of nutrient enriched water samples. We tested our hypothesis, which stated that when the amount of decomposers increase, then the dissolved oxygen concentration/ percentage in the different types of lake water,  will decrease because the decomposers (yeast) will consume oxygen during respiration. We did this by simply taking a 15 mL sample of each nutrient enriched water sample and adding 6 mL of yeast to each sample. When we waited for the yeast to settle, and started collecting the data for the percentage of dissolved oxygen. We concluded that our hypothesis was in fact supported by our experiment. Our experiment showed that when the algae die, they decompose and the nutrients that are in the organic matter are converted by microorganisms into inorganic form (Smith, 1999). When the algae decomposed this process consumes oxygen, which leads to a decrease in the concentration of the dissolved oxygen (Smith 1999).


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