What two substances are we concerned about over-nourishing our water sources and ultimately leading to eutrophication?

The correct answer is - nitrogen and phosphorus.

The nitrogen and the phosphorus are the two substances that cause most of the eutrophication. It is good for the organisms that live in the water when these two substances are available as they are very important nutrients, but if the optimal amount is surpassed than it can be devastating. When there's high quantities of nitrogen and phosphorus in the water it makes it much more rich in nutrients that it should be. That kind of situation is the best for the algae, which start growing all over the place, quickly covering the water and becoming the dominant life form. As that happens, the organisms that use photosynthesis will die out because the Sun will be blocked, and the algae  will also contribute to a big reduction of the oxygen in the water too. The end result being totally destroyed ecosystem that has been overtaken by algae.

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