Eutrophication, part of the natural aging process for bodies of water such as ponds or lakes, occurs due to a buildup of nitrates and phosphates over time. These compounds, often found in fertilizers, are plant nutrients that stimulate the growth of plants and algae in the water. As the amount of plant matter in the pond increases, more plants start to die off. Bacteria that decompose the dead plants also consume the oxygen in the water, so fish and other water animals soon die for lack of oxygen. The pond eventually becomes choked with plant matter and grows shallower. Ultimately, as shrubs grow where water once was, the pond dies. Which organizational structure does this text primarily use?