Nutrients, such as nitrogen and phosphorus, are essential for plant and animal growth, but the overabundance of certain nutrients in water can cause a number of adverse health and ecological effects. About 78% of the air that we breathe is composed of nitrogen gas, and in some areas of the United States, particularly the northeast, certain forms of nitrogen are commonly deposited in acid rain. Although nitrogen is abundant naturally in the environment, it is also introduced through sewage and fertilizers. Chemical fertilizers or animal manure is commonly applied to crops to add nutrients. Wastewater-treatment facilities that do not specifically remove nitrogen can also lead to excess levels of nitrogen in surface or groundwater. An excess of nitrogen in ponds and lakes can cause overstimulation of growth of aquatic plants and algae. The proliferation of these plants in a contained body of water has other secondary effects on the plant and animal life. They include all BUT ONE of the following conditions: A) The amount of dissolved oxygen drops below normal levels, resulting in fish kills.

B) Rapidly moving water, such as in a stream or river, tends to contain a lot of dissolved oxygen.

C) Excess organic material in lakes can cause eutrophication, which is an oxygen-deficient situation.

D) Decreased respiration efficiency of fish and aquatic invertebrates, leading to a decrease in animal and plant diversity.