Fertilizers provide essential nutrients for lawns and gardens, but these same nutrients can cause serious problems for the aquatic ecosystems of ponds, lakes and streams. Plants require relatively large quantities of nitrogen and phosphorus for optimal growth, so most general-purpose fertilizer products contain significant amounts of these two nutrients. But excess nitrogen and phosphorus that runs off into waterways can encourage imbalanced growth of aquatic organisms, leading to dangerously low levels of dissolved oxygen.

The term "fertilizer" can apply to any substance that supplies nutrients needed by lawn grasses, garden crops, fruit trees and other types of managed vegetation. Consequently, fertilizer products contain a wide variety of substances because plants need at least 17 elements for proper growth and reproduction. Most commercial fertilizers, however, supply the three major nutrients: nitrogen, phosphorus and potassium. Of these three, nitrogen and phosphorus pose the greater risk to waterways because they are limiting nutrients -- in other words, the growth of bacteria and plants is regulated by the limited quantities of nitrogen and phosphorus present in natural environments.

Many aquatic organisms, including fish, cannot survive without adequate levels of oxygen that has been dissolved in the water that surrounds them. Algae and other aquatic plants produce dissolved oxygen as a byproduct of photosynthesis, the process by which they make food from carbon dioxide and sunlight. An excessive population of algae, however, actually leads to oxygen depletion. A thick layer of algae on the top of a waterway can disrupt oxygen production by shading out larger photosynthetic plants. More importantly, excessive algal growth leads to an overabundance of dead algae, which must be decomposed by bacteria and fungi. This intense bacterial and fungal activity consumes oxygen and can seriously diminish or deplete the levels of dissolved oxygen in lakes, ponds and streams.

Bodies of water, like most of Earth's environments, are home to carefully balanced ecosystems in which various organisms interact with one another. Both natural and artificial conditions can disturb this balance, but the effects of artificial disturbances are often more pronounced. The relationship between algae and other aquatic organisms is an example of this balance. The limited availability of nutrients, such as nitrogen and phosphorus, helps to maintain the algae population at a level that contributes to proper amounts of dissolved oxygen. But algae thrive on surplus nitrogen and phosphorus from fertilizers. When the nutrients in fertilizers end up in waterways instead of on terrestrial plants, algae growth increases rapidly, creating an ecological imbalance that results in the depletion of dissolved oxygen.

The most important way to reduce the amount of fertilizer residue that enters waterways and promotes oxygen depletion is to avoid excessive and improper fertilization. Many commercial fertilizers contain soluble nitrogen, which readily leaches through the soil or runs off in irrigation water or rainfall if applied at the wrong time or at improper rates. Though phosphorus in soil is resistant to leaching, it can run off into waterways when applied in excess or when improper soil management allows for the erosion of phosphorus-rich soil particles. Another serious source of nutrient runoff is fertilizer that falls on non-absorbent surfaces such as sidewalks and driveways. These concentrated, readily available nutrients will be washed by rainfall into the storm drain and from there into lakes, rivers and streams.