restricted access VII.5 Managing Nutrient Mobilization and Eutrophication
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VII.5 Managing Nutrient Mobilization and Eutrophication D. W. Schindler OUTLINE 1. History of the term 2. The role of thermal stratification 3. Natural and cultural eutrophication 4. Ratios and sources of key nutrients 5. Whole-lake experiments and their role in eutrophication control policy 6. Nonpoint sources of nutrients 7. The trophic cascade 8. Internal recycling of phosphorus 9. Eutrophication in flowing waters 10. Eutrophication and the quality of drinking water 11. Eutrophication of estuaries Increasing the inputs of the nutrients phosphorus and nitrogen to freshwater bodies and estuaries causes increased growth of nuisance algae, termed eutrophication. In lakes, eutrophication can be prevented by controlling inputs of phosphorus. In estuaries, there is still controversy over whether nitrogen, phosphorus, or both must be controlled. GLOSSARY epilimnion. The uniformly warm upper layer of a lake when it is thermally stratified in summer. eutrophic. Eutrophic lakes are richly supplied with plant nutrients and support heavy plant growths. eutrophication. The complex sequence of changes initiated by the enrichment of natural waters with plant nutrients. hypolimnion. The uniformly cool and deep layer of a lake when it is thermally stratified in summer. mesotrophic. Mesotrophic lakes are intermediate in characteristics between oligotrophic and eutrophic lakes. They are moderately well supplied with plant nutrients and support moderate plant growth. oligotrophic. Oligotrophic lakes are poorly supplied with plant nutrients and support little plant growth. thermocline. Thermal or temperature gradient in a thermally stratified lake in summer. Occupies the zone between the epilimnion and hypolimnion. Eutrophication is the word used by scientists to describe the result of overfertilization of lakes with nutrients . The first symptom noticeable to casual observers is that the fertilized lakes turn green with plant growth. Paradoxically, we value the increased growth of plants that follows fertilization on land but abhor similar effects in our waters. 1. HISTORY OF THE TERM Eutrophication is derived from the German word eutrophe, which means ‘‘nutrient-rich.’’ The two nutrients that are responsible for increasing growth of algae and other aquatic plants are nitrogen and phosphorus. Eutrophic lakes typically have dense algal blooms. They can also have dense beds of rooted aquatic plants if the lakes have shallow areas with mud or sand bottoms. The term eutrophication was coined by the German wetland scientist C. A. Weber in 1907 to refer to the rich wetlands in areas of Europe that received nutrient runoff from surrounding lands. The term was first applied to lakes by Einar Naumann roughly a decade later. The term oligotrophic (nutrient poor) was applied to nutrient-poor lakes, which generally have clear water and deep waters that contain high concentrations of oxygen. Lakes that are between these two extremes are generally termed mesotrophic. All three categories of lakes can undergo eutrophication if nutrient concentrations are increased. Recently , extremely eutrophic lakes have been termed hypereutrophic. The early use of the terms was to refer to a lake’s appearance. Measurable indices of productivity, such as algal abundance, chlorophyll a, photosynthesis, or nutrient concentration were developed later and are usually used now to define trophic conditions in lakes (e.g., see table 1). The term eutrophication became widely used by limnologists (scientists who study lakes and other fresh waters) to describe the complex sequence of changes in aquatic ecosystems caused by an increased rate of supply of plant nutrients to water. The immediate response of an aquatic ecosystem to increased nutrients is an increase in photosynthesis and abundance of plants. This can give rise to increased productivity at all levels of the food chain, up to and including fish. But, as described in greater detail below, changes can also occur in the kinds of organisms inhabiting aquatic ecosystems during eutrophication to disrupt this transfer of energy up the aquatic food chain. 2. THE ROLE OF THERMAL STRATIFICATION In order to understand all of the changes caused by eutrophication, a working knowledge of a lake’s thermal characteristics is necessary. Some eutrophic lakes are deep enough to have a thermocline, a sharp boundary separating the warm upper waters of a lake (known as the epilimnion) from cold deep layers (termed the hypolimnion). This occurs because cooler waters are more dense than warmer waters. Most swimmers have experienced the thermocline as they pass suddenly from warm water to cold during a deep dive. The depth of a thermocline is determined by the wind velocity and the size of a lake. It can be as shallow as a few meters in small lakes to 15 meters...