Eutrophication (the overenrichment of aquatic ecosystems with nutrients leading to algal blooms and anoxic events) is a persistent condition of surface waters and a widespread environmental problem. Some lakes have recovered after sources of nutrients were reduced. In others, recycling of phosphorus from sediments enriched by years of high nutrient inputs causes lakes to remain eutrophic even after external inputs of phosphorus are decreased. Slow flux of phosphorus from overfertilized soils may be even more important for maintaining eutrophication of lakes in agricultural regions. This type of eutrophication is not reversible unless there are substantial changes in soil management. Technologies for rapidly reducing phosphorus content of overenriched soils, or reducing erosion rates, are needed to improve water quality. Limnologists have long studied the processes that cause some lakes to have low concentrations of algae (oligotrophic) and others to become highly turbid due to algae blooms, or eutrophic (1, 2). This research has led to understanding of eutrophication, a significant environmental problem. Consequences of eutrophication include excessive plant production, blooms of harmful algae, increased frequency of anoxic events, and fish kills. Economic losses attributed to eutrophication include costs of water purification for human use, losses of fish and wildlife production, and losses of recreational amenities (3). Eutrophication has become a global problem that is likely to intensify in coming decades because of increases in human population, demand for food, land conversion, fertilizer use, and nitrogen deposition (4).
Eutrophication of lakes is caused by overenrichment with nutrients, principally phosphorus (5). Excess phosphorus inputs to lakes usually come from sewage, industrial discharges, and runoff from agriculture, construction sites, and urban areas. Over time, many countries have regulated point sources of nutrients, such as municipal and industrial discharges. Nonpoint sources of nutrients, such as runoff from agricultural or urban lands, have replaced point sources as the driver of eutrophication in many regions (6). An important driver of nonpoint nutrient input is excessive application of fertilizer or manure, which causes phosphorus to accumulate in soils (7). Phosphorus-rich soils are washed into lakes, where some of the phosphorus dissolves and stimulates growth of phytoplankton and aquatic plants.
Oligotrophic conditions are usually stable, because the return of phosphorus from sediments is low, thereby limiting the growth of algae. Similarly, the eutrophic condition is stabilized by recycling of phosphorus from sediments within the lake. But many shallow (thermally unstratified) lakes display alternate stable states. One is a clear water state, with low algae but abundant rooted aquatic plants, whereas the other is a turbid state where shading by abundant algae suppresses rooted plants. Some lakes change between these states from time to time, whereas others persist for years in either the clear water or turbid state. The reasons for the differences in stability have been the subject of many investigations. In deeper (thermally stratified) lakes, the stabilization can involve several factors, including biogeochemistry of the deep layer of water (hypolimnion), temperature of the hypolimnion, shape of the lake basin, abundance of rooted plants, and food web structure. Regardless of lake depth, recycling can under some conditions maintain a persistent eutrophic regime. In principle, and sometimes in practice, the eutrophic regime can be destabilized by management interventions, thereby changing the lake toward the clear-water regime.                      Â
Lake eutrophication has proven to be a stubborn environmental problem. Instead of alternating regimes, many lakes remain eutrophic for extended periods of time. Causes of slow recovery, or nonrecovery, from eutrophication are multiple and not entirely understood. Persistent eutrophication could be due to internal recycling from a large pool of phosphorus in sediments, leading to alternative stable states. Chronic release of phosphorus from enriched soils may also explain persistent eutrophication. This paper evaluates the roles of internal recycling and slow dynamics of soil phosphorus, using a general model of phosphorus dynamics in stratified lakes and their watersheds. Results suggest that dynamics of soil phosphorus may control alternate stable states, potentially causing eutrophication to last for centuries. Viewed from the perspective of a human lifetime, eutrophication is often a one-way trip.
