Recent advances in the understanding and management of eutrophication

Major advances in the scientific understanding and management of eutrophication have been made since the late 1960s. The control of point sources of phosphorus reduced algal blooms in many lakes. Diffuse nutrient sources from land use changes and urbanization in the catchments of lakes have proved possible to control but require many years of restoration efforts. The importance of water residence time to eutrophication has been recognized. Changes in aquatic communities contribute to eutrophication via the trophic cascade, nutrient stoichiometry, and transport of nutrients from benthic to pelagic regions. Overexploitation of piscivorous fishes appears to be a particularly common amplifier of eutrophication. Internal nutrient loading can be controlled by reducing external loading, although the full response of lakes may take decades. In the years ahead, climate warming will aggravate eutrophication in lakes receiving point sources of nutrients, as a result of increasing water residence times. Decreased silica supplies from dwindling inflows may increasingly favor the replacement of diatoms by nitrogen-fixing Cyanobacteria. Increases in transport of nitrogen by rivers to estuaries and coastal oceans have followed increased use of nitrogen in agriculture and increasing emissions to the atmosphere. Our understanding of eutrophication and its management has evolved from simple control of nutrient sources to recognition that it is often a cumulative effects problem that will require protection and restoration of many features of a lake’s community and its catchment. Despite 20th century advances in understanding eutrophication, it remains one of the foremost problems in protecting freshwater and coastal marine ecosystems. Here, I trace advances in the science of eutrophication since 1967, when the U.S. National Academy of Sciences held an international symposium on the topic in Madison, Wisconsin, providing a summary of earlier work. That symposium was my own introduction to the eutrophication problem. A brief introduction to eutrophication—Eutrophication is a term that needs little introduction to limnologists. Hutchinson (1973) gave a clear history of the development and use of the term since Weber (1907) first coined the term to describe the appearance of wetlands. The early use of the term was largely descriptive, based on the appearance of lakes, hypolimnetic oxygen depletion, and key species of benthic macroinvertebrates. Needless to say, based on these criteria, many lakes proved to be difficult to categorize. Since the mid-1970s, the term appears to have taken on new meanings in a number of respects. As it became possible to measure the primary productivity of lakes directly, and the key role of human activities in a lake’s catchment and the concept of water renewal became known, the term began to be used in a more dynamic sense, implying changes in both within-lake processes and land–water interactions. As Wetzel (2001) points out, using production or biomass as the key variable in classifying lakes eliminates most of the difficulties encountered when attempting to use earlier classification schemes. 1 Corresponding author ([email protected])