increasing emphasis on research related to the effects of pollution on aquatic resources and on ways to restore and manage these resources;
increasing diversity in the disciplinary backgrounds of limnologists;
increasing variety in the types of lakes studied; and
increasing focus on other types of inland aquatic ecosystems (streams, wetlands, and reservoirs), broadening the field of limnology from its traditional focus on natural lakes.
Public concern about declining water quality and impaired ecological conditions in many aquatic resources caused by various human activities has resulted in greatly increased public expenditures for limnological studies in North America since the 1960s. Much of this funding has been associated with two major pollution problems: eutrophication and acid deposition, both of which generated large government-sponsored research programs for approximately 15-year periods (early 1960s to mid-1970s for eutrophication; late 1970s to early 1990s for acid deposition). A substantial portion of this research support was supplied by mission-oriented agencies such as the Environmental Protection Agency rather than by basic science agencies such as the National Science Foundation.
These large programs, although criticized by some scientists as inefficient (Roberts, 1987), resulted in significant practical advances. They defined the nature and extent of the problems, quantified sources of pollution, developed relationships for determining the responses of water bodies to various levels of pollution, and identified a variety of control and restoration measures. Beyond these practical results, the research supported or otherwise stimulated by these initiatives produced many conceptual advances and much new fundamental information about limnological processes, as well as advances or refinements in field and laboratory techniques. For example, eutrophication-related research led to improved understanding of aquatic food web interactions and to conceptual advances regarding the factors that control material and energy flows through aquatic food chains and webs. Similarly, research stimulated by concerns about lake acidification led to greatly improved understanding of the chemical and microbial processes affecting alkalinity and acid-base balances in dilute lake waters, new information about the biogeochemical cycling of sulfur in such systems, and advances in the understanding of mineral weathering rates. Acidification research also helped foster the development of techniques to understand the natural variability of ecosystems as a benchmark against which to measure the effects of human-caused stress. Both problems led to substantial advances in the ability to describe lake ecosystem processes mathematically and to develop predictive models of these systems. Finally, both led to improved understanding