Lakes as Ecosystems

W.M. Lewis    University of Colorado, Boulder, CO, USA

Introduction of the Ecosystem Concept

Scientific studies of lakes began as early as the seventeenth century, but at first were descriptive rather than analytical. Toward the end of the nineteenth century, measurements and observations on lakes became more directed. For example, the thermal layering of lakes was attributed to specific physical causes, and such phenomena as the movement of plankton in the water column were the subject of hypotheses that were tested with specific kinds of data.

Comprehensive studies of lakes began with the work of Alphonse Forel (1841–1912) on Lac Léman (Lake Geneva), Switzerland, as well as other Swiss lakes. In a three-volume monograph (Le Léman: 1892, 1895, 1904), Forel presented data on a wide variety of subjects including sediments and bottom-dwelling organisms, fishes and fisheries, water movement, transparency and color, temperature, and others. Thus Forel, who introduced the term ‘limnology’ (originally the study of lakes, but later expanded to include other inland waters), demonstrated the holistic approach for understanding a lake as an environmental entity, but without application of an explicit ecosystem concept.

The conceptual basis for studying lakes as ecosystems was first clearly given by Stephen Forbes (1844–1930; Figure 1) through a short essay, The Lake as a Microcosm (1887). Forbes was professor of biology at the University of Illinois in Champaign, IL, USA, and director of the Illinois Natural History Laboratory (subsequently the Illinois Natural History Survey), which was charged with describing and analyzing the flora and fauna of Illinois. Forbes realized that it was not possible to achieve a full understanding of a lake or, by implication, of any other environmental system such as a stream or forest, simply from knowledge of the resident species. Forbes proposed that the species in a particular environment, when interacting with each other and with the nonliving components of the environment, show collective (system) properties. The microcosm that Forbes described today would be called an ecosystem, although this term did not come into use until 48 years later through the work of the British botanist A. G. Tansley. By current usage, an ecosystem is any portion of the Earth’s surface that shows strong and constant interactions among its resident organisms and between these organisms and the abotic environment.

Forbes not only described accurately the modern ecosystem concept, but also identified ways in which critical properties of ecosystems could be measured and analyzed. He named four common properties of lakes, each of which provides a cornerstone for the study of lakes and other ecosystems, as shown in Table 1. Although Forbes’s concepts have been renamed, they are easily visible in the modern study of lakes as ecosystems.

Although the essay by Forbes now is considered a classic in limnology and in ecology generally, it caused no immediate change in the practices of limnologists or ecologists. Like many important discoveries in science, it was a seed that required considerable time to germinate.

In studies of Cedar Bog Lake, Minnesota, for his Ph.D. at the University of Minnesota, Raymond Lindeman (1915–1942; Figure 2) gave limnologists the clearest early modern example of the study of lakes as ecosystems. Rather than focusing on a particular type of organism or group of organisms, which would have been quite typical for his era, Lindeman decided that he would attempt to analyze all of the feeding relationships (‘trophic’ relationships) among organisms in Cedar Bog Lake. Thus, his Ph.D. work extended from algae and aquatic vascular plants to herbivorous invertebrates, and then to carnivores, and conceptually even to bacteria, although there were no methods for quantifying bacterial abundance at that time. Lindeman’s descriptions of feeding relationships were voluminous but straightforward to write up and publish, but he sought some more general conclusions for which he needed a new concept.

Lindeman took a postdoctoral position with G. Evelyn Hutchinson at Yale University in 1942. Hutchinson had become a limnologist of note through his quantitatively oriented studies of plankton and biogeochemical processes in the small kettle lakes near New Haven. He would in subsequent decades become the world’s most influential limnologist, and part of his reputation grew out of his contributions to the field of biogeochemistry, an important tool of ecosystem science.

Hutchinson made suggestions that no doubt were critical to Lindeman’s groundbreaking paper, The Trophic Dynamic Aspect of Ecology (1942, published in the journal Ecology), which now is recognized as a landmark in limnology and in ecology generally. Lindeman proposed a way of converting the tremendous mass of highly specific information for Cedar Bog Lake into a format that would allow comparisons with any other lake or even with other kinds of ecosystems. Building on the work of the German limnologist August Thienemann and the British ecologist Charles Elton, Lindeman organized the feeding relationships as a feeding hierarchy within which each kind of organism was assigned to a specific feeding level (trophic level). He then proposed that the feeding relationship represented by any given link in the food web be quantified as an energy flow. Thus, the total energy flow from level 1 (plants) to level 2 (herbivores) could be quantified as the summation of energy flows between all pairs of plants and herbivores; a similar estimate could be made for all other pairs of trophic levels. In this way, the flow of energy across the levels of the food web could be expressed in quantitative terms.

The first important conclusion from Lindeman’s energy-based approach was that each transfer of energy between trophic levels is governed by the second law of thermodynamics, which requires that significant energy loss must occur each time energy is transferred. Thus, Lindeman demonstrated why food webs have relatively few trophic levels: progressive dissipation of energy as it passes through the food web from the bottom (plants) to the upper levels (upper-level carnivores) ultimately provides insufficient energy for expansion of the food web to further levels. Also, analysis of a food web in this way sets the stage for calculating efficiencies of energy transfer, comparison of efficiencies across different ecosystem types, and the identification and analysis of bottlenecks restricting the flow of energy within the food web.

The contributions of G. Evelyn Hutchinson in the 1940s on the biogeochemistry of carbon in lakes also must be counted as landmarks in the development of ecosystem science. Even so, ecosystem science was scarcely represented in the research agenda of ecologists or in the academic curriculum as late as 1950. Penetration of the ecosystem thinking into research, teaching, and public awareness occurred first through the publication of a textbook, Fundamentals of Ecology (1953), written by Eugene P. Odum, and especially through the second edition of the same book (1959), written by E. P. Odum and Howard T. Odum. The Odums visualized ecology as best viewed from the top down, with ecosystems as a point of departure and studies of ecosystem components as infrastructure for the understanding of ecosystems.

The ecosystem perspective has not displaced the more specialized branches of ecology that deal with particular kinds of organisms or specific kinds of physical phenomena, such as studies of water movement, optics, or heat exchange. Rather, ecosystem science has had a unifying effect on studies of ecosystem components (Figure 3). Study of a specific ecosystem component produces not only a better understanding of that component, but also a better understanding of the ecosystem, which is a final objective for the science of an ecosystem type, such as lakes.

Metabolism in Lakes

The dominant anabolic component of metabolism in lakes is photosynthesis based on carbon...