Experimental Lakes Area
Experimental Lakes Area
The Experimental Lakes Area (ELA) in northwestern Ontario is in a remote landscape characterized by Precambrian bedrock, northern mixed-species forests, and oligotrophic lakes, bodies of water deficient in plant nutrients. The Canadian Department of Fisheries and Oceans began developing a field-research facility at ELA in the 1960s, and the area has become the focus of a large number of investigations by D. W. Schindler and others into chemical and biological conditions in these lakes.
Of the limnological investigations conducted at ELA, the best known is a series of whole-lake experiments designed to investigate the ecological effects of perturbation by a variety of environmental stress factors, including eutrophication, acidification , metals, radionuclides , and flooding during the development of reservoirs.
The integrated, whole-lake projects at ELA were initially designed to study the causes and ecological consequences of eutrophication. In one long-term experiment, Lake 227 was fertilized with phosphate and nitrate. This experiment was designed to test whether carbon could limit algal growth during eutrophication, so none was added. Lake 227 responded with a large increase in primary productivity by drawing on the atmosphere for carbon, but it was not possible to determine which of the two added nutrients, phosphate or nitrate, had acted as the primary limiting factor.
Observations from experiments at other lakes in ELA, however, clearly indicated that phosphate is the primary limiting nutrient in these oligotrophic water bodies. Lake 304 was fertilized for two years with phosphorus , nitrogen , and carbon, and it became eutrophic. It recovered its oligotrophic condition again when the phosphorus fertilization was stopped, even though nitrogen and carbon fertilization were continued. Lake 226, an hourglass-shaped lake, was partitioned with a vinyl curtain into two basins, one of which was fertilized with carbon and nitrogen, and the other with phosphorus, carbon, and nitrogen. Only the latter treatment caused an algal bloom . Lake 302 received an injection of all three nutrients directly into its hypolimnion during the summer. Because the lake was thermally stratified at that time, the hypolimnetic nutrients were not available to fertilize plant growth in the epilimnetic euphotic zone, and no algal bloom resulted. Nitrogen additions to Lake 227 were reduced in 1975 and eliminated in 1990. The lake continued with high levels of productivity by fixing nitrogen from the atmosphere.
Research of this sort was instrumental in confirming conclusively the identification of phosphorus as the most generally limiting nutrient to eutrophication of freshwaters. This knowledge allowed the development of waste management systems which reduced eutrophication as an environmental problem by reducing the phosphorus concentration in detergents , removing phosphorus from sewage, and diverting sewage from lakes.
Another well known ELA project was important in gaining a deeper understanding of the ecological consequences of the acidification of lakes. Sulfuric acid was added to Lake 223, and its acidity was increased progressively, from an initial pH near 6.5 to pH 5.0–5.1 after six years. Sulfate and hydrogen ions were also added to the lake in increasing concentrations during this time. Other chemical changes were caused indirectly by acidification: manganese increased by 980%, zinc by 550%, and aluminum by 155%.
As the acidity of Lake 223 increased, the phytoplankton shifted from a community dominated by golden-brown algae to one dominated by chlorophytes and dinoflagellates. Species diversity declined somewhat, but productivity was not adversely affected. A mat of the green alga Mougeotia sp. developed near the shore after the pH dropped below 5.6. Because of reduced predation, the density of cladoceran zooplankton was larger by 66% at pH 5.4 than at pH 6.6, and copepods were 93% more abundant. The nocturnal zooplankton predator Mysis relicta, however, was an important extinction . The crayfish Orconectes virilis declined because of reproductive failure, inhibition of carapace hardening, and effects of a parasite. The most acid-sensitive fish was the fathead minnow (Pimephales promelas ), which declined precipitously when the lake pH reached 5.6.
The first of many year-class failures of lake trout (Salvelinus namaycush ) occurred at pH 5.4, and failure of white sucker (Catastomus commersoni ) occurred at pH 5.1. One minnow, the pearl dace (Semotilus margarita ), increased markedly in abundance but then declined when pH reached 5.1. Adult lake trout and white sucker were still abundant, though emaciated, at pH 5.0–5.5, but in the absence of successful reproduction they would have become extinct. Overall, the Lake 223 experiment indicated a general sensitivity of many organisms to the acidification of lake water. However, within the limits of physiological tolerance, the tests showed that there can be a replacement of acid-sensitive species by relatively tolerant ones.
In a similarly designed experiment in Lake 302, nitric acid was shown to be nearly as effective as sulfuric acid in acidifying lakes, thereby alerting the international community to the need to control atmospheric emissions of gaseous nitrogen compounds.
[Bill Freedman Ph.D. ]
Freedman, B. Environmental Ecology. San Diego: Academic Press, 1995.
Schindler, D. W. "The Coupling of Elemental Cycles by Organisms: Evidence from Whole-Lake Chemical Perturbations." In Chemical Processes in Lakes, edited by W. Stumm. New York: Wiley, 1985.
Schindler, D. W., et al. "Long-Term Ecosystem Stress: The Effects of Years of Experimental Acidification of a Small Lake." Science 228 (June 21, 1985): 1395–1401.