Thermal Equilibrium Concept
Beginning in the 1950s, both scientists and the general public had begun to grow increasingly concerned about the effects of thermal pollution in streams and rivers. Initial laboratory experiments focused on the tolerances of individual species, but field research indicated that the correlation between survivorship and water temperature was more complicated than the limits established in the laboratory. In 1972 Bern Sweeney and Robin Vannote abandoned the idea of thermal limits and hypothesized that temperature changes altered the normal developmental and growth characteristics of a species, which in turn reduced its adult size and reproductive activity. Their Thermal Equilibrium Concept proposed two hypotheses:
- That for many cold-blooded aquatic animals, especially insects, a direct correlation exists between water temperature and reproductive potential; and
- That changing temperature cycles affect the geographic distribution of a species by gradually lowering its reproductive vitality.
Between 1980 and 1985 these ideas were put to a rigorous test on 25 river systems that stretched across the eastern Piedmont region of North America from Florida to Quebec. This remains the largest project ever undertaken at Stroud Water Research Center, and its results confirmed the essential tenets of both hypotheses. While many species evolve elaborate genetic mechanisms to cope with severe seasonal changes in temperature, such adaptations offer little protection against human activities. In a world increasingly intent on protecting its water, the Thermal Equilibrium Model provided a quantifiable way to measure the impact of pollution on stream life.