Light and heat properties are influenced by climate
and topography, and by a waterbody’s chemical composition,
suspended sediments, and primary productivity.
Water temperature directly regulates oxygen concentrations,
organism metabolism, and associated life
processes. The thermal regime greatly influences organismal
fitness and, by extension, the distribution of
species in both space (e.g., along latitudinal and altitudinal
gradients) and time (e.g., seasonal variation at
one location). In lakes particularly, the absorption of
solar energy and its dissipation as heat are critical to
development of thermal structure and water circulation
patterns (Wetzel 1983). These characteristics in turn
influence nutrient cycling, distribution of dissolved
gases and biota, and the behavioral adaptations of organisms.
Water temperature can change dramatically downstream
of dams (Ward and Stanford 1979). Mean
monthly temperatures ranged between 28C in winter
and 188C in summer in the Green River, Utah, USA,
before completion of the Flaming Gorge Dam in 1962.
After dam closure the water temperatures below the
dam exhibited a much reduced annual range of mean
monthly temperatures between 48C and 98C (Vinson
2001). Species richness declined and 18 macroinvertebrate
genera were extirpated; other species, notably
crustaceans, came to dominate the invertebrate fauna
(Vinson 2001). Aquatic insects have not recovered,
even after 20 yr of partial thermal restoration. Water
temperature also dropped in the Colorado River after
closure of the Glen Canyon Dam in 1963 and, along
with a dramatic increase in water clarity, this allowed
for development of a nonnative trout population and
an unusual food web more commonly found in Nearctic
regions (Shannon et al. 2001). Water clarity is now
routinely .7 m, whereas prior to dam closure the water