Introduction - Climate Change Impacts on Freshwater Ecosystems - page 7

to locally positive. In the absolute scale of temperature, water has its boiling and
freezing points very close to the mean surface temperature of the Earth. In its
evaporation and condensation, water is the operative liquid of the earth's
refrigerator. It follows that the denizens of freshwaters have had an evolutionary
history in which their habitats have rather frequently frozen solid or evaporated
to mud flats or rocky beds. Freshwater animals and plants are comparatively
young in evolutionary terms for they have had repeatedly to recolonize newly
constituted freshwaters from the land and the ocean following prolonged
glaciation, volcanic disruption or periods of great aridity. They are creatures of
continual disturbance (Milner 1996).
Some manifestations of this are that many aquatic insects and vascular plants
retain land characteristics as adults or where they flower, respectively; the
diversity of freshwaters is much lower, for example, lacking whole phyla, than
that of the oceans; freshwater organisms may have particularly high rates of
evolutionary change; resting spores and eggs to tide over inimical conditions are
common (Pennak 1985). Marine organisms, in contrast, almost universally lack
resting stages, for their medium, though changing in shape and depth, has
persisted as a body of water for nearly 4 billion years. The longevity of freshwaters
may sometimes be only weeks. The retention of adult flight allows movement for
insects that cannot persist as resting eggs, and apart from fish, almost all the
vertebrates associated with freshwaters are highly motile over land. Fish are
vulnerable for few can survive drought, though they are adept at migration
through river systems, using even the ocean as part of their life history in some
cases. Some crustaceans, however, may respond genetically and very rapidly to
thermal stress (van Doorslaer et al . 2007).
As climate changes, marine communities will have a continuity of habitat that
will accommodate major changes in distribution, though for sedentary organisms
like corals, the speed of change may cause severe difficulties. In contrast, land
communities, subjected to more frequent drought and without the buffering
medium of water, with its high specific heat, will be more vulnerable to extreme
temperatures. But the freshwater biota might adjust most readily to climate
change because of its preadaptation to disturbance. For them, however, there is a
further complication. Freshwaters most immediately and most graphically reflect
the many abuses an increasing human population, with its increasing demands
for resources, increasing production of waste and rapidly accelerating ability to
make changes through its technology, can impose. Freshwaters reflect all the
activities that go on in their catchments, which means the entire land surface.
Chemical and agricultural wastes, both dissolved and suspended, run into them
or rain onto them. Rivers have been repeatedly used as cheap pipes to remove
urban wastes. Floodplain wetlands have been embanked and drained so that their
fertile soils might be cultivated. Fish communities, the main source of animal
protein for many peoples, have been severely overfished. And the very ability of
freshwater communities to accommodate change has led to the persistence of
many introduced species that have sometimes become dominant and simplified
the communities that they have invaded. Not surprisingly, the Millennium
Ecosystem Assessment listed freshwaters as one of the most vulnerable of the
ecosystems it considered (Fig. 1.4). Exactly how freshwater habitats will change,