INT RODUCT ION
Fresh water: the essence of life Russel A. Mittermeier, Thomas M. Brooks, Tracy A. Farrell, Amy J. Upgren, Ian J. Harrison, Topiltzin Contreras-MacBeath, Richard Sneider, Fabian Oberfeld, Andrew A. Rosenberg, Fredrick Boltz, Claude Gascon, Olivier Langrand
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magine that Earth is a classroom globe about 30 centimeters (cm) across and just about half a millimeter (mm) thick. That half-mm is the space on Earth within which life lives— the biosphere. From space, our biosphere is blue. Almost 71% of the surface area of Earth is covered by water; about 15% of the biosphere. Fresh water passes through the biophysical environment (atmospheric, marine, terrestrial, surface, and subterranean) as a vapor, liquid, and solid. Groundwater recharge happens aboveground but flows underground, water evaporates from soils into the atmosphere and then falls again as precipitation, and transpiration from plants returns water to the atmosphere as well. Therefore, when discussing strictly fresh water, one must remember that it is a component of this much larger hydrological cycle—which is also connected to the carbon and nitrogen cycles, stimulating plant growth as part of terrestrial and freshwater ecosystems. Fresh water is the ultimate connector, from mountaintops to coral reefs. But freshwater systems must be managed and utilized differently from terrestrial and marine systems, partly because the diversity of species in freshwater systems is quite different. If we compare the volumes of water in the biosphere, we see that nearly all of it (97.5%) is saline—having salt concentrations of more than 0.5 grams per liter. The remaining 2.5% is “fresh water.” Water locked up in glaciers and permanent snow comprises 68.7% of the fresh water on Earth. Another 30.1% is found
deep underground, and a small percentage (0.86%) is frozen as ground ice and permafrost. This leaves just under 0.3% of fresh water, or 0.0075% of all water (fresh and salt). This final small percentage of fresh water covers about 0.8% of the total surface of Earth (i.e., only a little more than 4 million kilometers [km]2), or just under 3% of the terrestrial surface area on the planet. This is equivalent to less than one drop of water spread across our 30-cm classroom globe.
Opposite: European white water lily (Nymphaea alba) in a lake in Bohuslän, Sweden. —Magnus Lundgren, Wild Wonders of Europe
What is more, around 98% of that surface fresh water is found in lakes and swamps and other wetlands, with only a tiny fraction flowing in rivers and streams. In fact, of all the fresh water on the planet, a mere 0.26% is in lakes, 0.03% is in swamps and wetlands, and 0.006% is in rivers and streams. And yet, this minuscule fraction of a fraction teems with life. The fresh water on Earth is manifested through a tremendous diversity of habitats and supports a third of all vertebrate species—maybe 19,000 species in total—that live in it for some or all of their lifecycles (Baillie et al., 2004; and see chapter 1). Fresh water also hosts high proportions of many plant and invertebrate species, including such wellknown examples as dragonflies, mayflies, mollusks, crabs, and crayfish. These are the tip of the iceberg; much of aquatic life remains below the water surface. Our knowledge of freshwater biodiversity and our understanding of the importance of the ecological processes occurring in freshwater ecosystems are shockingly scanty; they have been referred to as
I NTRODUC TION
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