Physics. Fast electrons tie quantum knots.

888 PERSPECTIVES as much terrestrial carbon is received by inland waters as reaches the world's oceans (11). Calcareous catchments also release considerable carbon to lakes as bicarbonate and carbonate. In total, inland waters may bury about four times as much carbon as do the oceans (10). Much of the organic carbon received by lakes is mineralized and emitted to the atmosphere as carbon dioxide or methane. According to recent studies, inland waters— including eutrophic reservoirs and saline lakes as well as running waters—emit roughly as much carbon as is absorbed by the world's oceans (10, 12). These contributions are given little consideration in global climate models. Including lakes and reservoirs in models may shift estimates for many landscapes to greater sources of carbon dioxide. The dissolved organic carbon originating from terrestrial systems is usually colored and serves important functions in lakes. For example, it attenuates both damaging ultraviolet radiation and the longer wavelength solar radiation that regulates the thermal structure and physical habitat of the lake, thus changing nutrient and contaminant cycling. Dissolved organic carbon can be flocculated, contributing to carbon sedimen-tation, or mineralized through photolytic and biological processes (13). Widespread regional changes in dissolved organic carbon concentrations in inland waters remain poorly understood (14). The biodiversity of many freshwater habitats is gravely threatened. During the 20th century in North America alone, 123 species of freshwater animals were recorded as extinct, with 49% of mussels, 23% of gas-tropods, 33% of crayfishes, 26% of amphibians , and 21% of freshwater fishes imperiled by the end of the century (15). The Laurentian Great Lakes are in the process of an " invasional meltdown " (16), due largely to species imported in the ballast water of Eurasian ships. Reservoirs and other human-made impoundments, including constructed ponds and wetlands, are 2.4 to 300 times as likely to harbor invasive species as natural lakes (17). They accelerate the spread of invasive species by decreasing the distance to the nearest " stepping stone " of water (17). The Three Gorges Dam has allowed the spread of 55 invasive species throughout its 58,000 km 2 catchment, including the water hyacinth, regarded by many as the world's worst invasive species (18). Lakes also serve a sentinel function for many of the changes to forests and wetlands caused by a warming climate. For example, a 74 to 118% increase in the area burned by forest fires in Canada is …