Lake and stream ecosystems include many familiar places: large and small lakes, permanent and temporary ponds, and streams—from tiny, often temporary rivulets at headwaters to powerful floodplain rivers discharging from our continents. Freshwaters (lakes and rivers), which are so valuable for the sustainability of life as we know it, constitute only 0.0091% of the Earth's surface waters by volume. This is a great deal less than 0.5% for groundwater and 97.3% for oceans (Cole, 1994). Lakes and rivers are used intensively for recreation and are aesthetically valued. These values include fishing, hunting, swimming, boating, skating, and simply enjoying the view.

This section focusses on physical and biological processes and how they affect goods and services from lakes and streams that are regulated by these processes. The emphasis is on food, carbon, and biodiversity. Hydrology and water supply are covered in Chapter 4; oceans and coastal systems are covered in Chapter 6. Hydrological goods and services are covered largely in Chapter 4, but are considered here as an interactive influence of climate change on inland waters. Wetlands are covered in Section 5.8.

Products from lakes and streams include fisheries (fish, amphibians, crustaceans, and mollusks) and aquaculture; services include biodiversity, recreation, aesthetics, and biogeochemical cycling. A detailed list of services from freshwater ecosystems includes many items that are undervalued in economic terms and often are unrecognized and unappreciated—for example, their role in the carbon cycle.

Reported catches in freshwater and inland fisheries were 7.7 Mt of biomass in 1996 (FAO, 1999b) and 15.1 Mt from aquaculture. China accounts for 80% of aquaculture production. Actual landings in capture fisheries are believed to be two to three times larger owing to nonreporting (FAO, 1999c). About 100 fish species are reported in world catches—primarily cyprinids, cichlids, snakeheads, catfish, and barbs. Asia and China report the largest catches; Africa is second; North America ranks relatively low. River and large reservoir fisheries are important.

Recreational fish catches are included in the foregoing world catches and were reported to be 0.48 Mt in 1990 (FAO, 1992). This is likely to be an underestimate because only 30 (of 200) countries reported recreational catches. Recreational fishing is increasing in developed and developing countries. The number of anglers is estimated at 21.3 million in 22 European countries, 29.7 million in the United States, and 4.2 million in Canada (U.S. Fish and Wildlife Service, 1996; Department of Fisheries and Oceans, Canada, 1998). Total expenditures for recreational angling are in the billions of dollars—for example, $38 billion in the United States in 1996 (U.S. Fish and Wildlife Service, 1996) and $4.9 billion in Canada in 1995.

Freshwaters are known for high biodiversity and endemism owing to their island-like nature, which leads to speciation and reduces invasions of competitors and predators. Of about 28,000 fish species known on Earth, 41% are freshwater species and 58% are marine, of which 1% spend part of their lives in freshwater (Moyle and Cech, 1996). Individual east African Rift Valley lakes contain species flocks of almost 250 cichlid species. In Lake Baikal in Russia, 35% of the plants and 65% of the animals are endemic (Burgis and Morris, 1987). At the global level, biodiversity in many lakes has been decreasing in recent decades, with many species becoming extinct (Naiman et al., 1995b). The trend is likely to continue, with many species that now are listed as endangered or threatened becoming extinct (IUCN, 1996). The causes for these extinctions are likely to be related to the many pressures listed below.

Inland waters play a major role in biogeochemical cycling of elements and compounds such as carbon, sulfur, nitrogen, phosphorous, silica, calcium, and toxic substances. The general roles are storage, transformation, and transport (Stumm and Morgan, 1996). Storage is important because sediments and associated minerals accumulate in the bottom sediments of lakes, reservoirs, and floodplains. Transformation includes organic waste purification and detoxification of various human created compounds such as insecticides. Water movement redistributes these spatially.

Of special interest here is the role of freshwaters in carbon storage and CO₂ and methane (CH₄) release. Organic carbon from primary production in lakes and adjacent riparian lands accumulates in sediments; estimates are that 319 Mt yr^-1 are buried in the 3.03 million km² of small and large lakes, reservoirs, and inland seas worldwide (USGS, 1999; see also Stallard, 1998). This estimate excludes amounts for peatlands (96 Mt yr^-1). It is three times greater than estimates for the ocean in absolute terms (97 Mt yr^-1) even given the relatively small area of inland waters. Lakes also are a source of GHGs. Lakes become supersaturated with dissolved CO₂, and net gas exchange is from the lake to the atmosphere (Cole et al., 1994). For example, during summer in Lake Pääjärv, Finland, the amount of carbon from respiration in the water column was greater than that produced by phytoplankton and sedimentation combined (Kankaala et al., 1996). Pulses of CO₂ from the water column and CH₄ from sediments are released to the atmosphere during spring and fall mixing of the water column of dimictic lakes (Kratz et al., 1987; Riera et al., 1999; Kortelainen et al., 2001). Methane releases, especially from the littoral zone, can be significant in lakes and reservoirs (Fearnside, 1995, 1997; Alm et al., 1997a; Hyvönen et al., 1998).

Hydroelectric power plants generally are assumed to emit less CO₂ than fossil fuel plants. However, a hydroelectric reservoir may contribute more to GHGs over 100 years of operation than a fossil fuel plant that produces an equivalent amount of electricity (Fearnside, 1997). Emissions are likely to be high in the first few decades and then decrease. This is exemplified by a Brazilian hydroelectric reservoir that is simulated to release a large quantity of CO₂ during the first 10 years after filling (ca. 5-27 Mt CO₂ gas yr^-1) but relatively low amounts from years 30 to 100. Releases of CH₄ were simulated to be high for at least 100 years (about 0.05-0.1 Mt CH₄ yr^-1, with actual estimates for 1990 of 0.09 Mt) (Fearnside, 1995, 1997).

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