5.7. Lakes and Streams
Lake and stream ecosystems include many familiar places: large and small lakes,
permanent and temporary ponds, and streamsfrom 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.
5.7.1. Status of Goods and Services
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 unappreciatedfor 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 catchesprimarily 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
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 dollarsfor 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).