Page 413 - Environment: The Science Behind the Stories

and amphibians consume aquatic invertebrates and plants, and
birds such as kingfishers, herons, and ospreys dine on fish and
amphibians.

Lakes and ponds are ecologically
diverse systems

Lakes and ponds are bodies of standing surface water. Their
physical conditions and the types of life within them vary with
depth and the distance from shore. As a result, scientists have
(a) Braided river in Nebraska described several zones typical of lakes and ponds (Figure 15.7).
Around the nutrient-rich edges of a water body, the water
is shallow enough that aquatic plants grow from the mud and
reach above the water’s surface. This region, named the littoral
zone, abounds in invertebrates—such as insect larvae, snails,
and crayfish—on which fish, birds, turtles, and amphibians
feed. The benthic zone extends along the bottom of the lake or
pond, from shore to the deepest point. Many invertebrates live
in the mud, feeding on detritus or on one another. In the open
portion of a lake or pond, far from shore, sunlight penetrates
shallow waters of the limnetic zone. Because light enables
photosynthesis (pp. 49–50), the limnetic zone supports phyto-
plankton (algae, protists, and cyanobacteria; p. 93), which in
turn support zooplankton (p. 93), both of which are eaten by
fish. Within the limnetic zone, sunlight intensity (and there-
fore water temperature) decreases with depth. Clear water
allows sunlight to penetrate deeply, whereas turbid water
(b) Meandering river in Alaska
(water with suspended solids) does not. Below the limnetic
zone lies the profundal zone, the volume of open water that
sunlight does not reach. This zone lacks photosynthetic life
and is lower in dissolved oxygen than upper waters.
Ponds and lakes change over time as streams and runoff
bring them sediment and nutrients. Oligotrophic lakes and ponds,
which are low in nutrients and high in oxygen, may slowly give
way to the high nutrient, low oxygen conditions of eutrophic
water bodies (jump ahead to see Figure 15.23, p. 430). Even-
tually, water bodies may fill in completely by the process of
aquatic succession (p. 106). As lakes or ponds change over time,
species of fish, plants, and invertebrates adapted to oligotrophic
conditions may give way to those that thrive in eutrophic condi-
tions. These changes occur naturally, but eutrophication can also
result from human-caused nutrient pollution (pp. 126–127).
The largest lakes are sometimes known as inland seas.
North America’s Great Lakes are prime examples. Lake Baikal
in Asia is the world’s deepest lake, at 1637 m (just over 1 mile)
(c) Oxbow river in Arizona deep. The Caspian Sea is the world’s largest body of fresh
water, covering nearly as much area as Montana or California.
Figure 15.6 Rivers are classified according to their flow
across the landscape. The Platte River in Nebraska (a) shows an
example of a braided river, whereas the Wood River in Alaska Freshwater wetlands include marshes,
(b) meanders across the landscape. The Horseshoe Bend portion swamps, bogs, and vernal pools
of the Colorado River in Arizona (c) demonstrates an oxbow.
Wetlands are systems in which the soil is saturated with water
and which generally feature shallow standing water with
Rivers and streams host diverse ecological communi- ample vegetation. There are many types of wetlands, and most
ties. Algae and detritus support many types of invertebrates, are enormously rich and productive. In freshwater marshes,
from water beetles to crayfish. Insects as diverse as dragon- shallow water allows plants such as cattails and bulrushes to
flies, mayflies, and mosquitoes develop as larvae in streams grow above the water surface. Swamps also consist of shal-
412 and rivers before maturing into adults that take to the air. Fish low water rich in vegetation, but they occur in forested areas.

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