Page 449 - Environment: The Science Behind the Stories

Figure 16.11 Salt marshes occur in temperate intertidal Figure 16.12 Mangrove forests line tropical and subtropical
zones where the substrate is muddy. Tidal waters flow in chan- coastlines. Mangrove trees, with their unique roots, are adapted
nels called tidal creeks amid flat areas called benches, sometimes for growing in saltwater and provide habitat for many fish, birds,
partially submerging the salt-adapted grasses. crabs, and other animals.

Salt marshes line temperate shorelines have developed coastal areas. Shrimp farming in particular
has driven the conversion of large areas of mangroves. When
Along many of the world’s coasts at temperate latitudes, salt mangroves are removed, coastal areas lose the ability to slow
marshes occur where the tides wash over gently sloping sandy runoff, filter pollutants, and retain soil. As a result, offshore
or silty substrates. Rising and falling tides flow into and out of systems such as coral reefs and eelgrass beds are more readily
channels called tidal creeks and at highest tide spill over onto degraded. Moreover, mangrove forests protect coastal com-
elevated marsh flats (Figure 16.11). Marsh flats grow thick munities against storm surges and tsunamis. The 2004 Indian
with salt-tolerant grasses, as well as rushes, shrubs, and other Ocean tsunami (p. 60) devastated areas where mangroves had
herbaceous plants. been removed but caused less damage where mangroves were
Salt marshes boast very high primary productivity and pro- intact.
vide critical habitat for shorebirds, waterfowl, and many com-
mercially important fish and shellfish species. Salt marshes
also filter pollution and stabilize shorelines against storm Fresh water meets salt water in estuaries
surges. However, because people desire to live and do busi- Many salt marshes and mangrove forests occur in or near
ness along coasts, we have altered or destroyed vast expanses estuaries, water bodies where rivers flow into the ocean,
of salt marshes to make way for coastal development. When mixing fresh water with salt water. Estuaries are biologically
salt marshes are destroyed, we lose the ecosystem services productive ecosystems that experience fluctuations in salin-
they provide. When Hurricane Katrina struck the Gulf Coast, ity as tides and freshwater runoff vary daily and seasonally.
for instance, the flooding was made worse because vast areas Sheltered from crashing surf, the shallow water of estuaries
of salt marshes had vanished during the preceding decades as nurtures eelgrass beds and other plant life, producing abundant
a result of engineering of the Mississippi River, development, food and resources. For shorebirds and for many commercially
and subsidence from oil and gas drilling (Chapter 15). important shellfish species, estuaries provide critical habitat.
For fishes such as salmon, which spawn in streams and mature
Mangrove forests line coasts in the tropics in the ocean, estuaries provide a transitional zone where young
and subtropics fish make the passage from fresh water to salt water.
Estuaries everywhere have been affected by coastal devel-
In tropical and subtropical latitudes, mangrove forests replace salt opment, water pollution, habitat alteration, and overfishing.
marshes along gently sloping sandy and silty coasts. Mangroves Think of any major estuary in the United States, from New
are some of the few types of trees that are salt tolerant, and they York Harbor to Chesapeake Bay (Chapter 5) to Florida Bay to
have unique types of roots that curve upward like snorkels to the mouth of the Mississippi River (Chapter 15) to San Fran-
attain oxygen lacking in the mud or curve downward like stilts cisco Bay. They all have suffered pollution and other impacts
to support the tree in changing water levels (Figure 16.12). Fish, from human development. Coastal ecosystems have borne the
shellfish, crabs, snakes, and other organisms thrive among the brunt of human impact because two-thirds of Earth’s people
root networks, and birds feed and nest in the dense foliage of choose to live within 160 km (100 mi) of the ocean.
these coastal forests. Besides serving as nurseries for fish and
shellfish that people harvest, mangroves also provide materials Kelp forests harbor many organisms
that people use for food, medicine, tools, and construction.
From Florida to Mexico to the Philippines, half the Along many temperate coasts, large brown algae, or kelp,
448 world’s mangrove forests have been destroyed as people grow from the floor of continental shelves, reaching up toward

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