Figure 21.18: Many types of agriculture add pollutants to groundwater. ( 12 )nearby rivers, lakes and the oceans. Fertilizers contain nitrates that promote tremendous
plant growth in the water. Consequences of this accelerated plant growth include:
- Lakes, rivers and bays become clogged with a carpet of aquatic plants that block light
from entering the water. - Without light reaching plants in the water below, these organisms die.
- As the plants die, their decomposition uses up all the oxygen in the water. Without
enoughdissolvedoxygeninthewater,largenumbersofplants,fishandbottom-dwelling
animals die.
Every year you can seedead zones, hundreds of kilometers of ocean without fish or plant
life (Figure21.19). These dead zones occur in the Gulf of Mexico and other river delta areas
due to water polluted with fertilizers. In 1999, a dead zone in the Gulf of Mexico reached
over 7,700 square miles.
Ocean Water Pollution
Most (80 %) of ocean pollution comes as runoff from agriculture, industry, and domestic uses
(Figure21.20). These same kinds of runoff also pollute freshwater. The remaining 20% of
water pollution comes from oil spills and people dumping sewage directly into the water.
Coastal pollution can make coastal water unsafe for humans and wildlife. After rainfall,
there can be enough runoff pollution that beaches are closed to prevent the spread of disease
from pollutants.
A large proportion of the fish stocks we rely on for food live in the coastal wetlands. Coastal
runoff from farm waste often carries water-borne organisms that cause lesions that kill fish.
Humans who come in contact with polluted waters and affected fish can also experience