Visualizing Environmental Science

260 CHAPTER 10 Freshwater Resources and Water Pollution

age footostock/SuperStockMichael P. Gadomski/Science Source Images

c. and d. Aquatic ecologists understand the characteristics of oligotrophic and eutrophic lakes. c. An oligotrophic

lake has a low level of inorganic plant and algal nutrients; its fish species require cool, oxygen-rich water.

d. A eutrophic lake has a high level of these nutrients; its fish species tolerate warm, low-oxygen water.

a. and b. The average person looking at these two photographs would notice the dramatic differences between
them but wouldn’t understand the environmental conditions responsible for the differences. a. Shows Crater
Lake, an oligotrophic lake in Oregon; b. shows a small eutrophic lake in the Catskill Mountains, New York.
a. b.

c. d. High nutrient levels

Poor light penetration

Low dissolved oxygen

Shallow waters

High algal growth

Warm-water fish: Carp, bullhead,

catfish

Sand, silt,

or clay bottom

High dissolved oxygen

Deep waters

Low algal growth

Cool-water fish: Smallmouth

bass, lake trout, pike, sturgeon,

whitefish

Rock, gravel,

or sand bottom

Low nutrient levels

Good light penetration

WHAT A SCIENTIST SEES

Oligotrophic and Eutrophic Lakes

BOD measures how fast microorganisms remove oxygen

from a body of water. When dissolved oxygen levels are

low, anaerobic (without oxygen) microorganisms pro-

duce compounds with unpleasant odors, further deterio-

rating water quality.

Eutrophication: An Enrichment Problem Lakes,

estuaries, and slow-flowing streams that have mini-

mal levels of nutrients are considered unenriched, or

oligotrophic. An oligotrophic lake has cool, clear water

and s upports small populations of aquatic organisms

(see What a Scientist Sees, parts a and c). Eutrophication

is the enrichment of a lake, an estuary, or a slow-flowing

stream by inorganic plant and algal nutrients such as

phosphorus; an enriched body of water is said to be eu-

trophic. The enrichment of water results in an increased

photosynthetic productivity. The water in a eutrophic

lake is cloudy and usually resembles pea soup because of