Biomagnification is the rising concentration of a persistent toxin, such as PCBs, from one trophic level to the next.
Missing Word
Biomagnification
Biomagnification occurs because each consumer eats many contaminated organisms and stores the fat-soluble toxin rather than excreting it. Top predators therefore carry the highest concentrations.
Category: Ecosystem Dynamics
Question 2
Pregnant people are advised to avoid swordfish and shark, because these large predatory fish can carry high levels of mercury, a toxic metal.
Missing Word
mercury
Mercury builds up in aquatic food chains and reaches high levels in long-lived predatory fish. It can harm the developing nervous system, which is why the advice targets pregnant people and young children.
Category: Ecosystem Dynamics
Question 3
As water vapor cools high in the atmosphere, it changes into tiny liquid droplets that form clouds through condensation.
Missing Word
condensation
Condensation is the change of water from a gas to a liquid as it cools. In the water cycle it forms clouds, and the droplets eventually grow large enough to fall as precipitation.
Category: Water and Carbon Cycles
Question 4
Rhizobium, a genus of nitrogen-fixing bacteria, lives in nodules on the roots of legumes such as peas and beans.
Missing Word
Rhizobium
Rhizobium bacteria and their legume hosts are mutualists: the bacteria convert nitrogen gas into ammonia for the plant, and the plant supplies the bacteria with sugars. This is why legumes can enrich soil with nitrogen.
Category: Nutrient Cycles
Question 5
Bacteria and fungi convert the nitrogen in dead organisms and animal wastes into ammonium through ammonification.
Missing Word
ammonification
Ammonification is the release of ammonium from organic nitrogen as decomposers break down wastes and dead tissue. The ammonium can then be absorbed by plants or converted to nitrite and nitrate by soil bacteria.
Category: Nutrient Cycles
Question 6
Excess nitrogen and phosphorus from fertilizer runoff and sewage can trigger algal blooms through eutrophication, which can leave oxygen-poor dead zones.
Missing Word
eutrophication
Eutrophication is nutrient enrichment of a body of water that fuels excessive algal growth. When the algae die and decompose, dissolved oxygen falls, and fish and other animals can die.
Category: Nutrient Cycles
Question 7
Phosphorus cycles through ecosystems mainly as the phosphate ion, which organisms use to build nucleic acids, phospholipids and bone.
Missing Word
phosphate
The phosphate ion, PO4 3-, is the form in which phosphorus moves through soil, water and living things. It is released from rocks by weathering and is often a limiting nutrient.
Category: Nutrient Cycles
Question 8
Sulfur is part of cysteine, an amino acid whose sulfur atoms can form disulfide bonds that help proteins fold.
Missing Word
cysteine
Cysteine contains a sulfur-bearing side chain, and two cysteines can link through a disulfide bond that stabilizes a protein's shape. This is one reason sulfur is essential to living things.
Category: Nutrient Cycles
Question 9
A biome is a major type of ecosystem defined by its climate and dominant vegetation, such as tundra or tropical rainforest.
Missing Word
biome
Biomes are large-scale ecosystem types shaped mainly by temperature and precipitation. The same biome can occur on different continents wherever the climate is similar.
Category: Terrestrial Biomes
Question 10
In the Arctic tundra, permafrost, soil that stays frozen year-round, limits root growth and slows decomposition.
Missing Word
permafrost
Permafrost is permanently frozen ground beneath the surface. It keeps plants shallow-rooted and low-growing, and its cold slows the breakdown of dead material.
Category: Terrestrial Biomes
Question 11
Temperate forests in eastern North America are dominated by deciduous trees, which shed their leaves each fall.
Missing Word
deciduous
Deciduous trees drop their leaves before a cold winter and grow new ones in spring. Their thick leaf litter makes temperate forest soils rich in nutrients.
Category: Terrestrial Biomes
Question 12
Estuaries, where rivers meet the sea, contain brackish water, a mix of fresh and salt water whose salinity changes with the tides.
Missing Word
brackish
Brackish water is saltier than fresh water but less salty than seawater. Estuary organisms must tolerate shifting salinity, and salt-tolerant plants called halophytes grow along their edges.
Category: Aquatic Biomes
Question 13
The benthic realm of the ocean is the sea floor, stretching from the shoreline to the deepest ocean bottom.
Missing Word
benthic
The benthic realm includes all of the ocean bottom, while the pelagic realm is the open water above it. The benthic realm is rich in nutrients because dead organisms sink to it from above.
Category: Aquatic Biomes
Question 14
The intertidal zone, the strip of shore closest to land, is alternately covered by seawater and exposed to air as the tides rise and fall.
Missing Word
intertidal
Organisms in the intertidal zone must survive drying, temperature swings and pounding waves. Sea stars, barnacles and mussels are typical residents of rocky intertidal shores.
Category: Aquatic Biomes
Question 15
Where a slow river carries a heavy load of silt, the sediment settles at its mouth and builds a delta.
Missing Word
delta
A delta forms where a river slows as it enters a larger body of water and drops its sediment. The river's fresh water still mixes with seawater at its mouth, so a delta often borders brackish, estuary-like water.
Category: Aquatic Biomes