AP Environmental Science Quiz: Eutrophication

What this quiz covers

This quiz focuses on Eutrophication, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Environmental Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Nitrogen and phosphorus from fertilizer runoff (correct answer)
2. Carbon dioxide and oxygen from the atmosphere
3. Sodium and chloride from road salt
4. Calcium and silica from weathered rock

Explanation: Cyanobacterial blooms in ponds near agriculture are initiated by nitrogen and phosphorus from fertilizer runoff, key drivers of eutrophication. Atmospheric gases or salts do not trigger blooms. Choice A identifies the responsible inputs. Rock minerals support but do not initiate rapid blooms.

Question 2

1. Fewer and smaller algal blooms, leading to less decomposition-driven oxygen depletion (correct answer)
2. More frequent dead zones because fewer nutrients increase bacterial oxygen demand
3. No change in algal blooms because nutrients are not related to primary productivity
4. Immediate elimination of all algae because algae cannot survive without sewage

Explanation: Reducing sewage nutrients combats eutrophication by limiting algal growth. In the bay, lower N and P would lead to smaller blooms and less decomposition-driven hypoxia. Choice A predicts fewer dead zones as oxygen demand decreases. Recovery may take seasons as ecosystems adjust.

Question 3

1. Increase pesticide use to reduce algae-eating insects.
2. Add more fish to consume algae directly.
3. Reduce nitrogen and phosphorus inputs by limiting fertilizer application and establishing vegetated buffer strips. (correct answer)
4. Drain the lake each summer to expose algae to air.

Explanation: Eutrophication in the lake is driven by excess nitrogen and phosphorus from fertilizer runoff, leading to algal mats, oxygen depletion, and fish kills. To prevent this, addressing the root cause involves reducing nutrient inputs at the source. Limiting fertilizer application and establishing vegetated buffer strips along waterways can intercept and absorb nutrients before they reach the lake. This approach minimizes algal blooms by starving them of essential nutrients, thereby preventing subsequent oxygen crashes from decomposition. Adding fish or pesticides might temporarily control algae but does not solve the nutrient problem and could harm the ecosystem. Choice C targets the underlying issue effectively. Draining the lake is impractical and disruptive without addressing nutrients.

Question 4

1. Install riparian buffer vegetation and reduce fertilizer application rates to cut nutrient runoff. (correct answer)
2. Add chlorine to kill algae and increase nutrient availability.
3. Stock the reservoir with more predatory fish to increase nutrient inputs.
4. Increase shoreline lighting to promote nighttime photosynthesis.

Explanation: Eutrophication in the reservoir is fueled by nitrogen and phosphorus from lawn fertilizers, causing summer algal blooms. To target the source, reducing runoff through riparian buffers and lower fertilizer rates absorbs nutrients before they enter water. This prevents blooms by limiting nutrient availability. Chlorine or more fish address symptoms, not causes, and could harm ecology. Lighting does not affect photosynthesis significantly. Choice A directly mitigates the nutrient source. Other actions are ineffective or counterproductive.

Question 5

1. Higher dissolved oxygen in deep water due to increased mixing by algae
2. Reduced algal biomass because more nutrients reduce photosynthesis
3. Increased organic matter followed by oxygen depletion during decomposition, especially in deeper water (correct answer)
4. Permanent elimination of all bacteria because nutrients are toxic to decomposers

Explanation: Eutrophication from suburban runoff with phosphorus and nitrogen leads to summer algal blooms in reservoirs. This increases organic matter, which decomposes and depletes oxygen, particularly in deeper, stratified waters. Choice C matches this with increased biomass, decomposition, and hypoxia. Deep water is vulnerable due to limited reoxygenation.

Question 6

1. Oxygen produced and exchanged at the surface does not readily mix downward due to stratification, while decomposition consumes oxygen at depth (correct answer)
2. Algae only decompose in surface waters, so oxygen is consumed near the surface while deep water remains oxygen-rich
3. Nutrients sink and produce oxygen at the bottom, but surface water loses oxygen to the atmosphere
4. Fish move to the bottom and produce oxygen through respiration, preventing hypoxia at depth

Explanation: In stratified lakes, eutrophication causes surface blooms, but hypoxia develops at depth. Stratification blocks downward oxygen mixing, while bottom decomposition consumes DO. Choice A explains the oxygen gradient pattern. This is common in summer when thermal layers form.

Question 7

1. Increase dredging to deepen the estuary so that algae have more water volume to grow in.
2. Add more zooplankton to the estuary to increase grazing, without changing nutrient inputs.
3. Reduce nitrogen and phosphorus inputs by installing riparian buffer strips and improving fertilizer application practices in the watershed. (correct answer)
4. Increase nighttime aeration only during the algal bloom to raise oxygen while allowing nutrient loading to continue unchanged.

Explanation: Eutrophication occurs when excessive nutrients like nitrogen and phosphorus enter a water body, promoting algal blooms that can lead to oxygen depletion and dead zones. In this coastal estuary, chronic agricultural runoff supplies these nutrients, causing summer algal blooms followed by bacterial decomposition of dead algae, which consumes dissolved oxygen and creates hypoxic conditions near the bottom. A dead zone forms where oxygen levels are too low for most marine life to survive, disrupting the ecosystem. The most effective management action addresses the root cause by reducing nutrient inputs, such as through riparian buffer strips that filter runoff and improved fertilizer practices that minimize excess application. Choice C directly targets this by limiting nitrogen and phosphorus entering the watershed, thereby preventing excessive algal growth and subsequent hypoxia. Options like A (dredging) or D (aeration) treat symptoms without solving the nutrient problem, while B (adding zooplankton) might provide temporary grazing relief but does not stop ongoing nutrient loading. This approach emphasizes sustainable, preventive strategies over reactive fixes in managing eutrophication.

Question 8

1. Photosynthesis and air-water gas exchange increase oxygen near the surface, while decomposition of sinking organic matter consumes oxygen at depth. (correct answer)
2. Oxygen is heavier than water, so it sinks and is removed from surface waters first.
3. Nitrogen fertilizes deep-water plants only, which consume oxygen and create surface oxygen.
4. Phosphorus prevents oxygen from dissolving near the bottom by forming a surface film.

Explanation: Eutrophication creates vertical oxygen gradients in ponds due to nutrient-driven processes. Surface oxygen is high from algal photosynthesis and air exchange. At the bottom, decomposition of sinking organic matter from livestock nutrients consumes oxygen via bacterial respiration. Density or nutrient preferences do not explain the pattern. Choice A accounts for the distribution correctly. Phosphorus films or sinking oxygen are inaccurate.

Question 9

1. Bacterial decomposition of dead algae increases biochemical oxygen demand, reducing dissolved oxygen (correct answer)
2. Algae photosynthesize at night and consume oxygen faster than bacteria
3. Nutrients chemically bind oxygen molecules, removing them from the water
4. Saltwater contains less oxygen than freshwater, so the bay naturally becomes hypoxic each summer

Explanation: Eutrophication occurs when water bodies receive high levels of nutrients like nitrogen and phosphorus, often from sources such as sewage, promoting rapid algal growth. In this coastal bay, untreated sewage effluent provides these nutrients, resulting in a large algal bloom visible in satellite images. When the algae die, bacteria decompose the organic matter, increasing biochemical oxygen demand (BOD) and depleting dissolved oxygen, which creates a hypoxic dead zone. This hypoxia causes bottom-dwelling organisms to disappear as they cannot survive in low-oxygen conditions. Choice A accurately explains that bacterial decomposition is the direct cause of low DO, emphasizing the role of BOD in eutrophication's progression to dead zones. Understanding this process underscores the importance of nutrient management in preventing ecological harm.

Question 10

1. Cold temperatures increased oxygen solubility, causing algae to die; decomposers then produced oxygen, creating a dead zone.
2. Excess nutrients stimulated algal growth; when algae died, bacterial decomposition increased and consumed dissolved oxygen, leading to hypoxia and fish kills. (correct answer)
3. Nutrients directly poisoned fish; algae increased later because fewer fish grazed on algae.
4. Algal bloom increased photosynthesis at depth, raising DO near the bottom until fish suffocated from too much oxygen.

Explanation: Eutrophication is the process where excess nutrients, such as nitrates and phosphates from fertilizers, enter a water body and stimulate excessive algal growth. In this scenario, the spring runoff introduces high levels of these nutrients into the lake, leading to a rapid algal bloom that turns the surface green. As the algae die off and sink, bacteria decompose the organic matter, a process that consumes dissolved oxygen in the water. This bacterial decomposition is most intense in deeper waters where the dead algae accumulate, resulting in hypoxia or low oxygen levels near the bottom. Consequently, fish and other aquatic organisms that require oxygen suffocate, leading to fish kills observed near the shoreline. Choice B correctly describes this sequence, emphasizing the role of nutrient-stimulated algal growth followed by oxygen-depleting decomposition. Other choices misrepresent the mechanisms, such as suggesting direct poisoning or oxygen overproduction.

Question 11

1. Low oxygen caused algae to bloom; the bloom then removed nitrogen and phosphorus from the water, restoring oxygen levels
2. Excess nitrogen and phosphorus increased algal growth; when algae died, decomposers used oxygen during respiration, leading to hypoxia and fish kills (correct answer)
3. Increased algae directly produced toxins that consumed dissolved oxygen through photosynthesis, creating hypoxia
4. Stormwater diluted nutrients; algae declined; reduced decomposition lowered oxygen, causing fish kills

Explanation: Eutrophication is the process where excess nutrients, particularly nitrogen and phosphorus from fertilizer runoff, enter aquatic systems and trigger rapid algal growth. In this scenario, spring storms washed nitrate and phosphate from cornfields into the river, providing abundant nutrients for algae to bloom (turning the water green). When this dense algal bloom eventually dies, decomposer bacteria break down the dead algae through aerobic respiration, consuming large amounts of dissolved oxygen in the process. This oxygen depletion creates hypoxic conditions, especially near the bottom where dead algae settle, leading to fish kills. The correct answer B accurately describes this sequence: nutrients cause algal growth, then decomposition of dead algae depletes oxygen, resulting in hypoxia and fish deaths.

Question 12

1. Strong vertical stratification that limits mixing of oxygen-rich surface water with bottom water (correct answer)
2. Increased grazing by zooplankton that removes algae from the surface
3. High salinity that causes oxygen to be produced more rapidly by algae
4. Reduced nutrient inputs that prevent decomposition from occurring

Explanation: Eutrophication involves nutrient enrichment that fuels algal blooms, followed by oxygen depletion in deeper waters as dead algae decompose. In this estuary, fertilizer runoff provides N and P, leading to high surface algal biomass. Bottom waters become hypoxic in summer due to decomposition consuming oxygen. Strong vertical stratification, often from temperature or salinity differences, prevents oxygen-rich surface water from mixing downward, sustaining the hypoxia. Choice A correctly identifies this physical condition as key to maintaining low DO in bottom layers. This process illustrates how physical and biological factors interact in eutrophic systems to create persistent dead zones.

Question 13

1. As algae die, decomposers break down biomass and consume oxygen, increasing biochemical oxygen demand (correct answer)
2. Algae stop photosynthesis permanently once nutrients are abundant, so oxygen production ceases
3. Phosphate reacts with dissolved oxygen to form solid precipitates that settle to the bottom
4. Fish respiration increases because fish grow faster in warm water, and this alone causes hypoxia

Explanation: Eutrophication begins with nutrient inputs like phosphates from lawn fertilizers, triggering thick algal blooms in water bodies such as reservoirs. After the bloom peaks, algae die and sink, where decomposers break them down, consuming dissolved oxygen and causing sharp DO drops, especially at night and near the bottom when photosynthesis halts. This leads to hypoxic conditions that can stress or kill aquatic organisms. Choice A best explains this by noting that decomposition increases biochemical oxygen demand, directly linking the bloom to oxygen depletion. Recognizing this sequence helps in understanding why timing and location of DO measurements are crucial in monitoring eutrophication.

Question 14

1. Low oxygen caused algae to grow rapidly; the algal bloom then removed excess nutrients from the water
2. Excess nutrients increased algal growth; algae died and were decomposed by bacteria that consumed DO, creating hypoxia and a dead zone (correct answer)
3. Phosphates directly poisoned fish; dead fish decomposed and released oxygen, increasing DO near the bottom
4. Increased turbidity reduced sunlight; algae declined; DO rose due to reduced respiration, causing fish kills

Explanation: Eutrophication is the process by which excess nutrients, such as nitrates and phosphates from fertilizers, enter a water body and stimulate excessive algal growth. In this scenario, heavy rains wash these nutrients into the river, leading to a dense algal bloom that turns the water green. As the algae die off, bacteria decompose the dead organic matter, consuming dissolved oxygen (DO) in the process and creating hypoxic conditions, especially near the bottom where decomposition is intense. This oxygen depletion results in fish kills and the formation of a dead zone where aquatic life cannot survive. Choice B correctly describes this sequence: excess nutrients boost algal growth, followed by bacterial decomposition that reduces DO, leading to hypoxia. This explanation highlights how nutrient runoff disrupts aquatic ecosystems by altering oxygen dynamics.

Question 15

1. An increase in algal growth and reduced water clarity (correct answer)
2. An increase in dissolved oxygen at all depths due to bacterial respiration
3. A decrease in nutrient concentrations because algae convert nutrients into oxygen
4. A shift to fewer primary producers because nutrients inhibit growth

Explanation: Eutrophication starts with nutrient additions that boost primary production, reducing clarity. Hog farm runoff introduces N and P, likely causing algal growth and turbid water as blooms form. Choice A describes this initial change accurately. Later, decomposition may lead to hypoxia, but reduced clarity signals the onset.

Question 16

1. Algal growth reduces water clarity and light penetration, limiting photosynthesis by submerged plants. (correct answer)
2. Submerged plants consume all nutrients first, starving algae and causing plants to die.
3. Nutrients directly increase light penetration, causing plants to overheat.
4. Decomposers remove sunlight from the water column by respiration.

Explanation: During eutrophication from sewage nutrients, algal blooms increase turbidity, reducing light for submerged plants and causing their decline. Plants do not consume nutrients first or overheat. Choice A explains the reason. Decomposers do not remove sunlight.

Question 17

1. The storm diluted nutrients, causing algae to die; oxygen increased due to reduced respiration.
2. Nutrient inputs triggered a bloom; subsequent decomposition increased biochemical oxygen demand, reducing oxygen and causing foul odors. (correct answer)
3. Wastewater increased salinity, which directly killed fish and created odors; algae rose because of fewer fish.
4. Chlorophyll-a rose because oxygen was low; algae require hypoxia to grow.

Explanation: Eutrophication follows sewer overflows adding nutrients, leading to algal blooms (high chlorophyll-a) and later oxygen depletion from decomposition. The storm introduces nitrogen and phosphorus, triggering the bloom. As algae die, bacterial decomposition increases biochemical oxygen demand, dropping oxygen and producing decay odors. Dilution or salinity do not cause blooms or odors in this way. Choice B matches the sequence. Low oxygen does not promote algal growth.

Question 18

1. Dead algae sink and are decomposed by bacteria that use oxygen for respiration, lowering DO near the bottom. (correct answer)
2. Algae at the surface consume oxygen during photosynthesis, removing it from bottom waters.
3. Nitrates instantly bind with oxygen gas, preventing diffusion from air to water.
4. Phosphorus causes fish to breathe faster, using up all available oxygen in bottom waters.

Explanation: Eutrophication leads to dead zones when nutrient-rich runoff causes algal blooms, followed by oxygen-poor bottom waters. Dead algae sink, and bacteria decompose them using oxygen via respiration, depleting dissolved oxygen near the bottom. This hypoxia kills bottom-dwellers like clams. Surface algae produce, not consume, oxygen during photosynthesis. Nutrients do not bind oxygen directly. Choice A explains the mechanism accurately. Fish breathing or instant binding are not primary causes.

Question 19

1. Limited mixing between surface and deep water reduces oxygen replenishment while decomposition consumes oxygen at depth. (correct answer)
2. High wind speeds increase mixing, trapping oxygen below the thermocline.
3. Increased water clarity allows oxygen to escape more quickly from deep water.
4. Low nutrient levels prevent algal growth, which causes deep-water oxygen depletion.

Explanation: Eutrophication in stratified lakes involves nutrient-driven algal blooms, followed by oxygen depletion in deep layers. The thermocline limits mixing, preventing oxygen from surface waters (produced by photosynthesis) from reaching depths. Sinking dead algae decompose at depth, where bacteria consume oxygen without replenishment. This setup intensifies hypoxia in the hypolimnion. Wind or clarity do not trap or remove oxygen as described in alternatives. Choice A pinpoints the stratification's role. Low nutrients would prevent blooms, not cause depletion.

Question 20

1. A decrease in primary productivity due to nutrient scarcity.
2. An increase in algal growth and reduced water clarity. (correct answer)
3. A permanent increase in dissolved oxygen in deep water due to photosynthesis.
4. A shift from autotrophs to fewer decomposers because nutrients reduce bacterial growth.

Explanation: Eutrophication progresses with increasing nutrients from farmland runoff, leading to enhanced algal growth and decreased water clarity due to turbidity. This is the next likely change after nutrient levels rise. Oxygen may fluctuate but not permanently increase in deep waters. Decomposers increase, not decrease. Choice B identifies the imminent impact. Other options reverse the productivity or oxygen trends.