Eutrophication
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AP Environmental Science › Eutrophication
Following a period of heavy rainfall, a pond near row-crop agriculture develops a cyanobacterial bloom. Which pair of inputs is most directly responsible for initiating the bloom in the context of eutrophication?
Nitrogen and phosphorus from fertilizer runoff
Sodium and chloride from road salt
Carbon dioxide and oxygen from the atmosphere
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.
A coastal city upgrades sewage treatment and significantly reduces nitrogen and phosphorus entering a nearby bay. Which change is most likely over the next several seasons if eutrophication had been driving hypoxia?
No change in algal blooms because nutrients are not related to primary productivity
Immediate elimination of all algae because algae cannot survive without sewage
Fewer and smaller algal blooms, leading to less decomposition-driven oxygen depletion
More frequent dead zones because fewer nutrients increase bacterial oxygen demand
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.
A lake receives runoff from lawns and a golf course treated with fertilizer. The lake develops thick mats of algae, followed by a sharp drop in dissolved oxygen and a fish kill. Which initial change would best prevent the fish kill while addressing the root cause?
Drain the lake each summer to expose algae to air.
Increase pesticide use to reduce algae-eating insects.
Add more fish to consume algae directly.
Reduce nitrogen and phosphorus inputs by limiting fertilizer application and establishing vegetated buffer strips.
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.
A reservoir adjacent to suburban neighborhoods shows repeated summer algal blooms. Tests show elevated N and P from lawn fertilizer runoff. Which management action most directly targets the source of eutrophication?
Increase shoreline lighting to promote nighttime photosynthesis.
Install riparian buffer vegetation and reduce fertilizer application rates to cut nutrient runoff.
Stock the reservoir with more predatory fish to increase nutrient inputs.
Add chlorine to kill algae and increase nutrient availability.
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.
A reservoir downstream of suburban neighborhoods shows frequent algal blooms each summer. Water tests reveal elevated phosphorus from lawn fertilizers and nitrogen from pet waste. Which outcome is most consistent with eutrophication in this reservoir?
Reduced algal biomass because more nutrients reduce photosynthesis
Increased organic matter followed by oxygen depletion during decomposition, especially in deeper water
Higher dissolved oxygen in deep water due to increased mixing by algae
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.
In a stratified lake, nutrient inputs trigger an algal bloom at the surface. After the bloom, bottom waters become hypoxic, but surface waters still show moderate oxygen. Which explanation best accounts for this pattern?
Oxygen produced and exchanged at the surface does not readily mix downward due to stratification, while decomposition consumes oxygen at depth
Nutrients sink and produce oxygen at the bottom, but surface water loses oxygen to the atmosphere
Fish move to the bottom and produce oxygen through respiration, preventing hypoxia at depth
Algae only decompose in surface waters, so oxygen is consumed near the surface while deep water remains oxygen-rich
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.
A lake experiences eutrophication from agricultural runoff containing N and P. Which change would most likely occur first after a sudden increase in nutrient input?
A rapid increase in algal and cyanobacterial growth near the surface
A decrease in primary productivity because nutrients block sunlight
Immediate formation of a dead zone before any change in primary productivity
A long‑term increase in dissolved oxygen throughout the water column due to decomposition
Explanation
Eutrophication is characterized by nutrient overloads that accelerate primary production in aquatic systems. A sudden increase in N and P from agricultural runoff would first cause a rapid surge in algal and cyanobacterial growth near the surface, forming blooms. This precedes oxygen depletion, as decomposition follows the bloom's collapse. Choice A identifies this initial change accurately, focusing on the bloom as the starting point. Later stages involve sinking organic matter and hypoxia, but the bloom is the first visible response.
In an estuary, nutrient inputs from sewage lead to a large algal bloom. Which process most directly causes oxygen depletion when the bloom collapses?
Bacterial decomposition of algal biomass consumes dissolved oxygen
Nitrate converts to oxygen gas and escapes to the atmosphere
Phosphate blocks oxygen diffusion across the air-water interface
Increased photosynthesis by algae consumes dissolved oxygen
Explanation
Eutrophication in estuaries from sewage nutrients causes algal blooms, followed by collapse. Bacterial decomposition of the biomass directly depletes oxygen via respiration. Choice B identifies this key process in hypoxia development. Photosynthesis temporarily adds oxygen, but net loss occurs post-bloom.
A coastal estuary receives chronic nutrient inputs from upstream agricultural runoff rich in nitrate (N) and phosphate (P). Each summer, a large algal bloom forms, followed by a drop in dissolved oxygen near the bottom and the appearance of a seasonal “dead zone.” Which management action would most directly reduce the likelihood of hypoxia by addressing the root cause of eutrophication?
Reduce nitrogen and phosphorus inputs by installing riparian buffer strips and improving fertilizer application practices in the watershed.
Increase nighttime aeration only during the algal bloom to raise oxygen while allowing nutrient loading to continue unchanged.
Add more zooplankton to the estuary to increase grazing, without changing nutrient inputs.
Increase dredging to deepen the estuary so that algae have more water volume to grow in.
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.
In a eutrophic pond, dissolved oxygen is high near the surface but very low near the bottom. The pond receives nitrogen and phosphorus from nearby livestock operations. Which explanation best accounts for the vertical oxygen pattern?
Photosynthesis and air-water gas exchange increase oxygen near the surface, while decomposition of sinking organic matter consumes oxygen at depth.
Oxygen is heavier than water, so it sinks and is removed from surface waters first.
Nitrogen fertilizes deep-water plants only, which consume oxygen and create surface oxygen.
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.