Why does Eutrophication lead to aquatic oxygen depletion when algae produce oxygen during photosynthesis?

While living algae produce $O_2$ during daylight, excessive nutrient enrichment ($N, P$) triggers explosive algal blooms. When these massive blooms die, aerobic decomposers (bacteria) consume vast amounts of Dissolved Oxygen ($DO$) to break down organic biomass, causing $BOD$ to skyrocket and $DO$ to plunge to near zero, suffocating fish and aquatic fauna.

How do Chlorofluorocarbons ($CFCs$) act catalytic allies in stratospheric ozone depletion?

$UV$ radiation breaks down $CFCs$ in the stratosphere, releasing free chlorine radicals ($Cl^\bullet$). A single $Cl^\bullet$ radical reacts with ozone ($O_3$) to form $ClO^\bullet$ and $O_2$. The $ClO^\bullet$ then reacts with atomic oxygen ($O$) to regenerate the free $Cl^\bullet$ radical ($Cl^\bullet + O_3 \rightarrow ClO^\bullet + O_2$; $ClO^\bullet + O \rightarrow Cl^\bullet + O_2$). Because $Cl^\bullet$ is not consumed in the reaction, one single chlorine atom can destroy over $100,000$ ozone molecules.

What is the strategic trick to memorizing International Environmental Protocols for NEET?

Associate Montreal Protocol ($1987$, effective $1989$) strictly with Stratospheric Ozone Layer Protection and controlling $CFC$ emissions. Associate Kyoto Protocol ($1997$) with Greenhouse Gas emission reduction and Global Warming mitigation. Use the mnemonic: 'Montreal shields the Stratosphere ($O_3$), Kyoto cools the Globe (GHGs)'.

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Solved High-Yield Sample Questions

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Question #1NEET 2023

In an aquatic food chain, if the concentration of $DDT$ in water is measured at $0.003 \text{ ppb}$ , what will be its expected concentration in fish-eating birds at the top trophic level due to biomagnification?

Solution Locked

Step-by-Step Solution & Concept

Step 1

Identify sequential trophic levels in the aquatic food chain: Water ( $0.003 \text{ ppb}$ ) $\rightarrow$ Zooplankton ( $0.04 \text{ ppm}$ ) $\rightarrow$ Small Fish ( $0.5 \text{ ppm}$ ) $\rightarrow$ Large Fish ( $2 \text{ ppm}$ ) $\rightarrow$ Fish-eating Birds.

Step 2

Recognize that $DDT$ is a lipophilic, non-biodegradable toxicant that accumulates progressively in fatty tissues without being metabolized or excreted.

Step 3

At the apex trophic level (fish-eating birds), biological amplification increases $DDT$ concentration by nearly $10^7$ times, reaching $25 \text{ ppm}$ as per NCERT standard benchmarks.

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Question #2JEE Main 2023

A municipal sewage treatment facility releases effluent into a river. If $100 \text{ L}$ of river water sample consumes $0.6 \text{ g}$ of dissolved oxygen over $5 \text{ days}$ at $20^\circ \text{C}$ during incubation, calculate the Biochemical Oxygen Demand ( $BOD$ ) of the river water in $\text{mg/L}$ ( $\t\text{ppm}$ ).

Solution Locked

Step-by-Step Solution & Concept

Step 1

Recall the definition of $BOD$ : mass of consumed dissolved oxygen in milligrams ( $\text{mg}$ ) per liter ( $\text{L}$ ) of water sample during a $5\text{-day}$ incubation at $20^\circ \text{C}$ .

Step 2

Convert the total consumed oxygen mass from grams to milligrams: $0.6 \text{ g} = 0.6 \times 10^3 \text{ mg} = 600 \text{ mg}$ .

Step 3

Calculate $BOD$ per liter: $\text{BOD} = \frac{600 \text{ mg}}{100 \text{ L}} = 60 \text{ mg/L}$ (or $60 \text{ ppm}$ ).

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Mastering Environmental Issues: Formulas, Traps & Exam Strategy | ExamiFly

Chapter Overview

Core Subtopics & Syllabus Breakdown

Key Formulas & High-Yield Equations

Biochemical Oxygen Demand (BOD) Relationship

Sound Intensity Level in Decibels

Toxicant Biomagnification Factor

Stratospheric Ozone Thickness

Greenhouse Gas Relative Contribution

Common Mistakes & Traps to Avoid

Past Year Question (PYQ) Trends

Solved High-Yield Sample Questions

Frequently Asked Questions

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