Eutrophication
Eutrophication is the process by which a water body becomes overly enriched with nutrients — primarily nitrogen and phosphorus — leading to excessive growth of algae and aquatic plants, depleting dissolved oxygen, and causing the death of fish and other aquatic organisms. It most commonly results from agricultural runoff, sewage discharge, and industrial effluents that introduce nutrients into lakes, rivers, estuaries, and coastal zones. The subsequent algal blooms block sunlight, and when the algae die and decompose, microbial respiration consumes oxygen, creating hypoxic or anoxic "dead zones."
Key Formula
Dissolved Oxygen = Saturation DO − (BOD × Deoxygenation rate / (Deoxygenation rate + Reaeration rate))
LaTeX: DO = DO_{sat} - \frac{BOD \times K_d}{K_d + K_r}
| Symbol | Meaning | Unit |
|---|---|---|
| DO | Dissolved oxygen concentration | mg/L |
| DO_{sat} | Saturation dissolved oxygen | mg/L |
| BOD | Biochemical oxygen demand | mg/L |
| K_d | Deoxygenation rate constant | per day |
| K_r | Reaeration rate constant | per day |
Worked Example
Problem
A river has a BOD of 20 mg/L, a deoxygenation rate constant Kd of 0.3/day, and a reaeration rate constant Kr of 0.4/day. Saturated DO at 20°C is 9.1 mg/L. Estimate the steady-state DO.
Solution
Step 1: Apply the formula: DO = DO_sat − (BOD × Kd) / (Kd + Kr). Step 2: Numerator: 20 × 0.3 = 6 mg/L. Step 3: Denominator: 0.3 + 0.4 = 0.7. Step 4: Oxygen deficit term: 6 / 0.7 = 8.57 mg/L. Step 5: DO = 9.1 − 8.57 = 0.53 mg/L.
Answer
DO ≈ 0.53 mg/L (severely hypoxic — below the 2 mg/L threshold for most aquatic life)
Trophic States of Water Bodies Based on Nutrient Levels
| Trophic State | Total Phosphorus (µg/L) | Chlorophyll-a (µg/L) | Water Clarity (m) | Typical Condition |
|---|---|---|---|---|
| Oligotrophic | < 10 | < 2 | > 6 | Clear, low productivity, high DO |
| Mesotrophic | 10–35 | 2–9 | 3–6 | Moderate nutrients, moderate algae |
| Eutrophic | 35–100 | 9–25 | 1.5–3 | High algae, reduced DO, odour |
| Hypereutrophic | > 100 | > 25 | < 1.5 | Algal blooms, fish kills, anoxia |
Interactive Tools
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Related Terms
Acid Rain
Acid rain refers to any form of precipitation — rain, snow, fog, or dry deposition — with a pH lower than 5.6, resulting from the dissolution of sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) in atmospheric moisture to form sulphuric and nitric acids. These pollutants are primarily emitted by coal-burning power plants, vehicle exhausts, and industrial smelters, and can travel hundreds of kilometres before depositing. Acid rain damages forests, acidifies lakes and soils, corrodes buildings and infrastructure, and harms aquatic biodiversity.
Bioremediation
Bioremediation is the use of living organisms — primarily microorganisms such as bacteria, fungi, and algae — to degrade, neutralise, or remove toxic contaminants from soil, water, and air. It exploits natural metabolic processes to convert hazardous substances like petroleum hydrocarbons, heavy metals, chlorinated solvents, and pesticides into less harmful compounds. As a cost-effective and eco-friendly alternative to physical or chemical remediation methods, bioremediation is widely used at contaminated industrial sites, oil spill zones, and wastewater treatment facilities.
Biomagnification
Biomagnification (also called biological magnification) is the progressive increase in the concentration of a persistent, fat-soluble contaminant — such as DDT, mercury, PCBs, or dioxins — as it moves up the food chain from producers to top predators. Because these substances are stored in fatty tissues and are not easily metabolised or excreted, each successive trophic level accumulates higher concentrations, often by factors of 10 or more per level. This explains why apex predators such as eagles, sharks, and polar bears can have contaminant levels millions of times higher than ambient environmental concentrations.
From Greek "eutrophos" — "eu" (well, good) + "trophē" (nourishment, food). The term was introduced into limnology (the study of lakes) by the German biologist Einar Naumann in the early 20th century (around 1919) to describe nutrient-rich, highly productive lake conditions, later expanded to describe the problematic over-enrichment process.