Eutrophication
Eutrophication is a process where excess nutrients, primarily nitrates and phosphates, enter aquatic ecosystems, triggering a cascade of ecological changes.
- Eutrophication is the process by which a body of water becomes enriched with nutrients, particularly nitrates (NO₃⁻) and phosphates (PO₄³⁻), leading to excessive growth of algae and aquatic plants.
- This enrichment disrupts aquatic ecosystems, often resulting in oxygen depletion, loss of biodiversity, and decline in water quality.
Causes of Eutrophication
- Eutrophication can occur naturally (over centuries) or artificially due to human activity (cultural eutrophication).
- Anthropogenic (human-induced) eutrophication occurs much faster.
- Agricultural Runoff: Fertilizers applied to crops wash into rivers and lakes during rain, introducing nitrates and phosphates.
- Sewage Discharge: Untreated or partially treated domestic waste adds organic matter and nutrients.
- Detergents: Phosphate-based detergents contribute to nutrient loading.
- Industrial Effluent: Factories may release nitrogen compounds and organic pollutants.
In the Chesapeake Bay (USA), fertilizer and animal waste runoff caused persistent eutrophication and recurring “dead zones.”
The Process of Eutrophication
- Nutrient Enrichment: Runoff or discharge introduces nitrates and phosphates into the water.
- Algal Bloom: Rapid growth of algae and cyanobacteria due to increased nutrient availability.
- Light Limitation: Dense algal layers block sunlight, reducing photosynthesis in submerged plants.
- Plant Death: Submerged macrophytes die due to lack of light.
- Decomposition: Dead algae and plants decompose, consuming dissolved oxygen.
- Oxygen Depletion: Decomposition leads to hypoxia (low O₂) or anoxia (no O₂).
- Biodiversity Loss: Fish and other aerobic organisms die or migrate; anaerobic bacteria dominate.
- Toxic Gas Production: Anaerobic decomposition produces CH₄ (methane), NH₃ (ammonia), and H₂S (hydrogen sulfide).
Hypoxia
Hypoxia is a condition of severely reduced dissolved oxygen in a body of water.
Anoxia
Anoxia is the complete absence of dissolved oxygen.
In the Gulf of Mexico, dead zones have formed due to nutrient runoff from the Mississippi River, severely impacting marine life.
TipPositive Feedback Loop
Nutrient input ↑ → Algal growth ↑ → Death of organisms ↑ → Decomposition ↑ → Nutrient recycling ↑ → Continued algal growth ↑
Impacts on Ecosystem Services
1. Fisheries
- Hypoxia causes fish kills or migration.
- Surface-dwelling species like carp dominate, reducing biodiversity.
- Fish caught in eutrophic waters may contain toxins.
- Economic losses occur as fishermen travel farther offshore or catch declines.
The Gulf of Mexico Dead Zone, caused by nutrient runoff from the Mississippi River, results in hypoxic waters each summer, collapsing local fisheries.
2. Recreation
- Algal scum and foul odors reduce enjoyment of water-based activities (swimming, boating, fishing).
