8.5 - Eutrophication

Learning Objectives

I. Basics and Cultural Sources

• Eutrophication means “too much nourishment or food” (“eu” meaning much, “troph” meaning nourishment). It begins with an input of excess nutrients, primarily nitrogen and phosphorus, which are often limiting nutrients in aquatic ecosystems.

• Cultural eutrophication refers specifically to anthropogenic (human driven) nutrient pollution causing the process. Eutrophication can also occur naturally as bodies of water age and fill with sediment.

• Major human sources:
1. Agriculture: Runoff from synthetic fertilizers adds nitrate and phosphates. CAFOs release animal waste (manure) containing ammonia (nitrogen) and phosphate if manure lagoons flood or leach.
2. Wastewater treatment plants: Discharge can add nutrients (ammonia and phosphorus) if the plants fail to completely remove them. Heavy rain can cause facilities to flood and discharge raw sewage.
3. Detergents and soaps: These go down drains and contain phosphates that may contribute if not fully removed by treatment.

II. The Eutrophication Process and Dead Zones

1. Algae bloom: Algae quickly take up the excess nutrients and grow.

2. Sunlight blockage: The algae bloom blocks out sunlight, so plants beneath them die because they can’t photosynthesize.

3. Oxygen depletion (hypoxia): When the algae and plants die off, bacteria (decomposers) start to break them down. Since decomposition is an aerobic process, the bacteria use up a lot of dissolved oxygen (DO).

4. Positive feedback loop: The low DO levels mean more aquatic species die. More dead organisms mean more decomposition, which uses up even more oxygen, creating a positive feedback loop that drives conditions away from a stable state.

5. Dead zone: The resulting low oxygen condition is called hypoxia (hypo = low, ox = oxygen), which leads to a dead zone where aquatic life, which needs oxygen for cellular respiration, can’t be supported.

6. Fish tolerance: Dissolved oxygen is measured in parts per million (PPM). The minimum threshold for most fish populations to reproduce and sustain stability is 6 PPM. NOAA data from 2011 showed areas of the Gulf of Mexico dead zone had DO levels below 3 PPM, which makes stable fish populations impossible to support.

III. Oligotrophic vs. Eutrophic Waters

• Hypoxic waterways: Bodies of water that are low in dissolved oxygen. Scientists usually call water hypoxic when DO drops below 2 mg/L (2 ppm), the level where most fish and shellfish can’t survive for long. Water with no oxygen at all is anoxic.

• Example (Lake Erie and detergents): In the 1960s and 1970s, phosphates from laundry detergents and sewage caused huge algal blooms in Lake Erie. The lake improved after the US and Canada limited phosphorus in detergents and upgraded sewage treatment (Great Lakes Water Quality Agreement, 1972). Since the mid-1990s, blooms have come back, now mostly from phosphorus in farm runoff. In August 2014, toxins from a bloom reached the drinking water intake for Toledo, Ohio, and hundreds of thousands of people were told not to drink their tap water for about three days.

Big picture: Eutrophication, often driven by excess nitrogen and phosphorus from agricultural runoff and sewage, leads to explosive algae blooms that block sunlight and cause widespread plant death. The aerobic decomposition that follows consumes dissolved oxygen, which creates a dangerous hypoxic condition, the “dead zone,” and triggers a positive feedback loop of death and oxygen depletion.