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4.4.5 Eutrophication occurs when lakes, estuaries and coastal waters receive inputs of mineral nutrients, especially nitrates and phosphates, often causing excessive growth of phytoplankton.
Algal blooms only occur if phytoplankton growth had previously been limited by lower concentrations of phosphate and/or nitrate. Humans cause eutrophication when releasing detergents, sewage or agricultural fertilisers into water bodies.
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Eutrophication is what happens when a body of water receives too many mineral nutrients, mainly nitrates and phosphates, and responds with a burst of growth. The water can be a lake, an estuary (where a river meets the sea) or a stretch of coast.
The organisms that grow are mostly phytoplankton, microscopic plant-like organisms that float in the water and photosynthesise. Given plenty of nutrients, light and warmth they multiply fast, turning the water green or brown in what is called an algal bloom.
The nutrients come from people. The course names three sources: detergents (older washing powders were rich in phosphate), sewage (rich in both nitrogen and phosphorus) and agricultural fertilisers (washed off fields as run-off). You met all three as sources of water pollution last lesson.
Here is the part that catches people out. A bloom does not follow just because nutrients are added. It follows only if that nutrient was the one holding growth back in the first place.
Think back to limiting factors from Topic 2. The growth of phytoplankton is capped by whatever is in shortest supply. In most fresh water that limiting nutrient is phosphate. Add more phosphate and growth leaps, because the brake has been released. Add a nutrient that was already plentiful and little changes, because it was never the limiting factor.
This is why phosphate matters so much in lakes, and why, as you saw last lesson, it is the measurement managers watch most closely.
Task
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4.4.6 Eutrophication leads to a sequence of impacts and changes to the aquatic system.
Excessive growth of phytoplankton is typically followed by their death and, therefore, high rates of decomposition, rapid consumption of dissolved oxygen leading to hypoxia or anoxia in the water, and death of aquatic life that depends on dissolved oxygen.
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The bloom itself is not the disaster. The damage follows when the algae die, and it runs in a clear sequence.
1. The bloom dies. Phytoplankton are short-lived, and a huge bloom soon becomes a huge mass of dead organic matter sinking through the water.
2. Decomposers move in. Bacteria break down the dead algae. With so much food they multiply, and the rate of decomposition soars.
3. Oxygen is used up. Decomposition uses oxygen. As the bacteria work they strip dissolved oxygen, the oxygen gas in the water that animals breathe, out of the water faster than it can be replaced.
4. The water suffocates. Oxygen falls to very low levels (hypoxia) or runs out completely (anoxia).
5. Aquatic life dies. Fish and invertebrates that depend on dissolved oxygen can no longer breathe and die. A water body in this state is often called a dead zone.
This is the skill the course asks you to practise: turn the sequence into a systems model, a diagram of stores joined by flows, and find the positive feedback loop hidden inside it.
The stores (the boxes in your model):
The flows (the arrows between the boxes) are the processes you have just met: growth, death, decomposition, the use of oxygen, and death from low oxygen.
Task