Eutrophication worksheet - Free Printable

Let's solve this eutrophication exercise by matching each description with the correct image. Eutrophication is a process where water bodies become overly enriched with nutrients, leading to excessive plant and algae growth, which can ultimately harm aquatic life.

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🔹 Step-by-step Explanation of Eutrophication Stages:

1. Nutrients enter the water body
→ Fertilisers and nutrients from agricultural land are washed into rivers and lakes by rain runoff.

2. Algal bloom occurs
→ Excess nutrients (like nitrogen and phosphorus) cause rapid growth of algae and aquatic plants.

3. Light is blocked
→ Thick algal mats block sunlight, preventing photosynthesis in underwater plants → they die.

4. Oxygen depletion and death of organisms
→ Dead plants and algae are decomposed by bacteria, which consume dissolved oxygen → fish and other organisms die due to lack of oxygen.

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Now, let’s match the descriptions to the images based on what we see:

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🖼️ Image 1: Green, slimy surface covering a pond or lake

→ This shows algal bloom – thick green layer of algae covering the water.

✔ Match with:
> "Nutrients (a limiting factor) cause water plants and algae to grow exponentially."

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🖼️ Image 2: Aerial view of a lake with blue-green swirls (likely algal bloom)

→ This is another example of excessive algae growth, possibly cyanobacteria (blue-green algae).

✔ Match with:
> "Nutrients (a limiting factor) cause water plants and algae to grow exponentially."

Wait — both Image 1 and Image 2 show algal blooms? Let's check more carefully.

Actually, Image 1 is a close-up of algae-covered water, while Image 2 is an aerial shot showing large-scale algal bloom in a lake. Both represent the same stage: exponential growth of algae due to nutrient influx.

But since there are four descriptions and four images, we need to assign one per description.

Let’s look at all four images again:

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✔ Final Matching:

#### 🔹 Image 1 (Green algae covering water)
→ Shows dense algal growth.

➡️ Matches:
> "Nutrients (a limiting factor) cause water plants and algae to grow exponentially."

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#### 🔹 Image 2 (Aerial view of green-blue water in a lake)
→ Also shows algal bloom, but it may be earlier in the process or just another representation.

Wait — actually, this could be the same stage as above. But perhaps we should consider the context of the other images.

Let’s look at the third image:

#### 🔹 Image 3 (Muddy, brown river or stream)
→ This looks like runoff from land, carrying soil and fertilizers into water.

➡️ Matches:
> "Fertilisers and nutrients are washed off the land by rain into rivers and lochs."

Yes! This is the first stage — nutrient runoff.

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#### 🔹 Image 4 (Dead fish floating on water surface)
→ Clearly shows fish kills, which happen when oxygen levels drop.

➡️ Matches:
> "The decomposing organisms are broken down by bacteria and use up even more oxygen in the water. Death of the ecosystem."

This is the final stage of eutrophication.

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So now, only one description left:

> "Sunlight is prevented from entering the water column, photosynthesising plants and organisms die."

Which image matches this?

We already used:
- Image 3 → Runoff
- Image 4 → Fish death
- Image 1 → Algae bloom

So Image 2 (aerial view of green-blue water) might also be algal bloom, but let's think: does it show blocked sunlight?

Actually, both Image 1 and Image 2 show thick algal cover, which blocks sunlight.

But since Image 1 is clearly dense algae, it likely represents the algal bloom stage.

And Image 2 might be showing the result of algal bloom blocking light, causing plant death.

Wait — but Image 4 shows dead fish, so that’s after decomposition.

Let’s reorganize:

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✔ Correct Matching:

| Description | Image | Reason |
|-----------|-------|--------|
| Fertilisers and nutrients are washed off the land by rain into rivers and lochs. | Image 3 (muddy river) | Shows soil and runoff, likely carrying fertilisers into water. |
| Nutrients (a limiting factor) cause water plants and algae to grow exponentially. | Image 1 (green algae) | Dense algal bloom covers surface. |
| Sunlight is prevented from entering the water column, photosynthesising plants and organisms die. | Image 2 (aerial view of green water) | The algal bloom is so thick it blocks light; this leads to plant death. |
| The decomposing organisms are broken down by bacteria and use up even more oxygen in the water. Death of the ecosystem. | Image 4 (dead fish) | Fish are dead — clear sign of oxygen depletion. |

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✔ Final Answer:

1. "Fertilisers and nutrients are washed off the land by rain into rivers and lochs." → Image 3 (muddy river)

2. "Nutrients (a limiting factor) cause water plants and algae to grow exponentially." → Image 1 (green algae on water)

3. "Sunlight is prevented from entering the water column, photosynthesising plants and organisms die." → Image 2 (aerial view of green-blue water with algal bloom)

4. "The decomposing organisms are broken down by bacteria and use up even more oxygen in the water. Death of the ecosystem." → Image 4 (dead fish floating)

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🧠 Summary of Eutrophication Process:

1. Nutrient runoff (Image 3)
2. Algal bloom (Image 1)
3. Light blocked → plants die (Image 2)
4. Decomposition → low oxygen → fish die (Image 4)

✔️ This completes the matching exercise correctly.