Eutrophication in a jar: fertiliser run-off and algal growth · Concept Studio

The idea

Adding nutrients to still water increases producer growth, and the extra biomass changes the conditions for every other organism in the system.

Safety card

Low riskAn adult supervises

Hazards

Fertiliser concentrate is an irritant
Pond water pathogens

Controls

Gloves and eye protection when dosing; syringes used only for fertiliser
Wash hands
Jars lidded loosely to stop spills; water disposed of on soil

Note

Chemicals: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package (section 1, general information for all staff) and complete a RiskAssess risk assessment before the lesson.

What you need

Four 1 L clear jars per group filled with the same pond water (or dechlorinated water seeded with 50 mL of pond water)
A soluble garden fertiliser (liquid NPK plant food) and a 1 mL syringe to dose 0, 0.5, 1.0 and 2.0 mL per litre
A windowsill or lamp giving equal light to all jars; thermometer
A printed black-and-white card of decreasing font sizes held behind each jar as a turbidity gauge, or a phone light meter reading light passed through the jar
Optional: a dissolved oxygen test kit or probe for a reading early in the morning and another in mid-afternoon in the final week

How to do it

Fill the four jars, dose them 0, 0.5, 1.0 and 2.0 mL of fertiliser per litre, label and place them side by side in the light.
Every second day record the smallest font size readable through each jar (or the light meter reading), water temperature and a colour description.
Continue for three weeks, topping up evaporation with plain water.
In the final week take dissolved oxygen readings early in the morning and in the afternoon if a test kit or probe is available.
Plot turbidity gauge readings against time for each dose.
Dispose of the water on garden soil, not down a drain.

What you should see

Over the three weeks the dosed jars are expected to turn green and cloudy sooner and more strongly than the undosed jar, and the smallest readable font size rises as they cloud. The SAPS eutrophication notes warn that algae can arrive in some containers before others, so replicate jars and pooled class results matter. Where dissolved oxygen is measured, expect the greenest jar to show the largest difference between the early-morning reading (lower) and the afternoon reading (higher), because algae respire all night and photosynthesise by day. The learner knows it worked when the pooled class ranking of jars by clarity follows the ranking by dose.

What changes

What you change: Fertiliser dose (mL per litre)
What you measure: Turbidity (smallest font readable, or light transmitted) and colour; dissolved oxygen difference between early morning and afternoon
What you keep the same: Water source and volume
Light
Temperature
Jar size
Duration

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Fertiliser is good for a pond because it helps things grow.
Algae add oxygen, so a green pond always has more oxygen; overnight respiration lowers it.
Clear water has nothing living in it.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-LIV-01 SC4-WS-03 SC4-WS-06
Australian Curriculum v9AC9S7U02AC9S7I02AC9S7I05

Sources

The pages the author read to write this activity.