Grade 7 Science Fair Ms. Nwosu's class, spring 2026

How does warmer water affect coral?

Coral gets its color, and most of its food, from tiny algae living inside it. I wanted to find out what happens to those algae when the water heats up, so I tested them at three temperatures in my kitchen.

See my results Start with the research

Aisha R.
Student scientist, Grade 7

Sunlight shining down through clear blue water onto a healthy coral reef — A healthy reef needs sunlight, clear water and the right temperature.

I tested: 3 water temperatures
I measured: Algae color, for 12 days
I found: 57% color lost at 36°C

Table of contents

My project, step by step

Every science fair board follows the same path. Jump to any part, or scroll down and read it in order.

01 · Background research

What I learned before I started

Close-up of a cluster of colorful coral polyps with tiny tentacles

What is coral?

Coral looks like a rock or a plant, but it is an animal. Each piece is a colony of tiny animals called polyps, which are cousins of jellyfish and sea anemones. Reef-building corals make hard skeletons out of limestone, and over thousands of years those skeletons pile up into a reef.

Inside each polyp live millions of tiny algae called zooxanthellae (say it: zoh-uh-zan-THEL-ee). They use sunlight to make food and share it with the coral. They also give coral its bright colors.

A sea turtle resting on a coral reef in bright blue water

Why reefs matter

Reefs are sometimes called the rainforests of the sea. They are crowded, busy places, and lots of living things depend on them, including people.

Homes for sea life. Fish, turtles, crabs and sharks find food and shelter in the reef.
Coast protection. Reefs break up big waves before they reach the shore during storms.
Food and jobs. Hundreds of millions of people rely on reefs for fishing and tourism.
New medicines. Scientists study reef animals to find chemicals that could treat diseases.

What is coral bleaching?

When the water stays too warm for too long, coral gets stressed and pushes its algae out. Without them, the coral's tissue turns see-through and its white skeleton shows. That is bleaching.

Bleached coral is not dead yet, but it is starving. If the water cools down soon enough, the algae can come back. If it stays hot, the coral can die.

Healthy

Full of algae
Stressed

Algae leaving
Bleached

White and starving

Bleached white coral on a reef, its skeleton showing through

Did you know?

<1%

Coral reefs cover less than 1% of the ocean floor, but about a quarter of all ocean species live on or around them.
90%

The algae living inside coral can make up to 90% of the food a coral needs.
2,300 km

The Great Barrier Reef is about 2,300 km long. It is the biggest structure built by living things on Earth.
2024

NOAA confirmed the fourth global coral bleaching event in 2024, caused by record ocean heat.

02 · Hypothesis

My prediction

If the water is warmer, then the algae will lose more of their green color, because heat damages the chlorophyll they use to make food, just like it stresses the algae living inside coral.

I could not test real coral at home, and it would not be fair to the coral anyway. So I used green algae as a model, because the algae inside coral are what get pushed out during bleaching.

Independent variable: Water temperature; 24°C, 30°C and 36°C
Dependent variable: How green the algae stay; Scored 1 to 10 on my color chart
Kept the same: Jar size, amount of algae, amount of water, light, and number of days

03 · Materials and method

How I ran the experiment

Three glass flasks filled with different colored liquids on a white table

Materials

9 clear glass jars, 250 ml each
Green algae culture from a school science kit
3 plastic tubs used as water baths
2 aquarium heaters and a digital thermometer
LED desk lamp on a 12 hour timer
Homemade 10 shade green color chart
Lab notebook and a phone camera

Make a color chart
I glued 10 green paint chips in a row, from 10 (dark green) to 1 (almost clear), so I could score color the same way every time.
Fill the jars
I split the algae evenly into 9 jars with 200 ml in each. Every jar started at a score of 10.
Set up three water baths
One bath stayed at room temperature (24°C). The other two used aquarium heaters set to 30°C and 36°C.
Keep everything else the same
Three jars went in each bath, all under the same lamp at the same distance, with 12 hours of light a day.
Check the temperature
I checked each bath twice a day, before school and after school, and wrote it in my notebook.
Score the color
Every 3 days I held each jar against the chart in the same spot by the window and took a photo.
Do a blind check
On day 12 my brother scored the jars too, without knowing which temperature each one came from.
Work out the averages
I averaged the three jars at each temperature and turned the drop in score into a percent of color lost.

Safety: an adult set up the heaters and checked the cords, and I wore gloves and washed my hands after touching the algae.

04 · Results

The hotter the water, the more color the algae lost

Percent of color lost after 12 days

Average of 3 jars at each temperature. Every jar started at a score of 10.

Color scores on day 12 for each jar
Water	Jar A	Jar B	Jar C	Average	Color lost
24°C	9.5	9.0	9.5	9.3	7%
30°C	8.0	8.5	7.5	8.0	20%
36°C	4.5	3.5	5.0	4.3	57%

Swipe the table sideways to see every column.

What I noticed

The 24°C jars barely changed. They looked the same as day 1 to me.
The 36°C jars started fading by day 6 and were a pale yellow-green by day 12.
My brother's blind scores were within 1 point of mine for every jar.

Small fish swimming over a coral reef in deep blue water

Bleached coral is not dead yet. If the water cools down in time, the algae can move back in.

That is why every degree we keep the ocean from warming matters.

05 · Conclusion

Was I right?

Hypothesis supported

Yes. The algae in the warmest water lost the most color. At 36°C they lost 57% of their green, about eight times more than the jars at 24°C, which only lost 7%. Even the middle temperature, 30°C, almost tripled the color loss.

This matches what scientists see on real reefs. NOAA says coral can start to bleach when the water stays just 1°C above its usual summer high for a few weeks. My jars show the same pattern on a small scale: heat makes the algae fade, and the more heat, the faster it happens.

8×

more color lost at 36°C than at 24°C

+12°C

took the algae from 9.3 down to 4.3 on my chart

Looking up through branching staghorn coral toward the bright water surface

06 · Next steps

What I would do next

Every answer gave me two new questions. If I do this project again next year, here is what I want to try.

A
Use smaller temperature steps
Test 26, 28, 30 and 32°C, which are closer to what real reefs feel during a heatwave.
B
Test recovery
Move the faded 36°C jars back to 24°C and see if the green color comes back, like coral can recover.
C
Measure color with a sensor
Use a light sensor or a color app instead of my eyes, so the scores are more exact.

07 · Sources

Where my facts come from

I only used websites from science organisations and government agencies. I checked each fact on at least two of them.

08 · Feedback

Questions about my project?

Judges, classmates and family are all welcome to ask. I will answer at the science fair or in class with Ms. Nwosu.

Please only write your first name. Ms. Nwosu reads every question first.

How does warmer water affect coral?

My project, step by step

What I learned before I started

What is coral?

Why reefs matter

What is coral bleaching?

Did you know?

My prediction

How I ran the experiment

Materials

Make a color chart

Fill the jars

Set up three water baths

Keep everything else the same

Check the temperature

Score the color

Do a blind check

Work out the averages

The hotter the water, the more color the algae lost

What I noticed

Was I right?

What I would do next

Use smaller temperature steps

Test recovery

Measure color with a sensor

Where my facts come from

Questions about my project?

Saving Our Coral Reefs · Science project