How Can An Algal Bloom Be Harmful?

We’re in the middle of our Watershed Pollution Prevention class, and the term “Harmful Algal Bloom” has come up a few times. HAB’s don’t get their own lesson, so let’s talk a bit about what they are!

What’s an HAB?

HAB’s are classified when the bloom contains visible areas of blue-green algae, or cyanobacteria. That being said, cyanobacteria can vary in color, ranging from bright green, blue/green, and brown.

Small amounts of cyanobacteria exist is healthy waterways, but can grow out of control under these conditions:

• Excess nutrients

• Warm temperatures

• Low winds

• Lots of sunlight

You might notice it has streaks or looks like spilled paint.

Is all algae bad?

Not all algae is harmful. Like we learned in the Live Food lesson, algae is an important part of the food chain (Department of Environmental Conservation). Green algae might float, bubble, look silky, or form thick rafts, but they don’t produce cyanobacteria.

How can HABs harm humans?

HABs can harm people when they come in contact with it by touching it or ingesting it. You may notice fever-like symptoms, nausea/vomiting, diarrhea, or rashes (EPA).

How can HABs harm pets and wildlife?

Harmful cells from HABs can stick to animal fur and can be ingested when the animal cleans itself. Give your pet a bath with clean water and seek veterinarian advice if your pet shows signs of distress after a potential exposure (DEC). Symptoms of HABs exposure for dogs include:

• Excessive salivation or drooling

• Disorientation or inactivity

• Elevated heart rate and difficulty breathing

HABs can also impact entire ecosystems! Think about our trout for instance. HABs deplete oxygen (which we know our trout need!) and can kill crustaceans (like our daphnia) and small fish (like our trout). What happens to our predators, like birds or larger fish, if the bottom of the food chain collapses?

How can we prevent HABs?

The general public, especially those who live on the water, can do their part to reduce the likelihood of algae blooms by reducing the addition of nutrients (phosphorus and nitrogen) to waterbodies. The amount of nutrients can be decreased by:

• Limiting lawn fertilization

• Scoop the (dog) poop

• Maintaining septic tanks

• Installing and maintaining shoreline buffers

Water Quality and Nitrogen Cycle

By now, you and your students have run tens of water quality tests. Let’s take a moment to slow down and look at what’s going on and why.

Nitrogen Cycle

Your aquarium is producing “waste”, including fish poop, uneaten food, and dead fish. The Nitrogen Cycle is the biological process that converts toxic compounds into beneficial bacteria colonies:

Waste → Ammonia (toxic) → Nitrite (toxic) → Nitrate (okay)

In the beginning, we added a weekly dose of biological enhancer (MicrobeLift Special Blend) to help these beneficial bacteria colonies form in our coldwater ecosystem.

(If we had plants in the aquarium, they would absorb these nitrates and use them as nutrients, completing the cycle. Since we don't, we can do partial water changes regularly.)

The video here might be familiar to some of you. This example by Aquarium Co-op explains the Nitrogen cycle using m&m’s. You can use this video with your students, or another one that you may prefer more. 

How To Run A Full Water Quality Test

This video features our very own Phil Koon, showing the entire process of running a full water quality test. Ideally, we’re doing this every other day to have an accurate reading of our water trends.

Recording Water Quality Data

Not only do we want to be testing our data, but we want to be recording it as well. This helps us keep an accurate record, and can show the students these biological processes in real time. You can print out and keep this Trout Care Daily/Weekly Checklist nearby your aquarium for the students to help fill out. As the teacher, you will also be uploading this information into the 2025-26 Aquarium Monitoring Data Sheet, so the DCL team has access to this information and can help you troubleshoot any issues that may occur.

Recommended Water Testing Levels:

• Ammonia- Less than 1 mg/l (ppm).

• Nitrite- Near zero (ppm), once bacteria are established in the filter system

• Nitrate- Below 40 ppm (mg/l)

• pH- The pH scale runs from 1 (acid) to 14 (basic).  7 is “neutral”.  A pH of 7.4 – 8.4 is fine for our trout

Ammonia and Nitrite at high levels are toxic to your fish. Nitrate is not, it would take up to 200 ppm for nitrate to be toxic.

What To Do if Levels are Consistently High (Over Extended Testing):

• If ammonia or nitrite levels rise above 1 ppm for an extended period, contact us.  Reducing or temporarily pausing your feeding schedule is a good first step that you can take right away.  Depending on the conditions in your aquarium, additional filter media or aquarium cleaning/gravel vacuuming can also quickly reduce these waste products.

• Nitrate levels will steadily rise during the year.  Without the presence of plants in our aquarium, the nitrogen cycle is incomplete, and there is no place for nitrate to go.  This is not a threat to your trout, but high levels will encourage algae growth, odor, etc. The only way to reduce nitrate is with partial water changes.

More Troubleshooting:

Depending on where you are consistently seeing concerningly high levels in your aquarium, you may need to troubleshoot further as to what could be the cause. A really great resource for troubleshooting is this Pennsylvania Trout in the Classroom document. It provides a great overview of the Nitrogen Cycle, and if you skip over to pages 56-58, you can find causes and remedies for high Ammonia and Nitrite levels that may help target and fix your exact issue!

Cannibalism

Check out this photo from Elizabeth Ann Clune Montessori School, where you can see a tail fin sticking out of a trout's mouth!

You and your students know that the trout are carnivores, and this means they’ll even eat members of their own species. It will take them a while to fully eat what’s in their mouth, so you might see that fin throughout the rest of the school day. If you see something similar at your school, take a picture and send it in!

This is the circle of life, so even if it seems sad, it’s an opportunity to learn about our finned friends. The ones that want to move on from fish food to live food indicate their ability to survive in the wild. Keep an eye on the ones that grow bigger than the rest if you want to see it in action.

 What Else Do Trout Eat?

We’re wrapping up the Live Food Experiment this week, so let’s recap all the fun things we learned and how the experiments went for each school!

Our Data

Here are some observations the students made while running the experiment:

1. Did our teacher feed the fish already today?

2. Does the behavior of our trout seem calm or stressed?

3. What if we added an underwater camera to observe the fish, to reduce the anxiety of being watched?

4. Did it just spit up a daphnia?

5. Did it actually eat 45 daphnia, or has it been breathing?

6. It ate 0 daphnia today, does this mean it will never eat live food?

7. Why is the experiment exactly 6 minutes long?

Look at all this critical thinking! Your students were able to recognize different variables that could impact their experiment.

What Do Trout Eat?

The trout we raise are strictly carnivores, and are opportunistic feeders. This means, they’re able to adapt to eat available food. Here’s a list of food your trout might eat in nature:

• Macroinvertebrates 

• Dragonflies, stoneflies, caddisflies, mayflies, snails, worms, and more

• Other (smaller) fish and eggs

• This is why you might see cannibalism in the aquarium!

• Crustaceans

• Crayfish and scuds

• Amphibians

• Frogs and salamanders

• Mammals

• Mice and young mink

• Remember…our trout are opportunistic!

Common (incorrect) answers:

Krill

Krill was a common thought among students this year! Although they live in the ocean and, therefore, can’t be trout food, it’s not a bad guess. Krill are also zooplankton and crustaceans. This shows us that the students were able to recognize what kind of food trout might be attracted to.

Krill vs Daphnia

Algae

This answer tells us that students know trout search for something plentiful and smaller than themselves. This month, they learned that trout are carnivores, and don’t purposefully eat algae.

Rocks

Our trout don’t want to eat something that’s not nutritious!

Use the sheet below to play a game with your students! Would My Trout Eat This?

Daphnia!

Now that we have officially started our LIVE FOOD EXPERIMENTS, let’s take a closer look into the animal that makes it possible.

Daphnia are a small animal known as zooplankton. Their tiny size makes them a great snack for out trout, but also other creatures like macroinvertebrate larva. There are over 100 known species of daphnia! Trout in the classroom has used a few different kinds for our experiments over the years. These colonies can produce both sexually and asexually! Female daphnia can also release their eggs into the sediment where they sit dormant for upwards of 80 years before the right conditions allow them to hatch.

If your school hasn’t run the experiment yet, check out this video of the way they “jump” through the water!

So, what do Daphnia eat? Well, they’re herbivores! They eat phytoplankton, or “algae” that use the sun’s energy for photosynthesis. Phytoplankton come in all different shapes and colors, but they’re often GREEN.

Here is a picture of what I feed our colonies during the month! With a microscope, you can actually see their gut, which is clear, full of food. Daphnia are critical for controlling algal populations.

Phytoplankton can grow so intensely, they become known as an algal bloom. Check out these pictures in Owasko Lake (left) and Fall Creek (right). This can be exacerbated by human impact. Adding excess nutrients, such as fertilizer runoff or sewage/animal poop, have caused these blooms to worsen. Why is this bad?

• Harmful to human health

• Harmful to pet (including dog) health

• Depletes oxygen and kills fish and other organisms

• Blocks sun from reaching organisms deeper in the water

Daphnia are an amazing species. They are sensitive to pollution and, therefore, incredibly helpful in determining the health of a body of water.

Dapnia present? Healthy water! Daphnia are absent? Check out the water more.

This is known as an indicator species!

We might not spend the year studying daphnia, but they're helpful to learn about when considering the ecosystem for our trout. All of these factors come together to create a healthy habitat for us and the animals we live by.