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Lesson plan Grade 6 Science

Merrow Bay Nitrogen: A Needed Nutrient and a Pollutant

A 45-minute grade 6 lesson where students study a fictional bay to see how nitrogen feeds plants yet, in excess, fuels algae blooms that lower the oxygen fish need.

Objectives

  1. Students will explain how nitrogen acts as both a needed nutrient and a pollutant in a bay.
  2. Students will describe how an algae bloom lowers the dissolved oxygen that fish need.
  3. Students will use a dissolved-oxygen benchmark to judge whether bay water can support fish.

Vocabulary

Dissolved oxygen
Dissolved oxygen is the oxygen gas mixed into water that fish and other underwater animals breathe. Cold, moving water holds more oxygen than warm, still water.

Also: nutrient, nitrogen, algae bloom

Teacher overview

Nitrogen is a nutrient, which means living things need it to grow. Plants and algae use it to build their cells. This lesson uses the fictional Merrow Bay to show how the same nutrient can both help and harm. A small amount of nitrogen helps the bay's plants, while too much, washed in from farm fields and lawns, feeds a fast bloom of algae. When the algae die, microbes break them down and use up the dissolved oxygen in the water, and fish need dissolved oxygen above about 5 mg/L to stay healthy. Students read a short before-and-after dataset and decide whether the water can still support fish. The lesson corrects a common idea that a nutrient is always good; in the wrong amount, a nutrient becomes a pollutant.

Procedure

Hook (5 min): Hold up two jars, one with clear water and one tinted green. Ask students which bay they would rather swim or fish in, and why. Record their guesses about what made the water green.

Direct instruction (12 min): Explain that nitrogen is a nutrient plants and algae need. Draw the chain on the board for the fictional Merrow Bay: extra nitrogen from farms and lawns runs into the bay, algae grow fast, the algae die, and microbes use up the oxygen as they break the algae down. Introduce the benchmark that most fish need dissolved oxygen above 5 mg/L.

Guided practice (15 min): Project this fictional Merrow Bay dataset and read it together. Before the bloom, the dissolved oxygen was 8 mg/L. During the worst of the bloom, it dropped to 3 mg/L. Ask students to compare each reading with the 5 mg/L benchmark and decide, for each, whether fish could stay healthy. Have them explain what caused the drop, using the chain from direct instruction.

Class discussion (8 min): Ask how the bay could get less nitrogen in the first place. Guide students toward sources they can name, such as fertilizer running off fields and lawns, and toward simple fixes, such as buffer strips of plants along the shore.

Closure and exit ticket (5 min): On an index card, students answer one question: "Explain how nitrogen can be both helpful and harmful in Merrow Bay." Collect the cards and check that most students name both the plant-feeding role and the algae-bloom problem.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Before the bloom (8 mg/L): Fish could stay healthy, because 8 mg/L is above the 5 mg/L benchmark that most fish need.
  2. During the bloom (3 mg/L): Fish would be stressed or could die, because 3 mg/L is below the 5 mg/L benchmark, and the low oxygen came from microbes using it up as they broke down the dead algae.
  3. Exit ticket answer: A strong response explains that nitrogen is helpful because plants and algae need it to grow, and harmful because too much of it feeds a large algae bloom that, when it dies and decays, lowers the dissolved oxygen fish depend on.

Differentiation

Support

Give students a partly finished cause-and-effect chain (extra nitrogen, then algae grow, then blanks for the last two steps) to fill in during direct instruction, so the sequence is visible while they analyze the data.

Extension

Ask students to predict what a second dataset might show two weeks after the bloom clears, and to justify whether the dissolved oxygen would rise back above 5 mg/L and why.

ELL supports

Pre-teach "nutrient," "algae bloom," and "dissolved oxygen" with the two jars and simple pictures of each step in the chain. Provide sentence frames for the exit ticket: "Nitrogen helps because ___. Nitrogen harms because ___."

Extensions

Have students look for news of an algae bloom in a real lake or bay and describe, in their own words and without copying, how the reported causes match the Merrow Bay chain they studied.