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Lesson plan Grade 9 Science

Larkspur Lake Algae and Nutrients: A Grade 9 Ecology Lesson

A 50-minute grade 9 lesson where students trace nitrogen and phosphorus as both crop nutrients and lake pollutants, then evaluate cleanup steps for a fictional lake.

Objectives

  1. Students will explain how nitrogen and phosphorus act as both nutrients and pollutants.
  2. Students will trace the chain from nutrient runoff through an algae bloom to low dissolved oxygen.
  3. Students will evaluate a nutrient-reduction solution for a specific pollution source.

Vocabulary

Eutrophication
Eutrophication is what happens when a lake or bay gets overfed with nutrients: algae grows out of control, and when it dies and rots, the water loses the oxygen that fish need.
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, phosphorus, limiting nutrient

Teacher overview

Nitrogen and phosphorus are essential nutrients: crops need them to grow, and every living thing in a lake depends on them at low levels. The same nutrients become pollutants when too much washes into a lake, where they feed algae blooms that later die, decompose, and pull oxygen out of the water. This lesson uses the fictional Larkspur Lake to show how one substance can be a resource in a field and a problem in the water. Students trace the nutrient chain, learn why phosphorus is usually the limiting nutrient in fresh water, and evaluate a reduction solution for each of the lake's pollution sources.

Procedure

Hook (5 min): Describe a lake that turns bright green every August, with a mat of algae near the shore and dead fish reported by early September. Ask students what they think is feeding the algae, and where it might be coming from. Record ideas without confirming them.

Direct instruction (15 min): Define nutrients, and explain that nitrogen and phosphorus help plants and algae grow. Present the eutrophication chain step by step: extra nutrients wash in, algae grow quickly, the bloom dies, bacteria decompose it, and decomposition uses up dissolved oxygen. Note the water-quality benchmark that most fish need dissolved oxygen above 5 mg/L, so a bloom's aftermath can drop oxygen low enough to kill fish. Explain that in most fresh water phosphorus is the limiting nutrient, meaning the bloom is held in check by whichever nutrient runs out first, so cutting phosphorus is often the most effective single step.

Small-group analysis (15 min): Give each group a set of source cards for Larkspur Lake describing three nutrient sources: fertilizer runoff from a nearby farm, treated discharge from the town's wastewater plant, and runoff from fertilized lakeside lawns. For each source, groups name one reduction solution (for example, matching fertilizer to crop need, adding phosphorus removal at the wastewater plant, or restoring a vegetated shoreline) and rate how feasible and how effective it would be.

Share-out (8 min): Each group presents its highest-rated solution and the source it targets. As a class, discuss which solutions reduce the most phosphorus for the least cost and effort, and where two solutions might be needed together.

Formative check and exit ticket (7 min): Students answer three questions on an index card: first, explain how the same nutrient can be a resource and a pollutant; second, explain why cutting phosphorus is often the most effective way to slow a freshwater bloom; and third, trace how an excess of nutrients ends in low dissolved oxygen. Collect the cards and check that most students connect excess nutrients to oxygen loss.

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. Answer: A nutrient is a resource in a field and a pollutant in a lake.
    Nitrogen and phosphorus help crops and other plants grow, so they are valuable on farmland. The same nutrients become pollutants when excess amounts wash into a lake, where they feed algae blooms that later decompose and remove oxygen from the water. Accept any answer that names both the helpful role and the harmful role.

  2. Answer: Phosphorus is usually the limiting nutrient, so cutting it slows the bloom most.
    In most fresh water, algae growth is held back by phosphorus because it runs short before nitrogen does. Reducing the phosphorus supply removes the nutrient the bloom depends on most, which slows growth more than cutting a nutrient that is already plentiful. Accept any answer that connects the limiting nutrient to the greatest effect on the bloom.

  3. Answer: Excess nutrients lead to low dissolved oxygen through decomposition.
    The bloom grows, dies, and is broken down by bacteria, and that decomposition consumes dissolved oxygen. When oxygen falls below the level most fish need, fish can die. Accept any answer that traces nutrients to algae growth to oxygen loss.

Differentiation

Support

Provide the eutrophication chain as a labeled sequence of pictures (nutrients in, algae grow, algae die, oxygen falls, fish die) that students can point to while explaining. Give each group a completed example card for one source before they rate the other two.

Extension

Ask students to research, using teacher-provided sources only, why a lake can keep having blooms for years even after the outside nutrient supply is reduced, and connect their answer to phosphorus stored in the lake-bottom sediment.

ELL supports

Pre-teach "nutrient," "eutrophication," and "dissolved oxygen" with paired images and short definitions. Provide sentence frames for the exit ticket: "A nutrient is a resource when ___ and a pollutant when ___," and "Cutting phosphorus works best because ___."