Worksheet Grade 6 Science

Fertilizer Runoff and the Algae Bloom at Dovetail Pond

A grade 6 worksheet where students use percent-of-a-quantity math to trace how fertilizer runoff feeds an algae bloom and drops dissolved oxygen at a fictional pond.

Grade: Grade 6
Subject: Science
Material: Worksheet
Time: 30 min
Prep: No prep
Materials: None

Standards

MS-LS2-1 NGSS

Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.

How this material aligns: Items 2 through 6 have students analyze weekly coverage and nitrogen data to build evidence that added nutrients, a resource algae need to grow, drive the size of the algae population, exactly the resource-population link MS-LS2-1 requires.

All materials for MS-LS2-1 →

6.RP.A.3 Common Core Math

Use ratio and rate reasoning to solve real-world and mathematical problems, e.g., by reasoning about tables of equivalent ratios, tape diagrams, double number line diagrams, or equations, including finding a percent of a quantity and converting measurement units.

How this material aligns: Items 2, 3, 5, 6, and 10 require finding a percent of a quantity and completing a ratio table to model a proportional relationship, the unit-rate reasoning 6.RP.A.3 targets.

All materials for 6.RP.A.3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-16.

Objectives

Students will analyze how added nutrients change algae coverage and dissolved oxygen in a pond ecosystem.
Students will calculate percentages of a quantity and complete a ratio table describing pond conditions.

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, algae bloom, runoff

Teacher overview

Dovetail Pond, a fictional pond behind Dovetail Middle School, collects lawn runoff from the neighborhood on the hill above it every time a hard rain falls. This worksheet follows what happened after one such storm: nitrogen and phosphorus from fertilizer washed into the pond, algae multiplied because they suddenly had more of the resource they need to grow, and the pond's dissolved oxygen fell as the extra algae later died and decomposed. A common misconception is that algae blooms happen mainly because of warm weather or strong sunlight; the data in this worksheet instead point to added nutrients as the trigger, with weather only setting the pace at which an already-nutrient-rich pond blooms.

Fertilizer Runoff and the Algae Bloom at Dovetail Pond — Grade 6 Science

Name: ____________________________ Date: ______________

Student Worksheet

Dovetail Pond (a fictional pond) covers 2 acres. Use the data below for every item.

Baseline nitrogen level, measured before the storm: 1 mg/L.
Nitrogen level measured one week after the storm: 4 mg/L.
Dissolved oxygen (DO) before the algae bloom: 8 mg/L.
Dissolved oxygen after the bloom, once the extra algae died and decomposed: 3 mg/L.

In one or two sentences, explain why added nitrogen and phosphorus can be called a "resource" for algae, the same way food is a resource for animals.
One week after the storm, algae cover 20% of the pond's surface. How many acres does that represent?
Three weeks after the storm, algae cover 50% of the pond's surface. How many acres does that represent?
How many more acres are covered in algae in week 3 than in week 1? Use your answers to items 2 and 3.
The baseline nitrogen reading (1 mg/L) is what percent of the after-storm reading (4 mg/L)?
Suppose nitrogen entering the pond rises in proportion to the amount of runoff reaching it, and 1 truckload's worth of runoff adds 0.5 mg/L of nitrogen. Complete the ratio table.

Runoff reaching the pond (truckloads) Nitrogen added (mg/L)

1 0.5

2

4

8
Most fish need dissolved oxygen above 5 mg/L to survive. Does Dovetail Pond meet that level after the bloom? By how many mg/L does it fall short, if any?
Explain, in two to three sentences, the full chain of events from added nutrients to the fish-threatening drop in dissolved oxygen. Use the terms "nutrient," "algae bloom," and "dissolved oxygen" in your answer.
The parks department proposes dividing the pond into 4 equal zones and running an aeration system, a machine that adds oxygen to water, in only 1 of the 4 zones. If that zone's DO rises by 3 mg/L from the post-bloom level of 3 mg/L, what is the new DO level in that zone?
The other 3 zones stay untreated at 3 mg/L. What percent of the pond's total area still falls short of the 5 mg/L fish threshold?
Based on your answers to items 9 and 10, is treating only 1 zone out of 4 enough to protect fish across the whole pond? Explain your reasoning.

Runoff reaching the pond (truckloads)	Nitrogen added (mg/L)
1	0.5
2
4
8

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

Answer Key

Answer: Nutrients act as a resource for algae because algae need nitrogen and phosphorus to grow and reproduce, just as animals need food; when more of that resource becomes available, the algae population can grow larger and faster.
Answer: 0.4 acres.
20% of 2 acres = 0.20 × 2 = 0.4 acres.
Answer: 1.0 acre.
50% of 2 acres = 0.50 × 2 = 1.0 acre.
Answer: 0.6 acre.
1.0 acre (week 3) − 0.4 acre (week 1) = 0.6 acre more covered by week 3.
Answer: 25 percent.
1 mg/L ÷ 4 mg/L = 0.25, or 25%.
Answer: 1.0, 2.0, 4.0 mg/L.
The rate is 0.5 mg/L per truckload. 2 truckloads: 2 × 0.5 = 1.0 mg/L. 4 truckloads: 4 × 0.5 = 2.0 mg/L. 8 truckloads: 8 × 0.5 = 4.0 mg/L.
Answer: 2 mg/L short.
The threshold is 5 mg/L and the pond measures 3 mg/L after the bloom; 5 − 3 = 2 mg/L short of what most fish need.
Answer: Added nutrients let algae grow much faster than usual, producing a dense bloom; when that extra algae dies, bacteria that decompose it use up dissolved oxygen in the water, which can drop oxygen below the level fish need to survive.
Answer: 6 mg/L.
3 mg/L (post-bloom level) + 3 mg/L (rise from aeration) = 6 mg/L, which meets the 5 mg/L threshold.
Answer: 75 percent.
3 of the 4 zones (75%) remain untreated at 3 mg/L, which is below the 5 mg/L threshold.
Answer: No, treating only 1 zone is not enough. Three-quarters of the pond's area (75%) still sits below the 5 mg/L threshold fish need, so most of the pond remains unsafe for fish even after the single treated zone recovers.

Differentiation

Support

Provide a printed reference card showing "percent of a quantity" as a two-step process (convert the percent to a decimal, then multiply) with one worked example using numbers not found on the worksheet. Allow calculators for all percent and ratio-table items.

Extension

Ask students to figure out how many zones (out of the 4) would need aeration, at the same 3 mg/L rise per zone, before every zone in the pond meets the 5 mg/L fish threshold, and to justify their answer using the post-bloom starting level of 3 mg/L.

ELL supports

Pre-teach "nutrient," "algae bloom," "dissolved oxygen," and "runoff" with a simple diagram showing rain washing fertilizer off a lawn and into a pond, followed by algae growing and later dying. Allow item 1 and item 8 to be answered with a labeled diagram in addition to, or instead of, full sentences.