Graphic organizer Grade 8 Science
Nutrient Runoff and a Reservoir Fish Kill: A Cause-Effect Chain
A grade 8 cause-and-effect graphic organizer mapping how fertilizer runoff into a reservoir leads step by step to an algal bloom and a fish kill.
Objectives
- Students will sequence the cause-and-effect chain linking nutrient runoff to a reservoir fish kill.
- Students will explain how each stage in the chain changes a resource available to organisms in the reservoir.
- Students will identify dissolved oxygen as the resource that directly connects an algal bloom to a fish kill.
Teacher overview
This organizer is a six-stage, linear cause-and-effect chain that maps how nutrient pollution from farm fertilizer can lead to a fish kill in a reservoir, the kind of ecological sequence at the center of the middle school standard on resource availability. Students fill in what happens at each stage and how it changes a resource available to organisms in the reservoir, moving from added nutrients that feed algae to the dissolved oxygen that fish need to survive. As students work, press them to explain why each stage causes the next rather than treating the six stages as an unconnected list of reservoir facts; a strong answer for stage 3, for example, uses the word "because" and refers back to stage 2's nutrient levels rather than restarting the explanation from scratch.
Organizer
| Stage | What happens at this stage (fill in) | Where it leads |
|---|---|---|
| 1. Fertilizer runoff | → | |
| 2. Nutrient loading in the reservoir | → | |
| 3. Algal bloom | → | |
| 4. Bloom die-off | → | |
| 5. Oxygen depletion | → | |
| 6. Fish kill | (end of chain) |
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Completed Exemplar
| Stage | What happens at this stage | Where it leads |
|---|---|---|
| 1. Fertilizer runoff | Farms upstream of Cattail Hollow Reservoir (a fictional reservoir used for this organizer) apply nitrogen and phosphorus fertilizer to cropland each spring, and rain washes some of it off the fields and into streams that feed the reservoir. | → Carries added nutrients into the reservoir's water |
| 2. Nutrient loading in the reservoir | The nitrogen and phosphorus that reach the reservoir raise the concentration of nutrients in the water far above natural levels. | → Provides abundant food for algae |
| 3. Algal bloom | Because so much nutrient "food" is suddenly available, algae in the reservoir reproduce far faster than usual, quickly covering large areas of the surface in a dense bloom. | → Uses up the extra nutrients as the bloom keeps growing |
| 4. Bloom die-off | Individual algae in the bloom have short lifespans, so as older algae die faster than sunlight and remaining nutrients can replace them, large amounts of dead algae sink toward the bottom. | → Provides a sudden food source for decomposing bacteria |
| 5. Oxygen depletion | Bacteria that decompose the dead algae consume dissolved oxygen from the surrounding water as they break it down, and enough bacteria are at work that oxygen is used up faster than it can dissolve back in from the surface. | → Lowers the dissolved oxygen available to other organisms |
| 6. Fish kill | Most fish need dissolved oxygen above 5 mg/L to survive, so once oxygen in Cattail Hollow Reservoir falls below that level, fish and other aquatic animals die off or leave the area, and the utility drawing drinking water from the reservoir faces a harder, costlier treatment job. | (end of chain) |
Differentiation
Support (below grade level)
Give students a word bank of the six stage names (fertilizer runoff, nutrient loading, algal bloom, bloom die-off, oxygen depletion, fish kill) already matched to the correct row numbers, so they only need to write the "what happens" explanation in their own words rather than also recall the stage names from memory.
Extension (above grade level)
Ask students to research, using only class materials rather than outside sources, one method reservoirs or their surrounding watersheds use to reduce nutrient runoff before it reaches the water, such as a vegetated buffer strip along a stream, and add a seventh box to their chain showing exactly where that solution would interrupt the sequence.
ELL supports
Pair each stage name with a simple icon (a fertilizer bag, a green cloud for the bloom, a wilted-plant icon for die-off, a bubble icon for oxygen, a fish icon for the fish kill) and allow the "what happens" column to be completed with short phrases rather than full sentences.