PSOW – Practical 2.2G Measuring Primary Productivity with Teacher’s Notes

Description

IB ESS Practical 2.2G: Measuring Primary Productivity (HL only)

Give HL students hands-on experience of the classic light and dark bottle method for measuring photosynthesis, respiration, and primary productivity in aquatic ecosystems.

The method works by tracking dissolved oxygen (DO). Sealed bottles of aquatic plants are left under the same conditions, some clear and some wrapped in foil to block all light. Because the dark bottles cannot photosynthesize, they reveal respiration alone. Comparing them with the light bottles lets students calculate:

• Respiration = initial DO − dark bottle DO
• Net primary productivity = light bottle DO − initial DO
• Gross primary productivity = light bottle DO − dark bottle DO

This practical comes in two versions, so teachers can choose the one that suits their equipment:

• Version 1 — Standard equipment: students use six bottles (three light, three dark) with plant portions of equal mass, measure DO with a probe or chemical test kit, and express results per gram of plant material per 24 hours, with means and ranges
• Version 2 — Vernier data logging: students use Vernier biochambers and a LabQuest. Inverted bottles fitted with O₂ gas sensors log atmospheric oxygen every 5 minutes for 24 hours, alongside DO readings from a larger set of light and dark bottles. This lets students compare productivity measured in water and in air.

Version 2 also includes key vocabulary and background information explaining primary productivity, gross and net productivity, respiration rate, and dissolved oxygen. Both versions include write-in data tables.

The activity shows students why gross productivity cannot be measured directly: it can only be inferred by combining two measurements. Students who understand why the dark bottle is needed understand the difference between gross and net productivity properly.

IB ESS Internal Assessment Skills

The two versions are assessed against different IA criteria, giving teachers flexibility in which skills to target. The full rubrics are included with each.

• Version 1 uses Criteria C: Method and D: Treatment of Data. Students describe how they standardized plant mass, light, and temperature across all six bottles, show full calculations of respiration, NPP, and GPP with means and ranges, and identify the uncertainty of using oxygen change to represent biomass change.
• Version 2 uses Criteria E: Analysis and Conclusion and F: Evaluation. Students explain why the dark bottle measures only respiration, how the light bottle captures both photosynthesis and respiration, why atmospheric oxygen in the inverted bottles can measure productivity, and why DO is highest in the light bottles and lowest in the dark bottles.

The activity is strong preparation for aquatic and productivity-based IAs. The teacher’s notes explain why replicate bottles matter: DO readings are noisy, and single bottles can produce impossible negative values. They also note that unequal plant mass is the most common reason results fail to replicate.

Ideal for

• Teaching gross and net primary productivity
• Measuring photosynthesis and respiration
• Using the light and dark bottle method
• Measuring dissolved oxygen
• Using Vernier sensors and data logging
• Designing controlled experiments with replicates
• HL ESS classes
• Preparing students for the IB ESS Internal Assessment

Digital download. No physical product will be shipped.