Lab 8 – Solve the mystery of the dying crabs
Instructor Guide

In this activity, students will use OOI data on dissolved oxygen, seawater temperature, and wind direction to solve the mystery of the dying crabs along the Oregon coast at a water depth of about 25m. During the weeks that these data were collected, the area experienced Ekman transport-related upwelling of hypoxic/anoxic deeper water onto the shelf, which killed benthic marine life like crabs. By examining and interpreting relevant data, students will construct a scientific explanation of the events leading up to the death of the crabs.

Approximate time involved: 45-50 minutes for each of the three activities

Learning outcomes

  • LO1. Describe patterns in individual data sets and correlations between the different data types presented
  • LO2. Interpret the provided dissolved oxygen, temperature and wind speed data
  • LO3. Explain the relationship between wind direction and anoxic events on the Oregon Coast using evidence and relevant scientific concepts to support your conclusions.

 

Learning outcome Activity 1 Activity 2 Activity 3
Outcome 1 introduced introduced reinforced
Outcome 2 introduced reinforced reinforced
Outcome 3 introduced

Materials needed: none

What students should know before this activity

Students should have worked through the initial lab in this lab notebook that introduces them to basic data literacy skills, particularly reading time series charts and distinguishing trends in “messy data”. They also should have been previously introduced to the following basic concepts: wind direction and Ekman transport, upwelling/downwelling, and dissolved oxygen sources and variation within the ocean.

What instructors should know before this activity

Because this activity is based on a published case study, reading the associated paper may be a good start to teaching this lesson plan.

Optional Pre-Lab Activities:

Pre/post-lab Assessment Questions:

  1. Which of these are reasons that Dead Zones might form? Check all that apply.
    • Excess fertilizer run-off
    • Improperly disposed-of animal waste 
    • Increased run-off during storms
    • Upwelling of deep ocean water into shallow areas
    • Downwelling of shallow water into deep areas
  2. Compared to the shallow ocean, water in deeper parts of the ocean is often _____________ and contains _____________ dissolved oxygen.
    • Warmer, more
    • Warmer, less
    • Colder, more
    • Colder, less
  3. In the northern hemisphere, if the wind is blowing to the north just west of a coastline that runs north/south, shallow water currents will flow to the __________________, and _________________ will occur.
    • East, upwelling
    • East, downwelling
    • West, upwelling
    • West, downwelling
  4. An area in the ocean 400 meters below the surface has a DO level of 8 mg/L. What is the best explanation for this?
    • This is to be expected, due to the natural stratification of the ocean.
    • This must be a place where shallow water is downwelling into the deep ocean.
    • This must be an area of high productivity due to photosynthesis.

Scientific Background

During the time interval examined here, this area experienced pronounced differences in seawater temperature and DO that were related to wind-driven upwelling and downwelling. When the wind was blowing strongly towards the south, upwelling of cold, low-oxygen water occurred. When the winds calmed down or shifted towards the north, downwelling caused the seawater temperature and DO to both go up fairly abruptly.

Teaching Notes

Most Oceanography textbooks focus almost exclusively on coastal dead zones that are caused by excess nutrients from land. Depending on their background and prior knowledge, students are likely to be focused on an anthropogenic cause for the mystery of the dead crabs and may need guidance towards considering other potential explanations.

Students may have been exposed to the idea that more oxygen gas can be dissolved in cold water than warm water, which may lead them to think that the cold deep water may have more, rather than less oxygen in it. In this case, the relationship is the opposite of what students expect: the colder water has less, not more oxygen dissolved in it.

Extensions:

  • Because most case studies of Dead Zones are related to fertilizer and other waste run-off from land, students might make a concept map of the causes and effects of two cases of ocean anoxia, exploring similarities and differences.
  • Are these “naturally occurring” anoxic events becoming more common due to climate change? The paper that this activity is based on (see below) reports that these events have become more common in the last decade and a half. Although the evidence is not robust, students could construct scenarios of how a changing climate could be changing the ocean to make these events more likely. They could also suggest ways that OOI data could test their hypotheses, and (for advanced oceanography classes), search the actual OOI data and analyze it.

Associated Resources:

Answer Key

[Will be supplied to instructors]