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 Applied
Outcome 2 Introduced Guided practice Applied
Outcome 3 Introduced, guided practice

Materials needed

None

Lab 8 Student Answer Form

What students should know before this activity

Students should have worked through the Lab 2 activities that introduce them to basic data literacy skills, particularly reading time series charts and distinguishing trends in “messy data”. Recommended preparatory exercises are Lab 2.1 (time series) and Lab 2.5 (vertical sections).

The beginning of the lab includes a brief review, or introduction to needed concepts such as wind direction and Ekman transport, upwelling/downwelling, and dissolved oxygen sources and variation within the ocean. The content is reinforced with quick check questions. The introduction deliberately tries to explain the relevant background concepts as simply as possible for students who may be in a marine biology course and not exposed to physical oceanography. For these students, the lab intentionally does not name Ekman transport or attempt to explain the Coriolis effect. For a class that has gone through this material previously, you may ask them to remember the technical term for the described relationship between wind and current direction, or ask them to review all of the forces that cause Ekman transport.

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. For an advanced class, you might want the students to read this paper as an introduction to the lab, although it would give away the answers. It could also work as a follow-up activity.  Students do need to complete all three lab sections to understand the main point of the lab.

Scientific Background

During the time interval examined here, this area experienced pronounced differences in seawater temperature and dissolved oxygen (DO) that were related to wind-driven upwelling and downwelling. When the wind was blowing strongly towards the south (northerly wind), upwelling of cold, low-oxygen water occurred. When the winds calmed down or shifted towards the north (southerly wind), the relaxation of upwelling and possible onset of 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 (due to biological consumption).

Optional Pre-Lab Activities:
  • Show a map of dead zones around the world. Ask students to define “dead zone” and summarize what they know about the issue. What are some possible reasons that a dead zone might form?
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.
Extensions:
  • The students may not have noticed that there is a slight lag time between the onset of the change in wind direction and the arrival of cold, low-oxygen water. Why is this? Why does it take a little time for the upwelling to start?
  • 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.
  • For an oceanography class near the ocean, students could research the local seafood industry. What kinds of seafood are being harvested? Has the local seafood industry been impacted by dead zones? If so, was the cause of the dead zone the same or different?
  • 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.
  • For an advanced oceanography class, these data could illustrate the concept of “patchiness”. We tend to think of bodies of ocean water as large and homogenous, but within a larger body, there are likely to be areas of variability. Our monitor is fixed, so as a body of water flows past it, we are bound to see short-term variability as well as longer-term trends.
  • How are different marine organisms affected by these events?
    • Are some more vulnerable than others?
    • Which populations are especially vulnerable to long term population decreases, and which recover more quickly?

Associated Resources:

  • The Data Lab and this exercise were inspired by a case study in this paper.
  • Learn more about the Coastal Endurance array here.
  • A potential follow-up exercise that looks at how changing pH is affecting shellfish at the same location could keep the theme of how ocean water properties impact people: https://datalab.marine.rutgers.edu/explorations/2019/acidification.php?level=exploration (Note: this activity also includes a wind direction plot similar to the one in this lab exercise, but the data points are labelled differently. They are all labeled “north wind” and given negative or positive numbers. This could be confusing to students and should be pointed out.)