Lab 8 – Solve the mystery of the dying crabs

In 2017, crab fishing off the coast of Oregon was hurt when crab traps, which would normally be full of lively and tasty crabs, instead yielded dead and dying crabs. The Oregon Department of Fish and Wildlife put a remote camera on a crab trap near the OOI Oregon Inshore Mooring. Using data from the OOI and other sources, scientists worked together to try to determine the cause of the dying crabs. In this exercise we will look at OOI data for May and June of 2017 to recreate their findings. What was killing the crabs? What OOI oceanographic data can help solve this mystery?

Begin by watching this timelapse video of how this event affected the crabs:

In this exercise we will look at three data sets, all of which were collected over the same time period in 2017. First we will look at dissolved oxygen (DO) in ocean water at this site, which is measured in milligrams per liter (mg/L). Next we will examine seawater temperatures (T), and finally we will examine northward wind speed, which will help us determine if the wind was blowing more to the north or the south, and how strongly.

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.

Background Information

Review concepts: Animals that live in the ocean need oxygen to respire. Dissolved oxygen gas in the ocean can come from interactions between the ocean water and the atmosphere, or more commonly, from photosynthesis by phytoplankton, tiny organisms that harness the energy of sunlight to convert water and carbon dioxide gas to sugar molecules. The by-product of photosynthesis is oxygen gas, which can then dissolve into the ocean water and be used by animals to respire.

The presence of photosynthetic organisms is not the only factor in determining the level of oxygen in the ocean water. Temperature plays a role: cold water can hold more dissolved oxygen than warm water. Also, when dead organisms decay, this process uses up oxygen in the water.

In some places in the ocean, the amount of oxygen dissolved in the water drops to such low levels that animals cannot respire. This is called hypoxia. Sometimes, the water becomes completely without any dissolved oxygen, and this is called anoxia. Areas of the ocean that hypoxic/anoxic are referred to as “dead zones”.

Hypoxia and anoxia can happen for many reasons. It can be related to temperature. Very warm water can only hold low levels of dissolved oxygen. Sometimes, it is related to fertilizer washing into the ocean, encouraging the growth of phytoplankton. When the phytoplankton use up all the fertilizer, they die and as they decay, the decay process uses up the oxygen. This happens in the shallow water. But, hypoxia and anoxia can also be caused by water from the deep ocean upwelling, when water from the deep part of the ocean rises into the shallow part of the ocean. The opposite of this is downwelling, when shallow ocean water moves into the deeper part of the ocean.

Why would upwelling occur? Upwelling can be caused by shifts in the direction of the wind. As the wind blows over the surface of the ocean, it creates currents in the shallow ocean. But, because of the Coriolis effect, the resulting current does not flow in the same direction as the wind blows. In the northern hemisphere, the current flows towards 90 degrees clockwise of the direction that the wind was blowing towards. In the southern hemisphere, the current flows 90 degrees counter-clockwise of the direction that the wind is blowing towards.

If the shallow ocean currents cause the water to flow away from the coastline, then deeper water will upwell to take its place. If the shallow ocean currents cause the water to flow towards the coast, the shallow water will be forced down into the deep ocean, downwelling.

  • Key terms: dissolved oxygen, hypoxia, anoxia
  • Data collection: We will use data collected by the Ocean Observatories Initiative, an initiative that has stationed equipment for collecting data in different locations around the world. Our data come from the Coastal Endurance Array off the coast of Oregon (see map below).

The Coastal Endurance Array includes multiple oceanographic instruments attached along cables running from the surface down to the seafloor. The drawing below shows where the different data instruments are located in the ocean. Wind data in the activity were collected at the Shelf Surface mooring (2) and the dissolved oxygen and temperature data were collected at the Inner Shelf mooring (1) on the seafloor.

Activities in this Lab