Follow the Flow

In this series, educator Deb Greene walks us through some of the science on the ship. She explores the practices of science and how they fit in with the Next Generation Science Standards. Deb has taught and been involved in curriculum development in both the public and private sector for over 35 years. She currently works in Alaska with the Curriculum and Instruction Department for the Anchorage School District.


Part 6: Lessons in Physical Oceanography

Nick Beaird, a post doc in Emily Shroyer’s Physical Oceanography Lab in CEOAS at DSC04688.JPGOregon State University, is looking for answers. Scientists work on the premise that biology and chemistry don’t work in isolation. There are constant interactions between the natural and physical worlds. As physical scientists explore the idea of physical processes such as waves, currents, and winds, the influence of such factors on the basic productivity of the ecosystem can be examined in minute detail.

As scientists identify physical processes and relate them to the biology, they can look at and understand the physical processes currently affecting our oceans and then document physical changes as they happen. Knowing what current conditions are provides a baseline allowing scientists to predict and monitor changes. For example, wind blowing over an ocean covered in sea ice has a very different effect on wave action and coastal shorelines than wind blowing across open ocean waters.

As sea ice decreases, scientists are looking at the effects of increased winds on the ecosystem. We know that these changes are happening. Examining the implications of these basic physical processes, how they impact biology, and how we may expect them to change in the future is what Nick is on this science cruise for. His overarching question is, “What physical processes are influencing the productivity of the ecosystems in the Chukchi Sea?”

To find answers, Nick is measuring velocity and turbulence because turbulence mixes up the different layers of water, each with its own unique characteristics. The key is to measure exchange across the interface between layers. Some layers may provide nutrients for organisms; other layers are more suitable for organisms to thrive. So mixing is an integral part of the process to get nutrients to where the organisms are. The layered structure of the ocean can be modeled by students in class.

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Density Column

Start by using a discrepant event, showing students a multi-layered density column as away to engage a high level of student interest. As students ask questions, steer them in the direction of layering in the oceans as an intriguing way to get students to think about the oceans as being more than one stratum. Then allow students the opportunity to make their own density salt water columns using this Pearson College guide.

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Layered water samples

Another model students can simulate is the mixing of layered water. Balancing a very still tub of room temperature water on cups then, using a pipet, deposit red dye in a pool on the container’s bottom. Next, add a previous frozen blue ice cube to the top of the container. Carefully slide a cup of hot water under the bottom of the container so that it is positioned under the red pool of dye. If available, students can use temperature probes to monitor circulation currents. Once students have the technique down, they can begin devising their own experiments/models and asking their own questions relating to varying salinity in the water column. 

The instruments

There are two categories of things Beaird is measuring on this cruise: currents and turbulence. He brought along six different sensors to better understand the physics behind the biology and chemistry. Four of these instruments are attached to the SuperSucker and two are attached to the CTD. They include two AquaDopps (one of each on the SuperSucker and CTD), two Chipods (one of each on the SuperSucker and CTD), one Gust on the SuperSucker, and one ADCP on the SuperSucker.

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Nick Beaird removing his instruments after the last SuperSucker run. The blue-capped instrument in the front is an ADCP. The white instrument at the far end of the SuperSucker is the Gust. 

Currents

The Acoustic Doppler Current Profiler (ADCP) measures current. These sensors use acoustic doppler measurements to get a profile of the velocity at a range of about 50 meters in front of the instrument. It works in a similar way to sonar, sending out sound waves and measuring the reverberation that comes back. This gives us a medium scale picture of the currents in the water column – at least the ones that are sweeping by the SuperSucker and CTD – and tells us how quickly the water is moving around. This is a basic physical characteristic scientists want to know.

The AquaDopp measures current on a smaller scale. It’s similar to the ADCP, but looks at the single meter of water in front of the sensor instead of 50. This tells us something about little turbulent eddies at work that mix up the water column and potentially transport nutrients around. 

This also helps us predict whether turbulence may be in the water, since the AquaDopp measures small scale changes in velocity that are one of the main sources of turbulence.

Turbulence

Changes in temperature help Beaird understand the turbulence, or mixing, in the water. The Chipod and Gust are two instruments that measure very fast changes in temperature, and the rate of those changes. This is done with a tiny glass bead thermistor – essentially a tiny thermometer. This is a scale an order of magnitude smaller than what’s done on the CTD. These measurements are associated with really small scale eddies and whirls. They dissipate energy and transfer properties like nutrients across boundaries in the water. 

On this cruise Beaird spends much of his time adjusting his instruments, making sure he’s getting good data, and writing code to process that data. Much of Beaird’s work when he returns to OSU will be interpreting the results from this cruise, combined with data from the other teams.

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The white instrument with the “200m” orange tape at the front of the SuperSucker is the Gust, an instrument that measures informs Beaird about turbulence by measuring very fast changes in temperature.

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