Amanda Dice: Ending Week 1 at Line 8, August 26, 2017


NOAA Teacher at Sea

Amanda Dice

Aboard Oscar Dyson

August 21 – September 2, 2017

 

Mission: Juvenile Pollock Fishery Survey

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Oscar Dyson moves across the Shelikof Straight to collect the Line 8 samples

Geographic area of cruise: Western Gulf of Alaska

Date: August 26, 2017

Weather Data: 13.2 C, cloudy with light rain

Latitude 57 36.6 N, Longitude 155 .008 N

 

 

Science and Technology Log

As part of this survey, the scientists onboard collect data from what is known as “Line 8”. This is a line of seven sampling stations, positioned only a few miles apart, near the southern opening of Shelikof Straight between Kodiak Island and the Alaskan Peninsula. Water samples are taken at different depths at each sampling station to measure several different properties of the water. This study is focused on profiling water temperature and salinity, and measuring the quantities of nutrients and phytoplankton in the water.

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The CTD rosette is lowered into the water using a winch – as seen from above.

To collect this data, a conductivity and temperature at depth (CTD) instrument is lowered into the water. This instrument can take water samples at different depths, by using its eleven canisters, or Niskin bottles. The water collected in the Niskin bottles will be used to determine the nutrient quantities at each station. The rosette of Niskin bottles also has sensors on it that measure phytoplankton quantities, depth, temperature, and how conductive the water is. Scientists can use the readings from conductivity and temperature meters to determine the salinity of the water.

Each Niskin bottle has a stopper at the top and the bottom. The CTD goes into the water with both ends of each Niskin bottle in the open position. The CTD is then lowered to a determined depth, depending on how deep the water is at each station. There is a depth meter on the CTD that relays its position to computers on board the ship. The survey team communicates its position to the deck crew who operate the winch to raise and lower it.

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Niskin bottles are lowered into the water with the stoppers at both ends open.

When the CTD is raised to the first sampling depth, the survey crew clicks a button on a monitor, which closes the stoppers on both ends of Niskin bottle #1, capturing a water sample inside. The CTD is then raised to the next sampling depth where Niskin bottle #2 is closed. This process continues until all the samples have been collected. A computer on board records the depth, conductivity and temperature of the water as the CTD changes position. A line appears across the graph of this data to show where each sample was taken. After the Niskin bottles on the CTD are filled, it is brought back onto the deck of the ship.

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They let me take control of closing the Niskin bottles at the sampling depths!

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I used this screen to read the data coming back from the CTD and to hit the bottle to close each Niskin bottle. The purple horizontal lines on the graph on the right indicate where each one was closed.

Water is collected through a valve near the bottom of each Niskin bottle. A sample of water from each depth is placed in a labeled jar. This study is interested in measuring the quantity of nutrients in the water samples. To do this it is important to have samples without phytoplankton in them. Special syringes with filters are used to screen out any phytoplankton in the samples.

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Syringes with special filters to screen out phytoplankton are used to collect water samples from the Niskin bottles.

The “Line 8” stations have been sampled for nutrient, plankton, and physical water properties for many years. The data from the samples we collected will be added to the larger data set maintained by the Ecosystems and Fisheries-Oceanography Coordinated Investigations (Eco-FOCI), Seattle, Washington. This NOAA Program has data on how the marine ecosystem in this area has changed over the last few decades. When data spans a long time frame, like this study does, scientists can identify trends that might be related to the seasons and to inter-annual variation in ocean conditions. The samples continue to be collected because proper nutrient levels are important to maintaining healthy phytoplankton populations, which are the basis of most marine food webs.

 

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Collecting water samples from a Niskin bottle.

Personal Log

As we travel from one station to the next, I have some time to talk with other members of the science team and the crew. I have really enjoyed learning about places all over the world by listening to people’s stories. Most people aboard this ship travel many times a year for their work or have lived in remote places to conduct their scientific studies. Their stories inspire me to keep exploring the planet and to always search for new things to learn!

Did you know?

Niskin bottles must be lowered into the water with both ends open to avoid getting an air bubble trapped inside of them. Pressure increases as depth under water increases. Niskin bottles are often lowered down below 150 meters, where the pressure can be intense. If an air bubble were to get trapped inside, the pressure at these depths would cause air bubble to expand so much that it might damage the Niskin bottle!

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