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In search of the eastern tropical Pacific’s chlorophyll maximum

The views expressed are those of the author and are not necessarily those of Scientific American.


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shannon-tronickEditor’s Note: Journalist and crew member Kathryn Eident and scientist Jeremy Jacquot are traveling on board the RV Atlantis on a monthlong voyage to sample and study nitrogen fixation in the eastern tropical Pacific Ocean, among other research projects. This is the third blog post detailing this ongoing voyage of discovery for Scientific American.com.

The work of getting the conductivity, temperature and depth (CTD) measurement device safely on deck may be complete, but for many in the science party, the real work is now just beginning.

For Shannon Tronick, it’s critical that she gather her allotted water samples and get busy filtering. She’s measuring chlorophyll so she can plot where the chlorophyll peaks within the water column.

Once it’s safe to go out on deck, Tronick grabs her opaque plastic bottles and gets to work. First, she rinses out the bottles with water from the cast to make sure the sample doesn’t get contaminated. Then she fills her bottles, taking about one liter (at 10 depths) of 240 total liters collected in the cast, and heads inside to begin her analysis. 

In the lab, Tronick’s bench is outfitted with a series of what look like plastic funnels sitting on top of white filters no bigger than a quarter. Behind the filters, a vacuum pump sucks the water she pours in away from the setup, leaving only the chlorophyll behind.

The chlorophyll maximum is a commonly used data set for oceanographers working at sea. Since all plants, algae, and cyanobacteria use chlorophyll in the photosynthesis process, measuring the amount of chlorophyll present in a water sample helps scientists understand how much biology may be present within a given area.

The location of the chlorophyll maximum depends factors like how deep sunlight can penetrate the water column, and how well-mixed the water is. The maximum is usually found anywhere between 150 and 300 feet below the water’s surface, Tronick said. For example, as the ship has moved from 10 degrees South to 15 degrees South, the water has become more murky and she’s found the chlorophyll maximum closer to the surface.

At about the halfway point of the trip, Tronick has already spent many hours filtering for chlorophyll and has many more to go. But for the recent Princeton graduate with a master’s in geosciences, the opportunity to go to sea and participate in Chief Scientist Doug Capone’s project was one she couldn’t miss.

 

"It’s good field and research experience to see what everybody’s doing," she said. "I’m learning a lot of new techniques and seeing the interaction of a huge field project."

Once she’s filtered all the water, she needs to get the chlorophyll off the filters and into a solution that will allow her to run the samples through a machine. To do this, she slides the filter into a tube with acetone and leaves them in a refrigerator for a day. She’s following a protocol developed by her shipmate Jill Sohm, a post-doctorate who received her PhD in Capone’s lab at the University of Southern California.

Tronick’s not sure if she’ll go on for her PhD, or if she’ll join the job market. Her dream is work for NASA designing instruments that will help in both Earth-based and space exploration, she said.

"I’m interested in studying systems where organisms are stressed, and I’m interested in seeing organisms living on the edge in extreme locations." she said.

For Tronick, studying ocean environments holds a similar appeal to research in space, and she can see herself coming to sea again.

"When you think of the ocean you think we have so many things understood about it, because it’s here and it seems accessible. But there’s so much more ocean out there that we haven’t explored," she said. "If you’re not in this field, it seems basic, but when you come into the field of oceanography you realize how many questions have to be answered."

In the lab, once she’s gathered enough filters, Tronick is ready for the machines to read them. She’ll first put them in the spectrophotometer, a machine that assesses how much light the chlorophyll can absorb. Next, she’ll put her samples in the fluorometer and measure how much light, or fluorescence, the chlorophyll emits.

 

Finally, she plugs the information the machines give her into an equation, and voila! She has enough data to generate a graph showing the curve of chlorophyll present in the water column. Other scientists on board can now use this data and she can also compare the data to the readings gathered from the CTD sensors.

"It’s neat to see how you fit into the big picture and how a lot of different focuses can come together to make one big story," she said. "You’re really answering a lot of little questions to get at that one big question."

Image: Shannon Tronick filters water at her work bench in the lab on RV Atlantis.





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  1. 1. eco-steve 2:56 pm 02/16/2010

    Is the chlorphyl maximum directly related to the quantity of dissolved CO2 in the water, itself dependant on water temperature?

    Link to this

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