Diatoms and Dessert

I joined LDEO as a postdoctoral research scientist in January, and having spent most of my life in warm, sunny climates, was fearful of my first real winter in the Northeast. It has been one of the warmest winters in New York in recent history, however, and the boots, wool socks and insulated jacket that I bought have gone largely unused. The lovely spring weather in New York City as I prepared for this cruise was difficult to leave behind, and it will be nearly summer once we return. In the Bering Sea, it still feels like winter.

For the past two days we have sampled water out on deck with snowflakes falling from the sky. The air has become decidedly cooler, only 5 degrees Fahrenheit above freezing with significant wind chill at times. Seawater, of course, does not freeze until 28.4 degrees Fahrenheit, almost 4 degrees below freshwater, so we have yet to run into any ice problems. There may be ice ahead, however, which is impacting our initial cruise plan to head further north into the Bering Sea. Instead, we are completing a grid-like pattern of several stations to the southwest of Unimak Pass, having passed the Fox Islands, including Unalaska where our cruise will terminate, to our south last night as we transited to the first station. If you want to see exactly where we are, you can track the R/V Oscar Dyson here.

We had experienced relatively well-mixed water with very low concentrations of phytoplankton for a few days as we traveled through and around Unimak Pass. This meant we had to collect up to two gallons of water to concentrate enough phytoplankton on filters for later analyses. But yesterday we found ourselves back in water with higher phytoplankton concentrations and some interesting characters! One of the instruments that traveled to Alaska with us is the FlowCAM. The FlowCAM uses a blue laser to detect anything in the water that contains chlorophyll, the primary pigment of photosynthesis. Once a phytoplankton cell is detected, a camera in the FlowCAM takes a picture through a microscope objective, magnifying that cell many times its actual size. Other information, including size, is also collected from each cell. Together, the pictures and sizes of the phytoplankton allow us to generally estimate what types are dominant in a particular sample. For example, the sites we visited over the weekend were dominated by very small sized cells, mostly less than 15 micrometers in diameter or length, and with few distinct characteristics that would allow us to positively identify them.

Yesterday’s station, on the other hand, contained phytoplankton with a broader range of sizes, and some very beautiful members of the diatom class. Diatoms are very common phytoplankton in the world’s oceans and often dominate many of the spring blooms. Diatoms are unique in that they build glass houses for themselves, and they come in all shapes and sizes, existing as either single cells or joined up in colonies. They are typically thought to be tasty food items for many zooplankton and even small filter-feeding fish and shellfish. When a diatom bloom dies, the cells gum up and sink downwards. Zooplankton that feed and grow on diatoms release feces, which also sink. Through sinking, the carbon that diatoms fix through photosynthesis is ultimately exported to the deep ocean from where it cannot easily be returned to the atmosphere. The overall result: removal of carbon dioxide, a greenhouse gas, from the atmosphere.

It is not the same story with other, smaller types of phytoplankton. Instead of sinking deeper, many of these cells remain in the surface ocean. They also are not a preferred food for many zooplankton and therefore do not get exported in feces, as the diatoms do. This can result in an overall recycling of the carbon contained within their cells in the surface ocean and, ultimately, the atmosphere. Earlier studies suggest, and our own broad study confirms, that smaller phytoplankton are more dominant in warm year blooms in the Bering Sea. This presents a potential feedback loop: If a warming climate favors smaller phytoplankton, and smaller phytoplankton keep recycling their carbon back into the atmosphere, then we will continue to see increased carbon dioxide in the atmosphere, warming, and dominant small phytoplankton types, etc., etc. Such feedback loops are common in models for climate change, and they are part of what makes this global problem such an important one for scientists and humans to tackle.

The FlowCAM is made by Fluid Imaging out of Yarmouth, Maine, and we had the pleasure of visiting with them in March for a last minute tune-up of the instrument and refresher of our skills. I am pleased to report that it is operating quite nicely, so we will have to thank Damon and colleagues when we get back for their help. So far it has provided us with more data and images than we can possibly analyze while at sea, and we keep our fingers crossed that the information keeps flowing!

Last night we were mainly transiting to the grid we are now sampling, so a trip to the ship’s gym was on the schedule. The crew of the Oscar Dyson live on the ship for most of the year, and so it is outfitted with nearly everything you need to stay fit and happy, including a workout room, a large selection of movies, and a nicely sized library of books. And for me, when there is delicious homemade pie available at all hours, a little time on the elliptical machine or treadmill is a must!

Happy May Day! Time for science!