7 August, 2002
As we move out along the East Hanna Shoal transect, our work time hours shift. Because stations don't take 24 hours when they are shallow, our bottom sampling times moved from 2 AM to 10 PM to last night's work at 7 PM. It's often difficult to remember what day it is and whether it is AM or PM! As always, there are people in the lab at all times. The sampling and processing work always comes first. Meals and sleep follow after the work is done. I find that my journal writing schedule depends on my station schedule. When I have an early morning station (midnight or soon after) I try to get the journal written before I start the station. When the station comes soon after dinner, I usually finish my station work and write the journal in the morning. Yesterday (the 7th) I gathered information for my journal during the afternoon and sampled sediments after dinner. I'm now writing my journal on the morning of the 8th. For all that time, John Christenson has been awake working on his sampling procedure and the processing of his samples! It's that work that I'm going to tell you about.
Although I have said that we are always last at every station, that's not completely true. We go at the end because we muddy the water so much when we sieve out our van Veen grab samples. But, there is one more sediment group on board. (Laura and Karl occasionally sample sediments with their box core, but that is not the main focus of their work.) They are the ones that use the really big corer called the multicore. It's tall enough that they need a step ladder to hook it up to the cable that lowers it into the water. It takes eight cores at a time, each one taller and narrower than the ones we get with the Haps core. Allan Devol, John Christensen and Jason Masters make up this team. Allan is a professor of oceanography at the University of Washington in Seattle where Jason is an oceanographer. John is a research scientist at the Bigelow Laboratory in Booth Bay Harbor, Maine. Their work involves the denitrification rate. Let me give you some background, tell you how they sample, and try to show where it all fits into the big picture.
Nitrogen is a gas that exists in the air we all breath. It's also a very important element for plants, but they can only use it when it is in a different form (i.e. nitrate, nitrite, ammonia). The process of changing nitrogen gas into a form that plants can use is called nitrogen fixation. The amount of fixed nitrogen is very important for the plants (phytoplankton) in oceans. It can be a limiting factor for the phytoplankton which means that they can't grow if there's not enough fixed nitrogen. Fixed nitrogen can be added to the water by nitrogen fixing algae and it can be removed by a process called denitrification (the prefix de means to take away) in which nitrate (the fixed nitrogen) is changed back to nitrogen which escapes into the atmosphere. Most of the denitrification happens in the sediments of the shelf and the slope for a number of reasons. First of all, the amount of oxygen is very low in the sediments. That's because there is little circulation to them up and because the process of breaking down the dead material (decomposition) there uses up oxygen. When the amount of oxygen is low, bacteria in the sediments begin to use nitrate the same way other organisms use oxygen. When they do, they convert it to nitrogen gas that escapes into the water and eventually into the atmosphere. That's denitrification. This all happens mostly on the shelf and the slope because that's where most dead material settles to the bottom.
That's the background; here's the question John, Allan and Jason are trying to answer. Some scientists think there is more denitrification than nitrogen fixation in the ocean. If you understand the process outlined above, you'll realize that this means the oceans are losing more nitrogen than they are getting. (Remember that the ocean plants, the phytoplankton, need nitrogen.) There are other scientists who think that the nitrogen cycle (fixation and denitrification) is balanced. There is simply not enough data to know for sure which one is true. This is extremely important here in the Arctic Ocean because, although the Arctic Ocean makes up only 10% of the world's oceans, it contains 25% of the world's continental shelf and slope area. Whether the cycle is balanced or the Arctic Ocean is losing more nitrogen than it is getting is important if we begin to think of global climate change.
How does this team work? First they collect eight bottom cores at a time with the multicore. They then incubate (put them in an environment where things can grow naturally) for 24 - 48 hours. After that they measure the change in oxygen concentration, the change in nitrogen concentration, and the amount of nutrients present. To do all this they use a gas chromatograph, a mass spectrometer, or tiny oxygen electrodes. Each method checks the data in a slightly different way. They then take another one of their core samples and centrifuge it. Basically that means that they spin the sediments at high speed so the heavier mud stays on the bottom and the water remains on the top. Almost 90% of the sediments is water! They then analyze the remaining water for nutrients. In addition, John analyzes that water and the water from the incubated cores for CO2 (carbon dioxide). That gives them an idea of the activity that can take place in the sediments. They will also do a series of measurements on the remaining mud.
John also adds a substance called potassium bromide to the water over the incubated cores. The concentration of the potassium bromide will change based on the amount of animal activity that is taking place. It will diffuse into the sediments or be 'pulled into" the sediments by the activity of the animals in the mud. This last part is called macrobenthic irrigation (macro = large, benthic = animals on the bottom). When lots of animals are present, there's lots of exchange. And, to tie it all together, when lots of animals are present, and there's lots of exchange, that means there's lots of denitrification! Everything goes back to the central question.
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