Blog 9 – Biogeochemical Cycles

How fast is the ocean changing? Animals and plants in the ocean hold the key to this broad question. What do animals in the ocean eat? How much and at what time? Can they adapt to climatic changes? If so, how?

These are some of the questions that drive the research conducted on this vessel. In order to get a better understanding of the interactions between living organisms and their environments scientists look for clues in the biogeochemical cycles.

You have probably heard of the water cycle or hydrologic cycle. It is a never-ending process that takes water through many phases throughout the Earth’s spheres. These include the hydrosphere, atmosphere, geosphere, and biosphere. For example, surface water in a lake evaporates into the atmosphere. As water condenses, it forms clouds and then returns to land as precipitation (rain or snow).

Other very important cycles are called biogeochemical cycles, which include the carbon, nitrogen, and iron cycles. By studying how these elements move through Earth’s spheres through an oceanographer’s eye, we obtain clues about the changes that take place in the ocean over time.


Dr. Mike Stukel from Florida State University uses various methods to study the cycling of carbon in the ocean.

Dr. Mike Stukel, a Scripps graduate now based at Florida State University, seeks to better understand how plankton process carbon from the atmosphere into the ocean and the processes that also pump carbon from ocean surface waters into the deep ocean.

At the surface level of the ocean, phytoplankton use the carbon dioxide found in the atmosphere for photosynthesis. Then zooplankton consume the phytoplankton. Some of the carbon is used to make their bodies and some is released as waste. This waste can sink to the bottom of the ocean in the form of marine snow, so-called for its white snow-like characteristics. The carbon within the waste can be broken down by bacteria, remain at the bottom of the ocean for thousands of years, or it can be upwelled and used again.

One way to determine this rate of particle sinking is by studying the decay of elements such as Uranium- 238 and Thorium- 234. Thorium- 234 sticks to particles that sink to the bottom of the ocean. Through his experiments, Dr. Stukel can determine the rate that carbon is being taken out of the ocean.


Brandon Stephens, Ph.D. student at Scripps Institution of Oceanography filters sea water to collect organic material for later analysis.

Brandon Stephens, Ph.D. student at Scripps, is also interested in the carbon cycle. He focuses his research on dissolved organic matter. This matter that comes from all living things is made up of carbohydrates, amino acids, and lipids. His first step is to filter many gallons of sea water to collect enough organic material. The filter is removed and frozen. When Brandon returns to land he will analyze the filters by combustion. In other words he will burn each of the filters. By doing this, he will be able to trap the gas released as carbon dioxide and calculate the amount of carbon per liter of sea water.

Nitrogen is another key element studied here. Nitrogen is very important because it is used by every living organism and forms part of every cell in the human body. Almost 80 percent of the atmosphere is made up of nitrogen gas but we cannot use it. It has to be “fixed” into a suitable form by special types of plants or algae. Or the nitrogen in once-living organisms can be broken down into the correct form by even smaller organisms such as bacteria.


Alain de Verneil, a Scripps Ph.D. candidate, measures the amount of dissolved nitrogen in the ocean.


The deep pink colors show the higher concentrations of nitrogen at increasing depths.

Finally, the iron cycle is researched by Dr. Kathy Barbeau from Scripps Institution of Oceanography. This important element is required by almost all organisms in the ocean during respiration. It is an element necessary for all living things, including humans. On the earth’s crust iron is an abundant metal but in the ocean it is present only in very low concentrations.


Dr. Kathy Barbeau from Scripps Institution of Oceanography, studies the cycling of iron.

Collecting sea water samples and processing them in Kathy’s laboratory at sea is no easy task. One big concern for contamination of her samples is our ship! It is made completely out of steel, including the ship’s hull and structures on deck, as well as cables that may have rusted. This rust is a form of iron that could easily contaminate samples.


Trace metal rosette mostly made out of plastic. Its bottles, screws, and cables have a coating that lowers the risk of iron contamination.