Five Questions with Uta Passow, How An Oil Spill Affects the Movement of Carbon In the Ocean
There are millions of tiny drifting plants in the sunlit ocean, called phytoplankton. They produce oxygen that humans end up breathing in and provide food for animals in the plankton (the zooplankton). After death, some of the phytoplankton sink to deeper water, together with some of waste products of zooplankton—taking the carbon with them. Scientists call this transfer of carbon the biological carbon pump.
Uta Passow is a biological oceanographer at the University of California Santa Barbara where she studies how the biological carbon pump works, and how it moved oil released during the Deepwater Horizon spill in 2010.
What is the biological carbon pump?
The biological carbon pump is NOT a pump in the usual sense of the word. The term is used to describe the processes that move carbon from the surface ocean, where there is a lot of carbon, to the deep sea, where there is less.
Phytoplankton, the miniature algae that grow in the light-flooded surface ocean, use carbon dioxide and light to make organic carbon like sugars, fats and proteins through photosynthesis. Some of the phytoplankton get eaten by animals or decomposed by bacteria; when that happens, we say the carbon is respired and as a consequence it is released as carbon dioxide.
The ultimate fate of this carbon depends on the depths it reaches before being respired. If this happens in the surface ocean, it leads to no net change in the carbon budget (the total amount of carbon dioxide stored in the ocean). However, some fraction of the organic carbon sinks into deep water, either directly as phytoplankton or as feces of animals that ate the phytoplankton. It takes more than 100 years for deep-sea water (deeper than 1000 meters) to return to the surface, and so carbon dioxide that is released in the deep ocean is trapped there for 100 years or more.
In this way, the biological carbon pump can influence carbon dioxide concentrations in the atmosphere. So far the ocean has taken up about one-quarter of the fossil carbon released by humans since the industrial revolution.
How did you become interested in the ocean’s biological pump?
When I was a graduate student, scientists were just beginning to figure out that some of the algae and feces produced in the sun-lit upper ocean sink rapidly when they clump together. Individually, these microscopic particles sink very, very slowly and are usually eaten or decomposed before reaching even 100 meters depth. However, my PhD work showed that not all particles present in the surface ocean sink at the same rate—some sink as fast as 100 meters per day—and that the composition of particles in traps on the ocean floor differed from those in the surface ocean.
Both of these mysteries were at least partially explained when marine snow was described by two of my colleagues. Marine snow is made up of large and fast-sinking aggregations of microscopic algae, feces, and dead and decomposing animals. It is called marine snow because the particles look like falling snow to divers. I thought this was fascinating and began investigating how and when marine snow forms and sinks.
How do you track carbon movement in the ocean?
There are quite a few different ways to track carbon in the ocean. One way is to measure carbon in its different forms in the different parts of the ocean over time. We collect water and directly measure the dissolved carbon, and we filter this water to measure the carbon present in small particles like algae, feces, and tiny shells made by plankton. We use nets and underwater pumps to collect slightly larger organisms, and traps to collect sinking particles.
Other approaches rely on our understanding of the biology of the ocean. For example, we know the amount of photosynthesis conducted by marine algae is connected to the nitrogen that is available. So in some regions we can estimate photosynthesis by measuring the amount of nitrogen. Another example is that we can use the greenness of the ocean from satellite images to estimate the amount of plant productivity in the surface ocean. We also use small differences between atoms of the same element (called isotopes) to track and estimate specific parts of the flow of carbon in the ocean. All of these methods have inaccuracies; but because they all have different inaccuracies, taken together they give us a good picture of how carbon moves. That is why oceanography is a science that heavily relies on teamwork!
What does the biological pump have to do with the Gulf oil spill?
The biological pump works 24/7 everywhere, including the Gulf of Mexico. In hindsight, it makes perfect sense that the biological pump would move oil to the deep sea. But because usually one thinks of oil floating upward toward the surface of the ocean, oil spill responders had not really considered the importance of marine snow for the distribution of spilled oil.
Our scientific work on the consequences of the 2010 Deepwater Horizon spill made it clear that a significant amount of the leaked oil was transported to the seafloor, where it impacted corals, benthic fish and all the other animals living there. The oil became trapped in marine snow and then sank to deep water, where we caught it in our traps. Even now it is measurable on the seafloor where it accumulated.
Additionally, it appears that the oil itself strengthened the biological pump. This idea that oil led to the formation and sinking of marine snow at exceptionally high rates has sometimes been called the "dirty blizzard hypothesis." My colleagues and I are currently researching exactly what processes were of importance in creating this huge amount of sinking particles. Understanding the mechanisms that led to oil reaching the seafloor is important to plan the response to a future oil spill. As long as we drill for oil in increasingly challenging environments, spills will happen, even if we take all precautions possible.
Are there other ways that humans impact the workings of the biological pump?
Yes, almost anything that affects the ocean will have impacts on the biological carbon pump. Pollution, for example provides nutrients that frequently result in increased growth of phytoplankton (known as algae blooms). More phytoplankton means more carbon available to sink as marine snow. In areas with little water movement, this can lead to oxygen depletion and a so-called dead zone. The plume of the Mississippi creates such a region in the Gulf of Mexico, for example.
Ocean acidification and warming, which are direct consequences of increased carbon dioxide in the atmosphere, influence growth and reproduction of marine phytoplankton and animals that drive the biological pump. This means there could be changes in the functioning of the pump, although we do not yet understand the net effect.
These are two examples of inadvertent changes to the pump. But some scientists are also studying intentional changes to the pump. Because the biological carbon pump removes carbon from the atmosphere, some people think that increasing its strength could reduce the effects of climate change. A series of iron fertilization experiments were conducted to test this idea. However, they showed that the potential benefits of iron fertilization are likely to be small compared to the potential dangers of such a large-scale modification of the biological carbon pump.
Editor’s note: The research mentioned here related to oil impacts is funded by the Gulf of Mexico Research Initiative.The Ocean Portal receives support from the Gulf of Mexico Research Initiative (GoMRI) to develop and share stories about GoMRI and oil spill science. GoMRI is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. For more information, visit http://gulfresearchinitiative.org/.
Part 1 of a 6-part series describing WHOI's efforts to understand the scope and impact of the Deepwater Horizon oil spill in the Gulf of Mexico. "Oil Spill...