August 7, 2000
Recent climate model predictions indicate that rising levels of atmospheric carbon dioxide may trigger a dramatic shift in phytoplankton communities in the Southern Ocean, according to Kevin Arrigo, Assistant Professor of Geophysics at Stanford University.
Phytoplankton, plantlike organisms consisting mostly of algae and bacteria, are the foundation of the marine food chain. Drifting in a nearly invisible mass within the top surface layer of the ocean, they are thought to produce at least 40 percent of the food (i.e., organic carbon) made by photosynthesis on Earth each year. The presence and success of phytoplankton determine the biological characteristics of any ocean region.
Phytoplankton play a significant role in global climate. Since they use carbon dioxide for photosynthesis, they help keep atmospheric levels of the greenhouse gas in check. The larger the world's phytoplankton population, the more carbon dioxide gets pulled from the atmosphere, and the lower the average temperatures on Earth.
But not all phytoplankton are created equal. Arrigo found that some types of phytoplankton dominate waters that are shallow and highly stratified, while others prefer more deeply mixed waters. According to Arrigo, Phaeocystis antarctica (P. antarctica) and diatoms are the principal phytoplankton assemblages throughout Antarctic and Arctic waters. Diatoms, the dominant photosynthetic organisms in the world ocean, abound where freshwater from melting sea ice results in mixed layers that are more shallow and strongly stratified. P. antarctica, single-celled algae that can grow in lower light conditions, tend to populate more deeply mixed waters.
P. antarctica and diatoms differ geochemically, too, in that diatoms don't take up carbon dioxide as efficiently as P. antarctica. In fact, the ratio of carbon uptake to phosphorus uptake for P. antarctica is nearly twice as high as that for diatoms. "Should the phytoplankton community shift from P. antarctica to diatom dominance in response to enhanced upper ocean stratification, the capacity of the biological community to draw down atmospheric carbon dioxide could diminish dramatically," Arrigo said in a recent article in the journal Science.
Phytoplankton typically thrive at or near the sea surface where sunlight is most abundant. But since they can't swim, they are extremely sensitive to ocean dynamics. "Phytoplankton are at the mercy of the currents, so ocean physics determine how fast they will grow," Arrigo said.
Climate models indicate that over the next half century, increased precipitation in the Southern Ocean could have a significant effect on ocean dynamics, particularly ocean surface stratification. Ocean water typically consists of layers that differ in density, temperature, and salinity. The surface layer stratifies, or divides into layers, in response to atmospheric conditions. Stratification increases when surface waters warm, or when precipitation adds freshwater to the surface layer. "Increased stratification in the Southern Ocean is going to favor the diatoms, because they prefer to live in water that is not mixed very well," Arrigo said.
Focusing on the Ross Sea, a small region in the Southern Ocean, Arrigo and colleagues developed a model to examine the relationship between ocean physics and phytoplankton. "Historically, the Ross Sea has been considered one of the most productive regions of the Southern Ocean," he said. "It contains the largest and most persistent concentration of polynyas."
Polynyas, areas of open water surrounded by ice, form in areas where the wind blows the ice away or where warm water moves up from lower depths and melts the ice cover. "It's really difficult to get the models to form polynyas in the right locations, and the biology is very dependent on that," said Arrigo. When sea water freezes, it discharges salt into the ocean. Therefore, Arrigo's ocean circulation model uses sea ice concentration data to calculate salt and heat balances in the ocean, measurements that are essential to simulating water movement.
A long time-series of sea ice concentration data from microwave sensors, available from the National Snow and Ice Data Center DAAC, enabled Arrigo to examine how variations in sea ice cover affect biology. "We used the microwave data to help explain patterns we see in biology," said Arrigo. "Surprisingly, much of the productivity in the Southern Ocean is going on in the ice, not in the water."
Satellite data are an essential ingredient in Arrigo's primary production models, which use light, nutrients, and phytoplankton chlorophyll to estimate biological productivity. To determine phytoplankton concentrations, the research team analyzed ocean color data obtained from the Goddard Space Flight Center DAAC. Ocean color data are collected by satellite sensors that can distinguish variations in ocean color caused by chlorophyll and other plant pigments undetectable by the human eye.
Arrigo was intrigued to find that productivity in the Southern Ocean is higher than originally believed. "It's always been a paradox, because prior estimates of annual primary production in the Southern Ocean were insufficient to support the existing Antarctic food web," he said. The team's large-scale model projected productivity in the Southern Ocean to be four to five times higher than previous estimates made using in situ data.
According to Arrigo, biological responses to predicted changes in ocean dynamics have been poorly understood, but understanding the relationship between phytoplankton community structure and ocean dynamics is an essential part of modeling climate change. "For the first time, we can include the effect of this shift in phytoplankton species dominance in our large-scale models," said Arrigo.
Arrigo sees the Southern Ocean ecosystem as a biological hotbed that, due to its remote location, has been under studied by the research community. "In terms of global biology and chemistry, the Southern Ocean ecosystem is one of the richest regions in the world," said Arrigo. "Lots of people are studying the Pacific or the Indian Ocean, but not many people are studying the Southern Ocean and the Antarctic region, so there are opportunities to make significant impacts quickly," he said.