Decline In Microscopic Plant Life Could Affect The Atmosphere (VIDEO)

First Posted: Sep 25, 2015 10:52 AM EDT

The base of the marine food chain is in trouble, it seems, and that means that our atmosphere may be, too.

A recent study, published in Global Biogeochemical Cycles, by NASA has found a significant decline in certain microscopic plant life. This was the first study to look at global, long-term phytoplankton community trends using NASA's Ocean Biogeochemical Model, which recreates the conditions in the ocean, from its currents to the amount of sunlight and nutrients available in different ocean basins, according to a release.

From 1998 to 2012, there has been a one percent global decrease per year of the largest of the phytoplankton algae, known as diatoms. Significant losses have occurred in the North Pacific, North Indian and Equatorial Indian oceans. This reduction could reduce the amount of carbon dioxide that's pulled from the atmosphere and put into the ocean's depths for long-term storage.

"Phytoplankton need carbon dioxide for photosynthesis, just like trees," lead author and oceanographer Cecile Rousseaux, of Universities Space Research Association and NASA's Goddard Space Flight Center, said.

A phytoplankton bloom can span hundreds of miles, and can be seen from space. When they bloom, they take carbon dioxide (that has dissolved in cold ocean water) and convert it into organic carbon, the base of the marine food web, a form that animals can use as food to grow.

Then when the phytoplankton cell dies, it sinks to the ocean floor, taking with it the carbon in its body. Diatoms sink more quickly when they die than smaller types of phytoplankton. Some of them circulate back to the ocean's surface and fertilize another bloom, but the rest will rest on the sea floor, where they accumulate in sediment, to be stored for thousands, or millions, of years.

The decline in diatoms is one of several regional shifts observed in four types of phytoplankton in the 15-year study period, according to the study.

Rousseaux and her team took measurements of chlorophyll from NASA's Sea-viewing Wide Field of View Sensor (SeaWiFS) that flew above the earth on two separate satellites from 1997 to the present. The data shows the combined amount of chlorophyll of all types of phytoplankton.

There is one big challenge in studying phytoplankton. Satellites cannot always absolutely distinguish different types of phytoplankton, and samples from ships, and other direct measurements, obviously cannot observe the entirety of the globe's oceans.

"Inclusion of satellite data into this kind of biogeochemical modeling is really exciting," oceanographer Jeremy Werdell at Goddard, who was not involved with the study, said. "This kind of tool allows you to start exploring that problem in a way we're not able to do by using just a satellite alone, or just a model alone. By combining satellite data, models, and additional environmental information, you can start telling a more holistic story."

The diatom declines were found to be due to the mixed layer, or top layer, of the ocean becoming shallower. Taking into account variations of seasonal levels, the layer shallowed by 5.9 feet over the 15 years of the study.

The mixed layer draws up nutrients from a deeper layer of water below, then receives sunlight, which promotes phytoplankton growth. But, a shallower mixed layer has less volume, and therefore can hold fewer nutrients.

"The phytoplankton can run out of nutrients," said Rousseaux, which is what the model showed. One possible reason for the changes are altered winds, which can cause churning of the mixed layer.

Rousseau said that while the diatom decline is not yet considered severe, it is something to monitor in the future. Ocean conditions change, whether due to natural variation or climate change, and that may cause future declines.