By Kirsten Allen
A new study, published online in Proceedings of the National Academy of Sciences, has found that the rate of plankton production off the Northwest coast has decreased since the warming of the earth following the last ice age. However, the amount of organic material that ends its life cycle in the deep ocean has increased, suggesting that during the future warming of the climate we are inevitably going to face, the ocean may be more efficient at absorbing carbon dioxide from the atmosphere than previously believed.
Don’t breathe a sigh of relief quite yet; although that may be good news for the oxygen breathing among us, it raises concerns for the impact on marine life and there remains a major uncertainty on how life in the ocean will adapt to global warming and an increase of CO2. Scientists say that about one third of all CO2 emitted by burning fossil fuels is now in the ocean, which acts as sponge and absorbs carbon dioxide. “This is a good news/bad news situation,” said Alan Mix, an Oregon State University oceanographer and co-author on the study. “It helps to slow the rise of CO2 in the atmosphere, but it makes the ocean more acidic.”
The growth of microscopic plants, known as phytoplankton, near the surface of the ocean converts carbon dioxide into organic matter. When the plankton die, their remains either decompose at the surface, or sink to the bottom. The sinking of dead plankton is a very effective method of removing CO2 from the atmosphere. “It has been assumed that the amount of organic material that sinks to the sea floor would parallel that produced through photosynthesis near the sea surface,” said Mix, who is in OSU’s College of Earth, Ocean, and Atmospheric Sciences. “Surprisingly, our study found that even as plant growth decreased, past warming actually enhanced the biological export of carbon to the deep sea, at least in the northeast Pacific.”
Lead author Cristina Lopes, a visiting scientist at Oregon State who is based at the Instituto Português do Mar e da Atmosfera (IPMA, Portuguese Sea and Atmosphere Institute) in Portugal, and colleague Michal Kucera at the Center for Marine Environmental Sciences at Germany’s University of Bremen, applied a neural network method which is now used by financial and insurance industries, to analyze fossil diatoms off the Oregon coast, allowing them to calculate productivity of marine plankton during the last major global warming event. “Inspired by brain research, we adapted these machine learning methods to analyze the fossil record for a new view of how the ocean works,” Kucera said.
Rather than being exported to the deep ocean, researchers found that during the last ice age, carbon that was trapped in plankton off the coast of Oregon was mostly recycled. As the ocean began to warm during the waning ice age, plant growth decreased while carbon export increased. “This counterintuitive effect was driven by shift in ecosystems to one dominated by large diatoms,” Lopes said. “Those diatoms bloomed, than sank fast when they died.”
According to the researchers, these finding don’t necessarily mean the ocean can continue to absorb CO2 indefinitely. Additionally, although the export of carbon may help to slow down global climate change, it may worsen marine life conditions. As the sinking matter decomposes it consumes oxygen, which raises concerns since low oxygen dead zones have been appearing off the Oregon coast in recent years. “If these connections between warming and enhanced carbon export that we’ve found in past climate changes are triggered in the future, we can expect those marine dead zones to show up more frequently,” says Mix.