Spring Arrives Earlier in the Ocean Too

Warmer oceans are triggering phytoplankton to start their annual blooms up to four weeks earlier than usual—a signal of how climate change can have far-reaching impacts on marine ecosystems.

From 2003 to 2016, scientists at Woods Hole Oceanographic Institution monitored the growth of one of the most abundant phytoplankton in the ocean, a blue-green algae called Synechococcus, at a site off Martha’s Vineyard. In spring, Synechococcus blankets the ocean surface like grass on land, stimulating the base of the marine food chain.

In a study published October 2016 in the journal Science, the scientists reported that every one-degree rise in water temperature stimulated phytoplankton to reproduce sooner and more quickly, shifting the onset of blooms. The study’s lead author, Kristen Hunter-Cevera, recently earned her Ph.D. degree in the MIT-WHOI Joint Program in Oceanography.

To their surprise, the scientists found that as Synechococcus grew more quickly, they were consumed just as fast by tiny protozoa, viruses, or other single-celled organisms that prey on phytoplankton. This poses potential problems for whales, fish, seabirds, and other marine life up the food chain that base their migrations on where and when food is available. These animals may not readily adjust to disruptions in the long-established timing of blooms, Hunter-Cevera said.

“If ocean temperatures continue warming over the next century, we do expect impacts throughout ecosystems, eventually leading to shifts that could affect the livelihoods of larger species,” said WHOI biologist Heidi Sosik, Hunter-Cevera’s Ph.D. advisor, who initiated the long-term study.

The WHOI researchers were able to track the growth rates of Synechococcus, which are roughly one micrometer in diameter, by using an automated underwater instrument developed by Sosik and WHOI colleague Rob Olson, called FlowCytobot. It continually sampled seawater for 13 years, looking for specific characteristics of Synechococcus cells, including certain compounds that glow orange and red under laser light.

“We’re able to measure thousands and thousands of cells every hour, every day, every year, giving us a very high-resolution picture,” Sosik said. “There’s nothing like this out there.”

FlowCytobot could measure Synechococcus at such regular intervals for so long because it was installed at WHOI’s Martha’s Vineyard Coastal Observatory, an underwater platform for instruments off the island’s coast. Undersea cables running between the platform and a shore-based lab supply power to the instruments and send back real-time data.

Sosik said similar devices can be installed at locations around the world to monitor ocean ecosystem changes. “Synechococcus and other phytoplankton are sentinels,” she said. “They can tell us how an ecosystem is responding to shifts in climate.”

This research was funded by the National Science Foundation, the Gordon and Betty Moore Foundation, the National Aeronautics and Space Administration, the U.S. Department of Defense, and WHOI.