National Science Foundation
October 20, 2016
Researchers track effects of changing ocean temperature on
shows changes could have major effects on global marine ecosystems
Changes in ocean temperature
affect a key species of phytoplankton, according to scientists at the Woods
Hole Oceanographic Institution (WHOI) funded by the National Science Foundation
Their study, published in this week's issue of the journal Science,
tracked levels of Synechococcus -- a
tiny bacterium common in marine ecosystems -- near the coast of Massachusetts
over a 13-year period.
"Synechococcus and other phytoplankton are
sentinels," says WHOI biologist Heidi Sosik, who initiated the study.
"They tell us how an ecosystem is responding to shifts in climate."
As ocean temperatures increased during the study period, annual
blooms of Synechococcus occurred
up to four weeks earlier, researchers found, because cells divide faster in
warmer waters. Such shifts could have major effects on marine ecosystems
worldwide, according to Sosik.
If ocean temperatures continue warming over the next century,
some ecosystems could become dominated by small phytoplankton, eventually
leading to changes that could affect larger species like fish, whales and
birds, Sosik says.
That growth in smaller phytoplankton isn't a sure thing,
"Now that we have the appropriate technology to study
phytoplankton on time scales of hours to weeks, we're gaining a much better
understanding of what controls phytoplankton populations in coastal ocean
ecosystems," says David Garrison, program director in NSF's Division of
Ocean Sciences, which funded the research.
Although Sosik and colleagues saw that Synechococcus cells reproduced faster than usual as
conditions warmed, the overall size of the phytoplankton bloom didn't increase
much during the course of the study.
As the bacteria grew more quickly, tiny protozoa, viruses and
other single-celled organisms that prey on Synechococcus consumed them more quickly.
"That was a surprise to us," says Sosik. "We
didn't expect such a tight lockstep among Synechococcusand
its consumers as the spring bloom changed. It shows that the consumers are able
to keep up."
This balance of bacteria and consumers leads to a similar bloom
cycle year after year, but with a shift in timing earlier or later as the water
temperature changes, Sosik says.
"The question is: How stable is that balance?" asks
Kristen Hunter-Cervera, lead author of the paper and a graduate of the MIT-WHOI
Joint Program in Oceanography. "In the future, will consumer species be
able to keep up? A mismatch is a huge concern. If the bloom expands, or moves
earlier in the year, higher-level organisms that feed on those consumers at a
certain time of year might miss them entirely."
The team was able to determine division rates of Synechococcus by using an automated sensor called
"FlowCytobot," developed by Sosik and WHOI colleague Rob Olson.
FlowCytobot continually sampled seawater for 13 years.
The device looks for the physical characteristics of Synechococcus cells, which are roughly one micrometer
in diameter and contain compounds that glow orange and red under laser light.
Using this method to tally cells has allowed the researchers to
home in on just one species of phytoplankton among thousands in seawater -- the
first time such a long-term experiment has been able to do so.
"Looking at physiology at the species level is a holy grail
in marine ecology," Sosik says. "Each species interacts with its
environment in a different way, so to understand the effects of something like
temperature, it's critical to be able to study a single species. Doing that
every hour, every day, every year gave us a very high-resolution picture.
There's nothing like this out there."
The Gordon and Betty Moore Foundation, NASA, a National Defense
Science and Engineering graduate fellowship from the U.S. Department of
Defense, and WHOI also supported the work.