SCIENTISTS STUDY CORAL REEFS AND OCEAN ACIDIFICATION

FROM:  NATIONAL SCIENCE FOUNDATION
Coral reefs defy ocean acidification odds in Palau
Palau reefs show few of the predicted responses

Will some coral reefs be able to adapt to rapidly changing conditions in Earth's oceans? If so, what will these reefs look like in the future?

As the ocean absorbs atmospheric carbon dioxide (CO2) released by the burning of fossil fuels, its chemistry is changing. The CO2 reacts with water molecules, lowering ocean pH (making it more acidic) in a process known as ocean acidification.

This process also removes carbonate, an essential ingredient needed by corals and other organisms to build their skeletons and shells.

Scientists are studying coral reefs in areas where low pH is naturally occurring to answer questions about ocean acidification, which threatens coral reef ecosystems worldwide.

Palau reefs dodge ocean acidification effects

One such place is Palau, an archipelago in the far western Pacific Ocean. The tropical, turquoise waters of Palau's Rock Islands are naturally more acidic due to a combination of biological activity and the long residence time of seawater in their maze of lagoons and inlets.

Seawater pH within the Rock Island lagoons is as low now as the open ocean is projected to reach as a result of ocean acidification near the end of this century.

A new study led by scientists at the Woods Hole Oceanographic Institution (WHOI) found that coral reefs in Palau seem to be defying the odds, showing none of the predicted responses to low pH except for an increase in bio-erosion--the physical breakdown of coral skeletons by boring organisms such as mollusks and worms.

A paper reporting the results is published today in the journal Science Advances.

"This research illustrates the value of comprehensive field studies," says David Garrison, a program director in the National Science Foundation's Division of Ocean Sciences, which funded the research through NSF's Ocean Acidification (OA) Program. NSF OA is supported by the Directorates for Geosciences and for Biological Sciences.

"Contrary to laboratory findings," says Garrison, "it appears that the major effect of ocean acidification on Palau Rock Island corals is increased bio-erosion rather than direct effects on coral species."

Adds lead paper author Hannah Barkley of WHOI, "Based on lab experiments and studies of other naturally low pH reef systems, this is the opposite of what we expected."

Experiments measuring corals' responses to a variety of low pH conditions have shown a range of negative effects, such as fewer varieties of corals, more algae growth, lower rates of calcium carbonate production (growth), and juvenile corals that have difficulty constructing skeletons.

"Surprisingly, in Palau where the pH is lowest, we see a coral community that hosts more species and has greater coral cover than in the sites where pH is normal," says Anne Cohen, co-author of the paper.

"That's not to say the coral community is thriving because of the low pH, rather it is thriving despite the low pH, and we need to understand how."

When the researchers compared the communities found on Palau's reefs with those in other reefs where pH is naturally low, they found increased bio-erosion was the only common feature.

"Our study revealed increased bio-erosion to be the only consistent community response, as other signs of ecosystem health varied at different locations," Barkley says.

The riddle of resilience

How do Palau's low pH reefs thrive despite significantly higher levels of bio-erosion?

The researchers aren't certain yet, but hope to answer that question in future studies.

They also don't completely understand why conditions created by ocean acidification seem to favor bio-eroding organisms.

One theory--that skeletons grown under more acidic conditions are less dense, making them easier for bio-eroding organisms to penetrate--is not the case on Palau, Barkley says, "because we don't see a correlation between skeletal density and pH."

Though coral reefs cover less than one percent of the ocean, these diverse ecosystems are home to at least a quarter of all marine life. In addition to sustaining fisheries that feed hundreds of millions of people around the world, coral reefs protect thousands of acres of coastlines from waves, storms and tsunamis.

"On the one hand, the results of this study are optimistic," Cohen says. "Even though many experiments and other studies of naturally low pH reefs show that ocean acidification negatively affects calcium carbonate production, as well as coral diversity and cover, we are not seeing that on Palau.

"That gives us hope that some coral reefs--even if it is a very small percentage--might be able to withstand future levels of ocean acidification."

Along with Barkley and Cohen, the team included Yimnang Golbuu of the Palau International Coral Reef Center, Thomas DeCarlo and Victoria Starczak of WHOI, and Kathryn Shamberger of Texas A&M University.

The Dalio Foundation, Inc., The Tiffany & Co. Foundation, The Nature Conservancy and the WHOI Access to the Sea Fund provided additional funding for this work.

-NSF-

PLANKTON BLOOM FINDS WAY TO MOVE DOWN

FROM:  NATIONAL SCIENCE FOUNDATION
Spring plankton bloom hitches ride to sea's depths on ocean eddies

Eddies--whirlpools within currents--transport plankton downward from the ocean surface
Just as crocus and daffodil blossoms signal the start of a warmer season on land, a similar "greening" event--a massive bloom of microscopic plants, or phytoplankton--unfolds each spring in the North Atlantic Ocean from Bermuda to the Arctic.

Fertilized by nutrients that have built up during the winter, the cool waters of the North Atlantic come alive every spring and summer with a vivid display of color that stretches across hundreds and hundreds of miles.

North Atlantic Bloom turns ocean into sea of plankton

In what's known as the North Atlantic Bloom, millions of phytoplankton use sunlight and carbon dioxide (CO2) to grow and reproduce at the ocean's surface.

During photosynthesis, phytoplankton remove carbon dioxide from seawater and release oxygen as a by-product. That allows the oceans to absorb additional carbon dioxide from the atmosphere. If there were fewer phytoplankton, atmospheric carbon dioxide would increase.

Flowers ultimately wither and fade, but what eventually happens to these tiny plants produced in the sea? When phytoplankton die, the carbon in their cells sinks to the deep ocean.

Plankton integral part of oceanic "biological pump"

This so-called biological pump makes the North Atlantic Ocean efficient at soaking up CO2 from the air.

"Much of this 'particulate organic carbon,' especially the larger, heavier particles, sinks," says scientist Melissa Omand of the University of Rhode Island, co-author of a paper on the North Atlantic Bloom published today in the journal Science.

"But we wanted to find out what's happening to the smaller, non-sinking phytoplankton cells from the bloom. Understanding the dynamics of the bloom and what happens to the carbon produced by it is important, especially for being able to predict how the oceans will affect atmospheric CO2 and ultimately climate."

In addition to Omand, other authors of the paper are Amala Mahadevan of the Woods Hole Oceanographic Institution, Eric D'Asaro and Craig Lee of the University of Washington, and Mary Jane Perry, Nathan Briggs and Ivona Cetinic of the University of Maine.

The research was funded by the National Science Foundation (NSF).

"It's been a challenge to estimate carbon export from the ocean's surface waters to its depths based on measurements of properties such as phytoplankton carbon," says David Garrison, program director in NSF's Division of Ocean Sciences. "This paper describes a mechanism for doing that."

Tracking a bloom: Floats, gliders and other instruments

During fieldwork from the research vessels Bjarni Saemundsson and Knorr, the scientists used a float to follow a patch of seawater off Iceland. They observed the progression of the bloom by taking measurements from multiple platforms.

Autonomous gliders outfitted with sensors were used to gather data such as temperature, salinity and information about the chemistry and biology of the bloom--oxygen, nitrate, chlorophyll and the optical signatures of the particulate matter.

At the onset of the bloom and over the next month, four teardrop-shaped seagliders gathered 774 profiles to depths of up to 1,000 meters (3,281 feet).

Analysis of the profiles showed that about 10 percent had unusually high concentrations of phytoplankton bloom properties, even in deep waters, as well as high oxygen concentrations usually found at the surface.

"These profiles were showing what we initially described as 'bumps' at depths much deeper than phytoplankton can grow," says Omand.

Staircases to the deep: Ocean eddies

Using information collected at sea by Perry, D'Asaro and Lee, Mahadevan modeled ocean currents and eddies ("whirlpools" within currents) and their effects on the spring bloom.

"What we were seeing was surface water, rich with phytoplankton carbon, being transported downward by currents on the edges of eddies," Mahadevan says.

"Eddies hadn't been thought of as a major way organic matter is moved into the deeper ocean. But this type of eddy-driven 'subduction' could account for a significant downward movement of phytoplankton from the bloom."

In related work published in Science in 2012, the researchers found that eddies act as early triggers of the North Atlantic Bloom.

Eddies help keep phytoplankton in shallower water where they can be exposed to sunlight to fuel photosynthesis and growth.

Next, the scientists hope to quantify the transport of organic matter from the ocean's surface to its depths in regions beyond the North Atlantic and at other times of year and relate that to phytoplankton productivity.

Learning more about eddies and their link with plankton blooms will allow for more accurate global models of the ocean's carbon cycle, the researchers say, and improve the models' predictive capabilities.

-NSF-
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Cheryl Dybas, NSF

REPORT: CORRELATION BETWEEN GLOBAL CLIMATE AND ATMOSPHERIC CARBON DIOXIDE LEVELS

The phytoplankton Emiliania huxleyi offers new clues about climate past, present and future. Photo Credit:  Wikimedia.

June 6, 2012

Until now, studies of Earth's climate have documented a strong correlation between global climate and atmospheric carbon dioxide; that is, during warm periods, high concentrations of CO₂ persist, while colder times correspond to relatively low levels.

However, in this week's issue of the journal Nature, paleoclimate researchers reveal that about 12-5 million years ago climate was decoupled from atmospheric carbon dioxide concentrations. New evidence of this comes from deep-sea sediment cores dated to the late Miocene period of Earth's history.

During that time, temperatures across a broad swath of the North Pacific were 9-14 degrees Fahrenheit warmer than today, while atmospheric carbon dioxide concentrations remained low--near values prior to the Industrial Revolution.

The research shows that, in the last five million years, changes in ocean circulation allowed Earth's climate to become more closely coupled to changes in carbon dioxide concentrations in the atmosphere.

The findings also demonstrate that the climate of modern times more readily responds to changing carbon dioxide levels than it has during the past 12 million years.

"This work represents an important advance in understanding how Earth's past climate may be used to predict future climate trends," says Jamie Allan, program director in the National Science Foundation's (NSF) Division of Ocean Sciences, which funded the research.

The research team, led by Jonathan LaRiviere and Christina Ravelo of the University of California at Santa Cruz (UCSC), generated the first continuous reconstructions of open-ocean Pacific temperatures during the late Miocene epoch.

It was a time of nearly ice-free conditions in the Northern Hemisphere and warmer-than-modern conditions across the continents.

The research relies on evidence of ancient climate preserved in microscopic plankton skeletons--called microfossils--that long-ago sank to the sea-floor and ultimately were buried beneath it in sediments.

Samples of those sediments were recently brought to the surface in cores drilled into the ocean bottom.  The cores were retrieved by marine scientists working aboard the drillshipJOIDES Resolution.

The microfossils, the scientists discovered, contain clues to a time when the Earth's climate system functioned much differently than it does today.

"It's a surprising finding, given our understanding that climate and carbon dioxide are strongly coupled to each other," LaRiviere says.

"In the late Miocene, there must have been some other way for the world to be warm. One possibility is that large-scale patterns in ocean circulation, determined by the very different shape of the ocean basins at the time, allowed warm temperatures to persist despite low levels of carbon dioxide."

The Pacific Ocean in the late Miocene was very warm, and the thermocline, the boundary that separates warmer surface waters from cooler underlying waters, was much deeper than in the present.

The scientists suggest that this deep thermocline resulted in a distribution of atmospheric water vapor and clouds that could have maintained the warm global climate.

"The results explain the seeming paradox of the warm--but low greenhouse gas--world of the Miocene," says Candace Major, program director in NSF's Division of Ocean Sciences.

Several major differences in the world's waterways could have contributed to the deep thermocline and the warm temperatures of the late Miocene.

For example, the Central American Seaway remained open, the Indonesian Seaway was much wider than it is now, and the Bering Strait was closed.

These differences in the boundaries of the world's largest ocean, the Pacific, would have resulted in very different circulation patterns than those observed today.

By the onset of the Pliocene epoch, about five million years ago, the waterways and continents of the world had shifted into roughly the positions they occupy now.

That also coincides with a drop in average global temperatures, a shoaling of the thermocline, and the appearance of large ice sheets in the Northern Hemisphere--in short, the climate humans have known throughout recorded history.

"This study highlights the importance of ocean circulation in determining climate conditions," says Ravelo. "It tells us that the Earth's climate system has evolved, and that climate sensitivity is possibly at an all-time high."

Other co-authors of the paper are Allison Crimmins of UCSC and the U.S. Environmental Protection Agency; Petra Dekens of UCSC and San Francisco State University; Heather Ford of UCSC; Mitch Lyle of Texas A&M University; and Michael Wara of UCSC and Stanford University.