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WATER WORLD
Rising carbon dioxide levels, ocean acidity may change crucial marine process
by Staff Writers
Tallahassee FL (SPX) May 01, 2017


Ocean warming to cancel increased CO2-driven productivity
Adelaide, Australia (SPX) May 01, 2017 - University of Adelaide researchers have constructed a marine food web to show how climate change could affect our future fish supplies and marine biodiversity.

Published in Global Change Biology, the researchers found that high CO2 expected by the end of the century which causes ocean acidification will boost production at different levels of the food web, but ocean warming cancelled this benefit by causing stress to marine animals, preventing them using the increased resources efficiently for their own growth and development. The result was a collapsing food web.

"Humans rely heavily on a diversity of services that are provided by ocean ecosystems, including the food we eat and industries that arise from that," says project leader Professor Ivan Nagelkerken, from the University's Environment Institute.

"Our understanding of what's likely to happen has been hampered by an over-reliance on simplified laboratory systems centred on single levels of the food web. In this study, we created a series of three-level food webs and monitored and measured the results over a number of months to provide an understanding of future food webs under climate change."

The researchers constructed marine food webs based on plants which use sunlight and nutrients to grow (algae), small invertebrates that graze on the plants (such as shrimp), and fish that in turn prey on small invertebrates. They had 12 large aquaria with different species to mimic seagrass, open sand and rocky reef habitats, simulating tidal movements with circular currents.

The food webs were exposed to the levels of ocean acidification and warming predicted for the end of this century. Over several months, the researchers assessed the basic processes that operate in food webs like predation and growth of organisms.

"Elevated carbon dioxide concentrations boosted plant growth; more plant food meant more small invertebrates, and more small invertebrates, in turn, allowed the fish to grow faster," says PhD candidate Silvan Goldenberg, who is supervised by Professor Nagelkerken and Professor Sean Connell.

"However, ocean warming cancelled this benefit of elevated carbon dioxide by causing stress to the animals, making them less efficient feeders and preventing the extra energy produced by the plants from travelling through the food web to the fish. At the same time, fish were getting hungrier at higher temperatures and started to decimate their prey, the small invertebrates."

The researchers found that ocean warming would be an overwhelming stressor that made food webs less efficient, neutralised the 'fertilising' effect of elevated carbon dioxide and threw the fragile relationship between predators and prey off balance.

"The consequences for marine ecosystems are likely to be severe," says Professor Nagelkerken. "Oceans in the future may provide less fish and shellfish for us to eat, and larger animals that are at the top of the food web, in particular, will suffer. We hope this study will provide predictive understanding which is critical for effective fisheries management."

Research paper

Climate change may be putting cyanobacteria that are crucial to the functioning of the ocean at risk as the amount of carbon dioxide in the atmosphere increases and the acidity of ocean water changes.

In a paper published Thursday in Science, a team of researchers from Florida State University, Xiamen University in China and Princeton University argue that the acidification of seawater caused by rising carbon dioxide levels makes it difficult for a type of cyanobacteria to perform a process called nitrogen fixation.

Few people know much about a type of cyanobacteria called Trichodesmium, but this miniscule collection of cells is critical to the health of hundreds of species in the Earth's oceans. Through nitrogen fixation, Trichodesmium converts nitrogen gas into ammonia and other molecules that organisms are dependent on for survival.

Trichodesmium is thought to be responsible for about 50 percent of marine nitrogen fixation, so a decline in its ability could have a major ripple effect on marine ecosystems.

"This is one of the major sources of nitrogen for other organisms in the open ocean," said Sven Kranz, assistant professor of Earth, Ocean and Atmospheric Science at Florida State University and a co-author of this study. "If Trichodesmium responds negatively to the environmental changes forced upon the ocean by fossil fuel burning, it could have a large effect on our food web."

The effects of climate change on Trichodesmium have been studied extensively by scientists in labs across the globe but with widely different results. Some scientists found that increased carbon dioxide in ocean waters caused a decline in nitrogen fixation, while others saw huge increases. Because of the large role these bacteria play in the health of the Earth's oceans, Kranz and his colleagues sought to resolve the discrepancies.

Some of these discrepancies, they found, are based on the preparation of the water in which these organisms typically grow under laboratory conditions. For example, the researchers found contamination by elements such as ammonia or toxic elements like enhanced copper concentration.

"Any slight differences in the specific ingredients of the water - in this case artificial seawater that scientists prepare - can have a huge effect on the outcome," Kranz said.

A slight contamination can throw a huge wrench in the process, yet using this artificial seawater is common because not every lab has access to clean ocean water.

The authors also found that increased carbon dioxide could sometimes stimulate nitrogen fixation but this was offset by the negative effects of the increased ocean acidity.

Kranz began studying how increased carbon dioxide affects cyanobacteria as a researcher in Germany and then as a postdoctoral researcher with Francois Morel and Dalin Shi at Princeton University. Shi is now at Xiamen University and led the study with his research group there.

For this study, Kranz focused on the preliminary data collections and how the cyanobacteria reacted to changing concentrations of iron and carbon dioxide. Shi's group in China conducted further studies including protein analysis and replicated this work in the field, conducting experiments in the South China Sea in May 2016.

WATER WORLD
Vinegar offers hope in Barrier Reef starfish battle
Sydney (AFP) April 27, 2017
Coral-munching crown-of-thorns starfish can be safely killed by common household vinegar, scientists revealed Thursday in a discovery that offers hope for Australia's struggling Great Barrier Reef. The predatory starfish is naturally-occurring but has proliferated due to pollution and run-off at the World Heritage-listed ecosystem, which is also reeling from two consecutive years of mass cor ... read more

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