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Flowers & Power: Algae blooms caused by nutrient runoff are compounding ocean acidity problems

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The effects of carbon dioxide (CO­2) in Earth’s atmosphere are well documented and well understood: as a greenhouse gas, it traps heat from the Sun and contributes to rising temperatures. But CO2 is also making life less comfortable beneath the waves by contributing to rising ocean acidity, which can impact marine life and fisheries that support a $183 million industry in the U.S. Now new research shows that runoff from farms and discharge from sewers is adding CO2 to waterways by feeding algal blooms.

Burning fossil fuels for heat, electricity and transportation is one way CO2 gets into the air and water. In the new Environmental Science & Technology paper, William G. Sunda and Wei-Jun Cai explain that since the Industrial Revolution in the 18th and 19th centuries, the amount of carbon dioxide in the atmosphere has increased 40 percent. The oceans absorb about a third of that, which means that levels of CO2 in the ocean have been rising apace.

When CO2 dissolves in water, it reacts with water to form a number of related compounds, including carbonic acid. This makes ocean water more acidic, which can kill shellfish, deplete food stocks for larger predators, force fish populations to migrate and make it harder for corals to grow.

Another source of CO2 in the oceans is algae, which range from giant kelp and seaweed to microscopic versions that float with the currents. When there is an influx of nutrients, the latter can explode in number, forming so-called algal blooms. These population booms can form a green, yellow or brown scum on the water’s surface, with hundreds or thousands of algae in each teaspoon of water. Infamous red tides are one example. Fertilizer running off fields and organic matter discharged from sewers enter major waterways and end up in the oceans, where blooms can form.

These blooms quickly devour all the available oxygen and can kill off other ocean life, as has happened in the infamous “dead zone” at the mouth of the Mississippi. They also produce large amounts of CO2, contributing to the growing acidification problem.

Sunda and Cai took all this into account to build a computer model of how ocean acidification is likely to proceed. Another piece of the puzzle that they added into their model: ocean acidity is also affected by temperature, which is changing as more CO­2 enters the atmosphere. Between CO2 from the atmosphere and algal blooms, the authors’ model predicts that the oceans will get increasingly acidic.

The effects might be greatest for fisheries in places like the Gulf of Mexico and the Baltic Sea because of the large amount of nutrient input from coastal sources in those areas. The report singles out clams, oysters, scallops and mussels as populations that may be the most heavily impacted.

What other effects of ocean acidification concern you?

“Eutrophication Induced CO2Acidification of Subsurface Coastal Waters: Interactive Effects of Temperature, Salinity, and Atmosp...


Credit: National Oceanic and Atmospheric Administration