As we release more and more carbon dioxide from fossil fuel into the atmosphere, the world’s oceans become more and more acidic. Exactly how this will affect marine life remains unknown, but a paper published this week by marine chemists Keith Hester and his co-authors at the Monterey Bay Aquarium Research Institute is now shedding some light on how a change in acidity affects sound waves under water.

Beluga Whale

So, why is the speed of sound underwater of any interest to Monterey Bay Aquarium researchers? As sounds travel faster, the amount of background noise in the sea will increase and this could affect the behaviour of marine mammals. Many marine mammals, such as whales, dolphins, and porpoises, relay on sounds for communication and food location.

According to conservative projections by the Intergovernmental Panel on Climate Change (IPCC), the chemistry of seawater could change by 0.3 pH units by 2050. According to Hester and his colleges, such a change in acidity would allow sounds to travel up to 70 percent farther underwater in some areas, especially in the Atlantic Ocean. The paper also states that sound may already be travelling 10 percent farther in the oceans than it did a few centuries ago.

According to Hester et al, a change by 0.3 pH units by 2050 will have the greatest effect on sounds below about 3,000 cycles per second. This range includes most of the low frequency sounds that marine mammals are known to use, but it also includes a lot of sounds produced by human activity, such as boating, shipping, and certain military activities. As if acidification of the ocean wasn’t enough, the amount of underwater sound produced by human activities has increased dramatically over the last 50 years. So, even if acidification would make it possible for sound produced by marine mammals to travel farther than ever before, it might also cause these sounds to be effectively drenched by a cacophony of human generated low frequency noise. In such a noisy sea, a marine mammal’s ability to locate prey animals and a suitable mate and could be severely impinged on.

The paper will be published in the October 1, 2008 issue of Geophysical Research Letters.

Compared to just over a century ago, the pH-value of the sea’s surface water has gone down by 0.1 (i.e. 25 percent). This has caught the attention of Jon Havenhand and Michael Thorndyke, researchers at the University of Gothenburg, and they have together with colleagues in Australia studied if and how this decrease affects marine animals.

spatangus purpuerus – Sea urchin

As part of the study, Havenhand and Thorndyke used sea urchins of the species Heliocidaris erythrogramma to study reproduction. Sea urchins reproduce by releasing eggs that are fertilized out in the open water. In the study, Havenhand and Thorndyke studied breeding sea urchins in water where the pH-value had been lowered from its normal 8.1 down to 7.7. This might not sound as a significant drop, but a change from 8.1 to 7.7 means that the water becomes three times as acidic as before.

Havenhand and Thorndyke found out that in this changed environment, the sea urchins’ ability to reproduce was decreased by 25 percent. The low pH-value made the sperm swim slower than normally and move less effectively, which lowered the fertilization rate. But the problems didn’t stop here; when an egg was fertilized, the low pH-value could interfere with larval development and this too decreased the amount of eggs that actually developed into healthy sea urchin larvae.

More research is now needed to find out if these reproductive problems linked to acidification can be observed in other marine animals as well.