My series on ocean acidification continues this week with a look at the cutting-edge research of Jason Hall-Spencer from the University of Plymouth. I'll begin with a short video, part of CBC's One Ocean series, about Hall-Spencer at his main research site at the oceanic base of Mount Vesuvius.
There are only a few places in the ocean where pure carbon-dioxide (CO2) is pumped into the water at high enough levels to mimic conditions expected from ocean acidification, allowing for a glimpse into "futuristic" ecosystems. Underwater vents are generally poor places to study the effects of acidification due to complicating factors such as high temperature and hydrogen sulphide — so the unique characteristics of these sites are extremely valuable. Hall-Spencer has been surveying for several years, examining the differences in the aquatic communities along a CO2 gradient from acidified (pH 7.8) to normal levels (pH 8.1) nearby. He presented his latest results at the recent American Association for the Advancement of Science (AAAS) meeting in Vancouver.
Sign up for our newsletter
The bottom line of Hall-Spencer's results so far: acidification is bad news for biodiversity. A 30 per cent decrease in the number of species is observed at levels of acidification expected to occur across oceans later this century. Both calcifying and non-calcifying organisms (such as urchins, snails and fish) are adversely affected by acidification, allowing invasive algae to thrive in their place. Transformation of diverse benthic ecosystems into monocultures of invasive algae and seagrass is observed at CO2 vents in Mexico, New Guinea and the Mediterranean, a strong indication of what we can expect to see in the future. Although many large fish species are present in the acidified areas, they have not been observed to deposit their eggs in the algae and eelgrass, opting instead to lay eggs among coralline algae at normal pH levels.
Armed with the knowledge that acidified areas are very different than areas of normal pH, Hall-Spencer wanted to understand why the communities transform. What allows some organisms to thrive while others fail? He embarked on a set of transplant experiments, moving species between areas of high and low CO2 to observe reactions to altered conditions. Limpets make for an interesting study organism in this regard because their calcified shells grow by adding new material to the outer periphery. New shell growth in the acidified environment can therefore be directly compared to the shell that was present before. Although shell material created in the acidified environments was weaker, limpets maintained the ability to up-regulate calcium into their shells at the acidified sites during the winter months. In other words, they kept growing and didn't perish. At first, this appeared to be excellent news. However, the complete absence of limpets in acidified areas of his observational studies kept Hall-Spencer cautious enough to keep the experiments running for the duration of the year.
Good news turns bad
Despite the fact that in the winter months the limpets were able to continue growing at the acidified sites, the story completely changed in the summer. When exposed to the double whammy of both increased water temperature and decreased pH, all of the limpets died. These results are among the first to demonstrate that the effects of acidification may not become apparent until multiple stressors are involved. It is therefore critical to understand both the direct impacts of acidification (i.e., changes to shell characteristics) as well as the indirect effects (i.e., metabolic stress that causes organisms to be sensitive to other environmental changes). The work presented by Hall-Spencer at the recent AAAS meeting in Vancouver is just the tip of the iceberg. I look forward to hearing more results on the transplant experiments, because understanding the mechanisms by which ecosystems change is a critical part in predicting how oceans will look in the future.