Tatoosh and Ocean Acidification

Long-term studies are rare – the costs in time, effort, enthusiasm, persistence and funding are all formidable. But they are as valuable as they are rare.

One such study, stretching back five decades, is the research on the intertidal community of Tatoosh island, off the northwestern-most point of Washington State, at the mouth of the Strait of Juan de Fuca. Like other long-term studies it has depended on the initial and long-term research of a particular scientist and then his graduate students, and then theirs. In this case Robert Paine started the work, Timothy Wootton and Catherine Pfister are among his graduates students, and their graduate students continue to work with them on the island.

Tatoosh Island from the air – an old lighthouse, some steep cliffs, a few trees, and an extensive and accessible intertidal studied intensely since the 1960s (pbase.com)

The research on Tatoosh has given us insights into how predation and competition structure a community of species, including the concept of keystone species. Recently it has also provided critical evidence of current ocean acidification and correlated changes in the intertidal community.

Because the community has been so well studied for so long, changes in distribution, occurrence and sizes of individuals within populations are possible to recognize when they occur. For more than a decade now Pfister and Wootton have also measured ocean pH levels in great detail. What they are seeing is very troubling.

The intertidal of Tatoosh Island. Cape Flaherty on the Washington mainland is in the background (esa.org)

Concerning ocean pH, they have found that there is considerable diurnal and seasonal variation, a result of variation in sunlight (photosynthesis), darkness (respiration), temperature, phytoplankton abundance, and upwelling of the coastal waters, all of which modify CO2 levels, and hence pH, of the water. This in itself is really interesting, for the extent of the variation is certainly unexpected.

But they also have found a declining trend in ocean pH levels over the eight years of the initial study – 2000-2007. Allowing for the various sources of CO2 variation, and applying some sophisticated statistical tests, they have concluded that the decline in pH is correlated only with increased levels of atmospheric CO2.

In fact, pH dropped 0.045 units over the 8 years, 2.5 times faster than simulation models had predicted. Not good news, but good data, and the first of its kind outside of the tropics.

Species with calcareous shells or skeletons are particularly vulnerable to erosion as ocean pH drops. Over the same time period, several well-studied intertidal species with calcareous shells or skeletons – two species of mussels, and goose barnacles – declined in abundance and mean size, while non-calcareous algae increased in abundance.

Blue mussels Mytilus californianus have declined in abundance and size in the past decade, correlated with the decline in ocean pH (eeb.ucsc.edu)

Why the drop in pH is so great remains unexplained, but further research has addressed the question of whether such a drop in pH is just natural variation, or whether it is new. Mussel shells can last a long time after the animal inside dies, and their age can be determined. They also carry in them a record of the pH of the water they formed in. They provide an extraordinary record to compare with the present changes, dating back not just to the 1960s but as much as 1340 years ago to the middens left by the Makah who fished from the island in summer.

Shells of the intertidal shield limpet, Lottia pelta, though more difficult to analyse, confirm what the mussel shells have shown (wsu.edu)

And the conclusions? For the past decade the ocean waters around Tatoosh are acidifying at a rate faster than predicted. Nothing like this has occurred in the past 1300 years. We clearly don’t know enough yet about the causes, but the only strong correlation is with increased atmospheric CO2.

With the long-term studies of Tatoosh, we have a chance to detect such changes in water chemistry and community structure, and predict their occurrence elsewhere. That’s good science.

Meanwhile, we are warned once again. The emerging new world is going to look a lot different.

Populations of sea birds – murres and gulls – nesting on the cliffs of Tatoosh have also declined by 50% over the past decade (nytimes.com)

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