Migrating copepods may handle ocean acidification better

Copepod species in the Arctic Ocean that migrate daily between the surface and the depths will cope better with ocean acidification than species that tend to remain near the surface. That’s according to researchers from the UK and Canada, who reckon the crustaceans that venture deeper are adapted to deal with a wider range of acidities.

“We wanted to know if, …[as] happens with temperature, the size of the range of pH or carbon dioxide that an organism naturally experiences can help us to understand if that organism has mechanisms that enable it to cope with changing conditions over longer timescales,” Helen Findlay of Plymouth Marine Laboratory, UK, told environmentalresearchweb.

This did indeed seem to be the case. Two vertically migrating Calanus species survived artificially increased carbon dioxide conditions better than Oithona similis, which typically remains near the surface, Findlay and colleagues from the University of Exeter, the University of British Columbia, the University of Manchester, and Fisheries and Oceans Canada discovered.

Calanus glacialis and Calanus hyperboreus, both native to the Arctic, move up and down around 200 m in the water column each day, experiencing a range of carbon dioxide partial pressures of 140 µatmosphere. Oithona similis, which stays in the top 50 m of waters around the globe, sees a carbon dioxide range of less than 75 µatmosphere. In the Arctic Ocean pH reaches a minimum, and the partial pressure of carbon dioxide reaches a maximum, at a depth of roughly 100 m.

As atmospheric levels of carbon dioxide rise, the ocean is absorbing more of the gas, altering its carbonate chemistry and lowering pH.

“The Arctic is rapidly changing,” said Findlay. “It already has areas that are potentially corrosive to marine calcifying organisms – areas of low pH and high carbon dioxide. But few studies have managed to collect data in late winter. At this time there is not a lot of food available, there is limited light, and the conditions are harsh. So understanding what marine organisms do during these potentially stressful times is really valuable to give us insights into how they might cope with future stressors.”

The researchers took samples of water and zooplankton through a hole cut in the ice in Deer Bay in the high Canadian Arctic in March and April 2011. They placed a selection of copepods and their larvae in bottles holding seawater adjusted to contain partial pressures of carbon dioxide of 370, 700 and 1000 µatmosphere, to represent current, mid-future and high-future scenarios. The bottles were hung in the ocean beneath the sea ice for seven days.

The nauplii – copepod larvae – of both species, as well as adult Oithona, did not take to these changes in their surrounding chemistry.

“The smaller copepod [Oithona] was not able to cope as well as the larger [Calanus] copepods when we changed the pH conditions,” said Findlay. “Importantly, the large adults did ok but the nauplii larvae of the adults – small, less active and non-migrating – also did not cope well with changes in pH in experiments. So lifestyle, and experience, can be used to determine how sensitive these organisms are to changing conditions.”

The team also reviewed the literature on the impacts of ocean acidification on other copepods. “All of the copepod experiments to date that show no response to carbon dioxide/pH were carried out on adults that have some element of migration,” said Findlay. “The small species and the larvae are most at risk. This needs to be addressed because these small organisms make up a huge component of the Arctic food web, and Oithona is a globally important copepod. If there is going to be a loss of smaller species we need to understand what that means for the food web, for fish stocks and ultimately humans.”

The team, who carried out the study as part of the Catlin Arctic Survey, reported their results in PNAS.

Liz Kalaugher, Environmentalresearchweb, 10 January 2014. Article.

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