More animal plankton after iron fertilisation?

04 Jun 2014 | 188

In the wake of a controversial iron fertilisation 'experiment' off the coast of Canada, plankton scientists have noted a higher than average abundance of copepods - crustaceans that form an important food source for many marine creatures higher up food chains, including fish.

During the summer months of 2012 a large scale iron fertilisation experiment took place across a patch of sea west of Haida Gwaii, British Columbia, in contravention of international agreements. Beginning with 90 tons of iron sulphate in July and followed by 10 tons of iron oxide in August and some days later a further 10 tons of iron sulphate and 10 tons of iron oxide, the three part iron fertilisation experiment was carried out with a view to improving salmon catches which had been dwindling in recent years.

Iron fertilisation has been proposed as a way of mitigating climate change through capturing carbon dioxide (CO2) from the atmosphere as phytoplankton (microscopic plants near the sea's surface) capture the CO2 for their growth, much as land plants use CO2 in photosynthesis to build tissues, so sequestering atmospheric CO2. The idea at Haida Gwaii was that, in addition to CO2 sequestration, the added iron would cause the plankton to bloom over a large area providing food for small animal plankton (zooplankton), which in turn would be eaten by larger zooplankton and eventually fish, such as the salmon; so increasing stocks. Such experiments are highly controversial, and the Haida Gwaii fertilisation has come in for much criticism, to the point of being subject to investigation by Environment Canada.

Fortuitous sampling

However, the experiment did take place and coincidentally the Sir Alister Hardy Foundation for Ocean Science (SAHFOS) was carrying out routine plankton collections using its towed Continuous Plankton Recorder (CPR) in the area before and after the iron was spread across the sea. This enabled comparisons of pre- and post-fertilisation plankton catches, as well as a look back in time to previous years' results. Sure enough the results differed significantly from previous surveys across the same routes in previous years. Satellite images showed that a strong phytoplankton bloom followed the iron fertilisation in August and September 2012. What the satellites couldn't spot, but the CPR could sample, was that later in the autumn larger zooplankton - namely copepods - were recorded at levels higher than ever before in the CPR time series for this time of year, while both large phytoplankton and microzooplankton were the lowest recorded. This would likely be because the copepods and other crustaceans had been feeding on the glut of phytoplankton and smaller zooplankton.

Whilst lead scientist Dr Sonia Batten of SAHFOS cautions that there could be other factors at play in this complex area of the Northeast Pacific, she remains confident that the unusual plankton community was caused by an unusual event - probably the iron fertilisation. But she keeps an open mind on whether iron fertilisation could lead to increased fish stocks: "This was not the subject of our research and whilst there was an increase in copepods, which salmon feed upon, the relatively small area as well as other factors of salmon life histories has to be considered."

"What it does show, however", she continued, "was that the CPR, which follows regular routes and times from year to year, can pick up changes that satellites cannot, or would be missed by short term research cruises. The CPR's strength is the time series it builds and the consistency of its sampling and this event has proven its reliability providing an intriguing set of observations."

Professor Nicholas Owens, SAHFOS Director agrees: "This just goes to show that the CPR can build a baseline of data which can illuminate specific events while also contributing to a wider understanding of our ocean globally. Satellites can provide a broad overview, but only if there is no cloud cover, and cannot provide information about zooplankton; research vessels are expensive and limited in the time they can remain in one location; the CPR can collect data year in, year out over long periods. This event has really underpinned its value."

SAHFOS scientist, Dr Sonia Batten, is based in Nanaimo, British Columbia and can be contacted at soba@sahfos.ac.uk, +1250 756 7747

SAHFOS Director, Professor Nicholas Owens, is based in Plymouth, UK and can be contacted at njpo@sahfos.ac.uk, +1752 633281

NOTES:

  1. The scientific paper with a full explanation of methods and results is published this week in the Journal of Plankton Research
  2. Sir Alister Hardy Foundation for Ocean Science (SAHFOS) is an international charity that operates the Continuous Plankton Recorder (CPR) survey. The Foundation has been collecting data from the North Atlantic and the North Sea on biogeography and ecology of plankton since 1931. More recently, as the Foundation has become more involved in international projects, work has been expanded to include other regions around the globe. The results of the Survey are used by marine biologists, scientific institutes and in environmental change studies across the world. The CPR team is based in Plymouth, England and consists of analysts, technicians, researchers and administrators, who all play an integral part in the running of the Survey.

The Foundation is a charity and company limited by guarantee. It depends on voluntary cooperation of the international shipping community. A consortium of agencies from nine countries, the EU and international organisations provide financial support.

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