An article in press at Global Biogeochemical Cycles has shown that iron fertilisation can actually decrease the amount of carbon sinking to the ocean floor due to complex ecosystem processes.The iron fertilisation hypothesis was originally proposed as a rapid solution to climate change by increasing the photosynthetic uptake of CO2 by phytoplankton otherwise limited by their source of iron. Unfortunately, one of these climate change experiments was eaten by hungry crustaceans (see “Hungry Crustaceans Eat Climate Change Experiment”).

However, in another experiment, the scientists at the University of California at Berkeley continued to monitor the phytoplankton bloom and changes over an annual cycle with “Carbon Explorers”, floats that recorded data down to depths of 800 meters after the iron fertilisation experiment. These floats were placed both near and away from the iron induced phytoplankton blooms. Initially, these researchers discovered evidence in support of the Iron Hypothesis with a phytoplankton bloom leading to movement of carbon particles to at least 100m below the surface and this was reported in Science in April 2004.

Over the longer term the Carbon Explorers observed a different pattern which may be related to complex ecosystem processes that occurred during the following annual cycle. Despite the demise of the phytoplankton bloom the following winter, there was no carbon rain to match. In fact, there was greater particulate carbon falling at the site away from the original iron fertilisation. It turns out that the zooplankton survive the winter at depths below where the phytoplankton live due mixing of the oceans. Storms that cause this mixing create a conveyer belt of phytoplankton to the deeper dwelling zooplankton.

Larvae (zoea) of the spider crab (left) and the mitten crab (right) between 1 and 10 days old form part of the zooplankton ( 'hungry crustaceans').

If the water is continually mixed to depths with low light, then the phytoplankton do recuperate and the zooplankton eventually starve. At the site away from the iron fertilisation, the ocean mixing was intermittent and the phytoplankton were able to survive at the surface. The following spring, a bloom in phytoplankton fed the hungry zooplankton and led to increased carbon rain.

It seems that creating the right conditions for increasing oceanic carbon capture is in the hands of Poseidon and not something that can be easily predicted.

(Photograph courtesy of Flickr, zoea drawings from New Quay and UCSD)