Strength of the ‘biological pump’ overestimated

A traditional method of estimating how life in the oceans removes carbon from the atmosphere has been overturned.

Dr Andrew Yool of the National Oceanography Centre, Southampton, working with colleague Dr Adrian Martin and researchers from the University of Concepcion in Chile and Plymouth Marine Laboratory (Camila Fernandez and Darren Clark) calls into doubt one of the most popular means of assessing the strength of the so-called `biological pump’, which removes carbon dioxide from the air to the deep ocean.

Writing in the science journal Nature this week (Thursday 21 June, 2007) Dr Yool, a research fellow at the University of Southampton’s School of Ocean and Earth Science, says: “For more than 20 years scientists have been using a simple formula to calculate how much ‘marine snow’ – dead plankton and detritus – leaves the upper, sunlit ocean waters and sinks to the deep ocean. When studying climate change, knowing how much carbon reaches deep water this way is important for understanding how much more can be stored in the oceans. Because it is difficult and time-consuming to measure sinking in the open ocean, the strength of the biological pump has routinely been estimated using a simple formula known as the f-ratio.”

Dr Yool’s team has demonstrated that a key assumption often made in using the formula is incorrect. The formula was first published in Nature in 1979 and has since been used extensively by scientists to extrapolate from small-scale experimental results estimates of what happens in the wider ocean. By making use of both research cruise data from around the world and an ecological model of the ocean’s nitrogen cycle, Dr Yool’s team was able to look more carefully at this assumption.

Phytoplankton (microscopic plants that have been dubbed the ‘grass of the sea’) require both nitrogen and carbon to grow. When they die gravity causes them to sink – the biological pump effect. In surface waters nitrogen mostly exists as one of two forms, nitrate or ammonium, traditionally thought to have different sources: ‘new’ nitrate arriving in surface waters by the upwelling of deep, nutrient-rich water, while ‘recycled’ ammonium is being produced when plankton die and decompose in surface waters. This means that by quantifying the amount of nitrate taken up by phytoplankton to grow scientists can estimate the sinking flux, since at equilibrium what goes down must come up. The f-ratio, which compares this nitrate-fuelled production to total production (including that fuelled by ammonium) is routinely used by oceanographers for the above reason.

But, according to Dr Yool, making this measurement might be a waste of time. He says: “Upwelling of deep water is not the only source of nitrate. There are two groups of bacteria that, by a process known as nitrification, convert ammonium into nitrate. It’s only recently been discovered that they do this throughout the water column, from top to bottom. Previously it was thought to be relatively unimportant in surface waters and was largely ignored.”

Since the f-ratio was first devised scientists have been improving their techniques to measure nitrification, and its importance has begun to be reassessed. If near-surface nitrification is significant then the f-ratio cannot be used to estimate the strength of the biological pump.

To examine the importance of nitrification, the group lead by Dr Yool brought together almost all of its oceanic measurements. They then used these to configure a global ocean model which contained a simple representation of the plankton ecosystem, and “ran” this model for 20 years. By using a double-accounting trick this simulation found that throughout much of the world ocean a substantial fraction of the nitrate taken up in primary production is actually generated through recent nitrification near the surface, and not through the re-supply of nitrate from the deep ocean.

At the global scale, they found nitrification to be responsible for around half of the nitrate consumed by growing phytoplankton. For scientists working on research cruises this means that more difficult measurements of sinking ‘marine snow’ itself may have to replace the simpler f-ratio experiments.

Dr Yool concludes: “These results suggest that it is not possible to quantify accurately the biological pump using only nitrate and the f-ratio. Since many scientists predict that the pump's strength may decrease in the future in response to climate change or ocean acidification, this work highlights the importance of developing more accurate ways to measure it.”