SEASONAL VARIABILITY IN THE EFFICIENCY OF UPPER OCEAN CARBON EXPORT

The oceans are a major repository for atmospheric carbon. An important component of the global carbon cycle is the ocean's biological carbon pump (BCP), which is dominated by the sinking of organic particles from the surface ocean to its interior. Of the material generated via phytoplankton primary production in surface waters, most is recycled in the upper ocean. A small fraction is exported to the deep ocean and sequestered away from further contact with the atmosphere on timescales of hundreds to thousands of years. Both the size and efficiency of the BCP are predicted to decline globally in response to climate change, potentially resulting in reduced ocean carbon storage and hence increased atmospheric carbon dioxide levels. Therefore, accurately quantifying the magnitude and efficiency of the global BCP is essential to understanding the Earth's carbon cycle and the impact of continued anthropogenic inputs of carbon dioxide. However, current estimates of the strength of the BCP range 4-fold, suggesting that despite more than 30 years of study, no consensus on its magnitude has yet been reached. There is even more uncertainty about what controls the efficiency of the BCP and its variability on seasonal timescales.

Recently, a new parameterisation of the export ratio (which describes the efficiency of the upper ocean BCP) was developed by the PI and co-authors. This suggested that the BCP efficiency was substantially lower than previously thought. However, our parameterisation of the export ratio has relatively large uncertainty at cold sea surface temperature. The export ratio is thought to be driven in large part by the type of phytoplankton present in the upper ocean, because large, dense phytoplankton sink rapidly and export more efficiently than smaller plankton. Our hypothesis is that the variability in export ratio at low temperatures is due to strong seasonality in phytoplankton bloom evolution at high latitudes, driven by temporal shifts in phytoplankton community structure.

This project will assess how seasonal variability of the phytoplankton bloom alters the export ratio in the sub-Arctic through a combination of in situ and satellite data based studies. We propose to collate measurements of upper ocean particulate organic carbon flux and simultaneous phytoplankton community structure from two high latitude regions with suitably cold SST and strong variability in phytoplankton blooms. This project will use data to be collected on a UK Ocean Acidification Research Programme cruise to the Arctic (work already funded by NERC), and on a cruise to the Labrador Sea, for which funds are requested here. We propose to participate in an existing cruise in May 2012 (funded by DFO Canada) to take additional measurements of export flux and phytoplankton community structure. The variability in bloom stage and PCS encountered during the cruises will be used to determine the impact of seasonal gradients in bloom conditions on the export efficiency. We will then apply the understanding gained from the regional studies to a global database of export measurements, using satellite-derived data on sea surface temperature, bloom stage and phytoplankton community structure. We will then develop a revised parameterisation of the export ratio, including relevant seasonal information and in the final stage of the proposed work, apply our revised parameterisation globally to calculate a new estimate of the magnitude of the BCP. The project aims to gain understanding of controls on seasonal variability in the export ratio, and hence reduce uncertainty in the estimate of global BCP magnitude.

The key output of this project that is of interest to a wide audience is a deeper understanding of the role of the oceanic biological carbon pump in regulating Earth's climate. The results will be of use to climate scientists and biogeochemists, but perhaps of most direct interest to climate model developers. An improved parameterisation of the export ratio, resulting in reduced uncertainty in the magnitude of the biological carbon pump, will enable more accurate predictions of climate change impact on the global carbon cycle.

We can successfully engage the public and school children during the course of our project. The role of phytoplankton in regulating the atmospheric concentration of carbon dioxide is little appreciated by the general public. Our study can be used as a springboard for explaining the importance of phytoplankton to our planet's health.

This project also contains themes relevant to policy makers (e.g. through DEFRA, DECC), but downstream of the project. Results from the project will likely require the intermediate step of incorporating our results into coupled climate models to make predictions of oceanic response to global warming, before direct interaction with policy makers will be fruitful. We will however interact with intermediary bodies, such as the IPCC and UKCIP.