Over the past 200 years there has been a vast increase in anthropogenic emissions of carbon dioxide (CO2) and there is strong evidence that this increase is tightly coupled with a warming global climate. CO2 released into the atmosphere is either absorbed by the ocean or the terrestrial ecosystem or accumulates in the atmosphere. It is estimated that a quarter of anthropogenic CO2 emissions have been absorbed by the ocean, a quarter by the terrestrial ecosystem, with half remaining in the atmosphere. Uncertainty in how the ocean will respond to future CO2 emissions is one of the major concerns facing Earth System science.

Phytoplankton in the ocean account for ~50% of global net primary production, through the process of photosynthesis. Together with physical factors, phytoplankton help modulate the total CO2 concentration of the upper ocean and the rate at which CO2 exchanges between the ocean and the atmosphere occur. Phytoplankton consist of thousands of species which are organised into communities, with differing requirements for environmental resources and with differing roles in the ocean cycles of carbon and other elements. These communities vary in abundance, distribution and physiology, and in their response to changing environmental conditions. Understanding how phytoplankton community structure is responding to variability in climate is key to unravelling how the ocean will respond to future changes in climate in response to continued CO2 emissions.

The DECIPHER project aims to develop a new satellite ocean colour algorithm designed to retrieve the chlorophyll concentration of three phytoplankton groups in the surface ocean waters. The satellite algorithm is to be applied to a 10-year ocean colour dataset and validated using independent in situ data. Using time-series analysis, seasonal and inter-annual variations in the phytoplankton groups over the 10-year period will be analysed and compared with physical data derived from satellite (e.g. sea-surface temperature). Relationships between phytoplankton groups and the physical data will be analysed to investigate how phytoplankton groups may respond to future physical changes in the global ocean.

Changes in phytoplankton community structure are likely to alter the biological utilisation of CO2 and hence the driving potential for CO2 air-sea gas transfer, with implications for bio-chemical air/sea interactions. Through quantifying the relationship between phytoplankton community structure and physical forcing, and extrapolating these relationships using different climate projections, the DECIPHER project will aim to provide a model-based assessment and prediction of past, present and future states of the marine ecosystem.