CoccolitHophore controls on ocean ALKalinitY (CHALKY)

Each year in the North Atlantic Ocean, a key region for the global carbon cycle, immense areas of surface water turn turquoise in summer. This phenomenon relates to the growth and death of unique microscopic algae - coccolithophores. Coccolithophores cover their cells with scales of calcium carbonate (called coccoliths), produced internally and arranged into an exoskeleton around the cell. Under certain conditions, for example when nutrients are scarce or viruses infect cells, these coccoliths are shed in huge numbers. Due to their unique optical properties and immense abundance, they turn the water a milky turquoise colour and can be detected from space. These turquoise waters (termed 'white waters') are where coccoliths have accumulated in their trillions and have been considered as coccolithophore blooms.

Coccolithophores form coccoliths through calcification, which produces CO2 and reduces the pH of the ocean by consuming alkalinity. When coccoliths are lost from the surface ocean, it reduces the capacity of the ocean to absorb more CO2. In this way, 'white waters' are thought to lead to significant reductions in the ocean's carbon sink. However, we now suspect that these 'white waters' are not areas of intensive coccolithophore calcification or growth, rather they are regions of senescence and an accumulation of detrital material. Coccolithophores have been found to grow faster and calcify more outside of the 'white waters' and more recently we have found that they are also heavily grazed by small animals (zooplankton) who partly digest the calcium carbonate.

In this way, coccolithophore calcium carbonate appears to be recycled far more in surface waters than previously thought and the alkalinity they are associated with may be retained in the surface ocean. However, we have few coupled measurements of the balance of these different processes (growth, death and sinking) with which to take an informed view of how coccolithophores control ocean alkalinity. This represents a major uncertainty in the global marine C-cycle, with global C budgets and Earth System Models struggling to incorporate calcium carbonate or accurately replicate observations of seawater alkalinity.

The 'coccolithophore controls on ocean alkalinity' (CHALKY) project aims to fill this critical knowledge gap by quantifying the balance of coccolithophore production and loss processes and their impact on C-cycling and air-sea CO2 fluxes. Our assessment of ecological interactions and impacts on seawater chemistry will be carried out while improving in situ and remotely sensed optical detection of coccolithophores to allow us to use Earth Observation data to scale our insights to the global ocean and historically using existing satellite data sets.

CHALKY will, for the first time, concurrently quantify coccolithophore calcium carbonate production (consuming alkalinity), viral lysis (retaining alkalinity), zooplankton grazing (also retaining alkalinity) and sinking fluxes into the ocean's interior (removing alkalinity). We will look at the balance of these processes during the transition from late-spring to summer, when in situ and satellite data informs us that coccolithophores are most active. We combine a research cruise measuring these processes with autonomous platforms and state-of-the-art sensors measuring ocean chemistry and in situ optical properties. By quantifying the key growth and loss processes, within the context of seawater carbonate chemistry and C-cycling, CHALKY will inform a more accurate representation of how biology impacts the ability of seawater to absorb CO2, allowing closer matching of observations and models and inclusion of calcium carbonate in global C budgets.