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The effects of marine nitrogen-fixing cyanobacteria on ocean biogeochemistry and climate – an Earth system model perspective


Paulsen,  Hanna
Ocean Biogeochemistry, The Ocean in the Earth System, MPI for Meteorology, Max Planck Society;
IMPRS on Earth System Modelling, MPI for Meteorology, Max Planck Society;

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Paulsen, H. (2018). The effects of marine nitrogen-fixing cyanobacteria on ocean biogeochemistry and climate – an Earth system model perspective. PhD Thesis, Universität Hamburg, Hamburg. doi:10.17617/2.2598976.

Cite as: https://hdl.handle.net/21.11116/0000-0001-718D-1
Marine nitrogen (N2) fixing cyanobacteria provide a major supply of bioavailable nitrogen to the ocean’s euphotic zone. Furthermore, cyanobacteria organisms, largely positively buoyant, absorb light at the ocean surface and thereby modify the distribution of radiative heating in the water column. In this thesis, I investigate the role of marine cyanobacteria in the Earth system – both with respect to ocean biogeochemistry and with respect to the bio-geophysical feedback by light absorption – in present and high CO2 climate conditions by using the comprehensive Earth system model of the Max Planck Institute for Meteorology (MPI-ESM). To this end, I develop and implement a parameterization of prognostic N2-fixing cyanobacteria into the HAMburg Ocean Carbon Cycle model (HAMOCC), the global ocean biogeochemistry component of MPI-ESM. Including cyanobacteria as additional phytoplankton group considerably improves the representation of N2 fixation compared to the diagnostic approach used hitherto. Cyanobacteria growth (contributing ~7% to the global primary production) and N2 fixation (with a global value of ~135 Tg N yr −1) are confined to the tropical and subtropical ocean. Temperature, phosphate and iron limitation, which in addition to fixed nitrogen deficits determine N2 fixers’ growth, lead to a decoupling of N2 fixation from the upwelling areas of nitrogen-depleted water masses. Large-scale patterns of the relative abundance of surface phosphate to nitrate are improved in the new parameterization. The prognostic growth dynamics is capable of reproducing a reasonable seasonal variability of N2 fixation and furthermore enables the consideration of the potential response of N2 fixation to changing environmental conditions, such as seawater temperature, seawater pH and changes in atmospheric dust deposition. I furthermore include the prognostic cyanobacteria in the dynamic feedback from biological light absorption on the ocean heat budget in MPI-ESM. The simulations reveal that cyanobacteria shade and hence cool the subsurface water that feeds the shallow meridional overturning cells and that is upwelled at the equator and in the eastern boundary upwelling systems. This advective process outweighs the direct local heating by cyanobacteria light absorption and results in a net surface cooling effect in large parts of the tropical and subtropical ocean by up to 0.5 K. The regional surface cooling has implications for the climate mean state, such as a strengthening (~6%) and westward shift (~3 ◦ longitude) of the Walker circulation, as well as for climate variability, such as an increase in El Niño–Southern Oscillation variability (~16%). Including the dynamic feedback from bulk phytoplankton and cyanobacteria light absorption on the ocean heat budget in MPI-ESM reduces the tropical sea surface temperature bias and improves tropical Pacific variability compared to the standard model version which applies a globally constant light attenuation depth. Under rising CO2, i.e. in a scenario in which atmospheric CO2 increases by 1% per year, both phytoplankton groups (bulk phytoplankton and cyanobacteria) are projected to decrease in the tropical and subtropical ocean. The related increase in the penetration depth of light leads to upwelling of warmer subsurface water, which amplifies tropical surface warming regionally by up to 20% under quadrupling atmospheric CO2. In an additional scenario, in which potential physiological advantages of cyanobacteria under high CO2 are considered (such as a pH-dependent growth rate, temperature adaptation, and the uptake of dissolved organic phosphorus), cyanobacteria only regionally increase their abundance. This increase counteracts the decline in water turbidity in the eastern tropical Pacific and dampens the additional warming signal of the Pacific cold tongue. This thesis indicates the relevance of including N2-fixing cyanobacteria as phytoplankton functional type in Earth system models. First, cyanobacteria growth dynamics are needed to simulate N2 fixation and its potential future evolution. Second, cyanobacteria – and changes in cyanobacteria abundance – have a regulative effect on the tropical climate system via light absorption. Cyanobacteria hence introduce additional variability in the Earth system, especially in the tropical regions, which should indeed be accounted for.