Variable Stoichiometry Effects on Glacial/Interglacial Ocean Model
Biogeochemical Cycles and Carbon Storage
Realistic model representation of ocean phytoplankton is important for
simulating nutrient cycles and the biological carbon pump, which affects
atmospheric carbon dioxide (pCO2) concentrations and, thus, climate.
Until recently, most models assumed constant ratios (or stoichiometry)
of phosphorous (P), nitrogen (N), silicon (Si), and carbon (C) in
phytoplankton, despite observations indicating systematic variations.
Here, we investigate the effects of variable stoichiometry on simulated
nutrient distributions, plankton community compositions, and the C cycle
in the preindustrial (PI) and glacial oceans. Using a biogeochemical
model, a linearly increasing P:N relation to increasing PO4 is
implemented for ordinary phytoplankton (PO), and a nonlinearly
decreasing Si:N relation to increasing Fe is applied to diatoms (PDiat).
C:N remains fixed. Variable P:N affects modeled community composition
through enhanced PO4 availability, which increases N-fixers in the
oligotrophic ocean, consistent with previous research. This increases
the NO3 fertilization of PO, the NO3 inventory, and the total plankton
biomass. Surface nutrients are not significantly altered. Conversely,
variable Si:N shifts south the Southern Ocean’s meridional surface
silicate gradient, which aligns better with observations, but depresses
PDiat growth globally. In Last Glacial Maximum simulations, PO respond
to more oligotrophic conditions by increasing their C:P. This
strengthens the biologically mediated C storage such that dissolved
organic (inorganic) C inventories increase by 34-40 (38-50) Pg C and
0.7-1.2 Pg yr-1 more particulate C is exported into the interior ocean.
Thus, an additional 13-14 ppm of pCO2 difference from PI levels results,
improving model agreement with glacial observations.